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The Cornubian Batholith, SW England: D/H and 180/160 Studies of Kaolinite and Other Alteration Minerals

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The Cornubian Batholith, SW England: D/H and 180/160 Studies of Kaolinite and Other Alteration Minerals The Cornubian batholith, SW England: D/H and 180/160 studies of kaolinite and other alteration minerals S.M.F. SHEPPARD SUMMARY D/H and 1sO/leO ratios were determined on thermal fluids. Kaolinites from the major china minerals and whole-rock samples of fresh clay deposits are of weathering origin. They granites, granites altered during greisenization are isotopically consistent with having formed and kaolinization of the Hercynian Cornubian in a tropical to warm temperate climate during batholith, detrital kaolinitic sediments of the the Cretaceous-Tertiary. Although intense Bovey Formation, and modem meteoric supergene kaolinization was probably localized waters. The granites are high-lSO rocks with by earlier post-magmatic processes, there is no 8180 = Io.8 to I3.2 %o due to melting, assimi- evidence that china clay kaolinites originally lation and/or exchange with argillaceous-rich formed during hydrothermal activity and sub- metasediments at depth. Vein-controUed grei- sequenfly underwent post-formational isotopic sening was dominated by meteoric-hydro- exchange. T H ~ C H I N A C L A Y D E YO SI T S in the granites of SW England are one of the world's most important sources of high quality kaolinite. In addition to kaolinization, the Cornubian batholith is associated with important Sn-Cu-Pb-Zn-W mineraliza- tion. The genesis of kaolinite in altered granites which are associated with miner- alization has often been a source of controversy. Both a low temperature supergene and/or deep weathering origin (Hickling I9o8, Coon I9I I, Konta I969) and a hydrothermal origin (Collins i878 , I9o9, Exley I959, I964, Bristow I969, Ed- monds et al. 1969) have been advocated for the kaolinite; the latter is more gener- ally accepted. This paper is primarily concerned with the application of hydrogen and oxygen isotopes to investigate the genesis of kaolinite from these china clay deposits; it forms part of a broader isotopic study of the Cornubian batholith and its associated alteration products. Savin & Epstein (I97O) demonstrated that there is a well defined relationship between the D/H and xso/lsO isotope ratios in kaolinites formed in a weather- ing environment. This principle was applied by Sheppard et al (I969) to dis- tinguish between low temperature supergene and hypogene or hydrothermal clays in porphyry copper deposits. Subsequent studies have supported this approach and have also shown that extensive post-depositional hydrogen and oxygen iso- topic exchange in kaolinite at low temperatures is generally unimportant (Law- rence & Taylor i97i , Sheppard & Gustafson I976, Lombardi & Sheppard I976 ). Because of the variety of complex alteration processes associated with the Cornu- bian batholith some preliminary isotopic data are also presented on the granites and their vein-halo alteration assemblages or greisens to set limits on (I) the isotopic composition of magmatic-hydrothermal waters that possibly evolved from oTl geol. Soc. Lond. vol. x33, x977, pp. 573-59 I, 5 figs., x table. Printed in Great Britain. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/6/573/4885514/gsjgs.133.6.0573.pdf by guest on 02 October 2021 574 S. M. F. Sheppard the crystallizing magmas, and (2) the origin and histories of the fluids during greisening. I. Geological relations of the Cornubian batholith Five major and a number of minor post-kinematic composite granitic plutons intrude the dominantly argillaceous sediments and volcanics of Devonian and Carboniferous age (Fig. I). These country rocks had been metamorphosed under low grade greenschist-facies conditions. The plutons are considered to represent cupolas of the 29 ° Ma Hercynian Cornubian batholith which trends WSW for over 25 ° km in SW England (Bott et al. I958, Edmonds et al. I969). The granites are dominantly biotite-muscovite adamellites which are notably enriched in B, Li, and F (Exley & Stone I966 ). Among accessory minerals tourmaline, topaz, andalusite, cordierite and garnet are of note. Biotite-bearing xenoliths occur widely and locally are common. White orthoclase megacrysts, formed at a late- or post-magmatic stage (Stone & Austin i96I), are widely distributed and are par- ticularly abundant (up to ,~ 25 % by volume) in the big-feldspar granites. In two complexes--St Austell and Tregonning-Godolphin--lithionite granites are im- portant with albitic plagioclase, mica in the siderophyllite-protolithionite- zinnwaldite series, topaz and fluorite (Exley i959, Stone I976 ). Subsequent alteration processes, tourmalinization, greisenization, veining with Cu-Sn-Pb-Zn mineralization and kaolinization were widespread (Dines I956 ). .--. N =T ~" /o Terticlry /s / LE3 " " ....... ~c _ ~ Major porphyry 0-° 6ctz ~ , ', 2Ok,,, l tldykes J 0 z, 8 1Z miles J FIG. x. Generalized geological map of SW England showing part of the Cornubian batholith and its postulated margins (after Bott et al. I958) and the location of samples (see Fig. 2 for St Austell granite). Abbreviations for granites are: A = St Austell, B ---- Bodmin, C = Carnmenellis, D -- Dartmoor, L = Land's End, TG = Tregonning-Godolphin. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/6/573/4885514/gsjgs.133.6.0573.pdf by guest on 02 October 2021 Cornubian batholith, SW England 575 Kaolinization, although widespread in all of the plutons, is particularly well developed in the roof zones, or locaUy at the present surface of the intrusions (Bristow 1969). Areas of extremely intense kaolinization tend to occur in those parts of the roof zones which have quartz or greisen vein swarms, such as in the lithionite granites of St AusteU (Fig. 2). However, there are many quartz-tour- maline veins and greisens that grade into non-kaolinized 'unaltered' granite, e.g. St Michael's Mount, Simms lode, Geevor (Wilson 1972 ). Some of the clay deposits are thought to be funnel- or trough-shaped and up to 200 m or more deep. They are exploited commercially to depths of 125 m or so. Secondary (supergene) alteration in the metalliferous hydrothermal deposits locally extends to depths of about 300 m (Edmonds et al. 1969). The age of kaolinization of the granites is uncertain. Petrographical evidence indicates that kaolinization occurred after the greisening and tourmalinization processes (Exley 1959). Much of the kaolinization was probably Tertiary or earlier because near the edge of the Dartmoor Granite at Bovey Tracey (Fig. I) a deep tectonic basin is filled with Eocene to Miocene sediments including kaolinite rich beds derived at least in part from the Dartmoor Granite (Scott 1929, Edwards 1976 ). The Dartmoor granite was unroofed by late Permian time (Dangerfield & Hawkes i969). CE? N O China Ctay pits Q Kaolinized granite //" Tin lodes 5A-1~ "':-.-..-. :.. SA-4 SA-9 Trethosa I m St Austett 0 Scale Skm I F , , , I FIO. 2. The St. Austell granite showing the distribution of the major granite types, tin lodes, and kaolinization (after Ussher et al. 19o9, Exley 1959, Bristow I969). Also shown are the locations of the samples. Abbreviations for the granite types are: a = megacrystic biotite-muscovite granite; b = megacrystic lithium mica granite; c = lithium mica granite; d = fluorite granite. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/6/573/4885514/gsjgs.133.6.0573.pdf by guest on 02 October 2021 576 S. M. F. Sheppard TAB LE I: Sample location, description and isotopic analysis A. St. Austell Granite, Cornwall (see Fig. 2 and Exley, 1959). Mineral ~D ~18C MeBacrystic lithium mica granite SA-2 Carbeanpit; kaolinite after feldspar; 10 m below surface K -66 18.5 K* 19.5 SA-4 Dorothypit; kaolinite after feldspar in tourmaline granite; surface K -64 19.5 SA-9 Littlejohn pit; kaolinite from 1 cm wide gash vein; 20 m below sur~ce K -65 20.4 SA-IO Littlejohn pit; kaolinite from borehole sample; 73 m below surface K -62 20.C SA-II Goonbarrowpit; unaltered granite from pit bottom WR 13.2 Bi -78 8.6 B-I B1ackpoolpit; kaolinite after feldspar; lO0 m below surface K -62 19.3 Lithium mica granite SA-5 Hendrapit; kaolinite after feldspar; 6 m below surface K -61 18.3 K* 20.3 KC-I Kernickpit; kaolinite after feldspar; 90 m below surface K -60 19.I K* 20.C KC-3 Kernick pit; kaolinite after feldspar; 90 m below surface K -61 17.9 K* 19.3 Qtz 13.C T-I Trethosapit; kaolinite after feldspar; 125 m below surface K -60 18.9 T-3 Trethosapit; kaolinite from pegmatitic vug; 125 m below surface K -60 19.9 T-6 Trethosapit; sericitic halo adjacent to 20 cm wide quartz-toqrmaline vein Set -36 9.1 T-8-I Trethosa pit; pervasively altered granite Lt mica -49 10.9 Megacrystic biotite-muscovite 9ranite SA-7 HelmanTor; unaltered granite WR 12. Bi -65 7. SPS - Kaolinite SPS High quality kaolinite produced from St. Austell area by English China Clays Group -61 19. B. Other Unaltered and Altered Granites and Sediments, Southwest England (see Fig.l) Land's End granite LE-3 Bostrazepit; kaolinite after'feldspar; 60 cm from greisen vein; 6 m K -61 18. below surface Qtz 12. LEG-I Geevor mine; unaltered granite, ~ I00 m from contact; 14 level, -330 m below O.D. WR -42 !2o I 6ClO Lamornaquarry; unaltered big feldspar biotite-muscovite granite Qtz 13.1 Tregonning-Godolphin granite 6C9 Tregonningnon-megacrystic lithium mica granite; Tregonning Hill Qtz 14.: Carnmenellis-Carn Brea granite Carn-I Quarry 1 km north of Constantine (Rice, 1973; No.12, Fig.2) WR -57 12 .I SC-I SouthCrofty mine; altered granite, 450 m below O.D. Qtz 13., GC-2 GreatCondurrow mine; unaltered granite, 25 m below surface Qtz 122 Musc -41 10., Bi -49 7.; WR 11 .: Bodmin granite 6C22 Hantergantick quarry; unaltered biotite-muscovlte granite WR 13.q Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/6/573/4885514/gsjgs.133.6.0573.pdf by guest on 02 October 2021 Cornubian batholith, SW England 577 2.
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