~erm~hronolo~ of the Cornub~~ batholith in southwest ...

Gwehintica et C~~~hirnic~ Acia Vol5.7.pp.I817-8I35 ~1~7037/93~.+~.a0
Copyri#O+t1993 PergamPomnsLtd. Printed in U.S.A.

~erm~hronolo~ of the Cornub~~ batholith in southwest Engine
Implications for pluton emplacement and protracted hydrothermal mineralization

J. T. CHESLEY, I,*A. N. HALLIDAY,’ L. W. SNEE,* K. MEZGER,1*3T. J. SHEPHERD,~and R. C. SCRIVENER’
’ ~~rnent of Geological Sciences, Unive~~ of Michigan, Ann Arbor, MI 48 109, USA
* USGS, Denver, CO 80225, USA
3 Max-Plank-Institut f& Chemie, &a&raw 23, D-6500 Mainz, Germany
4 British Geological Survey, Keyworth, Nottingham NG12 SGG, UK
’ British Geological Survey, Exeter EX4 6BX, UK

(Received May 25, 1992, accepted in r~~sed~~rm October29, 1992)

Abstract-The metalliferous ore deposits of southwest England are associated with biotite-muscovite

granites that intruded upper Paleozoic sediments and volcanic rocks at.the end of the Hercynian Orogeny.

The hydrothermal minerhation can be subdivided into four stages: ( 1) exoskams; (2) high-temperature

tin and tungsten oxide-bearing sheeted greisen bordered veins and S&earing tourmaline veins and

breccias, (3) polymetallic q~-tou~~n~~o~te-s~~defluo~t~~ng fissure veins, which repent

the main episode of economic mineraliz&on; and (4) late-stage, low-temperature polymetallic fluorite

veins. U-Pb dating of monazite and xenotime and 40Ar/ 39Ardating of muscovite were used to determine

emplacement ages and cooling times for individual plutons within the Comubian batholith, as well as

separate intrusive phases within the plutons. In addition, 40Arf 39Ar ages from hornblende and secondary

muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relationship between

pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmatism

was protracted from -300 Ma down to -275 Ma with no evidence of a major hiatus. There is no

systematic relation between the age of a pluton and its location within the batholith. The U-PI) ages for

separate granite phases within a single pluton are resolvable and indicate that magma emplacement within

individual plutons occurred over periods of as much as 4.5 myrs. Felsic porphyry dike emplacement was

coeval with plutonism, but continued to -270 Ma. The geochronologic data suggest that the Cornubian

batholith originated from repeated melting events over 30 myrs and was formed by a series of small

coalescing granitic bodies. Cooling rates of the main plutons are unrelated to emplacement age, but

decrease from the southwest to the northeast from -210°C myr-’ to -60°C myr-’ with a mean of

100°C myr-’ . These slow cooling rates appear to reflect the addition of heat from multiple intrusive

episodes. The mineraiization history is distinct for each pluton and ranges from coeval with, to up to 40

myrs younger than the cooling age for the host pluton. Stage 2 mineralization is broadly synchronous

with the emplacement of granite magmas, is dominated by fluids expelled during crystallization, and

may be repeated by the emplacement of younger magmas within the same pluton. Sm-Nd isochrons for

fluorite from stage 3 ~l~e~lic minerali~tion give ages of 259 + 7, 266 +: 3 and 267 + 12 Ma,

postdating stage 2 mineralization by up to 25 myrs within the same deposit. The similarity in age of the

main polymetallic mineralization hosted by the oldest and youngest plutons, suggests that this stage of

mineralization is unlikely to be related to hydrothermal circulation driven by the emplacement and

cooling of the host granite. The mineralization is more likely the product of regional hydrothermal

circulation driven by heat from the emplacement and crystallization of younger buried pulses of magma.

INTRODUCTION hydrothermal circulation associated with the deposition of
major ore deposits.
PITCHER ET AL. ( 1985 ) , defined a pluton as “a certain unity
of structure characterizing magmas emplaced within a single, The generally accepted model for granite-related hydro-
~ntinuous outer contact: so defined plutons can have resulted thermal mineralization assumes that pluton emplacement,
from single or multiple injection.” Many examples of piutons hydrothe~~ circulation, and mineral deposition take place
of a composite nature are available. Although there should over a geologically insignificant period of time (<lo6 yr)
be no genetic or chronologic connotation in the use of the (e.g., SKINNER, 1979; CATHLES,198 I, 1990; NESBIIT, 1990).
term pluron, such implications are common in the literature. This conclusion is based on (a) quantitative models which
Modern hip-p~ision ge~hronology may permit the direct suggest that in the presence of hydrothermal convection a
resolution of the emplacement history of individual plutons. pluton will cool in as little as lo4 to 10’ years ( CATHLES,
In addition, ages for the different stages of mineralization 198 1, 1990; NORTON and KNIGHT, 1977; NORTON and
may be used in conjunction with the emplacement and cool- TAYLOR, 1979); (b) the existence of porphyry copper deposits
ing history of the batholith to elucidate the chronology of on Pacific islands that formed over one to two million years
(SKINNER, 1979); (c) calculations of the lifetime of active
* Present address: Department of Geosciences, University of Ar- geothermal systems (e.g., WHITE and HEROPCXJLOS1, 986 );
izona, Tucson, AZ 85721, USA. and (d) isotopic age determinations of host plutons and as-
sociated mineralization which have been unable to resolve
differences in age (e.g., SKINNER, 1979; NORTON and

1817

1818 J. T. Chesley et al.

Perrrm-Triassicred beds
Devonian
Argilliles and Turbfftes
Ca~onl~rous
U~i~en~~~ Sediments and Vofcanfcs
Intrusfve Rocks
MafiiVofcanics
Coarse-GrainecfGranites (CGBG)
Medium-Fine Grained Granites
Oulline of the Cornubian Batholith‘-
Pre-BathoffthMetamorphic Rocks

Plutons -
(
CE lands End
T Tregonning-Godolphin
C Carnmeneffiffi
CM
cf3
SA
B
G
D
H

f Old Gunnisfake Mine

Wheat Remfry :e-Mine
i
Wheat Jane Mine
\ L

South Crafty Mine /
\
‘Hemerdon W-Mine

G&&arrow Clay Pit

04O
kilometers

50-h

L(b

FIG. I. (a) Generalized geologic map of the Comubian batholith in southwest England, modified from EDMONDS
et al. ( 1969) and WILLB-RICHARDSand JACKSON( 1989) and (b) sample location map.

CATHLES, 1979). However, the results from recent geochro- be an oversimplification and that granite-related mineraiiza-
tion may postdate the host granite pluton by 5 to 35 myrs.
nologic studies at the Panasqueira Sn-W deposit, Portugal
(SNEEet al., 1988) and the Cornubian metalliferous-province, Although models have been proposed for long-lived fluid cir-
England ( HALLIDAY, 1980; DARBYSHIRE and SHEPHERD, culation driven by elevated radioelement heat production in
1985; CHESLEY et af., 199 1) suggest that this hypothesis may southwest England (SIMPSON et al., 1979; DURRANCE et al.,

Magmatic activity and hydrothermal mineralization 1819

1982 ) , these models can only exprain lower temperature &rid WHrrE ( 1974), They intrude folded, weak& rn~rno~h~ De-
circulation and cannot account for the 200 to 400°C fluids
observed at either southwest England or at Panasqueira vonian and ~ar~niferous sedimentary and mafic igneous rocks, re-
(SHEPHERDet al., 1985 ). If hydrothermal circulation is indeed
long-lived or significantly postdates known plutonic activity, suhing in a thermal overprint and metasomatism of the country rocks
then we should reexamine some of the fund~en~ inter-
pretations regarding pluton formation and granite-related which produced a range of lithologies including homfels, tourmal-
minerahzation. For exampIe, what are the time spans and
mechanisms for the generation and emplacement of com- inized slates, and garnet-pyroxene-amphibole skams.
posite intrusives and silicic magmas? How important is the
duration of batholith and pluton emplacement in the gen- Several workers have classified the granites in the Comubian
eration of an ore deposit? What is the driving force for hy-
drothermal ci~ulation? is hydrothermal circulation episodic batholith on the basis of mineralogy, texture, and chemistry (e.g.,
or continuous?
HAWKESand DANoERnnD, 1978; EXLEYand STONE,1982; EXLEY
The classic Cornubian Sn-bearing granites of southwest et al., 1983; SforJE and EXLEY,1985;HILLand MANNING, 1987).
England (Fig. la) have provided a natural labomtory for the
development of many early theories of granite emplacement Xenoliths of granitic to granodioritic composition are found within
and related mineral deposition (e.g., PRKE, 1778; DE LA
BECHE, 1839; DAVISON, 1927; DINES, 1956). Welf-docu- Sameof the main plutons and are considered to representthe disrupted
mented geologic and paragenetic reIations make the Cor-
nubian batholith an ideal setting for a detailed geochronologic remains of earlier episodes of plutonism. Approximately 90% of the
study of the relation between mineralization and plutonism.
exposedgranite isa caarse-grainedbiotite-muscovitegranite (CGBG),
In this study, U-W isotopic ages of monazite and xenotime
are compared with @Ar/-Ar ages of hornblende and primary with turbid megacrysts of potassium-feldspar in a finer grained, equi-
muscovite (muscovite was determined to be primary using
the criteria of MrtLER et aI., 198 1) from skams and individual gram&r matrix of quartz, plagioclase, biotite, muscovite, and tour-
plutons within the Comubian batholith in order to assess
both the magmatic history of the batholith and the cooling maline, ~lthou~ voIumetricahy minor, four fater episodes of granite
rate of individual plutons. In addition, 40Ar/ 3gA~muscovite magmatism have been recognized (EXLEY et al., 1983): fine-granted
ages from mineralized veins and Sm-Nd isochron ages of
fluorite from the main stage of minemli~tion are used to biotite granite; megacrystic Li-mica granite; fine-granted L&mica
quantify the lifetime of the Comubian mineralizing sys-
tem( s). None of the current models for granite emplacement gmnite (topaz granite of MANNINGand HILL, 1990), and fluorite
and miner&z&ion adequately explain the emplacement,
cooling, and mineralization histories established by these iso- granite. The different granite lithologies are not observed within each
topic age determinations.
phtton, but all of the plutons are composite in nature.
The Comubian batholith wasemplaced at the end ofthe Hercynian
Orogeny (Carboniferous to Permian), which involved the continental Grosscutting some of the major granite ptutons are a series of ver-
collision of what is now northern and southern Europe (e.g., STONE tical to near vertical feisic porphyry dikes, iocahy &led eLvans. DAM
and EXLEY, 1985). The batholith has a strike length of 250 km and
a maximum width of 50 km, extending from Dartmoor in the north- GERFIELDand HAWKESf 1969) suggested that the dikes represent
east to the Isles of Scitly in the southwest (Fig. ta). There are five
major plutons and a number of satellite granite bodies which comprise contemporaneous felsic volcanism. Other workers have considered
the majority of the exposed batholith. The batholith consists pre-
dominantly of coarse-grained, peraluminous, biotite, and biotite- the elvans to be the last stage of magmatism in the Comubian batholith
muscovite granites similar to the S-type granites of CHAPPELLand
and separate the elvans (280-270 Ma) from the plutons (290-280
Ma) in time (DARBYSHIREand SHEPHERQ1985), or suggest that

the &vans represent crystagizedand degassed products extracted from

the main CGBG residuum derived from deep within the batholith

t IACKSCJNet al., 198% WILLIS-RKHARDSand JACK.%~NI,989 ). Mf

nor and trace element chemistry suggestthat the elvans are genetically
linked to the CGBG granites (DARBYSWREand SHEPHERDI,985 ).
Many authorsproposed a temporal link between the mineralization
and the emplacement of the elvans (e.g., HALLIDAY,1980, JACXWN
et at., 1982, 1989; DARBYSH~RaEnd SHEPHERD1,985), becausethe

elvans utilize the same fracture systems as the rn~ne~~i~ fluids

and indicate the presence of a magmatic heat source young enough

to produce high-temperature tin mineralization.

Mineralization is associated with multiple, steeply-dipping fractures

and lodes that crosscut both the granites and country rocks. The

lodes are as much as several km in length and average 0.5-J m in

width (JACKSONelal.,1989). Ore-bearing lodes typically exhibit

evidence of muhipte episodes of monetizing fluids, accompanied

by subtle changes in fluid chemistry. Early workers attributed mineral

zoning to simpie cooling of a fluid from a single emanative center

(eg,, DINES, t956), but later workers recognized the overlapping

polyascendent nature of the mineralization and the telescoped zoning
of the ores (e.g., GARNETT, 1965; JACKSONet al., 1982). Presently,

mast researchers recognize four major stages of granite-related min-
eralization (e.g., STONEand EXLEY, 1985; SHEPHERDet al., 1985;
Table I ):

1) The earliest recognized minemii~ation features are exoskarns

produced by metasomatism of peiitic shales and metabasalts dur-
ing granite intrusion (JACKSONet al., 1982; VAN MARCKE DE
LUMMENand VERKAEREN1, 985; SCMVENERet al. 1987). EXO-

Stage 1 Stage 2 Stage 3
Skarns.
Pegmatitic She&d Greisen &-Bearing Late Potymetallir Sulfide
Fe, Cu, Sn Deposits. BorderedVeins. Polymetallic Fissure VehlS.
“CroSScourSC”
TOurmafi~ Veins. Predominantly North-South.
Predominantly East-
Qz.B,ar., Dol., t&c., Fluor.
Bearing veins. Wat.

§n,WfMo B&W Sn, &o. Pb, 2% As,
Fe
Metats

Qz., Fsp., Chl.,
Hem., Fluor.

2tw - 4000

1820 J. T. Chesley et al.

skarn minerals include garnet, pyroxene, epidote, Cl-rich am- essary to obtain an accurate measurement of ( 1) the lifetime of mag-
matism within the batholith as well as individual plutons and (2)
phiboles, malayaite, vesuvianite, siderite, and axinite. The skams the time taken for the plutons to cool below temperatures where they
were no longer capable of driving 200-4OO”C mine~izing fluids.
contain economic concentrations of Sn, Cu, Fe, and As; fluid
The closure temperature for d&rent isotopic systems in dilferent
inclusion hom~en~tion temperatures (7-h) have been reported minerals provides a quantitative evaluation of the cooling history of
to mnge from 260-475’C. (JACKSONet al., 1982; SCRIVENERet an igneous or metamorphic temne. In order to resolve the cooling
rate of a pluton, the pluton must have cooled slowly enough so that
al., 1987). the elapsed time between ages for different minerals with different
closure temperatures is greater than the analytical uncertaintyin the
2) An earlyepisodeof mineralized pegmatites, greisen-bordered veins age. A detailed explanation of closure temperature theory and models
is available in DOLBON( 1973), GANGULYand Ruiz (1987) and
and Sn-tourmaline veins and breccias containing predominantly MCDOUGALLand HARRISON( 1988) .

tin and tungsten oxides; 2-sranges from 300°-5OO’C. Tourmaline Normally, U-Pb ages from zircons are used to provide the best
is coeval with the eassiterite and is the main gangue mineral. age of emplacement for a magmatic body. However, the Comubian
granites were formed by melting of a source with a significant sedi-
SHEPHERDet al. ( 1985) recognized a bimodal population of fluid mentary component (e.g., HAMIVONand TAYLOR,1983), therefore,
the zircons will likely contain inherited radiogenic Pb (see also Fig.
inclusions, which was interpreted as unmixi~ of early high-tem- 2). Monazite appears to lose Pb by volume diffusion during high-
temperature metamorphic or melting events (PARRISH,1990). Hence,
perature magmatic fluids into a low-salinity, high COz, W it Sn- inherited Pb is seldom observed in granites. Although there have
been no rigorous experimental determinations of the closure tem-
bearing thud and a high-salinity, low CO,, Sn-bearing fluid. perature for Pb, PARRISH( 1990) has estimated the closure temper-
ature to be 725” + 25°C from a variety of studies in metamorphic
3) The main episode of mineralization consists of polymetallic and igneous terranes. Estimates for the closure temperature for Pb
in xenotime are scarce, but it is consideredto be --6SO*C by HEAMAN
quartz-toutmaline-chlorite-sulfide-fluotite-hearing fissure veins and PARRISH( 199I). The presence of primary andalusite and mus-
co&e suggest that crystallization temperatures were ~750°C (e.g.,
which trend predominantly east-west. The veins contain Sn, Cu, KERRJCK, 1990). Therefore, a more accurate and reliable emplace-
Pb, Zn, Fe, and As-subides & SnOz and give T, values of 200- ment age can be obtained from U-Pb dating of monazite and xe-
notime. The U-Pb data for Th-rich minerals such as monazite com-
380°C (JACKSON.1977: JACKSONet al.. 1982: SHEPHERDet al.. monly am reversely discordant (e.g., PARRISH, 1990). This discor-
dance is generally thought to be the result of incorporation of excess/
i985; SHEPHERDand ~RIVENER, I987 ).Stage 3 veins represent unsuppo~~ %Pb that Formed from *?Th in~~mted during the
formation of the mineral (ScX&ER, 1984). In most cases, it has
the major period of economic mineralization in Cornwall. Gangue been demonstrated that the 20’Pb/2s5U ages can be accepted as geo-
logically correct ( SCHKRER,1984; PARRISH,1990). Therefore, only
minerals are dominantly chlorite with minor tourmaline. The the 207Pb/23sUage of the monazite is used for a thermochronologic
comparison of ages in this study. Monazite has been shown to retain
mineralogical zoning, high fluid inclusion homogenization tem- Pb during low-temperature melting events (COPELANDet al., 1988).
To evaluate whether any differences between the U-P6 monazite and
peratures and structures of these ore bodies, together with their *Ar/39Ar muscovite ages could be the result of inherited Pb, replicate
U-Pb analyses were made of different size euhedral grains from the
apparent spatial relations with the granite led to the traditional same monazite sample (Table 2). The ages are in good agreement
for the different size fractions and do not indicate inheritance of Pb
view that this s&ge of rnine~li~~on was related to emanative in any sample.

centers within the host pluton (DINES, 1956). Muscovite K-Ar closure temperatures have been estimated from
the literature to be 320” * 40°C (SNEE, 1982). SNEEet al. f1988)
4) The last stage of mineralization consists of polymetallic (Pb, Zn, inde~ndently estimated a similar tem~ratu~ for closure of mus-
covite to Ar diffusion of -32S”C, based on fluid incfusion filling
Ag, and U) quartz-fluorite-barite-sulfide veins, locally termed temperatures at the Panasqueira mine. Using regressions of both lab-
oratory and geologic data, HARRISON( 1981) and MCDOUGALLand
crosscourses. Ruid inclusions yield r, values of 105-180°C HARRISON( 1988) calculated a K-Ar closure temperature of 525”
rt 25°C for hornblende in a pluton cooling at a rate of fOO”C myr-’
(JACKSON, 1977; JACKSONet al., 1982; SHEPHERDet al., 1985; and demonstrated that the closure temperature For rapidly cooled
homblendes ( 1000°C myr-‘ f would be higher (-580°C). One of
SHEPHERDand SCRIVENER,1987). These fluids are similar in the strengths of eAr/39Ar dating lies in the ability to resolve an age
using step-wise degassing to determine a pfateau or the average ofa
composition to Na-Ca-Cl-rich deep sedimentary brines and have portion of an age spectrum whose ages overlap at the 2~ level. The
been suggested to be the result of fluid expulsion from adjacent majority of the samples presented here exhibit an age plateau using
the widely accepted and published definition for the term (e.g., DAL-
Mesozoic sedimentary basins (ALDERTON,1975,1978; SHEPHERD RYMPLEand LANPHERE,1974, RECK et a!., 1977). In order to com-
pare the 40Ar/39Ar plateau-ages with those of other muscovites and
et al., 1985). U-Pb ages of monazite or xenotime, the plateau criteria have been
expanded to conform to those of SNEEet al. ( 1988) and the errors
An economically important feature of the southwest England are reported at the 95% confidence level. Samples that failed the
granites is the widespread but localized presence of intense hydro- criteria for a plateau were also reduced using the isochron (YORK,
thermal kaolinization (china clay). Kaolinization is centered on the 1969) and correlation diagram methods ( RODD~CKet al., 1980).
western portion of the St. Austeli granite, southern Dartmoor granite
and smaller workings on other plutons. The age and genesis of the Several biotite Fractions were analyzed for 40Ar/39Ar. Most of the
kaolin deposits is still a matter of debate (e.g., SHEPPARD, 1977; data define hump-shaped age-spectra that do not fit the criteria For
DURRANCEet al., 1982; EXLEYet al., 1983), but they are commonly a plateau. The total gas age is similar to the plateau-ages for corre-
thought to be the result of meteoric fluid circulation during the late sponding muscovites, but in most cases it is slightly older. Many of
stages of cooling of the Comubian batholith. the biotites exhibit weak chloritization in thin section; despite efforts
to obtain clean biotite separates, chlorite may be present in the sub-
Stage 1, 2, and 3 mineralization have been directly attributed to
the intrusion and cooling of the Comubian batholith. Although the
stage 4 low-tern~mtu~ fluids may not be linked directly to intrusion
and cooling of the batholith, a model involving low-temperature,
regional fluid-convection driven by the high-heat production of the
granite batholith has been proposed (DURRANCEet al., 1982).

Previous geochronologic studies of the Comubian batholith and
associated mineral deposits are numerous and exhibit a high variability
in the measured ages (see WILLIS-RICHARDSand JACKSQN, 1989,
for summary of ages). A detailed Rb-Sr study of the major and minor
piutons by DARBYSHIREand SHEPHERD( 1985, 1987) established a
two-stage emplacement for the batholith, with an age of 290-230
Ma for major granite magmatism and an age of 280-270 Ma for the
minor rhyolite porphyry dikes and chemically specialized granites.
Mineralization ages from the Sn-bearing lodes also exhibit a high
degree of scatter ranging from 294-259 Ma (see WILLIS-RICHARDS
and JACKSON, 1989, for summary of ages). Rubidium-strontium
fluid inclusion ages of 269 + 4 Ma (DA&SHIRE and SHEPHERD,
1985f and Sm-Nd fluorite aaes 259 + 7 Ma CCHESLEYet al.. i 99 1)
have been reported for the St&e 3 mineralization in the SoutbCrofiy
mine and postdate the host Cammenellis pluton by 30-40 myrs,
suggesting that mineralization must be Linkedto later igneous activity.
“Ar/39Ar and K-Ar ages for clay concentrates suggest that lower
temperature hydrotherrmu solutions were active during the Mesozoic
and Cenozoic (JACKSOeNt al., 1982).

SCXENTIFIC ~ET~O~~Y

To determine the emplacement and cooling history of the Cor-
nubian batholith and its effect on hydrothermal circulation, it is nec-

Magmatic activity and hydrothermal mineralization 1821

- 2M3Pb 0049 subsequently washed in boiling deionized water for 2-3 h to remove
surface contamination. Representative subsamples of the hand-
23qJ . picked separate were examined on the microprobe with Energy Dis-
persive System (EDS) and Back-Scattered Electron (BSE) imaging
0.042 to assure identity and purity. After spiking with a mixed *3sU-~sPb
tracer the samples were dissolved in distilled 6 N HCl or concentrated
L’‘260 Din7 = Dartma+187 -e HF and 7 N HNO,, in a 3 ml Teflon@ vial inside a Rrogh-style
Diw - Datlmoa-188 monazite Teflon@ vessel at 210°C for 3-5 days. The Pb. was purified using
standard HBr-HCl chemistry on BioRad AGl-X8 200-400 mesh
H-H-monaziie resin. The U was purified using HCI-HNOs-HZ0 chemistry on the
same columns (TILTON, 1973; MA~N~N, 1986;MANHESet al.,
0.048 l- I I I I , I I I, I 1984; PARRISHet al., 1987). Samples were corrected for common
Pb using the feldspar analyses Of HAMFTON and TAYLOR( 1983) for
0.30 0.32 0.34 the different plutons in the Comubian batholith. Due to the high
206Pb/M”Pb ratios of most samples the effect of the common Pb cor-
=Pb rection is negligible. The measured ratios were corrected for a total
procedural Pb blank of 40-200 pg. Isotopic compositions of Pb and
23s~ U were determined in the static mode on a VG Sector mass spec-
trometer equipped with an ion~unting Daly detector. The repro-
Fro. 2. Concordia diagram for monazite, xenotime and zircon ducibility of the =‘Pb/z%~ ratio for the standard was 0.06% (au).
separates from plutons in the Comubian batholith. Error ellipses are The U total process blanks were less than 10 pg during the period of
drawn using 2a error, analysis. The 2u uncertainty of the U-I% and zo7Pb/206wages include
the reproducibility of the standard, common Pb, and blank corrections
microscopic level. The UDAr/39Aranalyses of biotite samples are re- as well as within-run uncertainties and the uncertainties in the U/
ported, but are not used for interpretation of the thermal history of Pb ratio of the spike.
the Comubian plutons.
Minerals for *Ar/39Ar analysis were separated from the same
Sample locations am plotted on an outline map of the Comubian crushed samples used for U-Pb destinations using standard metb-
granites (Fig 1b). A list of sample locations and lithologic descriptions ods. Subsequently, the separates were hand-picked under a binocular
can be found in Appendix I. microscope to better than 99% purity. Between 10 and 40 mg of each
sample was enclosed in an Al-foil capsule and placed along with
ANALYTICAL TIXXINIQUES MMhb- 1 hornblende standard (SAMSONand ALEXANDER,1987 ),
CaFz and K2S04 in evacuated quartz vials. The samples were irra-
Monazite, zircon, and xenotime were separated from crushed diated in two separate packages at 1 MW for 30 and 40 hours, re-
granite specimens using standard separation techniques. Between spectively, while being contin~~y rotated in the central thimble of
twenty and fifty grains were hand-picked under a binocular micro- the TRIGA research reactor at the USGS in Denver, Colorado ( DAL-
scope on the basis of size, color, lack of inclusions and shape and RYMPLEet al., 1981) . The MMhb 1 monitor was distributed evenly
within the quartz vials in order to obtain a precise monitoring of
both the horizontal and radial flux gradients; every sample was ad-
jacent vertically to at least one standard. Data from MMhb 1 monitors
show an error in the J-value of 0.1% ( t a) for both packages. The
samples were anafyzed in the static mode on a MAP 2 15 mass spec-
trometer with a Nier-type source at the USGS in Denver (GEISSMAN

Table 2. U-Pb Analytical Results Abde n~ttw

S*mple S LllLOlgf Dl*meter U Pb 2+Spb 2SSp b 2”Pb 20 2*Sp)j 20 2S7pb 2a 2SSpb AgutNa)
IMlrer8lq 0-w ppm ppm K I,cp 1%) 233fJ (*f 23s~
Lands Eml zrlpb i%) 23SU 20 2S7pb 2a 2S7pb
flC-1353-W
JfG135L191 q 2rdP b
JTC.1363-89
llGML89 FciBo (Xc) so-m c 51u 274 3916 0.34% 0.05 I85 ( 0.08 ) 0.0442 ( 0.25 ) 0.3166 f 0.26 1 279.3 0.7 ) 279.3 I 0.6 1 279.2
Cwsmenclllr 2210s 959 19535 o.Olx7 O.OJll4 ( 0.07 ) 0.0442 i 0.25 j 0.3164 i 0.26 j 279.3 0.7 ) 279.2 [ 0.6 ) 278.6
rrC-s3-89 W] I30-200c. 2579 413 1664 3.172 o.ow7 ( 0.10 ) 0.0435 ( 0.25 ) 0.3109 ( 0.27 ) 274.9 0.7 ) 274.9 ) 0.6 ) 275.3
CaE uw ( 0.10 ) 0.0436 ( 0.27 ) 0.3105 ( 0.29 ) 0.7 ) 274.6 1 0.7 ) 268.8
20.50, c 6778 RI 2495 2.350 0.05162 275.3
1&l 6fuoo.e

ffiffi IMZI 329 46.0 761.2 2.6% 0.05223 f 0.20 ) OB429 ( 0.26 ) 0.3095 f 0.31 ) 271.3 0.7 ) 273.8 [ 0.8 ) 295.5
O.OS2l7 ( 0.12 f 0.0452 ( 0.24 ) 0.3254 ( 0.28 ) 285.2 0.7 ) 266.1 f 0.7 ) 293.0
lu7S 234 1236 4.477 O.OS252 ( 0.1s f 0.0467 ( 0.25 ) 0.3351 ( 0.29 f 294.4 0.7 ) 293.5 [ 0.7 f 286.4
4M6 874 157s 3.345 0.05223 ( 0.24 ) 0.0466 ( 0.29 ) 0.3357 ( 0.37 ) 293.8 0.8 ] 294.0 [ 1.0 ] 295.7
6069 1414 2612 4.925

fTC.276M-89 ikbi 782 130 3443 3.220 0.0517l ( 0.10 ) 0.044S ( 0.26 ) 0.3200 ( 0.28 ) 283. I 0.7 ) 282.0 ( 0.7 ] 272.9
fTC276LS9 281.6 0.7 ) 281.6 ) 0.7 ) 282.0
t&l rm sin 1622 3.3% 0.05192 ( 0.10 ) 0.0146 ( 0.25 ) 0.31% ( 0.27 ) 281.8 0.7 ) 281.7 ] 0.7 ) 280.9
JTG276G89 69s4 451 2719 0.6087 OMl89 ( 0.08 ) 0.0446 ( 0.26 ) 0.3197 ( 0.27 ) 0.8 ) 284.3 1 0.9 ) 307.9
I7c~276T-89 ra 670.0 MD7836 0.05249 281.5 0.7 ) 277.8 f 0.8 1 292.0
JTC-276B-89 3EL 15.5 9S6.3 O.lKss2 0.65215 ( 0.19 ) aD446 ( 0.28 ) 0.3230 ( 0.35 ) 276.1 0.9 1 278.7 1 0.9 ) 292.2
Badmln Moor E w 10.7 I423 O.&X%9 0.65215 ( 0.17 ) 0.0437 f 0.7.S ) 0.3146 ( 0.32 ) 277.1
731 31.1 f 0.12 ) oa439 ( 0.34 f 0.3158 ( 0.37 )
JIG312-89 IW
-aG 3014 571 678.0 3.444 0.05205 ( 0.14 ) o.D463 ( 0.27 ) 0.3324 ( 0.30 ) 291.9 0.8 ] 291.4 [ 0.8 I 287.9

2574 3% 2211 2.664 O.OSIM ( 0.10 ) 0.0440 ( 0.32 ) 0.3151 ( 0.34 ) 278.1 [ 0.9 ) 278.2 [ 0.8 1 278.f
7% 128 146.6 2.639 O.OSl8S ( 0.31 ) 0.0445 ( 0.25 ) 0.3183 ( 0.47 ) 280.8 ] 0.7 )
280.6 ] I.2 I 279.0
( 0.M f 0.3270 ( 0.32 )
Hemadoo Cala (MS) 2MQI 1663 174 1215 3.067 0.03229 f 0.11 ) 0.0453 285.9 [ 0.1 ) 287.3 f 0.8 ) 2911.3

nwt8EP) bblbgp%;U3BlT=PtSS-&mW~~

S-e-0 ~lnEthoo0
2@+b@f’b
lqddlbh.
-lad mdos. AU aboc imU@ mdm cmmacd for blankspike. ammo1 hd md fmaimuim.

1822 J. T. Chesley et al.

Lands End

!80
260
280
260
280
260

260

I II I I II II II
0 20 40 60 60 100

Percentage 3gArKReleased
(4

Carnmenellis

280
260

260

280 R2480

260

280
260

280
260

260

JTC-242a CGBG

III

0 20 40 60 80 100 20 40 60 80 100

Percentage 3gAr, Released
(b)

FIG. 3a-g. 40Ar/3PAr release spectra for primary and secondary muscovite (must), biotite (bio) and hornblende
(hbld) from plutons, skarns and mineralized veins in southwest England. CGBG = Coarse.-grained biotite-granite;
FGBG = line-grained biotite granite; EBG = equigranular biotite granite; GBV = greisen bordered vein; TB= total
gas age; TP= Total plateau age; all spectra are plotted using the 20 error in the age; see text and Table 3 for explanation.
Data for the Ar analyses will be available as a U.S. Geological Survey Bulletin (in prep.).

Magmatic activity and hy~othe~a~ mineralization 1823

St Austell

280 20 40 60 80 100 20 40 60 80 100
(ct
260
Bodmin Moor
D
5260

8 260
a
E
2 280
%.
Q.260
a

280

260

0

280 JTG3m.es JTC.200.33

280 -T,, II 288.1 f0.9 Tp1283.3 f0.9

k II I 1I muvI
0300 I I I I I I 8 Q1
280 /*
60 GBV
260%
.nxx~z-ea JTC.10788
LsI Tp -293.1 f 0.8
1, I 287.1 f 0.9 mur
- 80 100 , L, I t I I 1

LI I 20 40 60 80 100

20 40

Percentage 39Ar, Released
W @I

‘Dartmoor CQEG

Skvn

280 _ JTC449 I I I LI I _ Tp.279.9f1.0 Hemerdon

260 - Thbpl-d28Q3f1.0 CGBG I I t I I tI_

. *I El8MS Comdy~cicGr~ g CQBG

280 - JTC-199.99 - Tg-272.3f0.9

Tp-270.Sf0.8 - NIC

rmy:

260 JTC-18749 #, 3$I / I I **I “r, , , I * & I L
- 271.5 f 0.9
280 _ CGBG FGBG Dika E GBV
26Oi- Tmp ust
_-- JTTpC.2-7271.21.89kt.0 5 260 r, - 296.2 * 0.9
0 20 40 42 260. . MM
I Wo, 0 I IIII
60 80 100 20 40 II II I II II I
60 60 100 20 40 60 80 100
Percentage 39Ar, Released
Percentage 39Ar, Released

(f) (s)

FIG. 3. (Continue)

et al., 1992 ) . Analyses were made on a single faraday colkctor ; de- ANALYTICAL RESULTS
tection limit during the period of analvses was 2 X lO_” moles Ar.
Corrections were apPliedfor inter5enGea from the reaction products The results of U-Pb, *Ar/39Ar, and Sm-Nd analyses are
of Ca, Cl, and K and from the decay of “Ar. Uncertainties for all presented in Tables 2, 3, and 4. The U-Pb data are plotted
aaes are auoted at the 20 confidence level and include the estimated on a standard concordia diagram (Fig. 2); a U-Pb analysis
errorin j. is considered concordant only if there is substantial overlap
with concordia at the 2u level. Argon age-spectra are plotted
The techniques for the Sm-Nd 5uorite analyses are given in CHES- in Fig. 3 and the Sm-Nd isochrons are plotted in Fig. 4. A
LEY et al. ( I99 1). All data were reduced using the decay constants summary of the U-Pb, *Ar/39Ar, and Sm-Nd data from this
and isotopic abundances recommended by STEIGER and JAGER study for the Comubian ba~olith is presented in Fig. 5.
( 1977) and an age of 520.4 Ma for MMhb-1 (SAMSONand ALEX-
ANDER, 1987). Isochrons, concordia plots, and weighted average cat-
cuiations were performed using ISOPLOT (LUDWIG, 199 1).

I824 J. T. Chesley et al.

0.5123 Ma and 272.9 + 0.8 Ma, respectively. The difference in U-
Pb ages, the estimated lower closure temperature (-650°C)
f-.P for xenotime compared to monazite (HEAMAN and PARRISH,
B 1991), and the similarity in 40Ar/39Ar ages suggest that the
muscovite from the fine-grained granite has been reset or was
0 above the Ar closure temperature at the time the main CGBG
was intruded. Combined with reports from a drill hole which
MSWD I 0.514 passed vertically through the fine-grained granite into the
main CGBG (REID and FLETT, 1907), these results indicate
0.1 0.2 0.3 that the fine-grained granite is probably a roof pendant of an
earlier igneous phase. A total of five muscovite samples from
‘47Sm/‘UNd the Lands End pluton were analyzed by the 40Ar/39Ar
method: two from the main CGBG ( 136, LE47625), and
FIG. 4. Sm-Nd isochron diagram for the Geevor Mine D-lode. the fine-grained granite dikes (94,247); one from fine-grained
The fluorites are plotted as 2a mean error bars; seetext for discussion. granite ( 135 ). All of the samples overlap within error and
yield a weighted average age of 272.4 + 0.4 Ma. The similarity
Lands End Pluton in cooling ages throughout the pluton, in the aureole and at
depth may indicate regional cooling of the Lands End area
Two samples were analyzed for U-Pb from the Lands End possibly produced through cooling promoted by hydrother-
pluton: a fine-grained biotite-granite (sample 135) and a mal convection. A cooling rate of -210°C myr-’ can there-
coarse-grained biotite-granite (CGBG), typical of the Lands fore be calculated from the age difference ( 1.9 myr) between
End pluton (sample 136). The fine-grained biotite granite, closure of monazite to Pb at -725’C and muscovite closure
4 X 6 km in outcrop, is considered to be either a younger to Ar at -325°C. If the fine-grained granite dikes are coge-
intrusion equivalent to the fine-granted granites observed to netic with the crosscutting granite observed in the Geevor
crosscut the CGBG at depth in the Geevor Mine (MOUNT, mine, then a minimum emplacement age for the fine-granted
1985 ), or alternatively a roof pendant of earlier granite within granite is implied.
the main CGBG of the Lands End pluton. The weighted
average zo7Pb/235U age is 279.3 ? 0.4 Ma for xenotime from A comparison of our results with the Rb-Sr data provided
the fine-grained granite ( 135) and 274.8 f 0.5 Ma for mon- by DARBYSHIREand SHEPHERD,( 1985,1987) for the batho-
azite from the main CGBG ( 136). Although the U-Pb ages lith provides a test of their original interpretation and con-
differ by approximately 4 myrs, the corresponding muscovites
give indistinguishable 40Ar/39Ar plateau-ages of 272.3 + 0.9

300
290

270

260

250

..an - - CRC

m Emplacement Age (Monarite or Xenotime U-Pb). m Stage-l Skarn (Hornblende “Ar/“Ar)

m Cooling Age (Muscovlte’h~Ar). P Stage-2 Veln mineralization (Muscovite “Arl %r)

@ Elvans (Inferred age Yuscovlte fr/ =Ar) Stage3 Veln mineralization (fluorite, Sm-Nd)

1 I

FIG. 5. Compilation of all geochronologic data (this study) for the Cornubian batholith. The symbols include the
20 error in the age.

Magmatic activity and hydrothe~al mine~li~tion 1825

tributes to the understanding of the relationship between Rb- likely due to the incorporation of excess ‘%. The weighted
Sr, U-Pb, and 40Ar/39Ar isotopic systems. They obtained a average of the m7Pb jZ3%,Jages is 293.7 + 0.6 Ma and is taken
whole-rock Rb-Sr isochron age of 268 + 2 Ma for the Lands as the best estimate for the emplacement age. Data for two
End granite. Based on a Rb-Sr whole-rock isochron age for separates of monazite from sample 53 are discordant. How-
the Wherry elvan of 282 -+6 Ma, which is thought to crosscut ever, the 207Pb/206Pb ages of -293 and -295 Ma are in
the metamorphic aureole of the Lands End granite and a Rb- excellent agreement with the emplacement age determined
Sr mineral-age of 279 + 4 Ma from a crosscutting greisen for the Cammenellis granite. These monazites most likely
bordered quartz vein in the granite ( HALLIDAY, 1980), represent partial disturbance associated with the stage 2 and
DARBYSHIREand SHEPHERD(1985) concluded that the Rb- 3 hip-tem~rature hydrothe~~ fluids at the South Crofiy
Sr systematics of the Lands End granite had been reset from Mine.
an older age (290-280 Ma). However, the younger monazite
and xenotime U-Pb ages and agreement with the 40Ar/39Ar The muscovites from samples 8 1 and 242 give ages of 290.7
muscovite ages with respect to closure temperature, suggest ? 0.9 and 290.0 + 0.8 Ma, respectively, and correspondingly
that the Lands End granite was not reset from an older age. yield a cooling interval of 3.1 and 3.3 myrs ( - 125°C myr-’ )
from emplacement to muscovite closure for Ar. Sample 239
Four samples from mineralized veins or lodes which con- is from a tine-grained granite dike and yields an age of 288.7
tain secondary muscovite were analyzed from the Geevor + 0.8 Ma. Samples 57 and 83 were collected in the Cam
Mine and the Bostraze china clay pit. The secondary mus- Brea granite in close proximity to elvan dikes in the South
covite is generally anhedral and occurs as a replacement of CrofIy mine and give younger @Arf 39Arplateau-ages of 286.5
K-feldspar, as replacement rims on biotite and within quartz- t- 0.9 Ma and 285.4 + 0.9 Ma, respectively. Both ages overlap
tourmaline veinlets. A secondary muscovite from a high and are younger than the cooling age of -290 Ma for the
temperature (400-46O”C, JACKSON et al., 1982), sheeted Cammenellis pluton suggesting that they were reset by the
greisen-bordered vein in the Bostraze clay pit ( 177) gives a emplacement of the elvan. Sample 243 from the Cam Marth
plateau-age of 272.3 -+ 0.9 Ma (Table 3), which is indistin- granite gives a plateau age of 286.0 f 0.9 Ma. This age suggests
guishable from the muscovite cooling ages for the host main a separate cooling history postdating that of the Carnmenellis
CGBG (272.7 + 0.8 Ma). Although the sample passes the pluton by about 4 myrs. The idea of a separate intrusion is
criteria for a plateau, the total spectrum exhibits some features supported by the field relations (HILL and MACALISTER,
that may indicate disturbance (Fig. 3a), possibly by the lower 1906) and the more evolved geochemistry (CHAROY, 1986)
temperature clay forming event. The age most likely repre- of the Carn Marth granite. Sample R2480 is from an equi-
sents a minimum age for the stage 2 mineralization contem- granular granite of unknown dimension intersected in the
poraneous with the cooling of the host granite below mus- Hot Dry Rock deep borehole (Rosmanowas quarry) in the
covite closure to Ar. Secondary muscovite from altered host Carnmenellis granite. The muscovite gives a plateau age of
CGBG on the contact of the Simms lode ( 107), the CGBG 285.7 + 0.9 Ma suggesting significantly slower cooling or a
on the hanging wall contact of the Coronation lode ( 147) separate pulse of granite magma.
and the crosscutting, fine-grained granite along the Coro-
nation lode ( 129) in the Geevor mine yield slightly younger In contrast to the Lands End granite, it is clear that the
plateau-ages of 270.4 f 0.9 Ma, 270.5 f 0.8 Ma, and 268.4 Cammenellis pluton underwent a more protracted cooling
rt 0.8 Ma, respectively, and are distinct in age from primary history which was affected by multiple intrusive events post-
muscovites from the host pluton. The overlapping nature of dating initial emplacement and cooling. The geochronology
the minemli~tion f GARNETT, 1965; JACKSONet al., 1982) supports field observations by GHOSH ( 1934) who suggested
and the distinct ages imply that stage 2 and stage 3 miner- several discrete episodes of granitic magmatism in the Cam-
alization was long-lived and occurred over a minimum period menellis area. As in Lands End, there is an excellent agree-
of 2-4 myrs. The age for stage 3 polymetallic mineralization ment between the *Ar/3gAr plateau-ages (289.2 f 0.8 and
is corroborated by a fluorite Sm-Nd isochron age of 267 290.3 f 0.9 Ma) and the Rb-Sr mineral isochron ages (287
-?r12 Ma (2~) (Fig. 4). The above results are also consistent -+ 2 Ma. and 290 4 2 Ma) for samples Cl and C6 on Cam-
with a previously determined Rb-Sr isochron mineralization menellis from the study of DARBYSHIRE and SHEPHERD
age of 270 + 2 Ma (JACKSON et al., 1982). ( 1985), but the whole-rock Rb-Sr isochron age (285 + 19
Ma) has a large uncertainty. The 40Ar/39Ar cooling age of
Carnmenellis Pluton the Carn Marth granite is significantly younger than the Rb-
Sr whole-rock age (298 + 6 Ma) from DARBYSHIREand
The Carnmenellis pluton is composed of several distinct SHEPHERD( 1987). The Cam Marth pluton contains abun-
phases ofgranite int~sion (GHOSH, 1934) and has two small dant xenocrysts of earlier granite, therefore the Rb-Sr age
satellite intrusions, the Cam Marth, and Cam Brea granites may reflect ~on~mination from the earlier event.
(Fig. la). The Cam Brea granite is observed to be in contact
with the Carnmenellis pluton at depth (TAYLOR, 1969) and Secondary muscovite taken from a stage 2 tourmaline-
the Cam Marth granite is considered a separate intrusion quartz vein from drill core (44) in the South Crafty Mine
(HILL and MACALISTER,1906). Two monazite samples from gives a 40Ar/39Ar plateau age of 286.6 ? 0.8 Ma, significantly
quarries ( 8 1,242) in the Cammenellis granite and one from younger than that for the magmatic muscovites in the Cam-
the South CroRy Mine in the Carn Brea granite (53) were menellis pluton (Table 3), but in excellent agreement with
dated by the U-Pb method (Table 2; Fig. 2). The age of the inferred age for elvan emplacement in the South Crofty
sample 81 is concordant, but thk analysis of sample 242 is Mine and the cooling ages for the Cam Marth and Rosma-
reversely discordant beyond analytical unce~inty, most nowas intrusives. Muscovite from a vein selvage in a base-
metal pyroxene-garnet-bea~ng skarn intersected -900 m

1826 J. T.Chesley et al.

Table 3. doAr/agAr Data

Sample II Lfthology Total PI~lc~u EWW %39Ar Steps in
Mincr~l’ Gas Age Age (2U)‘t in p*1tru pl~lcsu

Lmlds End PGBGD m 271.5 271.8 f 0.8 88.9 6
Gmdtrd KxiG m 272.1 272.3 f 0.9 91.1 6
JTC-p4-89 CGBG m 272.9 f 0.8 89.0 6
JTC-135-89 KWGD m 272.5 272.5 f 0.8 81.4 6
n-C-136-89 m 271.7 272.5 zt. 0.8 88.5 6
JTC-247-89 273.3
LB-E47625 270.4 * 0.9 92.4 4
268.4 i. 0.8 88.6 5
ITC-107-89 GBV m 269.9 270.5 i 0.8 88.8 6
Jlx-129-89 GBV m 267.2 212.3 fz 0.8 84.5 6
JTC-147-89 GBV m 270.0
JTC-177-89 GBV m 272.4

Cwnmencllis

Granites m 285.6 286.5 * 0.8 94.8 5
II-C-57-89 b 289.9 * 0.9
JTC-El-69 m 290.5 290.7 f 0.9 89.4 6
JTG81-89 m 287.7 288.1 * 0.8 81.5 5
ITC-83-89 b 289.4 i 1.0
m 288.5 288.7 i 0.8 88.9 6
ITC-239-89 m 290.0 290.0 i 0.8 89.9 7
3X-239-89 b 290.0 292.1 f 1.0 75.2 3
J?C-24289 m 285.9 286.0 i 0.9 94.8 8
JTC-242-89 m 283.7 285.7 f 0.8 80.6 6
” 288.7 289.2 f 0.8 85.8 7
JTC-243-89 m 290.5 290.3 i 0.9 80.4 6
R2480
CS* GBV m 285.2 286.6 i 0.8 86.4 6
C6* GBV m 280.7 * 0.9
Mimrsliutim
l-r-44-89
IX-80.89
Sl. Auslell

Granites CLMG b 277.4 276.9 i 0.9 92.0 8
JTC-29-89 CLMG m 216.7 276.7 f 0.8 88.3 8
JTC-29-89 CCBG m 276.5 * 0.9
JTC-30-89 CGBG b 271.8 278.3 f I.0 90.2 7
JX-276-89 CGBG m 278.1 273.9 f 0.8 86.8 6
JTC-276-89 RMG m 273.8 f 0.8
JTC-289-89 m 267.1 270.3 f 0.9 70.6 5
FG-1 FIG 276.8 92.0 7
f 0.8
Mineralization f 1.2

JTC-2X4-89 GBV m 270.4

G-5@ GJW m 276.6

Bodmin Moor

Granite? CGBG m 281.2 287.1 * 0.9 92.9 6
I’K-312-89 CGBG m 288.2 288.1 f 0.6 86.6 6
JTC-309-89
283.3 f 0.9 95.3 7
Gunnirlske CGBG m 282.5
m 280.7 283.1 i 0.9 53.8 4
JTC-200-89
Mineralization
ITC-197-89

Dertmoor

Granites Fe-Skim hb 280.0 280.3 i 1.0 84.0 4
JTC-489 m 270.5 i 0.8
ITC-185-89 CY3BG m 271.0 271.5 t 0.8 95.0 5
JTC-187-89 CCBG m 275.3 275.9 * 1.0 720 6
JTC-188-89 CMG m 272.3 f 0.8

JTC-189-89 FGBGD b 277.1 i 1.0
Mindimicm 277.3 * 1.0
JTC-212-89
Repeat 212-89

Hcmcrdon

Mina~lilization

IX-318E-89 SM m 271.0 277.8 f 0.8 91.6 5
biotitc-uanitc dike,
JK!-319C-89 GBV m 286.2 * 0.9

“CGEG=Coarsc-mined biotircmud. pOJ3G=F~dncd biotitwmmitc KZBGD=F~nnined

Magmatic activity and hydrothermal mineralization 1827

Table 4. Sm-Nd Data for Fiuorlte From Geevor Mine. younger Li-mica granite. Sample 289 is from the fine-granted
biotite granite at the Wheal Martin deposit and gives a pla-
Sample sm Ud ‘4’S= “‘N d Error teau-age of 273.9 f 0.8 Ma, which appears to represent slower
lr4Nd - (2cr lo+ cooling or a younger magmatic event than the emplacement
(PC=) (ppm) of either the Li-mica granite or the main CGBG. Muscovite
from the fluorite granite ( FG- 1) yields a disturbed age spec-
JTC-270.14 25.3 129 0.1182 0512139 (7) trum and a total gas age of 267.7 + 0.9 Ma. In thin section,
(8) the muscovite is ragged and appears to have reacted with
JTC-1’10.lb I.pJ 123 0.1nn aSl!ZZO9 later fluids to form interstitial fluorite, topaz, and secondary
muscovite. The Ar-release spectrum appears to have two sep-
JTC-270.3~ 0.819 2.51s 0.1968 oLm273 arate release plateaus containing 39% and 4 1% of the gas and
may retlect a mixing of the two separate muscovite popula-
JTC-270.4d 2.64 6.43 0.2484 OS2368 tions. Therefore, the muscovite total gas age only represents
a minimum cooling age. The isochron for sample FG- 1 yields
below the Wheal Jane Mine (6 km northeast of the Cam- an identical age for all points and a maximum age of 271.0
menellis pluton), gives a total gas age of 280.7 + 0.8 Ma, but + 0.4 Ma (2~) using the oldest steps from the spectrum. The
shows an Ar loss profile with no plateau and therefore only U-Pb ages are younger than previous estimates (285 to 30 1
represents a minimum age. An isochron plot of the oldest Ma) for the age of emplacement of the main coarse-grained
age spectrum steps yields a maximum age of 283.5 + 0.3 Ma biotite St. Austell granite (HARDING and HAWKES, 1971;
(2~). Skarn-type mineralization is considered the earliest BRAY and SPOONER,1983; DARBYSHIREand SHEPHERD,
mineralization related to an intrusive event and should 1985).
therefore define a minimum age for the intrusion. Fluorite
Sm-Nd isochrons for the stage 3 mineralization in the South Sample G-5 is from a greisen-bordered vein in the Goon-
Crofty and Wheal Jane Mines define ages of 259 + 7 Ma and barrow Pit, gives a 40Ar/‘9Ar plateau-age of 276.8 f 1.2 Ma
266 + 3 Ma (2a), respectively (CHESLEYet al., 1991). The (2~) (L. W. Snee, unpubl. data), and is in excellent agreement
stage 3 mineralization ages are in good agreement with fluid with the cooling age for the muscovite in the Goonbarrow
inclusion Rb-Sr isochron age of 269 + 4 Ma ( DARBYSHIRE Pit. Kaolinization is considered to be a younger, low-tem-
and SHEPHERD, 1985) for a Sn-bearing quartz vein from the perature process (SHEPPARD, 1977; BRISTOW and EXLEY,
South Crafty mine. In the Wheal Jane mine, elvans clearly 1988); however, it appears to have little or no effect on the
predate the emplacement of stage 3 mineralization ( HOLL, Ar systematics of the primary or secondary muscovites. Sam-
1988). At both South Crafty and Wheal Jane the age of main ple 284 is from the Wheal Remfry tourmaline breccia and
stage 3 mineralization clearly postdates the granite emplace- yields a *Ar /39Ar plateau-age of 270.3 + 0.8 Ma. The Wheal
ment in the Cammenellis region by 20 to 40 myrs and is Remfry breccia is a Sn-tounnaline breccia and postdates
significantly younger than any documented magmatism in greisen vein mineralization on the basis of crosscutting re-
the Comubian batholith. lations (BRAY and SPOONER, 1983 ) and contains large in-
clusions of elvan material. The field relations and geochro-
St. Austell Pluton nology suggest elvan emplacement was no later than 270 Ma.

The St. Austell pluton contains multiple episodes of granite Bodmin Moor Pluton
intrusions, the largest kaolin deposits in southwest England
and is the only pluton to contain all of the minor granite Monazite from sample 3 I2 is from the CGBG major gran-
phases as defined by EXLEY et al. ( 1983) and STONE and ite phase of the Bodmin pluton and yields a 207Pb/235U age
EXLEY ( 1985). Three monazite and three zircon fractions of 291.4 ? 0.8 Ma. The corresponding muscovite gives a
were analyzed from sample 276 of the main CGBG in the plateau age of 287.1 f 0.9 Ma and thus a cooling rate of
St. Austell pluton (Table 2; Fig. 2). A U-Pb analysis of one 95°C myr -’ . The cooling age is in excellent agreement with
of the monazite splits is reversely discordant, but the “‘Pb/ sample 309 which yields a plateau age of 288.1 _+ 0.8 Ma
235Uage overlaps with the age of the other two concordant (Table 3). The 40Ar/39Ar ages are in excellent agreement
size fractions and all three give a weighted average “‘Pb/ with Rb-Sr biotite ages 287 +- 2 Ma ( DARBYSHIREand SHEP-
235Uage of 28 1.8 + 0.4 Ma (2~). The agreement in age from HERD, 1985) ; however, the Rb-Sr whole-rock age (275 f 9
the different size fractions indicates that there is no inherited Ma) is in poor agreement with the U-PI, age.
Pb. The zircon fractions were chosen on the basis of their
euhedral form, pencil shape and clarity, all samples had a Gunnislake Pluton
length to width ratio of between 5 to 10: 1 suggesting the
zircons probably grew from the granite melt. Data for all Gunnislake is a minor CGBG satellite intrusion located
fractions are discordant, and judging from their 207Pb/206Pb between the Bodmin Moor and the Dartmoor plutons and
ages of 307 to 292 Ma these fractions contain inherited Pb gives a 40Ar/39Ar plateau age of 283.1 + 0.9 Ma for sample
predating the St. Austell pluton. The 40Ar/39Ar data from 200. Sample 197 is from a minor greisen bordered quartz
sample 276 gives a plateau age of 278.3 + 0.8 Ma, suggesting vein in the Old Gunnislake mine and yields a plateau age of
that the sample cooled to below the Ar closure temperature 283.2 & 0.9 Ma, in excellent agreement with the muscovite
for muscovite within 3.5 myrs (- 115°C myr-‘). Samples cooling age of the intrusion.
29 and 30 are heavily altered CGBG and Li-mica granite
from the Goonbarrow china clay pit and give indistinguish-
able plateau ages of 276.9 f 0.8 Ma and 276.7 f 0.9 Ma,
respectively. Sample 29 is slightly younger than the unaltered
CGBG granite in sample 276 and most likely represents pro-
tracted cooling or resetting caused by the intrusion of the

1828 J. T. Chesleyet al.

Dartmuor Pluton (Fig. 3g) suggesting subsequent reheating. An isochron of the
data gives a minimum age of 290.2 + 0.4 Ma and should
Two monazite samples were analyzed from quarries in the therefore be considered a minimum age and is consistent
Dartmoor pluton which is the largest pluton in the batholith. with the age for the Hemerdon pluton inferred from the
Monazite samples 187 and 188 are both concordant and give monazite data. Sample 318E, a secondary muscovite from
zo7Pb/235U ages of 278.2 f 0.8 Ma and 280.4 + 1.2 Ma, the Hemerdon granite, gives a plateau age of 277.8 it. 0.8 Ma
respectively. A hornblende (sample 4) from the metamorphic (Table 3). This age is coincident with the time of emplace-
aureole of the Dartmoor piuton (2 km east of sample 188 ) ment and cooling of the neighboring Dartmoor pluton and
gives a 40Ar/39Ar plateau age of 280.3 +: 1.0 Ma (Table 3; may represent formation of secondary muscovite by high-
Fig. 3f), which is confirmed by an isochron analysis. The temperature fluids and volatiles expelled during initial crys-
hornblende is from a garnet pyroxene hornblende magnetite tallization.
skarn with fluid inclusion Th = 350 to 475’C (SCRIVENER
et al., 1987). Because of the high blocking temperature of ORIGIN AND EMPLACEMENT
hornblende, the 40Ar/39Ar age should also reflect the em- OF THE CORNUBIAN BATHOLITH
placement age of the granite and is in excellent agreement
with the U-Pb age (280.4 * 1.2 Ma). Corresponding mus- Surprisingly, only a very limited amount of information
covites from samples 187 and 188 yield plateau ages of 27 1.5 is available on the time span over which granite magmas
k 0.8 Ma and 275.9 _t 1.OMa, respectively. An isochron plot coalesce to form composite plutons and batholiths. The
of the latter at 275.7 5 0.3 Ma (2~) is in excellent agreement. monazite and xenotime ages from this study clearly dem-
These samples indicate cooling rates between -60” and 85°C onstrate that granite emplacement in the Comubian batholith
myr-' . The difference in cooling rate between samples may continued over a minimum of 25 myrs (Fig. 5). There is no
reflect the location of sample 187 farther away from the pluton apparent systematic relation between the age of a pluton and
contact compared to sample 188. The cooling rates are also location within the ba~oli~ (Fig. 5). Furthermore, the in-
significantly slower than those ofthe other phttons and suggest dividual plutons contain different intrusions that are resolv-
the Dartmoor pluton may be exposed at a deeper level. The able with high-precision geochronology. The U-Pb ages give
other two samples of muscovite ( 185, t 89) exhibit disturbed definitive evidence for magma emplacement over time spans
age-spectra and give total gas ages of 270.5 It 0.8 Ma and of up to 4.5 myrs and the 40Ar/ 3qAr ages can be used to infer
272.3 +r 0.8 Ma. An isochron analysis of sample 185 yields that emplacement and cooling of separate bodies within a
an age of 276.2 + 0.4 Ma which may yield a minimum cooling pluton may have continued for as much as 10 myrs. These
age. There are two generations of muscovite evident in thin observations are in good agreement with the concept ofcom-
section, one primary and the other secondary from the posite plutons suggested by earlier workers (e.g., GHOSH,
breakdown of primary muscovite, potassium-feldspar, and 1934; EXLEV and STONE, 1982). The longevity of silicic
biotite and both age-spectra appear to have two separate pulses magma chambers has been modeled by HUPPERTand SPARKS
of gas release. The multiple emplacement ages and disturbed ( f 988 ) and SPARKSet al. ( L990). They suggested that magma
spectra are consistent with field relations which suggest that bodies of a size similar to those ofComw~1 would be expected
the Dartmoor pluton comprises several episodes of mag- to solidify within a period of less than lo5 years. Therefore,
matism (BRAMMALL and HARWOOD, 1932; SCRIVENER, the existence of composite intrusions with life spans of several
1982). The U-Pb and 40Ar/ 39Ar ages are in excellent agree- million years requires repeated mehing events (HUPPERT and
ment with both the isotope dilution Rb-Sr whole-rock (280 SPARKS, 1988) or the existence of a deeper magmatic system
4 1 Ma) and mineral isochron ages (278 * 2 Ma) from the that remained molten because of a continuous supply of heat
same quarry as sample ( 188) of DARBYSHIREand SHEPHERD from mantle-derived magmas ( HALLIDAYet al., 1989; HAL-
l.II)AY, 1990).
(1985).
The elvan dikes do not appear to be the result of separate
Hemerdon Pluton magmatism that postdates major granitic intrusions as has
been suggested ( DARBYSHIREand SHEPHERD,1985; JACKSON
Hemerdon is a small, highly altered, W-bearing satelhte et al., 1989; WILLIS-RICHARDSand JACKSON, 1989). The
intrusion on the south flank of the Dartmoor bathoIith. The 286 Ma 40Ar/3YArage inferred for the efvan empIa~ment at
U-Pb data for a monazite from sample 3 18Eof the Hemerdon Carnmenellis, the 282 Ma age of the Wherry elvan (DAR-
granite is discordant (Fig. 2) and has a zo7Pb/206Pb age of BYSHIRE and SHEPHERD, 1985) and the 270 Ma minimum
298.3 +- 2.3 Ma (Table 2). This age is consistent with the age for the elvan at St. Austell suggest that elvan emplacement
older Rb-Sr whole-rock isochron age of 304 -r:23 Ma (DAR- is probably coincident with major magmatism and that the
BYSHIREand SHEPHERD, 1987). However, due to the exten- ages depend on the location in the batholith. The data dem-
sive hydrothermal alteration and potential for excess Z3”Th onstrate that younger magmatism took place across the entire
in the monazite, both ages should be regarded with caution. batholith. It is therefore conceivable that the emplacement
of early plutons formed a barrier which prevented the younger
Samples 3 18E and 3 19C are from a drill core at the Hem- granites from penetrating to the levels of the present-day sur-
erdon W-mine and both exhibit complete replacement of the face. The elvans may represent dikes tapping into the upper
coarse-g&ted biotite granite by secondary muscovite quartz portions of buried magmas shortly after empiacement, and
and kaolinite. Sample 319C, a secondary muscovite from a they could possibly be feeders for contempor~eous feksic
greisen-bordered quartz vein, yields a total gas age of 286.2 volcanism ( DANGERFIELD and HAWKES, 1969).
5 0.9 Ma and the age-s~ct~rn exhibits an Ar loss profile

Ma~atic activity and hy~the~~ mine~i~tion 1829

There are three main schools of thought regarding the gen- ing a discrete evolution after extraction from a comparable
esis of the granites of the Comubian batholith. Several workers source, leading to a separate emplacement and cooling his-
have suggested that melting was initiated in response to the tory.
crustal thickening by thin-skinned thrusting from the south
during the Variscan orogeny (e.g., SHACKLETONet al., 1982; COOLING HISTORY
PEARCEet al., 1984). SHACKL_ETQeNt al. ( 1982) also pointed
out that the crust in the region is too thin (~32 km) to generate The isotopic ages presented here provide not only evidence
the granites by melting in the zone where they now occur of protracted magmatism, but of a resolvable cooling history
and suggested the granites were generated in the south, and as defined by the difference between the 207Pb/23sU monazite
injected to the north, along southward dipping structures as ( T, - 725’C) or xenotime (T, - 65O’C) ages and the 40Ar/
a single sheet-like granite mass with separate diapiric up- 39Ar muscovite ages ( T, - 325’C). In order to interpret the
wellings forming the exposed plutons. Other workers have age differences of minerals rigorously, it is necessary to assess
suggested that the granites were derived from the mantle via all of the uncertainties including the decay constants, spike
fractional c~s~li~tion of a more matic parent and crustal calibration, and accepted age of the fIux monitor (in the case
assimilation (e.g., SIMPSONet al., 1979; WATSONet al., 1984). of 40Arf39Ar) that could have introduced a systematic bias
A third possibility requires an elevated geothermal gradient, into either the U-Pb or 40Ar/39Ar results. Apart from the
possibly due to basaltic underplating ( HUPPERT and SPARKS, obvious use of internationally accepted values for the decay
1988), which resulted in regional melting of the lower crust constants (STEIGER and JKGER, 1977) and flux monitors
(LEAT et al., 1987). (SAMSONand ALEXANDER,1987), a further check is provided
by the 40Ar/39Ar analysis of an amphibole from the Haytor
The ages presented here, together with a consideration of skam, in the aureole of the Dartmoor pluton. The amphibole
the chemistry and isotopic compositions of the granites (e.g., yields a plateau age of 280.3 & 1.O Ma, in excellent agreement
SHEPPARD, 1977; DARBYSHIREand SHEPHERD,1985,1987) with the U-Pb monazite age of 280.4 t 1.2 from a nearby
provide powerful cons~ints on the origin of the magmas. granite sample ( 188). In addition, these ages are in good
The first question to address is whether the batholith was a agreement with a whole-rock Rb-Sr age of 280 t 1 Ma de-
large interconnected reservoir or was built progressively from termined by isotope dilution for the Dartmoor pluton (DAR-
separate pulses of melting. The data presented here dem- BYSHIREand SHEPHERD, 1985). Similarly, the 40Ar/39Ar
onstrate unequivocally that the batholith was formed by the muscovite ages are in excellent agreement with isotope di-
coalescing of separate granite intrusions over a >25 myr in- lution Rb-Sr biotite-isochron ages from the same localities
terval ( Fig. 5 ) . The absence of any relationship between the in the plutons and on the same samples used in the original
age of the plutons, their location in the batholith, and the study of DARBYSHIREand SHEPHERD( 1985). The internal
duration of emplacement of both the ba~olith and the in- consistency of the 40Ar/39Ar method is observed in samples
dividual plutons are irreconcilable with models involving 242 and C6, which were both collected from the same quarry
thin-skinned tectonics transporting magma from the south. and yield ages of 290.0 + 0.8 Ma and 290.3 f 0.9 Ma, re-
A large interconnected magma system at shallow levels would spectively.
be expected to crystallize in a short time span and not over
the 25 myr period measured. Cooling times for the individual plutons, from emplace-
ment down to the closure of muscovite to Ar, range between
Models involving fractionation of mantle-derived magmas 1.9 and 6.7 myrs with a mean of 3.9 myrs (- 100°C myr-' )
are difficult to reconcile with the large volume of highly frac- (Fig. 6). These rates reflect the calculated times that elapsed
tionated granites in the batholith, the lack of more ma& between monazite and muscovite closure to diffusion; the
endmembers and the high 6 ‘so and %r /“Sri values ( SHEP- actual cooling rate would initially be rapid, followed by slow
PARD, 1977; HAWKERand DANGERFIELD,1978; DARBYSHIRE asymptotic cooling towards an ambient value (e.g., NORTON
and SHEPHERD,1985 ) . We consider models involving crustal and KNIGHT, 1977; CATHLES, 1981; HARRISON and
melting initiated by mantle-derived magmatism to be a more MCDOUGALL, 1980). These cooling rates are much slower
likely scenario; an elevated heat flux produced through un- than those predicted by thermal modeling of cooling silicic
derplating of basaltic magmas could account for semi-con- magmas ( SPERA, 1980; HUPPERT and SPARKS, 1988).
tinuous granite magmatism over a >25 myr interval. MAN- Though there is no apparent correlation between cooling rate
NING and H!LL ( 1990) suggested that the late topaz granites and age of a pluton, there appears to be a decrease in the
observed at St. Austell were the result of melting of refractory overall cooling rate from Lands End in the southwest to
residue after partial melting and removal of the biotite-granite Dartmoor in the northeast (Fig. 6).
melt and are independent of fractional crystallization within
the batholith. The timing ofemplacement between the biotite- Factors that could affect the granite cooling rates include:
granites and the topaz granites would require a long-lived 1) initial magmatic temperature; 2) heat production of the
elevated heat source in the same area. granite; 3) thermal conductivity of the wall rocks; 4) mass
of magma; 5) geometry of the granite body; 6) location of
A more detailed evaluation of mechanisms of batholith the sample within the pluton; 7) repeated igneous intrusion
formation requires discussion of the chemical ~m~sition into the same area; 8) cooling from meteoric fluids; and 9)
for the granites and is beyond the scope of this paper. How- depth of emplacement. Because of the similar composition
ever, it is clear from the data presented here that the Cor- of the granites and country rocks across the area of the batbo-
nubian batholith should be considered as a series of small lith, the initial temperature, heat production, and thermal
anastomosing igneous bodies with each individual body hav-

J. T. Chesley et al.

50” 1979; CATHLES, 198 1, 1990; FURLONGet al., 199 1). During
initial intrusion, permeability will be very low, fluids will be
3 unable to circulate and cooling will be primarily by conduc-
tion (NORTON, 1990; NORTON and CATHLES, 1979;
r” CATHLES, 198 1, 1990; FURLONG et al., 199 1). As the magma
P crystallizes and fractures, fluids can circulate through the
pluton in proportion to the extent of fracturing. In the pres-
100" s ence of hydrothermal circulation, models have been used to
ez suggest that cooling would be complete in as little as IO3 to
t IO5 years (SPERA, 1980; NORTON and CATHLES, 1979;
CATHLES, I98 1). Shortcomings of such models lie in the
200" z inability to incorporate multiple intrusive events and the effect
v of poorly known pluton geometry.

FIG. 6. Cooling interval ofindividual plutons within the Comubian Modeling of the Carnmenellis pluton by WILLIS-RICHARDS
batholith. The cooling interval is defined as the difference in age and JACKSON( 1989) in the absence of hydrothermal con-
between the closure of monazite to the diffusion of Pb (-725°C) vection suggests that the upper portions of the pluton would
and the closure of muscovite to Ar (-325°C). cool rapidly, whereas the deeper portions of the pluton (> I5
km) would remain molten after 15 to 20 myrs and would be
conductivity of the wall rocks would be expected to produce a source for the mineralization and elvan dikes. Their model
only second-order effects. The presence of dikes, sheeted vein is based on geophysical estimates of the pluton shape and
systems, and large roof pendants in the Lands End pluton size and the assumption that the emplacement of the Carn-
indicates proximity to the upper portions of the pluton, while menellis pluton is as a single large magmatic body. In contrast,
stable isotope analyses of fluid inclusions suggest interaction the field evidence (GHOSH, 1934), geochemistry (DARBY-
with meteoric fluids during early mineralization (JACKSON SHIREand SHEPHERD, 1985, CHAROY, 1986), and geochro-
et al., 1982 ). Both factors would promote rapid cooling (e.g., nology support a multiple intrusive history. Hence, plutonism
SPERA, 1980, CATHLES, 198 1). The Dartmoor pluton is the may be better represented as a series of successive smaller
largest pluton in the batholith and contains several episodes intrusions staggered in time, underneath and beside the main
of magmatism. The slow cooling rate (60” to 80°C myr-‘) Carnmenellis granitic body. The depth of formation and ge-
disagrees with models of rapid cooling for the Dartmoor ometry of these smaller plutons would dramatically influence
granite supposedly caused by rapid unroofing ( WILLIS-RICH- their crystallization and cooling history by controlling the
ARDS and JACKSON, 1989). The sample locations are near timing and extent ofvolatile exsolution, interaction with me-
the margins of the pluton and therefore would be expected teoric fluids, and contribute to the total heat budget of the
to give maximum cooling rates. Gravity models of the batho- system. The model of WILLIS-RICHARDS and JACKSON
lith indicate a deep fault controlled trough just west of the ( 1989) also fails to explain the lack of similar protracted
Dartmoor pluton and a decrease in the estimated batholith cooling and mineralization histories for other plutons within
thickness in the Dartmoor area (WILLIS-RICHARDS and the batholith and why major economic mineralization post-
JACKSON, 1989). The Dartmoor granite is characterized by dates the Carnmenellis pluton by 30 to 40 myrs.
coexisting very high salinity and dense vapor-rich fluid in-
clusions, consistent with fluids in equilibrium with a granite MINERALIZATION
melt (RANKIN and ALDERTON,1985). The same assemblages
are also typical of the Birch Tor quartz-cassiterite tourmaline The overall mineralization history with respect to the Cor-
veins and greisen-bordered quartz veins of the Hemerdon nubian batholith lasted over 25 myrs (Fig. 5). However, the
mine of high-temperature magmatic-hydrothermal origin mineralization history for each pluton appears to be distinct.
(SHEPHERD et al., 1985). In view of the S-deficient nature The stage I skarn mineralization age is in good agreement
of the mineral assemblages, these observations strongly sug- with the U-Pb emplacement age for the associated pluton as
gest that the Dartmoor mineralizing fluids were primarily would be expected. Modeling of Sn-W-bearing fluid chemistry
magmatic and did not experience significant mixing with suggests that high-temperature, low f& and low pH fluids
meteoric, sulfur-bearing fluids during emplacement and are required for the transport of tin (e.g., HEINRICH, 1990;
cooling. The long cooling history may also reflect heat loss WILSON and ELJGSI-ER1,990). In addition, at low fo,, sulfur
primarily by conduction to the wall rocks and not due to will partition more readily into the magma than into the
heat loss from hydrothermal circulation. exsolved aqueous phase (BURNHAM and OHMOTO, 1980),
which is consistent with the sulfur-deficient nature of the
There are few direct measurements of the cooling rates of stage 2 fluids. According to the models acidic Sn-bearing fluids
plutons (from emplacement to -300°C) to compare with expelled from the crystallizing magma will be buffered by
the results of the present study. Most of the information and the reaction:
understanding of plutonic cooling rates are derived from
quantitative modeling of igneous cooling by diffusive and 3KAlSi908 + 2HCl = KAl,Si,O,,(OH), + 6Si02 + 2KCl
convective heat transfer through porous media (e.g., JAEGER.
1964: NORTON and KNIGHT, 1977; NORTON andC,b~li~~~. and

3NaA1Si308 + 2HCl + KC1

KAl,Si,O,,,(OH )> + 6SiOZ + 3NaCI

Magmatic activity and hydrothermal mineralization 1831

forming secondary muscovite (e.g., HEINRICH, 1990). The 1300-295 Ma 1 - y 1275-26M5 a] c
age of stage 2 mineralization is best represented by 40Ar/39Ar
secondary greisen-muscovite ages, and in the case of Bostraze 295-275 Ma1 B Present Day1
and Goonbarrow agree within the limits of statistical reso-
lution for the cooling ages of the plutonic muscovite. These FIG.7. Schematic illustrationdepictingthe emplacement history
ages define a minimum age for the first pulses of magmatic of the plutons in the Comubian batholith and relationship to min-
fluid and stage 2 mineralization during the initial crystalli- eralization. (A) Early emplacement of granite intrusions within the
zation and cooling of the pluton. In several cases, the sec- Comubian batholith; (B) Emplacement of the bulk of the exposed
ondary muscovite age is significantly younger than the cooling Comubian batholith from -295 Ma down to -275 Ma; (C) Em-
age of the host pluton (2-5 myrs), and may be correlated placement of chemically specialized granites; unexposed granite in-
with a nearby magmatic event. The long cooling intervals trusions and stage-III mineralization; see text for detailed explanation.
for each pluton, with the exception of Lands End, suggest
that meteoric fluids did not significantly interact with the Further cooling of the granites and expulsion of magmatic
plutons during emplacement and initial cooling. fluids continued, with the location of fractures and deposition
of stage 2 mineralization controlled by the pattern of pre-
The age of stage 3 mineralization is represented by the Sm- existing fractures and the regional tectonic framework. Be-
Nd fluorite-isochron ages and the Rb-Sr fluid inclusion age tween 275 and 265 Ma the chemically specialized granites
(269 f 4 Ma; DARBYSHIREand SHEPHERD,1985). In the and late elvans were emplaced, and the magmatic fluid system
case of Cammenellis stage 3 mineralization postdates mag- collapsed allowing external fluids into the upper portions of
matism by 25-40 myrs. Although some samples display an the plutons (Fig. 7~). Circulation of external fluids leached
argon loss profile suggesting disturbance by a younger event. metals and sulfur from the country rocks and plutons. These
The secondary muscovite ages from stage 2 mineralization fluids mixed with the late magmatic fluids expelled from the
do not appear to record the age of the younger lower-tem- buried crystallizing magmas initiating stage 3 mineralization.
perature stage 3 fluids. The development of the stage 3 min- Continued circulation of external fluids was driven by the
eralization of similar age in the oldest (Cammenellis) and elevated regional geothermal gradient and latent heat of crys-
youngest (Lands End) plutons suggest this was a regional tallization derived from deeper magmatism. Deposition of
event and not just centered on a single pluton. Stage 3 min- stage 3 hydrothermal mineralization was controlled by pre-
eralizing fluids may thus reflect regional hydrothermal cir- existing fractures and by overpressuring leading to hydro-
culation driven by the elevated heat flux produced by the fracturing, rupture, mineral deposition, and resealing of the
emplacement and crystallization of younger pulses of granite system. Stage 4 mineralization commenced after the em-
magmatism (~270 Ma), or fluid circulation driven by the placement and cooling of the batholith was completed and
elevated regional geotherm produced by the long-lived, mul- was triggered by a change in the regional tectonic stresses
tiply overlapping, plutonic episodes. Additional heat sources (e.g., SHEPHERDand SCRIVENER,1987).
will certainly prolong the duration of fluid circulation such
that the fluid flow would be realigned with respect to the new DISCUSSION
heat source ( CATHLES,1990; NORTON, 1990). Focusing of
hydrothermal circulation by younger plutons may account The lack of available high-precision geochronologic in-
for the asymmetric zoning of mineralization on the sides of vestigations of ore deposition make extension of this model
the plutons. to other tin deposits, much less other granite-related hydro-
thermal ore deposits, somewhat tenuous. However, several
CHRONOLOGY OF EVENTS important observations should be made. The multiple intru-
sive nature of granite batholiths are well documented (e.g.,
Integration of the geochronology with field relations and PITCHER et al., 1985), and supporting evidence exists that
granite chemistry requires an emplacement and mineraliza- many plutons are composed of discrete litbologic units dis-
tion history that differs significantly from traditional views tinguishable in age (e.g., GEISSMAN et al., 1992); however,
(e.g., STONE and EXLEY,1985; DARBYSHIREand SHEPHERD, the shape and size of individual intrusions is often poorly
1985; JACKSONet al., 1989) and must involve long-lived known. The spatial and temporal multiplicity of the magmas
multiple intrusive episodes, each with its own mineralization will lend itself to independent crystallization, fluid expulsion,
history but overlapping and overprinting earlier mineralizing fluid mixing, and mineral deposition histories.
events. The onset of major granitic magmatism in Cornwall
began between 300 and 295 Ma (Fig. 7a). During cooling Recent two-dimensional models have been used to sim-
and crystallization of the magma, high-temperature magmatic
fluids were released. A high geothermal gradient, emplace-
ment of younger granite magmas and slow cooling served to
prevent external fluids from entering the system (Fig. 7a).
Major granitic intrusion continued in different regions of the
batholith from -295 Ma to -275 Ma (Fig. 7b). Emplace-
ment of elvan dikes was coincident with magmatism and
continued to at least 270 Ma. Formation of stage 1 skam
mineralization was directly related to the timing of intrusion.

1832 J. T. Chesley et al.

ulate the temporal and spatial variations in the geothermal provided the samples used in her original study, unpubl~h~ data
gradient of regions where the distribution and rates of em- and useful discussion with respect to her Rb-Sr analyses. JTC would
placement of igneous heat sources are important (BARTON like to thank S. Harlan and R. Yeoman who ran the day shift for
and HANSON, 1989; HANSON and BARTON, 1989). Such 40Ar/39Ar analysis at the U.S. Geological Survey in Denver. The
models have been used to demonstrate that the background work presented represents a portion of the senior author’s Ph.D dis-
geothermal gradients between intrusions may be elevated by sertation at the University of Michigan. This research was supported
5” to 30°C km-‘, and depending on the abundance, depth by National Science Foundation grants EAR 86 1606I, 8804072 and
of emplacement and timing of subsequent intrusions can re- 9004413 to A.N.H. and by the Denver U.S. Geological Survey 40Ar/
main elevated for significant time intervals (several tens of j9Ar Geochronology project. We are indebted to W. Kelly, S. Kesler,
millions of years) before returning to normal geothermal J. O’Neil, J. Mauk, M. Ohr, A. Anibas, A. Goode and M. Miller for
gradients. discussion and/or critical review of the original manuscript. This
paper is published with the approval of the Director of the British
The existence of large regional hydrothe~~ circulation Geological Survey (NERC) and the Director of the U.S. Geological
systems associated with the emplacement ofgranite ~tholiths Survey. Thou~tful reviews for GCA by K. Ludwig H. Stein, J. Ruiz,
(e.g., TAYLOR, 1978; CRISSand TAYLOR, 1983) and regional and I. Fanning greatly improved the manu~pt.
metamorphism (e.g., WOOD and WALTHER, 1986; FERRY
and DIPPLE, 199 I ) is well documented. The exact conditions Edtoriui handling: K. R. Ludwig
that permit these large circulation systems and the timing of
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Magmatic activity and hydrothermal mineralization 1835

APPENDIX I

Ssmpk # Loeatba Sampk Description

Lands End

Granites

JTC-94-89 Geevor Mine15 w 4 CmssalL FGBG dike io metammphic -le. SW 37s 345

JTC-135-89 Cestlaen-Dines Quarry. FGBG SW489344

JTC-136-89 New Mill Quarry. CGBG SW 459 342

ITC-247-89 Robins Rocks et F%mhmeor Pt. FGBG dike in maemorphic eunwle. SW 42.5 380

L&R47625* Rosewall Hill @any. CGBG SW 497 391

Mineralization

JTC-107-89 Geevor Mine 14 level. GBV in CGBG wallrock, Simms lode. SW 375 345
JTC-129-89 Geevor Mine 15 level. GBV in FGBG waUm&, Cmamtion lode. II
JTC-147-89 Geevor Mine 15 level. GBV in CGBG wJlmck, Coronatim lode.
,.

ITC-177-89 Bostmze China Clay Pit. GBV in CGBG wellrock. SW 385 315

Carnmeneaii

Granites

JTC-57-89 South Cmfty 360 Sub-level. CGBG et elvan contaa. SW675411

ITC-81-89 Bosaban Qoeny. CGBG SW 730 301
ITC-81-89 II * *

ITC-83-89 South Crafty 310 level. CCBG 3.5 m from elwn. SW 675 411

ITC-239-89 PMRyIlQwrry. FGBG dike. SW 737 332
ITC-239-89 I, * SW 737 332
ITC-242-89 SW 737 332
JTC-242-89 Pen Ryn Quarry. CGBG SW 737 332
* II

ITC-243-89 Abandoned Quamy on Cent Matth. CGBG SW720410
R2480* SW 736 347
CS-FD* Rosememwas Quarry Deq Bae Hole. EBG SW 758 345

camseQwltalTy. CGBG

C6-FD* Pen Ryn Quarry. CGBG SW 737 332
Mineralization

lTC-44.89 south crony 380 level. Indrillcore91mbetweea4and8lodes. SW675411
ITC-80-89 Wheal Jane Deep Bore Hole. Basemetal-bearing &am in drill hole. SW 775 425

St. Aurlell

Granites

ITC-29-89 Goonbamw tit. Ii Mice Granite sx 005 570
JTC-29-89 0, II ,I
ITC-30-89
II CGBG I0

KC-27689 Luxulyen Quany. CGBG sx 053 591
JTC-27689 ,, CGBC II

ITC-289-89 Wheal Martyn. FGBG SW 995 555
FG-I*** Goonwan end Rostowmck. SW 940 555
Hard purpleFIG.

Mineralization

JTC-284-89 Wbeal Remfry Breccie. Toormeline bea%. SW 925 975
c-s** Goonbatmw Pit. Greiim bordered vein. sx 005 570

Bodmin Moor

Granite.5

ITC-309-89 DeLMk Quarries-St Bred. CGBG sx 10175s
JTC-312-89 Bodmin Mow. CGBG in road-cut. SX 162 742
Gunnislake

Granites

ITC-200-89 Old Gunnisleke Mine. CGBG SX 435 725

Mineralization

ITC-197-89 Old Gunnislake Mine. GBV in CGBG w&o&. SX 435 725
Dartmoor

Granites

JTC4-89 Haytor Mine. Game&pymxate-amphibolebearing &am. sx 780 775
JTC-185.89 MerrivaIe Qlwy. CGBG sx 545 745
ITC-187-89 Yellowmeade Farm Quarry. CGBG sx 565 73s
JTC-188-89 Haytor Quarry. CGBG sx 75s 774
ITC-189-89 Blocking&one Query. CMG SX 784 857
Mineralization

ITC-212.89 East Vitafer Mine. CGBG SX 705 825
Hemerdon

Mineralization

JTC-318E-89 Hemerdm W-Mine. Altered CGBG in drill core. SX 57,0, 585
JTC-3 19C-89 Hemerdm W-Mine. GBV in drill core.

CGBG=Couse-grained biotite granite, FGBG=Fitwgnbted biotitegmoite. EBDEqui.wu-lar biotite mite

CLMG=Coam.-grained li-mica granite, FlMOFiie-gained Li-mica granite. CMG=Carse megaaystic Sranite.

RG=Fluorite Granite, GBVzGreisen bordered vein.

* Samplesfrom Derbyshire and Shepherd (1985). l * Colkcted by H. Bell. *** Collected by C. Smale