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Journal of he Geological Society, London, Vol. 151, 1994, pp. 587-590. 2 figs. Printed in Northern Ireland

margin of the CreditonTrough (Shepherd, unpublished data). Thus there is evidence to suggest that many styles of Timing and significance of crosscourse low temperature mineralization in SW are related mineralization in SW England to high salinity fluids with a distinct basinal brine affinity. Moreover data for otherUK deposits (Table 1) indicate that the phenomenon is not necessarily restricted tothe R. C. SCRIVENER’, D. P. F. metalliferous areas of SW England, but may be an intrinsic DARBYSHIRE’ & T. J. SHEPHERD3 feature of late and post-Variscan mineralization proximal to l British Geological Survey, St Just, 30 Pennsylvania Permo- basins. Road, EX4 6BX, UK 2 NERC Isotope Geosciences Laboratory, Kingsley Chronology of mineralization. While the crosscourse Dunham Centre, Keyworth NG12 5GG, UK mineralization is demonstrably later than main stage 3British Geological Survey, Kingsley Dunham mineralization (De la Beche 1839; Dines 1956; Hosking 1964), its precise age has remained a matter of speculation. Centre, Keyworth NG12 5GG, UK Collins (1912) and Hill & MacAlister (1906) proposed Mesozoic and Tertiary ages respectively, although no firm evidence was cited. More recently, Durrance et al. (1982) Rb-Sr isotope analysesofinclusion fluids fromquartz have ascribed crosscourse mineralization to ‘an influx of Mesozoic demonstratedTriassica age (236f3Ma) forN-S-trendmg sea water’;a viewwhich clearly conflicts with the fluid Pb-Zn-F vein mineralization in the Tamar Valley district of the inclusion data cited above. Cornubian orefield. Consideration of the stratigaphy and structure Radiometric dating of ore deposits from SW England has of thePermo-Triassic basins both onshore and in the English mostly been targeted at understanding the age of mainstage Channel suggest that this mineralization results from the formation mineralization in relation tothe emplacement of the offractures during regional extension-driven subsidence, and the subsequent ingress of basinal brines to the Variscan basement. Cornubiangranites between 300Ma and 270Ma (Darby- shire & Shepherd 1985, 1987). In addition to the results for In addition to tin and copper, the metalliferous province of the granites,Darbyshire & Shepherd (1985) gave a fluid SW England has yielded considerable tonnages of lead and inclusion Rb-Sr isochron age of 269 f 4 Ma forthe zinc. Much of the lead and zinc was won from N-S-trending mainstage__ tin mineralization at South Crofty Mine. quartz f fluorite-barite hydrothermal vein systems, whereas barxer studies by Halliday (1980) and Bray & Spooner the predominant E-W-trending veins carried tin and copper (1983) reported K-Ar and Rb-Sr mineral ages for a range of deposits.This distinction hasled to the use of the terms granite-related ore deposits in Cornwall. Metal-bearing ‘crosscourse’ and ‘main stage vein’ respectively for the two pegmatites and greisens yielded ages from 285 Ma to groups of deposits.Some N-S veins carry iron ores or 280 Ma, while feldspars from main stage polymetallic veins assemblages including uraniumminerals; the term ‘cross- gave a mean age of 270 Ma. The general scarcity of mineral course’ is also applied to theseand to barrenstructures phasessuitable forradiometric dating in the later which intersect and, in some cases considerably displace, the sulphide-rich stages of the main stage veins, prompted main stage veins more or less at right angles to their strike. Chesley et al. (1991) to attempt Sm-Nd dating of fluorite from tin-bearing structures. Isochron ages of 259 7 Ma and Spatial separation of crosscourse veins from centres of f 266 3 Ma respectively were obtained from suites of fluorite tin-copper mineralization has led tothe theory that they f from South Crofty Mine and Wheal Jane. A similar age of represent an outer, cooler, zone of granite-related deposits 263 8 Ma was obtained by Darbyshire & Shepherd (1990) (e.g. Dines 1956). Recent fluid inclusion data(Alderton f for fluorites from Pendarves Mine. 1978, Shepherd & Scrivener 1987) have demonstrated that Previousradiometric dating crosscourse mineraliza- certain N-S deposits, including theBere Alston (Tamar of tion has been restricted to U-Pb and Pb-Pb determinations. Valley) veins (Fig. l), which arethe subject of this Moorbath (1962) obtained ages in the range 280 20 Ma for study,and Menheniot(Liskeard area) veins , wereformed f agroup of galena specimens from SW England which from low temperature, high salinity Na-Ca-C1 fluids included materialfrom the Menheniotand Bere Alston enriched in CaCI2. In contrast, E-W lead-zinc veins were veins. Uranium minerals froma number of localities in formed from low salinity, sodium chloride-dominated brines and Cornwall were analysed by Pockley (1964) and (Alderton 1978) generally similar to, but of somewhat lower Darnley et al. (1965). The results of these studies tentatively temperature than the brines responsible for polymetallic suggested periods of hydrothermal mineralization at 290 Ma, sulphide ores in E-W-trending veins elsewhere in SW England (Bull 1982; Scrivener et al. 1986). c. 225 Ma and 50-60 Ma. Based on fluid inclusion evidence and REE analyses for TamarValley veins. In selecting the Tamar Valley veins, the host fluorites,Shepherd & Scrivener (1987) concluded Bere Alstondistrict, for geochronological study, two that the low-temperature, high salinity, calcium chloride- important criteria were satisfied; namely, distal location with rich crosscourse fluids were formational brines that had been respect tothe nearest granite, and well-documented expelled from adjacent sedimentary basins. A similar relationship to main stage veins. Furthermore, they have hypothesis was proposed forthe origin of calcium been the subject of detailed fluid inclusion and mineralogical chloride-rich brines responsible for gold-palladium mineral- study (Bull 1982; Shepherd & Scrivener 1987). ization in veins cutting reef limestone at Hope’s The Tamar veins extend southwards from Calstock for a Nose, Torquay (Scrivener et al. 1982). Calcium chloride-rich distance of about 6 km (Fig. 1) and comprise two parallel fluids have also been recognized in association with systems about 1 km apart, cutting folded and faulted slates, low-temperaturereplacement manganese mineralization shales and of Late Devonian Lateto hosted in Permiansandstone and at the southern age. A paragenetic study by Bull (1982) 587

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Table 2. Rb-Sr data for inclusion fluids in quartz and for fluorite from the Tamar Valley crosscourse veins Sam ple LocationSample "'Rb/"%r R7Sr/R6Sr

SW-82-6 Buttspill Fluorite 0.71289 SW-88-6 Buttspill Quartz 1.837 0.71909 SW-88-8 S Tamar Consols Quartz 0.733776.217 SW-88-9 S Tamar Consols Quartz 0.714820.555 CG-la Furzehill Quartz 0.720652.327 CG-3a Furzehill Quartz 0.714550.499

Analyses were carried out at the NERCIsotope Geosciences Laboratory,London, following the method of Darbyshire & Shepherd (1985). Isotope measurements were made on a VG Isotopes MM30 mass spectrometer. Analyses of the NBS 987 Sr isotope standard yielded a mean "Sr/%r value of 0.71038 f O.ooOo2. Analytical uncertaintiesare estimated at 0.02% for R7Sr/R6Srand 1.0% for "Rb/"Sr. Fig. 1. Distribution of basinal brine mineralization localities in SW England mentioned in the text and in Table 1. Harland et al. (1990) for the Ladinian-Carnian stage boundary. indicated that fluorite andsphalerite are deposited atan early stage followed by galena. Minor amounts of calcite are Permo-Triassicbasin development. In the light of the also presenttogether with tetrahedrite, chalcopyrite, Triassic age indicated for the Tamar valley crosscourse veins arsenopyrite, pyrargyrite,pyrite and marcasite. Fluid and the regional distribution of such deposits, the geological inclusion studies(Alderton 1978; Bull 1982; Shepherd & conditions at the time of their emplacement may be revealed Scrivener 1987) on fluorites from both Tamar systems by examiningstratigraphy and structure in the Permo- demonstrate a very restricted range of salinities (19-27 wt% Triassic basins adjacentto the Variscan fold belt. To the NaClequivs) and homogenization temperatures(Th northand south of the landmass of the SW England 110-170°C). Compositions are in the range11-15wt% peninsula respectively, are the submarine Bristol Channel NaCl and 9-13 wt% CaCl, equivalents. and Plymouth Bay basins (Brooks & Al-Saadi 1977; Evans 1990). Onshore,the western part of the Wessex Basin Fluid inclusion Rb-Sr results. To determine the age of the comprisesa substantial sequence of and Triassic Tamar crosscourse mineralization, use was made of the fluid continental redbeds,the New Red (NRS) that inclusion Rb-Sr method described by Darbyshire & extendsfrom southand eastDevon northwards tothe Shepherd (1985). Specimens of quartz were collected from Bristol Channel. spoil dumpsat Buttspill [SX 437 6781, Furzehill [SX 437 Early work by Ussher (1906), with subsequent 6561, andSouth Tamar Consols [SX 437 6451, a N-S lithostratigraphical refinements by Selwood et al. (1984), distance of 3 km, along the eastern vein system. The quartz Bristow & Scrivener (1984) and Scrivener & Edwards was selected from well-crystallized material associated with (1990), described the NRS of SW England in terms of two sulphides, avoiding cryptocrystalline varieties or quartz with major sedimentary cycles, each of which fines upwards from inclusions of fluorite. and/or conglomerates,through sandstone, to Fivespecimens were preparedfor analysis and results mudstone. are shown in Table 2. The R7Sr/R6Srvalue for afluorite Above the Carboniferous-NRS , the specimen from Buttspill was also determined.The data sandstones and breccias are interbedded with basaltic and yielded an isochron age of 236 f 3 Ma (Fig.2). This lamprophyric lavas of the Exeter Volcanic Rocks (EVR). establishes a Mid- to Late Triassic age for the TamarValley Miller et al. (1961) obtaineda K-Ar age forthe EVR crosscoursesystem, close tothe age of 235 Ma given by minette lava flow at Killerton Park, nearExeter of

Table 1. Summary fluid inclusion duru for low lemperorure minerol deposits spaliully relaled 10 Permo-Trimsic bminr m England and Waks Location Mineralization Temperature Salinity NaCI/CaC12 Th"C Wt% NaCl (estimated) Tamar Valley' Pb-Zn-Ba-F 110-l70 19-27 1.1 Teign Valley2.' Pb-Zn-Ba IW160 15-23 2.1 South Crofty4 F IS180 7- 18 1 .n Crediton Trough5 Mn

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285 f 12 Ma (recalculated from 279 f 12 Ma to allow for a mineralization age of 236 f 3 Ma reported here relates to change in decay constants and a 2-U error). However, a new the circulation of ore forming fluidsin the 40Ar/3'Ar analysis of biotite from the Killerton minett;. has Devonian/Carboniferous folded basement. These fluids are yielded an age of 290 f 1 Ma (J. Chesley, pen. comm.), quite distinct from those associated with earlier granite- which constrains the earlier dataand providesa related deposits but similar in overall salinity and calcium chronostratigraphical marker. enrichment to present-day brines found in deep sedimentary The time at which NRS sedimentationcommenced basins (Carpenter et al. 1974; Kharaka et al. 1987). The remains unproven, but it is constrained by the youngest widespreadoccurrence of such brines elsewhere in the strata affected by the Variscan folding, which belong to the region suggests thereforethat they were sourced froma Westphalian C stage (c. 305 Ma in Harland et al. (1990)) of similar reservoir or reservoirs over a considerable period. the Upper Carboniferous (Freshney et al. 1979), and by the Accumulation of the Aylesbeare Group mudstones in 290 Ma age for the Killerton lava which is underlain by NRS the Early Triassic would have acted as an aquiclude above sandstoneand breccia. Abovethe EVR basalts of the the underlying and more permeable Permian sandstones and Exeter area, Warrington & Scrivener (1990) have reported a breccias. Henson (1971) has suggested that a direct source sequence of breccias and sandstone of Late Permian age of detritalmaterial from the Cornubian Batholith (Kazanian to Tatarian, 250-260 Ma). contributing tothe NRS sediments was cut off by Above the basal breccias of the NRS sequence of the Aylesbeare Group times dueto thespread of lacustrine Exeterarea are the predominantlyaeolian conditions across the whole of the region. This is confirmed SandstoneFormation and, higher still, the Aylesbeare in Evans (1990), reporting onthe of the English Group, which comprises a c. 400 m thick sequence of red Channel and its westernapproaches; thick sequences of mudstone. Thequartzite pebbleconglomerates of the mudstone and siltstone in the Melville Basin (up to 600 m) Budleigh Salterton Pebble Beds are without biostratigraphi- and with less certainty, in the Plymouth Bay Basin are cal evidence of age, but vertebrate remains in the overlying correlated with the Aylesbeare Group.The extent of the Otter Sandstone (Milner et al. 1990) indicate an Anisian age former cover of the Variscan basement by NRS strata in SW for the upper partof that formation. The middle part of the England is not certain; the presence of NRS breccias in the Mercia MudstoneGroup (Keuper Marl) has yielded Cawsand-Withnoe area of east Cornwall and of the NRS miospores (Warrington 1971) indicative of the Carnian stage outlier at Portledge in north Devon strongly suggest that the of the Triassic system. The crosscourse vein event at Permo-Triassic cover was formerly much more extensive 236 f 3 Ma is constrained, in terms of stratigraphy to a and has subsequently been removed by erosion. Locally ex- positionbetween the fossiliferous horizons of theOtter tensive reddening of the Devonian and Carboniferous strata Sandstone (Anisian) andthe middle part of the Mercia in SW England has for long (e.g. De la Beche 1839) been Mudstone succession (Carnian).The Ladinian-Carnian regarded as evidence of the former extent of NRS cover. (235 Ma) stage boundary probably lies in the lower part of The fracturing leading to fluid migration responsible for the Mercia Mudstone. the Tamar Valley mineralization is considered to be due to the c. 1 km of extension driven fault-controlled subsidence, Discussion. While there is no evidencefor mineralization described by Evans (1990) as commencing during the early resulting from the movement of basinal fluids in pre-Exeter Triassic in the offshore basins of the northern part of the Volcanic Rock times(older than 290Ma), the work of English Channel.A complex N-Sof normalfaults Shepherd et al. (1985), Scrivener et al. (1986) and Shepherd downthrowing tothe west, and affecting stratafrom the & Scrivener (1987) demonstrates the involvement of such Variscan basement tothe Otter Sandstone is shown by brines by mixing with magmatic and meteoric fluids Whittaker (1985) at the western edge of the Wessex Basin responsible for mainstage Sn-Cu mineralization (c. 285- and these demonstratethe continuation of an E-W 260 Ma) . extensional regime throughout much of the Triassic of the The crosscourse mineralization in the Tamar Valley and region. elsewhere in SW England, reflects a period of E-W crustal In conclusion, the relationship of crosscourse mineraliza- extension leading to the development of regionally extensive tion tothe development of theNRS succession inSW N-S fracture systems. Two important pre-requisites for the England can be summarized as follows. formation of these deposits within these fracture systems are (1) An age of 236 rt 3 Ma has been established for the the availability of metal-complexing fluids, and metalliferous crosscourse mineralization in the Tamar Valley. This age is source-rocks. Scrivener et al. (1989) drew attention to the close to the accepted age of 235 Ma forthe chronostrat- significance of volcanic-related Devonianand Lower igraphical boundary of the Ladinian/Carnian stages of the Carboniferousstratiform mineralization in relation tothe Mid- to Late Triassic distribution of younger hydrothermal veins, in particular the (2)commonA feature of crosscourse and other larger crosscourse systems. For the Tamar and Teign vein low-temperature deposits (<200 "C) is their association with systems, both of which produced substantial amounts of lead highly saline, calcium chloride-rich brines which originate and silver,metals may havebeen derived in partfrom within Permo-Triassic sedimentary basins stratiformenrichments in the LowerCarboniferous host (3) TheTamar Valley crosscourse mineralization took rocks, which include basic volcanic rocks,cherts and place afterthe deposition of a substantial thickness sulphidic black shales. (probably >l500 m) of NRS sedimentary rocks including the It is clearfrom the stratigraphicalaccount above that thick and laterally extensive mudstones of the Aylesbeare fluids evolving in the NRS basin, as developed onshore in Group (c. 400 m). Basinal brines probably formed during SW England, could have been available throughout much of the earlier stages of NRS basin development,and are periodbetween the base of the Permian (290Ma) and at considered to have been involved in main stage granite- least theUpper Triassic (205 Ma).The Tamar Valley related hydrothermal vein mineralization

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Received 25 January 1994; revised typescript accepted 25 February 1994

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