Gwehintica et C~~~hirnic~ Acia Vol.57. pp. I8 17-I 835 ~1~7037/93~.~+.a0 Copyri#+tO 1993 PergamonPms Ltd. 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 October 29, 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/ 39Ar dating 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 The generally accepted model for granite-related hydro- of structure characterizing magmas emplaced within a single, thermal mineralization assumes that pluton emplacement, ~ntinuous outer contact: so defined plutons can have resulted hydrothe~~ circulation, and mineral deposition take place from single or multiple injection.” Many examples of piutons over a geologically insignificant period of time (copper deposits may be used in conjunction with the emplacement and cool- on Pacific islands that formed over one to two million years ing history of the batholith to elucidate the chronology of (SKINNER, 1979); (c) calculations of the lifetime of active geothermal systems (e.g., WHITE and HEROPCXJLOS,1986 ); and (d) isotopic age determinations of host plutons and as- * Present address: Department of Geosciences, University of Ar- sociated mineralization which have been unable to resolve izona, Tucson, AZ 8572 1, USA. 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-Bathoffth Metamorphic Rocks

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

f Old Gunnisfake Mine

:e-Mine Wheat Remfry i

L Wheat Jane Mine \ / 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- nologic studies at the Panasqueira Sn-W deposit, Portugal tion may postdate the host granite pluton by 5 to 35 myrs. ( SNEE et 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 vonian and ~ar~niferous sedimentary and mafic igneous rocks, re- suhing in a thermal overprint and metasomatism of the country rocks observed at either southwest England or at Panasqueira which produced a range of lithologies including homfels, tourmal- (SHEPHERDet al., 1985 ) . If hydrothermal circulation is indeed inized slates, and garnet-pyroxene-amphibole skams. long-lived or significantly postdates known plutonic activity, Several workers have classified the granites in the Comubian then we should reexamine some of the fund~en~ inter- batholith on the basis of mineralogy, texture, and chemistry (e.g., pretations regarding pluton formation and granite-related HAWKESand DANoERnnD, 1978; EXLEYand STONE,1982; EXLEY et al., 1983; SforJE and EXLEY, 1985;HILL and MANNING, 1987). minerahzation. For exampIe, what are the time spans and Xenoliths of granitic to granodioritic composition are found within mechanisms for the generation and emplacement of com- Same of the main plutons and are considered to represent the disrupted posite intrusives and silicic magmas? How important is the remains of earlier episodes of plutonism. Approximately 90% of the duration of batholith and pluton emplacement in the gen- exposed granite is a caarse-grained biotite-muscovite granite (CGBG), with turbid megacrysts of potassium-feldspar in a finer grained, equi- eration of an ore deposit? What is the driving force for hy- gram&r matrix of quartz, plagioclase, biotite, muscovite, and tour- drothermal ci~ulation? is hydrothermal circulation episodic maline, ~lthou~ voIumetricahy minor, four fater episodes of granite or continuous? magmatism have been recognized (EXLEY et al., 1983): fine-granted The classic Cornubian Sn-bearing granites of southwest biotite granite; megacrystic Li-mica granite; fine-granted L&mica England (Fig. la) have provided a natural labomtory for the gmnite (topaz granite of MANNING and HILL, 1990), and fluorite granite. The different granite lithologies are not observed within each development of many early theories of granite emplacement phtton, but all of the plutons are composite in nature. and related mineral deposition (e.g., PRKE, 1778; DE LA Grosscutting some of the major granite ptutons are a series of ver- BECHE, 1839; DAVISON, 1927; DINES, 1956). Welf-docu- tical to near vertical feisic porphyry dikes, iocahy &led eLvans. DAM mented geologic and paragenetic reIations make the Cor- GERFIELDand HAWKES f 1969) suggested that the dikes represent contemporaneous felsic volcanism. Other workers have considered nubian batholith an ideal setting for a detailed geochronologic the elvans to be the last stage of magmatism in the Comubian batholith study of the relation between mineralization and plutonism. and separate the elvans (280-270 Ma) from the plutons (290-280 In this study, U-W isotopic ages of monazite and xenotime Ma) in time (DARBYSHIREand SHEPHERQ1985), or suggest that are compared with @Ar/-Ar ages of hornblende and primary the &vans represent crystagized and degassed products extracted from muscovite (muscovite was determined to be primary using the main CGBG residuum derived from deep within the batholith t IACKSCJNet al., 198% WILLIS-RKHARDSand JACK.%~N,I989 ). Mf the criteria of MrtLER et aI., 198 1) from skams and individual nor and trace element chemistry suggestthat the elvans are genetically plutons within the Comubian batholith in order to assess linked to the CGBG granites ( DARBYSWREand SHEPHERD,I985 ). both the magmatic history of the batholith and the cooling Many authors proposed a temporal link between the mineralization rate of individual plutons. In addition, 40Ar/ 3gA~muscovite and the emplacement of the elvans (e.g., HALLIDAY,1980, JACXWN et at., 1982, 1989; DARBYSH~REand SHEPHERD,1985), becausethe ages from mineralized veins and Sm-Nd isochron ages of elvans utilize the same fracture systems as the rn~ne~~i~ fluids fluorite from the main stage of minemli~tion are used to and indicate the presence of a magmatic heat source young enough quantify the lifetime of the Comubian mineralizing sys- to produce high-temperature tin mineralization. tem( s). None of the current models for granite emplacement Mineralization is associated with multiple, steeply-dipping fractures and miner&z&ion adequately explain the emplacement, 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 cooling, and mineralization histories established by these iso- width (JACKSON elal., 1989). Ore-bearing lodes typically exhibit topic age determinations. 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 The Comubian batholith was emplaced at the end ofthe Hercynian polyascendent nature of the mineralization and the telescoped zoning Orogeny (Carboniferous to Permian), which involved the continental of the ores (e.g., GARNETT, 1965; JACKSONet al., 1982). Presently, collision of what is now northern and southern Europe (e.g., STONE mast researchers recognize four major stages of granite-related min- and EXLEY, 1985). The batholith has a strike length of 250 km and eralization (e.g., STONEand EXLEY, 1985; SHEPHERDet al., 1985; a maximum width of 50 km, extending from Dartmoor in the north- Table I ): 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 1) The earliest recognized minemii~ation features are exoskarns the majority of the exposed batholith. The batholith consists pre- produced by metasomatism of peiitic shales and metabasalts dur- dominantly of coarse-grained, peraluminous, biotite, and biotite- ing granite intrusion (JACKSONet al., 1982; VAN MARCKE DE muscovite granites similar to the S-type granites of CHAPPELL and LUMMENand VERKAEREN,1985; SCMVENERet al. 1987). EXO-

Stage 1 Stage 2 Stage 3 Skarns. Pegmatitic She&d Greisen &-Bearing Late Potymetallir Sulfide Deposits. BorderedVeins. Polymetallic Fissure VehlS. Veins. “CroSScourSC” TOurmafi~ Predominantly East- Predominantly North-South. Bearing veins. Wat. Fe, Cu, Sn §n,WfMo B&W Sn, &o. Pb, 2% As, Metats Fe Qz., Fsp., Chl., Qz.,Bar., Dol., t&c., Fluor. 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- phiboles, malayaite, vesuvianite, siderite, and axinite. The skams matism within the batholith as well as individual plutons and (2) contain economic concentrations of Sn, Cu, Fe, and As; fluid the time taken for the plutons to cool below temperatures where they inclusion hom~en~tion temperatures (7-h) have been reported were no longer capable of driving 200-4OO”C mine~izing fluids. to mnge from 260-475’C. (JACKSONet al., 1982; SCRIVENERet The closure temperature for d&rent isotopic systems in dilferent al., 1987). minerals provides a quantitative evaluation of the cooling history of 2) An earlyepisode of mineralized pegmatites, greisen-bordered veins an igneous or metamorphic temne. In order to resolve the cooling and Sn-tourmaline veins and breccias containing predominantly rate of a pluton, the pluton must have cooled slowly enough so that tin and tungsten oxides; 2-s ranges from 300°-5OO’C. Tourmaline the elapsed time between ages for different minerals with different is coeval with the eassiterite and is the main gangue mineral. closure temperatures is greater than the analytical uncertainty in the SHEPHERDet al. ( 1985 ) recognized a bimodal population of fluid age. A detailed explanation of closure temperature theory and models inclusions, which was interpreted as unmixi~ of early high-tem- is available in DOLBON( 1973), GANGULY and Ruiz (1987) and perature magmatic fluids into a low-salinity, high COz, W it Sn- MCDOUGALL and HARRISON( 1988 ) . bearing thud and a high-salinity, low CO,, Sn-bearing fluid. Normally, U-Pb ages from zircons are used to provide the best 3) The main episode of mineralization consists of polymetallic age of emplacement for a magmatic body. However, the Comubian quartz-toutmaline-chlorite-sulfide-fluotite-hearing fissure veins granites were formed by melting of a source with a significant sedi- which trend predominantly east-west. The veins contain Sn, Cu, mentary component (e.g., HAMIVONand TAYLOR,1983), therefore, Pb, Zn, Fe, and As-subides & SnOz and give T, values of 200- the zircons will likely contain inherited radiogenic Pb (see also Fig. 380°C (JACKSON. 1977: JACKSONet al.. 1982: SHEPHERDet al.. 2). Monazite appears to lose Pb by volume diffusion during high- i985; SHEPHERDand ~RIVENER, I987 ). Stage 3 veins represent temperature metamorphic or melting events (PARRISH,1990). Hence, the major period of economic mineralization in Cornwall. Gangue inherited Pb is seldom observed in granites. Although there have minerals are dominantly chlorite with minor tourmaline. The been no rigorous experimental determinations of the closure tem- mineralogical zoning, high fluid inclusion homogenization tem- perature for Pb, PARRISH( 1990) has estimated the closure temper- peratures and structures of these ore bodies, together with their ature to be 725” + 25°C from a variety of studies in metamorphic apparent spatial relations with the granite led to the traditional and igneous terranes. Estimates for the closure temperature for Pb view that this s&ge of rnine~li~~on was related to emanative in xenotime are scarce, but it is consideredto be --6SO*C by HEAMAN centers within the host pluton (DINES, 1956). and PARRISH( 199 I). The presence of primary andalusite and mus- 4) The last stage of mineralization consists of polymetallic (Pb, Zn, co&e suggest that crystallization temperatures were ~750°C (e.g., Ag, and U) quartz-fluorite-barite-sulfide veins, locally termed KERRJCK, 1990). Therefore, a more accurate and reliable emplace- crosscourses. Ruid inclusions yield r, values of 105-180°C ment age can be obtained from U-Pb dating of monazite and xe- (JACKSON, 1977; JACKSONet al., 1982; SHEPHERDet al., 1985; notime. The U-Pb data for Th-rich minerals such as monazite com- SHEPHERDand SCRIVENER, 1987). These fluids are similar in monly am reversely discordant (e.g., PARRISH, 1990). This discor- composition to Na-Ca-Cl-rich deep sedimentary brines and have dance is generally thought to be the result of incorporation of excess/ been suggested to be the result of fluid expulsion from adjacent unsuppo~~ %Pb that Formed from *?Th in~~mted during the Mesozoic sedimentary basins (ALDERTON,1975,1978; SHEPHERD formation of the mineral (ScX&ER, 1984). In most cases, it has et al., 1985). been demonstrated that the 20’Pb/2s5U ages can be accepted as geo- An economically important feature of the southwest England logically correct ( SCHKRER,1984; PARRISH, 1990). Therefore, only granites is the widespread but localized presence of intense hydro- the 207Pb/23sU age of the monazite is used for a thermochronologic thermal kaolinization (china clay). Kaolinization is centered on the comparison of ages in this study. Monazite has been shown to retain western portion of the St. Austeli granite, southern Dartmoor granite Pb during low-temperature melting events (COPELANDet al., 1988). and smaller workings on other plutons. The age and genesis of the To evaluate whether any differences between the U-P6 monazite and kaolin deposits is still a matter of debate (e.g., SHEPPARD, 1977; *Ar/39Ar muscovite ages could be the result of inherited Pb, replicate DURRANCEet al., 1982; EXLEYet al., 1983), but they are commonly U-Pb analyses were made of different size euhedral grains from the thought to be the result of meteoric fluid circulation during the late same monazite sample (Table 2). The ages are in good agreement stages of cooling of the Comubian batholith. for the different size fractions and do not indicate inheritance of Pb Stage 1, 2, and 3 mineralization have been directly attributed to in any sample. the intrusion and cooling of the Comubian batholith. Although the Muscovite K-Ar closure temperatures have been estimated from stage 4 low-tern~mtu~ fluids may not be linked directly to intrusion the literature to be 320” * 40°C (SNEE, 1982). SNEE et al. f1988) and cooling of the batholith, a model involving low-temperature, inde~ndently estimated a similar tem~ratu~ for closure of mus- regional fluid-convection driven by the high-heat production of the covite to Ar diffusion of -32S”C, based on fluid incfusion filling granite batholith has been proposed (DURRANCEet al., 1982). temperatures at the Panasqueira mine. Using regressions of both lab- Previous geochronologic studies of the Comubian batholith and oratory and geologic data, HARRISON( 198 1) and MCDOUGALLand associated mineral deposits are numerous and exhibit a high variability HARRISON( 1988) calculated a K-Ar closure temperature of 525” in the measured ages (see WILLIS-RICHARDSand JACKSQN, 1989, rt 25°C for hornblende in a pluton cooling at a rate of fOO”C myr-’ for summary of ages). A detailed Rb-Sr study of the major and minor and demonstrated that the closure temperature For rapidly cooled piutons by DARBYSHIREand SHEPHERD( 1985, 1987) established a homblendes ( 1000°C myr-‘ f would be higher (-580°C). One of two-stage emplacement for the batholith, with an age of 290-230 the strengths of eAr/39Ar dating lies in the ability to resolve an age Ma for major granite magmatism and an age of 280-270 Ma for the using step-wise degassing to determine a pfateau or the average ofa minor rhyolite porphyry dikes and chemically specialized granites. portion of an age spectrum whose ages overlap at the 2~ level. The Mineralization ages from the Sn-bearing lodes also exhibit a high majority of the samples presented here exhibit an age plateau using degree of scatter ranging from 294-259 Ma (see WILLIS-RICHARDS the widely accepted and published definition for the term (e.g., DAL- and JACKSON, 1989, for summary of ages). Rubidium-strontium RYMPLEand LANPHERE,1974, RECK et a!., 1977). In order to com- fluid inclusion ages of 269 + 4 Ma (DA&SHIRE and SHEPHERD, pare the 40Ar/39Ar plateau-ages with those of other muscovites and 1985 f and Sm-Nd fluorite aaes 259 + 7 Ma CCHESLEYet al.. i 99 1) U-Pb ages of monazite or xenotime, the plateau criteria have been have been reported for the St&e 3 mineralization in the SoutbCrofiy expanded to conform to those of SNEEet al. ( 1988) and the errors mine and postdate the host Cammenellis pluton by 30-40 myrs, are reported at the 95% confidence level. Samples that failed the suggesting that mineralization must be Linked to later igneous activity. criteria for a plateau were also reduced using the isochron (YORK, “Ar/39Ar and K-Ar ages for clay concentrates suggest that lower 1969) and correlation diagram methods ( RODD~CKet al., 1980). temperature hydrotherrmu solutions were active during the Mesozoic Several biotite Fractions were analyzed for 40Ar/39Ar. Most of the and Cenozoic (JACKSONet al., 1982). data define hump-shaped age-spectra that do not fit the criteria For a plateau. The total gas age is similar to the plateau-ages for corre- SCXENTIFIC ~ET~O~~Y sponding muscovites, but in most cases it is slightly older. Many of To determine the emplacement and cooling history of the Cor- the biotites exhibit weak chloritization in thin section; despite efforts nubian batholith and its effect on hydrothermal circulation, it is nec- to obtain clean biotite separates, chlorite may be present in the sub- Magmatic activity and hydrothermal mineralization 1821

subsequently washed in boiling deionized water for 2-3 h to remove surface contamination. Representative subsamples of the hand- picked separate were examined on the microprobe with Energy Dis- persive System (EDS) and Back-Scattered Electron (BSE) imaging to assure identity and purity. After spiking with a mixed *3sU-~sPb tracer the samples were dissolved in distilled 6 N HCl or concentrated HF and 7 N HNO,, in a 3 ml Teflon@ vial inside a Rrogh-style Teflon@ vessel at 210°C for 3-5 days. The Pb. was purified using 2M3Pb standard HBr-HCl chemistry on BioRad AGl-X8 200-400 mesh - 0049 23qJ . resin. The U was purified using HCI-HNOs-HZ0 chemistry on the same columns (TILTON, 1973; MA~N~N, 1986;MANHES et al., 1984; PARRISH et al., 1987). Samples were corrected for common Pb using the feldspar analyses Of HAMFTON and TAYLOR( 1983) for 0.042 the different plutons in the Comubian batholith. Due to the high

Din7 = Dartma+187 -e 206Pb/M”Pb ratios of most samples the effect of the common Pb cor- L’‘260 Diw - Datlmoa-188 monazite rection is negligible. The measured ratios were corrected for a total H-H-monaziie procedural Pb blank of 40-200 pg. Isotopic compositions of Pb and I I I I , I I I, I 0.048 l- U were determined in the static mode on a VG Sector mass spec- 0.30 0.32 0.34 trometer equipped with an ion~unting Daly detector. The repro- ducibility of the =‘Pb/z%~ ratio for the standard was 0.06% (au). =Pb The U total process blanks were less than 10 pg during the period of 23s~ analysis. The 2u uncertainty of the U-I% and zo7Pb/206w ages include the reproducibility of the standard, common Pb, and blank corrections Fro. 2. Concordia diagram for monazite, xenotime and zircon as well as within-run uncertainties and the uncertainties in the U/ separates from plutons in the Comubian batholith. Error ellipses are Pb ratio of the spike. drawn using 2a error, Minerals for *Ar/39Ar analysis were separated from the same crushed samples used for U-Pb destinations using standard metb- ods. Subsequently, the separates were hand-picked under a binocular microscopic level. The UDAr/39Aranalyses of biotite samples are re- microscope to better than 99% purity. Between 10 and 40 mg of each ported, but are not used for interpretation of the thermal history of sample was enclosed in an Al-foil capsule and placed along with the Comubian plutons. MMhb- 1 hornblende standard (SAMSON and ALEXANDER, 1987 ), Sample locations am plotted on an outline map of the Comubian CaFz and K2S04 in evacuated quartz vials. The samples were irra- granites (Fig 1b). A list of sample locations and lithologic descriptions diated in two separate packages at 1 MW for 30 and 40 hours, re- can be found in Appendix I. spectively, while being contin~~y rotated in the central thimble of the TRIGA research reactor at the USGS in Denver, Colorado ( DAL- RYMPLEet al., 198 1) . The MMhb 1 monitor was distributed evenly ANALYTICAL TIXXINIQUES within the quartz vials in order to obtain a precise monitoring of both the horizontal and radial flux gradients; every sample was ad- Monazite, zircon, and xenotime were separated from crushed jacent vertically to at least one standard. Data from MMhb 1 monitors granite specimens using standard separation techniques. Between show an error in the J-value of 0.1% ( t a) for both packages. The twenty and fifty grains were hand-picked under a binocular micro- samples were anafyzed in the static mode on a MAP 2 15 mass spec- scope on the basis of size, color, lack of inclusions and shape and trometer with a Nier-type source at the USGS in Denver (GEISSMAN

Table 2. U-Pb Analytical Results Abde n~ttw AgutNa) S*mple S LllLOlgf Dl*meter U Pb 2+Spb 2SSp b 2”Pb 20 2*Sp)j 20 2S7pb 2a 2SSpb 20 2S7pb 2a 2S7pb IMlrer8lq 0-w ppm ppm K I,cp zrlpb i%) 23SU 1%) 233fJ (*f 23s~ q 2rdP b Lands Eml flC-1353-W 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 JfG135L191 W] I30-200.c 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 JTC.1363-89 CaE uw 20.50, c 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 llGML89 1&l 6fuoo.e 6778 RI 2495 2.350 0.05162 ( 0.10 ) 0.0436 ( 0.27 ) 0.3105 ( 0.29 ) 275.3 0.7 ) 274.6 1 0.7 ) 268.8 Cwsmenclllr rrC-s3-89 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 lu7S 234 1236 4.477 O.OS2l7 ( 0.12 f 0.0452 ( 0.24 ) 0.3254 ( 0.28 ) 285.2 0.7 ) 266.1 f 0.7 ) 293.0 4M6 874 157s 3.345 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 6069 1414 2612 4.925 0.05223 ( 0.24 ) 0.0466 ( 0.29 ) 0.3357 ( 0.37 ) 293.8 0.8 ] 294.0 [ 1.0 ] 295.7

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 fTC.276M-89 ikbi rm sin 1622 3.3% 0.05192 ( 0.10 ) 0.0146 ( 0.25 ) 0.31% ( 0.27 ) 281.6 0.7 ) 281.6 ) 0.7 ) 282.0 fTC276LS9 t&l 69s4 451 2719 0.6087 OMl89 ( 0.08 ) 0.0446 ( 0.26 ) 0.3197 ( 0.27 ) 281.8 0.7 ) 281.7 ] 0.7 ) 280.9 JTG276G89 ra 3EL 15.5 670.0 MD7836 0.05249 ( 0.19 ) aD446 ( 0.28 ) 0.3230 ( 0.35 ) 281.5 0.8 ) 284.3 1 0.9 ) 307.9 I7c~276T-89 w 10.7 9S6.3 O.lKss2 0.65215 ( 0.17 ) 0.0437 f 0.7.S ) 0.3146 ( 0.32 ) 276.1 0.7 ) 277.8 f 0.8 1 292.0 JTC-276B-89 E 731 31.1 I423 O.&X%9 0.65215 f 0.12 ) oa439 ( 0.34 f 0.3158 ( 0.37 ) 277.1 0.9 1 278.7 1 0.9 ) 292.2 Badmln Moor JIG312-89 -aG IW 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 Hemadoo nwt8EP) Cala (MS) 2MQI 1663 174 1215 3.067 0.03229 f 0.11 ) 0.0453 ( 0.M f 0.3270 ( 0.32 ) 285.9 [ 0.1 ) 287.3 f 0.8 ) 2911.3 bblbgp%;U3BlT=PtSS-&mW~~ ~lnEthoo0 S-e-0 lqddlbh. 2@+b@f’b -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 I I I II II II I 0 20 40 60 60 100

Percentage 3gArK Released (4 Carnmenellis

280

260

260

280 R2480

260

280

260

280

260

260

JTC-242a CGBG

I I I 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

260

D 5260

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

280

260 0 20 40 60 80 100 20 40 60 80 100 (ct

Bodmin Moor

280 JTG3m.es JTC.200.33 280 T,,- II 288.1 f0.9 Tp1283.3 f0.9 muvI I I I 1 I I I I I I I 8 Q 1 k 0300 GBV

280 .nxx~z-ea JTC.10788 1, I 287.1 f 0.9 Tp -293.1 f 0.8 260% - mur L I I / * L s I , L , I t I I 1 20 40 60 80 100 20 40 60 80 100 Percentage 39Ar, Released W @I ‘Dartmoor Skvn CQEG 280 _ JTC449 Tp-28Q3f1.0 260 - hbld _ Tp.279.9f1.0 Hemerdon * I I I I L I I I I t I I t I _ El8MS JTC-199.99 CGBG Comdy~cicGr~ g CQBG . Tp-270.Sf0.8 - Tg-272.3f0.9 280 - rmy: - NIC

260 # , 3 $ I / I I * * I “r, , , I * & I L JTC-18749 CGBG FGBG Dika E GBV - 271.5 f 0.9 280 _ mustTp 5 260 26Oi- -- _ JTC-212.89Tp.277.1 kt.0 42 260. . r, - 296.2 * 0.9 MM I Wo, 0 I I I I I I I I I I I I I I I 0 20 40 60 80 100 20 40 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 aaes are auoted at the 20 confidence level and include the estimated presented in Tables 2, 3, and 4. The U-Pb data are plotted error in j. on a standard concordia diagram (Fig. 2); a U-Pb analysis The techniques for the Sm-Nd 5uorite analyses are given in CHES- is considered concordant only if there is substantial overlap LEY et al. ( I99 1). All data were reduced using the decay constants with concordia at the 2u level. Argon age-spectra are plotted and isotopic abundances recommended by STEIGER and JAGER in Fig. 3 and the Sm-Nd isochrons are plotted in Fig. 4. A ( 1977) and an age of 520.4 Ma for MMhb-1 (SAMSONand ALEX- ANDER, 1987). Isochrons, concordia plots, and weighted average cat- summary of the U-Pb, *Ar/39Ar, and Sm-Nd data from this cuiations were performed using ISOPLOT (LUDWIG, 199 1). study for the Comubian ba~olith is presented in Fig. 5. I824 J. T. Chesley et al.

Ma and 272.9 + 0.8 Ma, respectively. The difference in U-

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

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 The muscovites from samples 8 1 and 242 give ages of 290.7 and xenotime U-Pb ages and agreement with the 40Ar/39Ar ? 0.9 and 290.0 + 0.8 Ma, respectively, and correspondingly muscovite ages with respect to closure temperature, suggest yield a cooling interval of 3.1 and 3.3 myrs ( - 125°C myr-’ ) that the Lands End granite was not reset from an older age. 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 39Ar plateau-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- In contrast to the Lands End granite, it is clear that the nation lode ( 129) in the Geevor mine yield slightly younger Cammenellis pluton underwent a more protracted cooling plateau-ages of 270.4 f 0.9 Ma, 270.5 f 0.8 Ma, and 268.4 history which was affected by multiple intrusive events post- rt 0.8 Ma, respectively, and are distinct in age from primary dating initial emplacement and cooling. The geochronology muscovites from the host pluton. The overlapping nature of supports field observations by GHOSH ( 1934) who suggested the minemli~tion f GARNETT, 1965; JACKSONet al., 1982) several discrete episodes of granitic magmatism in the Cam- and the distinct ages imply that stage 2 and stage 3 miner- menellis area. As in Lands End, there is an excellent agree- alization was long-lived and occurred over a minimum period ment between the *Ar/3gAr plateau-ages (289.2 f 0.8 and of 2-4 myrs. The age for stage 3 polymetallic mineralization 290.3 f 0.9 Ma) and the Rb-Sr mineral isochron ages (287 is corroborated by a fluorite Sm-Nd isochron age of 267 -+ 2 Ma. and 290 4 2 Ma) for samples Cl and C6 on Cam- -?r 12 Ma (2~) (Fig. 4). The above results are also consistent menellis from the study of DARBYSHIRE and SHEPHERD with a previously determined Rb-Sr isochron mineralization ( 1985), but the whole-rock Rb-Sr isochron age (285 + 19 age of 270 + 2 Ma (JACKSON et al., 1982). Ma) has a large uncertainty. The 40Ar/39Ar cooling age of the Carn Marth granite is significantly younger than the Rb- Carnmenellis Pluton Sr whole-rock age (298 + 6 Ma) from DARBYSHIRE and 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 Secondary muscovite taken from a stage 2 tourmaline- with the Carnmenellis pluton at depth (TAYLOR, 1969) and quartz vein from drill core (44) in the South Crafty Mine the Cam Marth granite is considered a separate intrusion gives a 40Ar/39Ar plateau age of 286.6 ? 0.8 Ma, significantly (HILL and MACALISTER, 1906). Two monazite samples from younger than that for the magmatic muscovites in the Cam- quarries ( 8 1,242) in the Cammenellis granite and one from menellis pluton (Table 3), but in excellent agreement with the South CroRy Mine in the Carn Brea granite (53) were the inferred age for elvan emplacement in the South Crofty dated by the U-Pb method (Table 2; Fig. 2). The age of Mine and the cooling ages for the Cam Marth and Rosma- sample 81 is concordant, but thk analysis of sample 242 is nowas intrusives. Muscovite from a vein selvage in a base- reversely discordant beyond analytical unce~inty, most metal pyroxene-garnet-bea~ng skarn intersected -900 m 1826 J. T.Chesley et al.

Table 3. doAr/agAr Data Lfthology Total PI~lc~u EWW %39Ar Steps in Sample II Mincr~l’ Gas Age Age (2U)‘t in p*1tru pl~lcsu

Lmlds End Gmdtrd JTC-p4-89 PGBGD m 271.5 271.8 f 0.8 88.9 6 JTC-135-89 KxiG m 272.1 272.3 f 0.9 91.1 6 n-C-136-89 CGBG m 272.5 272.9 f 0.8 89.0 6 JTC-247-89 KWGD m 271.7 272.5 f 0.8 81.4 6 LB-E47625 m 273.3 272.5 zt. 0.8 88.5 6

ITC-107-89 GBV m 269.9 270.4 * 0.9 92.4 4 Jlx-129-89 GBV m 267.2 268.4 i. 0.8 88.6 5 JTC-147-89 GBV m 270.0 270.5 i 0.8 88.8 6 JTC-177-89 GBV m 272.4 212.3 fz 0.8 84.5 6 Cwnmencllis Granites II-C-57-89 m 285.6 286.5 * 0.8 94.8 5 JTC-El-69 b 289.9 * 0.9 JTG81-89 m 290.5 290.7 f 0.9 89.4 6 ITC-83-89 m 287.7 288.1 * 0.8 81.5 5 ITC-239-89 b 289.4 i 1.0 3X-239-89 m 288.5 288.7 i 0.8 88.9 6 J?C-24289 m 290.0 290.0 i 0.8 89.9 7 JTC-242-89 b 290.0 292.1 f 1.0 75.2 3 JTC-243-89 m 285.9 286.0 i 0.9 94.8 8 R2480 m 283.7 285.7 f 0.8 80.6 6 CS* ” 288.7 289.2 f 0.8 85.8 7 C6* m 290.5 290.3 i 0.9 80.4 6 Mimrsliutim l-r-44-89 GBV m 285.2 286.6 i 0.8 86.4 6 IX-80.89 GBV m 280.7 * 0.9 Sl. Auslell Granites JTC-29-89 CLMG b 277.4 i 0.9 JTC-29-89 CLMG m 216.7 276.9 f 0.8 92.0 8 JTC-30-89 CCBG m 276.5 276.7 * 0.9 88.3 8 JX-276-89 CGBG b 271.8 f I.0 JTC-276-89 CGBG m 278.1 278.3 f 0.8 90.2 7 JTC-289-89 RMG m 273.8 273.9 f 0.8 86.8 6 FG-1 FIG m 267.1 f 0.9 Mineralization JTC-2X4-89 GBV m 270.4 270.3 f 0.8 70.6 5 G-5@ GJW m 276.6 276.8 f 1.2 92.0 7 Bodmin Moor Granite? I’K-312-89 CGBG m 281.2 287.1 * 0.9 92.9 6 JTC-309-89 CGBG m 288.2 288.1 f 0.6 86.6 6 Gunnirlske JTC-200-89 CGBG m 282.5 283.3 f 0.9 95.3 7 Mineralization ITC-197-89 m 280.7 283.1 i 0.9 53.8 4 Dertmoor Granites JTC-489 Fe-Skim hb 280.0 280.3 i 1.0 84.0 4 ITC-185-89 m 270.5 i 0.8 JTC-187-89 CY3BG m 271.0 271.5 t 0.8 95.0 5 JTC-188-89 CCBG m 275.3 275.9 * 1.0 720 6 JTC-189-89 CMG m 272.3 f 0.8 Mindimicm JTC-212-89 FGBGD b 277.1 i 1.0 Repeat 212-89 277.3 * 1.0 Hcmcrdon Mina~lilization IX-318E-89 SM m 271.0 277.8 f 0.8 91.6 5 JK!-319C-89 GBV m 286.2 * 0.9 “CGEG=Coarsc-mined biotirc mud. pOJ3G=F~dncd biotitwmmitc KZBGD=F~nnined biotitc-uanitc dike, 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 sm Ud ‘4’S= “‘N d Error biotite granite at the Wheal Martin deposit and gives a pla- Sample (PC=) (ppm) lr4Nd - (2cr lo+ teau-age of 273.9 f 0.8 Ma, which appears to represent slower

JTC-270.14 25.3 129 0.1182 0512139 cooling or a younger magmatic event than the emplacement JTC-1’10.lb I.pJ 123 0.1nn aSl!ZZO9 of either the Li-mica granite or the main CGBG. Muscovite JTC-270.3~ 0.819 2.51s 0.1968 oLm273 (7) JTC-270.4d 2.64 6.43 0.2484 OS2368 (8) from the fluorite granite ( FG- 1) yields a disturbed age spec- trum and a total gas age of 267.7 + 0.9 Ma. In thin section, the muscovite is ragged and appears to have reacted with below the Wheal Jane Mine (6 km northeast of the Cam- later fluids to form interstitial fluorite, topaz, and secondary menellis pluton), gives a total gas age of 280.7 + 0.8 Ma, but muscovite. The Ar-release spectrum appears to have two sep- shows an Ar loss profile with no plateau and therefore only arate release plateaus containing 39% and 4 1% of the gas and represents a minimum age. An isochron plot of the oldest may retlect a mixing of the two separate muscovite popula- age spectrum steps yields a maximum age of 283.5 + 0.3 Ma tions. Therefore, the muscovite total gas age only represents (2~). Skarn-type mineralization is considered the earliest a minimum cooling age. The isochron for sample FG- 1 yields mineralization related to an intrusive event and should an identical age for all points and a maximum age of 271.0 therefore define a minimum age for the intrusion. Fluorite + 0.4 Ma (2~) using the oldest steps from the spectrum. The Sm-Nd isochrons for the stage 3 mineralization in the South U-Pb ages are younger than previous estimates (285 to 30 1 Crofty and Wheal Jane Mines define ages of 259 + 7 Ma and Ma) for the age of emplacement of the main coarse-grained 266 + 3 Ma (2a), respectively (CHESLEY et al., 1991). The biotite St. Austell granite (HARDING and HAWKES, 1971; stage 3 mineralization ages are in good agreement with fluid BRAY and SPOONER,1983; DARBYSHIRE and SHEPHERD, inclusion Rb-Sr isochron age of 269 + 4 Ma ( DARBYSHIRE 1985). and SHEPHERD, 1985) for a Sn-bearing quartz vein from the Sample G-5 is from a greisen-bordered vein in the Goon- South Crafty mine. In the Wheal Jane mine, elvans clearly barrow Pit, gives a 40Ar/‘9Ar plateau-age of 276.8 f 1.2 Ma predate the emplacement of stage 3 mineralization ( HOLL, (2~) (L. W. Snee, unpubl. data), and is in excellent agreement 1988). At both South Crafty and Wheal Jane the age of main with the cooling age for the muscovite in the Goonbarrow stage 3 mineralization clearly postdates the granite emplace- Pit. Kaolinization is considered to be a younger, low-tem- ment in the Cammenellis region by 20 to 40 myrs and is perature process (SHEPPARD, 1977; BRISTOW and EXLEY, significantly younger than any documented magmatism in 1988); however, it appears to have little or no effect on the the Comubian batholith. Ar systematics of the primary or secondary muscovites. Sam- ple 284 is from the Wheal Remfry tourmaline breccia and St. Austell Pluton yields a *Ar /39Ar plateau-age of 270.3 + 0.8 Ma. The Wheal The St. Austell pluton contains multiple episodes of granite Remfry breccia is a Sn-tounnaline breccia and postdates intrusions, the largest kaolin deposits in southwest England greisen vein mineralization on the basis of crosscutting re- and is the only pluton to contain all of the minor granite lations (BRAY and SPOONER, 1983 ) and contains large in- phases as defined by EXLEY et al. ( 1983) and STONE and clusions of elvan material. The field relations and geochro- EXLEY ( 1985). Three monazite and three zircon fractions nology suggest elvan emplacement was no later than 270 Ma. were analyzed from sample 276 of the main CGBG in the St. Austell pluton (Table 2; Fig. 2). A U-Pb analysis of one Bodmin Moor Pluton of the monazite splits is reversely discordant, but the “‘Pb/ 235U age overlaps with the age of the other two concordant Monazite from sample 3 I2 is from the CGBG major gran- size fractions and all three give a weighted average “‘Pb/ ite phase of the Bodmin pluton and yields a 207Pb/235U age 235Uage of 28 1.8 + 0.4 Ma (2~). The agreement in age from of 291.4 ? 0.8 Ma. The corresponding muscovite gives a the different size fractions indicates that there is no inherited plateau age of 287.1 f 0.9 Ma and thus a cooling rate of Pb. The zircon fractions were chosen on the basis of their 95°C myr -’ . The cooling age is in excellent agreement with euhedral form, pencil shape and clarity, all samples had a sample 309 which yields a plateau age of 288.1 _+ 0.8 Ma length to width ratio of between 5 to 10: 1 suggesting the (Table 3). The 40Ar/39Ar ages are in excellent agreement zircons probably grew from the granite melt. Data for all with Rb-Sr biotite ages 287 +- 2 Ma ( DARBYSHIREand SHEP- fractions are discordant, and judging from their 207Pb/206Pb HERD, 1985) ; however, the Rb-Sr whole-rock age (275 f 9 ages of 307 to 292 Ma these fractions contain inherited Pb Ma) is in poor agreement with the U-PI, age. predating the St. Austell pluton. The 40Ar/39Ar data from sample 276 gives a plateau age of 278.3 + 0.8 Ma, suggesting Gunnislake Pluton that the sample cooled to below the Ar closure temperature for muscovite within 3.5 myrs (- 115°C myr-‘). Samples Gunnislake is a minor CGBG satellite intrusion located 29 and 30 are heavily altered CGBG and Li-mica granite between the Bodmin Moor and the Dartmoor plutons and from the Goonbarrow china clay pit and give indistinguish- gives a 40Ar/39Ar plateau age of 283.1 + 0.9 Ma for sample able plateau ages of 276.9 f 0.8 Ma and 276.7 f 0.9 Ma, 200. Sample 197 is from a minor greisen bordered quartz respectively. Sample 29 is slightly younger than the unaltered vein in the Old Gunnislake mine and yields a plateau age of CGBG granite in sample 276 and most likely represents pro- 283.2 & 0.9 Ma, in excellent agreement with the muscovite tracted cooling or resetting caused by the intrusion of the cooling age of the intrusion. 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, respectively. A hornblende (sample 4) from the metamorphic the Hemerdon granite, gives a plateau age of 277.8 it. 0.8 Ma (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; Fig. 3f), which is confirmed by an isochron analysis. The may represent formation of secondary muscovite by high- 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.O Ma, 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 HUPPERT and 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, the existence of composite intrusions with life spans of several matism (BRAMMALL and HARWOOD, 1932; SCRIVENER, 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 from mantle-derived magmas ( HALLIDAYet al., 1989; HAL- same quarry as sample ( 188) of DARBYSHIREand SHEPHERD l.II)AY, 1990). (1985). The elvan dikes do not appear to be the result of separate magmatism that postdates major granitic intrusions as has Hemerdon Pluton been suggested ( DARBYSHIREand SHEPHERD,1985; JACKSON Hemerdon is a small, highly altered, W-bearing satelhte et al., 1989; WILLIS-RICHARDS and JACKSON, 1989). The intrusion on the south flank of the Dartmoor bathoIith. The 286 Ma 40Ar/3YAr age inferred for the efvan empIa~ment at U-Pb data for a monazite from sample 3 18E of 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 Samples 3 18E and 3 19C are from a drill core at the Hem- of early plutons formed a barrier which prevented the younger erdon W-mine and both exhibit complete replacement of the granites from penetrating to the levels of the present-day sur- coarse-g&ted biotite granite by secondary muscovite quartz face. The elvans may represent dikes tapping into the upper and kaolinite. Sample 319C, a secondary muscovite from a portions of buried magmas shortly after empiacement, and greisen-bordered quartz vein, yields a total gas age of 286.2 they could possibly be feeders for contempor~eous feksic 5 0.9 Ma and the age-s~ct~rn exhibits an Ar loss profile volcanism ( DANGERFIELD and HAWKES, 1969). 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_ETQNet 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 The ages presented here, together with a consideration of by the 40Ar/39Ar analysis of an amphibole from the Haytor the chemistry and isotopic compositions of the granites (e.g., skam, in the aureole of the Dartmoor pluton. The amphibole SHEPPARD, 1977; DARBYSHIREand SHEPHERD, 1985,1987) yields a plateau age of 280.3 & 1.O Ma, in excellent agreement provide powerful cons~ints on the origin of the magmas. with the U-Pb monazite age of 280.4 t 1.2 from a nearby The first question to address is whether the batholith was a granite sample ( 188). In addition, these ages are in good large interconnected reservoir or was built progressively from agreement with a whole-rock Rb-Sr age of 280 t 1 Ma de- separate pulses of melting. The data presented here dem- termined by isotope dilution for the Dartmoor pluton (DAR- onstrate unequivocally that the batholith was formed by the BYSHIRE and SHEPHERD, 1985). Similarly, the 40Ar/39Ar coalescing of separate granite intrusions over a >25 myr in- muscovite ages are in excellent agreement with isotope di- terval ( Fig. 5 ) . The absence of any relationship between the lution Rb-Sr biotite-isochron ages from the same localities age of the plutons, their location in the batholith, and the in the plutons and on the same samples used in the original duration of emplacement of both the ba~olith and the in- study of DARBYSHIREand SHEPHERD( 1985). The internal dividual plutons are irreconcilable with models involving consistency of the 40Ar/39Ar method is observed in samples thin-skinned tectonics transporting magma from the south. 242 and C6, which were both collected from the same quarry A large interconnected magma system at shallow levels would and yield ages of 290.0 + 0.8 Ma and 290.3 f 0.9 Ma, re- be expected to crystallize in a short time span and not over spectively. the 25 myr period measured. Cooling times for the individual plutons, from emplace- Models involving fractionation of mantle-derived magmas ment down to the closure of muscovite to Ar, range between are difficult to reconcile with the large volume of highly frac- 1.9 and 6.7 myrs with a mean of 3.9 myrs (- 100°C myr-' ) tionated granites in the batholith, the lack of more ma& (Fig. 6). These rates reflect the calculated times that elapsed endmembers and the high 6 ‘so and %r /“Sri values ( SHEP- between monazite and muscovite closure to diffusion; the PARD, 1977; HAWKERand DANGERFIELD,1978; DARBYSHIRE actual cooling rate would initially be rapid, followed by slow and SHEPHERD, 1985 ) . We consider models involving crustal asymptotic cooling towards an ambient value (e.g., NORTON melting initiated by mantle-derived magmatism to be a more and KNIGHT, 1977; CATHLES, 1981; HARRISON and likely scenario; an elevated heat flux produced through un- MCDOUGALL, 1980). These cooling rates are much slower derplating of basaltic magmas could account for semi-con- than those predicted by thermal modeling of cooling silicic tinuous granite magmatism over a >25 myr interval. MAN- magmas ( SPERA, 1980; HUPPERT and SPARKS, 1988). NING and H!LL ( 1990) suggested that the late topaz granites Though there is no apparent correlation between cooling rate observed at St. Austell were the result of melting of refractory and age of a pluton, there appears to be a decrease in the residue after partial melting and removal of the biotite-granite overall cooling rate from Lands End in the southwest to melt and are independent of fractional crystallization within Dartmoor in the northeast (Fig. 6). 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 A more detailed evaluation of mechanisms of batholith of magma; 5) geometry of the granite body; 6) location of formation requires discussion of the chemical ~m~sition the sample within the pluton; 7) repeated igneous intrusion for the granites and is beyond the scope of this paper. How- into the same area; 8) cooling from meteoric fluids; and 9) ever, it is clear from the data presented here that the Cor- depth of emplacement. Because of the similar composition nubian batholith should be considered as a series of small of the granites and country rocks across the area of the batbo- anastomosing igneous bodies with each individual body hav- lith, the initial temperature, heat production, and thermal J. T. Chesley et al.

1979; CATHLES, 198 1, 1990; FURLONG et al., 199 1). During initial intrusion, permeability will be very low, fluids will be 50” unable to circulate and cooling will be primarily by conduc- 3 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 100" s pluton in proportion to the extent of fracturing. In the pres- ez t ence of hydrothermal circulation, models have been used to 200" z suggest that cooling would be complete in as little as IO3 to v IO5 years (SPERA, 1980; NORTON and CATHLES, 1979; CATHLES, I98 1). Shortcomings of such models lie in the inability to incorporate multiple intrusive events and the effect of poorly known pluton geometry. Modeling of the Carnmenellis pluton by WILLIS-RICHARDS and JACKSON ( 1989) in the absence of hydrothermal con- FIG. 6. Cooling interval ofindividual plutons within the Comubian batholith. The cooling interval is defined as the difference in age vection suggests that the upper portions of the pluton would between the closure of monazite to the diffusion of Pb (-725°C) cool rapidly, whereas the deeper portions of the pluton (> I5 and the closure of muscovite to Ar (-325°C). km) would remain molten after 15 to 20 myrs and would be a source for the mineralization and elvan dikes. Their model is based on geophysical estimates of the pluton shape and conductivity of the wall rocks would be expected to produce size and the assumption that the emplacement of the Carn- only second-order effects. The presence of dikes, sheeted vein menellis pluton is as a single large magmatic body. In contrast, systems, and large roof pendants in the Lands End pluton the field evidence (GHOSH, 1934), geochemistry (DARBY- indicates proximity to the upper portions of the pluton, while SHIRE and SHEPHERD, 1985, CHAROY, 1986), and geochro- stable isotope analyses of fluid inclusions suggest interaction nology support a multiple intrusive history. Hence, plutonism with meteoric fluids during early mineralization (JACKSON may be better represented as a series of successive smaller et al., 1982 ). Both factors would promote rapid cooling (e.g., intrusions staggered in time, underneath and beside the main SPERA, 1980, CATHLES, 198 1). The Dartmoor pluton is the Carnmenellis granitic body. The depth of formation and ge- largest pluton in the batholith and contains several episodes ometry of these smaller plutons would dramatically influence of magmatism. The slow cooling rate (60” to 80°C myr-‘) their crystallization and cooling history by controlling the disagrees with models of rapid cooling for the Dartmoor timing and extent ofvolatile exsolution, interaction with me- granite supposedly caused by rapid unroofing ( WILLIS-RICH- teoric fluids, and contribute to the total heat budget of the ARDS and JACKSON, 1989). The sample locations are near system. The model of WILLIS-RICHARDS and JACKSON the margins of the pluton and therefore would be expected ( 1989) also fails to explain the lack of similar protracted to give maximum cooling rates. Gravity models of the batho- cooling and mineralization histories for other plutons within lith indicate a deep fault controlled trough just west of the the batholith and why major economic mineralization post- Dartmoor pluton and a decrease in the estimated batholith dates the Carnmenellis pluton by 30 to 40 myrs. thickness in the Dartmoor area (WILLIS-RICHARDS and JACKSON, 1989). The Dartmoor granite is characterized by MINERALIZATION coexisting very high salinity and dense vapor-rich fluid in- The overall mineralization history with respect to the Cor- clusions, consistent with fluids in equilibrium with a granite nubian batholith lasted over 25 myrs (Fig. 5). However, the melt (RANKIN and ALDERTON, 1985). The same assemblages mineralization history for each pluton appears to be distinct. are also typical of the Birch Tor quartz-cassiterite tourmaline The stage I skarn mineralization age is in good agreement veins and greisen-bordered quartz veins of the Hemerdon with the U-Pb emplacement age for the associated pluton as mine of high-temperature magmatic-hydrothermal origin would be expected. Modeling of Sn-W-bearing fluid chemistry (SHEPHERD et al., 1985). In view of the S-deficient nature suggests that high-temperature, low f& and low pH fluids of the mineral assemblages, these observations strongly sug- are required for the transport of tin (e.g., HEINRICH, 1990; gest that the Dartmoor mineralizing fluids were primarily WILSON and ELJGSI-ER,1990). In addition, at low fo,, sulfur magmatic and did not experience significant mixing with will partition more readily into the magma than into the meteoric, sulfur-bearing fluids during emplacement and exsolved aqueous phase (BURNHAM and OHMOTO, 1980), cooling. The long cooling history may also reflect heat loss which is consistent with the sulfur-deficient nature of the primarily by conduction to the wall rocks and not due to stage 2 fluids. According to the models acidic Sn-bearing fluids heat loss from hydrothermal circulation. expelled from the crystallizing magma will be buffered by There are few direct measurements of the cooling rates of the reaction: plutons (from emplacement to -300°C) to compare with the results of the present study. Most of the information and 3KAlSi908 + 2HCl = KAl,Si,O,,(OH), + 6Si02 + 2KCl understanding of plutonic cooling rates are derived from and quantitative modeling of igneous cooling by diffusive and 3NaA1Si308 + 2HCl + KC1 convective heat transfer through porous media (e.g., JAEGER. 1964: NORTON and KNIGHT, 1977; NORTON andC,b~li~~~. KAl,Si,O,,,(OH )> + 6SiOZ + 3NaCI Magmatic activity and hydrothermal mineralization 1831

c forming secondary muscovite (e.g., HEINRICH, 1990). The 1300-295 Ma 1 - y 1275-265Ma] age of stage 2 mineralization is best represented by 40Ar/39Ar secondary greisen-muscovite ages, and in the case of Bostraze and Goonbarrow agree within the limits of statistical reso- lution for the cooling ages of the plutonic muscovite. These ages define a minimum age for the first pulses of magmatic fluid and stage 2 mineralization during the initial crystalli- 295-275 Ma1 B Present Day1 zation and cooling of the pluton. In several cases, the sec- ondary muscovite age is significantly younger than the cooling age of the host pluton (2-5 myrs), and may be correlated with a nearby magmatic event. The long cooling intervals for each pluton, with the exception of Lands End, suggest that meteoric fluids did not significantly interact with the plutons during emplacement and initial cooling. FIG. 7. Schematic illustration depicting the emplacement history The age of stage 3 mineralization is represented by the Sm- of the plutons in the Comubian batholith and relationship to min- Nd fluorite-isochron ages and the Rb-Sr fluid inclusion age eralization. (A) Early emplacement of granite intrusions within the (269 f 4 Ma; DARBYSHIREand SHEPHERD,1985). In the Comubian batholith; (B) Emplacement of the bulk of the exposed case of Cammenellis stage 3 mineralization postdates mag- Comubian batholith from -295 Ma down to -275 Ma; (C) Em- matism by 25-40 myrs. Although some samples display an placement of chemically specialized granites; unexposed granite in- trusions and stage-III mineralization; see text for detailed explanation. argon loss profile suggesting disturbance by a younger event. The secondary muscovite ages from stage 2 mineralization do not appear to record the age of the younger lower-tem- perature stage 3 fluids. The development of the stage 3 min- Further cooling of the granites and expulsion of magmatic eralization of similar age in the oldest (Cammenellis) and fluids continued, with the location of fractures and deposition youngest (Lands End) plutons suggest this was a regional of stage 2 mineralization controlled by the pattern of pre- event and not just centered on a single pluton. Stage 3 min- existing fractures and the regional tectonic framework. Be- eralizing fluids may thus reflect regional hydrothermal cir- tween 275 and 265 Ma the chemically specialized granites culation driven by the elevated heat flux produced by the and late elvans were emplaced, and the magmatic fluid system emplacement and crystallization of younger pulses of granite collapsed allowing external fluids into the upper portions of magmatism (~270 Ma), or fluid circulation driven by the the plutons (Fig. 7~). Circulation of external fluids leached elevated regional geotherm produced by the long-lived, mul- metals and sulfur from the country rocks and plutons. These tiply overlapping, plutonic episodes. Additional heat sources fluids mixed with the late magmatic fluids expelled from the will certainly prolong the duration of fluid circulation such buried crystallizing magmas initiating stage 3 mineralization. that the fluid flow would be realigned with respect to the new Continued circulation of external fluids was driven by the heat source ( CATHLES,1990; NORTON, 1990). Focusing of elevated regional geothermal gradient and latent heat of crys- hydrothermal circulation by younger plutons may account tallization derived from deeper magmatism. Deposition of for the asymmetric zoning of mineralization on the sides of stage 3 hydrothermal mineralization was controlled by pre- the plutons. existing fractures and by overpressuring leading to hydro- fracturing, rupture, mineral deposition, and resealing of the system. Stage 4 mineralization commenced after the em- CHRONOLOGY OF EVENTS placement and cooling of the batholith was completed and was triggered by a change in the regional tectonic stresses Integration of the geochronology with field relations and (e.g., SHEPHERDand SCRIVENER,1987). granite chemistry requires an emplacement and mineraliza- tion history that differs significantly from traditional views DISCUSSION (e.g., STONE and EXLEY,1985; DARBYSHIREand SHEPHERD, 1985; JACKSONet al., 1989) and must involve long-lived The lack of available high-precision geochronologic in- multiple intrusive episodes, each with its own mineralization vestigations of ore deposition make extension of this model history but overlapping and overprinting earlier mineralizing to other tin deposits, much less other granite-related hydro- events. The onset of major granitic magmatism in Cornwall thermal ore deposits, somewhat tenuous. However, several began between 300 and 295 Ma (Fig. 7a). During cooling important observations should be made. The multiple intru- and crystallization of the magma, high-temperature magmatic sive nature of granite batholiths are well documented (e.g., fluids were released. A high geothermal gradient, emplace- PITCHER et al., 1985), and supporting evidence exists that ment of younger granite magmas and slow cooling served to many plutons are composed of discrete litbologic units dis- prevent external fluids from entering the system (Fig. 7a). tinguishable in age (e.g., GEISSMAN et al., 1992); however, Major granitic intrusion continued in different regions of the the shape and size of individual intrusions is often poorly batholith from -295 Ma to -275 Ma (Fig. 7b). Emplace- known. The spatial and temporal multiplicity of the magmas ment of elvan dikes was coincident with magmatism and will lend itself to independent crystallization, fluid expulsion, continued to at least 270 Ma. Formation of stage 1 skam fluid mixing, and mineral deposition histories. mineralization was directly related to the timing of intrusion. Recent two-dimensional models have been used to sim- 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 and useful discussion with respect to her Rb-Sr analyses. JTC would gradient of regions where the distribution and rates of em- like to thank S. Harlan and R. Yeoman who ran the day shift for placement of igneous heat sources are important (BARTON 40Ar/39Ar analysis at the U.S. Geological Survey in Denver. The and HANSON, 1989; HANSON and BARTON, 1989). Such work presented represents a portion of the senior author’s Ph.D dis- models have been used to demonstrate that the background sertation at the University of Michigan. This research was supported geothermal gradients between intrusions may be elevated by by National Science Foundation grants EAR 86 1606 I, 8804072 and 9004413 to A.N.H. and by the Denver U.S. Geological Survey 40Ar/ 5” to 30°C km-‘, and depending on the abundance, depth j9Ar Geochronology project. We are indebted to W. Kelly, S. Kesler, of emplacement and timing of subsequent intrusions can re- J. O’Neil, J. Mauk, M. Ohr, A. Anibas, A. Goode and M. Miller for main elevated for significant time intervals (several tens of discussion and/or critical review of the original manuscript. This millions of years) before returning to normal geothermal paper is published with the approval of the Director of the British Geological Survey (NERC) and the Director of the U.S. Geological gradients. Survey. Thou~tful reviews for GCA by K. Ludwig H. Stein, J. Ruiz, The existence of large regional hydrothe~~ circulation and I. Fanning greatly improved the manu~pt. systems associated with the emplacement ofgranite ~tholiths (e.g., TAYLOR, 1978; CRISS and TAYLOR, 1983) and regional Edtoriui handling: K. R. Ludwig metamorphism (e.g., WOOD and WALTHER, 1986; FERRY and DIPPLE, 199 I ) is well documented. The exact conditions that permit these large circulation systems and the timing of REFERENCES fluid flow with regard to the emplacement and cooling of the ALDERTOND. H. M. ( 1975) Fluid inclusion studies in SW England. batholiths are poorly understood. The presence of long-lived Ussher Sot. Proc. 3, 2 14-2 I 7. geothermal anomalies and large-scale fluid circulation that ALDERTOND. H. M. ( 1978) Fluid inclusion data for lead-zinc ores from SW England. Inst. Mining Metallurgy Trans. 87, 132-l 35. could be modified depending on the relation and timing of BARTONM. D. and HANSONR. B. 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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 I, * ITC-239-89 SW 737 332 ITC-242-89 Pen Ryn Quarry. CGBG SW 737 332 * II JTC-242-89 SW 737 332 ITC-243-89 Abandoned Quamy on Cent Matth. CGBG SW720410 R2480* Rosememwas Quarry Deq Bae Hole. EBG SW 736 347 CS-FD* camsewQltalTy. CGBG SW 758 345 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. Hard purpleFIG. SW 940 555 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 570 585 JTC-3 19C-89 Hemerdm W-Mine. GBV in drill core. ,, CGBG=Couse-grained biotite granite, FGBG=Fitwgnbted biotitegmoite. EBDEquiwular. - 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