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Timing and Significance of Crosscourse Mineralization in SW

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Timing and Significance of Crosscourse Mineralization in SW 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 Printed in Northern Ireland margin of the CreditonTrough (Shepherd, unpublished data).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 England 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-Triassic basins. Road, Exeter 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 , were formed 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 Devon 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 Devonian 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 sandstones of Late Devonian Lateto hosted in Permiansandstone and breccia at the southern Carboniferous age. A paragenetic study by Bull (1982) 587 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/4/587/4889939/gsjgs.151.4.0587.pdf by guest on 28 September 2021 588 R. C. SCRIVENER, ET AL 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 adjacent to 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 Permian and Triassic Tamar crosscourse mineralization, use was made of the fluid continental redbeds,the New Red Sandstone(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
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