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 mineralization. The genesis of kaolinite in altered granites which are associated with mineralization 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 generally 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 weathering 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 isotopic 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.

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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 particularly abundant (up to ,~ 25 % by volume) in the big-feldspar granites. In two complexes--St Austell and Tregonning-Godolphin--lithionite granites are important 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

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...... ~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.

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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-tourmaline 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~

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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.

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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

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2. Sample selection Sample descriptions and locations are given in Table i and on Figs. I and 2. Twelve kaolinites and two kaolinite-rich samples come from three types of occurrence. (I) Ten are from intensely altered granites where the kaolinite is, in large part, pseudomorphic after feldspar. These rocks are white and generally very friable. They constitute the bulk of what is mined for china clay. There is a certain variation in their textures reflecting both variations in the parent granite and different intensities of kaolinization. The samples come from the present surface and up to I25 m down into kaolinized granite. Although some of the samples come from close to quartz veins with well defined phyllic haloes (T-i, LE-3) , most are from pervasively altered granite tens of metres away from major veins. Quartz is the major primary mineral preserved from the granite. Pale brown or white micas including sericite or illite are present in most of these kaolinized granites. Orthoclase is present in the less intensely kaolinized granites. (2) In two samples, kaolinite is not demonstrably replacing an earlier mineral; T-3 is massive kaolinite filling a vug in a pegmatitic clot, SA- 9 is from a gash vein which cuts intensely kaolinized granite. (3) Two whole-rock samples (BT-8, BT-26) come from kaolinite-rich Tertiary sediments of detrital origin which were derived from the Dartmoor area. In addition to the analysis of igneous quartz from five altered granites, eight

TABLE I : continued

.Dartmoor granite 6D2 Merrivale quarry; unaltered biotite-muscovite granite WR 10.8

DI-2 Lee Moor, Whitehill; sericitised muscovite granite Qtz 13.6 Musc -44 ll.7 Ser -32 ll.8 Cligga Head granite CH-2 Greisenhalo, 3 cm wide, about very weak quartz vein Musc -28 11.I Sediments 6C12 Shale; Mylor beds I km east of Porthleven WR 13.4

JT35 Sericitised shale adjacent to vein, Mulberry mine WR -45 14.5

BT-8 Teignbridge mine; Bovey Beds; 23 m below surface WR -59 17.4

BT-26 Teignbridge mine; Bovey Beds; 23 m below surface WR -57 17.9

Water samples

SAW-1 Carthew; stream on St. Austell granite (Collected 7/68) W -36 5~3

SAW-2 Trethosa pit. St. Austell granite; spring at bottom of pit (collected 7/68) W -35 - 6,0 DW-I Cadover bridge, Dartmoor; stream on Dartmoor granite (collected 7/68) W -37 - 5.6

Abbreviations: K = kaolinite; WR = whole rock; Bi = biotite; Qtz = quartz; Set = sericite; Musc = muscovite; W : water

* 6180 kaolinite corrected approximately for impurities (see text).

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'fresh' granites were analysed to characterize the granitic host rocks. Like most 'fresh' granites from Cornwall and Devon, there is evidence for minor incipient alteration in these samples. Because of the size of the Cornubian batholith and extent of alteration and mineralization, it is not yet known whether these few samples are representative of the complex as a whole. Further study is in progress to clarify this point. Samples of muscovite/sericite from feldspar destructive alteration haloes about veins (greisens) come from the St Austell (T-6)and Cligga Head ((]H-2) granites, and from a vein in the metasediments at the Mulberry mine (JT-35 , Sheppard et al. (I97Z)). Sericitization of the Dartmoor granite in D z-2 is pervasive and not from a well defined halo. Sericite is used in a general sense to describe the fine- grained dioctahedral mica or 'mica-type' minerals. The Devonian shale (6(]I2) comes from the very outer fringe of the contact metamorphic aureole about the Tregonning-Godolphin granite. Meteoric waters were also collected from a spring and two rivers on the St Austell and Dartmoor granites. 3. Experimental procedures Standard mineral separating procedures were used, including hand picking under a binocular microscope. The purity of some kaolinites was improved using water

+ I ' I ' I ; I ' I ' I i Sandstones, shales & pelitic schists I' Ii 6C12

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to o Grenville il II II , i11111 Jllil ,i t

New Hampshire J Granites Red Lake, Ontario i

Illl Ill ! I I I I I I I Tuscan Igneous Province IIII III I i I I I i I I

granites l I i I l I , I , I , I l, 8 10 12 14 16 18 20 $180(%ol FIo. 3. Whole rock ~lSO values of Cornubian granites compared with pertinent data from other studies.

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sedimentation techniques. XRD methods indicate that trace amounts of quartz and/or micas and sericite are present in most kaolinite samples, and minor impurities in some (SA-2, SA-5, KC-I, KC-3). The effect of these impurities is not critical to this study (see above). Hydrogen and oxygen were extracted quantitatively from the mineral and water samples by well established analytical methods (Bigeleisen et al. 1952, Epstein & Mayeda I953, Clayton & Mayeda 1963, Sheppard & Taylor 1974). All adsorbed and/or interlayer water was removed from the clays and micas at temperatures up to 2oo°C while under vacuum. Isotope ratios were determined on H~ and COs gases in McKinney-Nier type mass spectrometers. Reproducibility was generally o. I to o.2 per mil for oxygen and I to 2 per rail for hydrogen On duplicate analyses. The data (Table i) are reported as 8-values defined as the difference in D/H or 180/leO ratio of the sample relative to standard mean ocean water (SMOW), in parts per thousand. Analysis of NBS-28 gives 9.60%o.

4. Isotopic results

A) THE CORNUBIAN BATHOLITH The whole-rock oxygen isotope analyses of the fresh granites, coming from the Land's End, Carnmenellis-Carn Brea, Tregonning-Godolphin, St Austell, Bod- rain and Dartmoor granites, are compared in Fig. 3 with whole rock data for other granites. Some of our whole-rock data are calculated from data for igneous quartz samples which were separated from the unaltered and altered granites, taking the quartz/whole-rock fractionation as 1.2 per rail. The quartz samples from the pervasively sericitizcd and kaolinizcd granites arc considered to bc re- lated to the isotopic composition of the granite prior to alteration because textur- ally the quartz is igneous and quartz is a most resistant mineral to isotopic exchange even in a hydrothermal environment (Clayton et al. 1968, Taylor I968 , Sheppard et al. i97i ). The Cornubian granites are all enriched in 180, some by up to 3 per rail, relative to Ha group plutonic subsolvus granites of Taylor (1968). The high 180/ 1sO ratios are observed in quartz from both the altered and unaltered granites. The mineral-mineral fractionation data (e.g. GC-2, Table i) indicate that high temperature magmatic type fractionations are preserved. The 180-rich characteristics cannot be due to post-magmatic alteration and exchange processes. Up to this i968 compilation, the great majority of unaltered plutonic granites were observed to have ~80 values in the range 8- 4 to IO.2 (Ha group). Ex- cluding altered or gneissic granites HH group granites (~180 > IO.2) were known from New Hampshire (a biotite-muscovite granite) and the Prccambrian Red Lake granite, Ontario (Fig. 3). Other HH group granites are now known. The Cretaceous biotite granites of Japan (Matsuhisa et al. I972 , Honma & Sakai 1975) and the Loon Lake biotite granite (monzonitic) in the Grenville of Ontario (Shieh et al. 1976 ). Additionally, studies of the asO-rich rhyolitic volcanic complexes of Tuscany, Italy convincingly demonstrate that 180-rich magmas can exist (Taylor & Turi I976 , Lombardi & Sheppard i976 ).

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A few epi- and mesozonal granites have a marginal zone which has been enriched in ~80 to values greater than IO.2%o (Shieh & Taylor 1969, Turi & Taylor 1971 ). These are not included on Fig. 3 because such extreme ~so enrichment is generally restricted to within a few tens of metres of the contact with zoO-rich country rocks. The enrichment reflects exchange with the neighbouring z sO-rich metasediments. Although most of our kaolinized ~sO-rich granites are probably from the marginal roof zones of the plutons, not all of the other ~SO-rich Cornu- bian samples come specifically from the very outer margins of the plutons. The D/H ratios of the granites, derived from the whole rock, muscovite and biotite data, are in the range --42 to --78 per rail (Fig. 4). Most are within the 'normal' igneous range but some of the muscovite-rich granites are slightly enriched in deuterium (Taylor 1974).

(B) PHYLLIC ALTERATION The D/H and zsO/Z60 ratios of secondary muscovites and sericites coming from alteration haloes about veins in the granites and metasediments are plotted on Fig. 4. Some additional data on sericites from the Simms lode, Geevor mine are

I I I I I I SMOW 0- .~ • Kaolinite 0 Sericite Kaolinite- sericite [] Muscovite [] Biotite -2o. ~ Alteration micas x Whole Rock + Water

i SS ~ ®4 0 /

v -4t - /° Present .-'o -- ', °o; -- r'~ /" tff', , , / Meteoric ,,- ; ,,.o....,, tO r Waters ~lg [] ~ Bovey t ~ Tracey I I I Granites x~ ,,a~,, .- / .q, -60 I I"

China Clays i~'~ tI /*" t S.W England /~.~

-80 I I I I I i -5 0 5 10 15 20 25 6;8 0 (%o) F zG. 4. Plot of SD versus 3tsO values of minerals and waters from granites, alteration haloes, china clay deposits, sedimentary clay deposits of Bovey Tracey and modern meteoric waters. The kaolinite line and meteoric water line are given for reference.

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also included (Jackson, Halliday, Sheppard & Mitchell, unpublished results). The two reference lines shown are the meteoric water line (Craig I96I ) and the kaolinite line defining the isotopic variations in kaolinite from surface weathering environments (Savin & Epstein I97o ). The restricted field for alteration micas lies in between the two reference lines. The sericites from the altered granites are similar to or enriched in deuterium relative to the igneous muscovites. As noted by Sheppard et al. (197 i), sericites from Cornwall are enriched in both D and 180 relative to previously analysed hydrothermal sericites.

(C) KAOLINITES All of the kaolinites given in Table I have a very narrow range of both 8D and 8180 values. Although the amount and nature of the impurities that are present in most of the china clay samples are not known precisely small approximate 8180 corrections have been applied in Table I to those samples with less than 90 % kaolinite. Impurities are largely inherited igneous quartz, feldspar and micas whose average composition is taken to be I2.5~oo. No 8D correction is necessary because kaolinite is overwhelmingly the dominant source of hydrogen in these samples. The corrected kaolinite data have an even smaller range of 180/IEO ratios. These kaolinite data, corrected where necessary, are plotted on Fig. 4. All of the china clay kaolinites, whether from the St Austell or Land's End granites, plot in a very restricted field by the reference kaolinite-weathering line. Two whole-rock ball clay samples from Bovey Tracey (BT-8, BT-26) are also plotted on Fig. 4. They lie close to the china clay field but are enriched in D and depleted in 180 relatively. Mineralogically these ball clays consist of 60-70% kaolinite, I5-25 % mica and 7-I 1% quartz (Edwards I976 ). Isotopically these clays are consistent with their mineralogy if they are mixtures of average china clay (e.g. kaolinite SPS), igneous quartz and muscovite or sericite-illite similar isotopically to those analysed here. The higher 8D values of the ball clays reflect the small but significant hydrogen contribution from the micas.

5. Discussion

A) THE GRANITES AND THE ISOTOPIC COMPOSITION OF MAGMATIC WATER The thirteen whole-rock and quartz analyses of the granites and their regional distribution (Fig. I) imply that a substantial part of the exposed granites must be in Taylor's (i968) HH group of 180-rich granites. These high 180/160 ratios must be characteristic of the granites at the time of their crystallization. They cannot be due to local 180 enrichment processes whose effect is restricted to within a few metres of the contacts. Hence, the 180/1~O ratios of the granites are either primary, reflecting properties of the source region, or are a consequence of large scale assimilation and/or exchange processes between an initially lower-180 magma and 180-rich country rocks at the present or deeper levels.

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Most of the 180-rich granites previously reported occur in amphibolite-facies terrains where the granites were involved in the regional metamorphic processes. 180 enrichment processes are advocated such as large scale exchange assimilation and/or mixing between the magma and its 180-rich country rocks (Matsuhisa et al. I972 , Honma & Sakai i975, Shieh et al. I976 ). Primary HH group magmas like those in the Tuscan Igneous Province (Fig. 3) attained their extremely high 180 characteristics either during the melting of 180-rich argillaceous metasediments (some altered volcanics can be similarly 180-rich (Taylor I968)) or through exchange and/or assimilation processes with 180-rich sediments prior to their emplacement at a higher level (Taylor & Turi I976 , Lombardi & Sheppard I976). For the Cornubian granites large scale exchange at the present level seems unlikely because of the very low metamorphic grade of the surrounding country rocks. Evidence for assimilation is widespread but when the chemical and possible isotopic compositions (Fig. 3) of the country rocks are considered it is doubtful whether this is a sufficient process. Although further information is required to evaluate the relative importance of these mechanisms to generate 1sO-rich granites, what is of immediate concern here is that the magmas were 1sO-rich at the time of crystallization. There- fore, magmatic waters which evolved from the Cornubian granites in the regions affected by subsequent alteration processes were up to 3 per mil enriched in 180 relative to primary magmatic waters (Fig. 5). Because most plutonic and volcanic igneous rocks have a very narrow range of 8D and 8180 values ('normal' igneous rocks), primary magmatic waters were defined to have an equally restricted range of 8D values (--4 ° to --8o) and 8180 values (+6.0 to +9"5) (Sheppard et al. i969, Taylor I974, Sheppard I977). In Fig. 5, a field is labelled Cornubian magmatic waters. This represents the isotopic composition of waters in equilibrium with the analysed granites at magmatic temperatures and hence the isotopic composition of magmatic-hydrothermal solutions prior to any post- magmatic exchange or mixing processes.

B) HYDROTHERMAL MINERALIZATION The origin of hydrothermal fluids (magmatic, meteoric, etc.) can be determined, in general from a knowledge of the hydrogen and oxygen isotope composition of the fluids (Taylor I974, Sheppard I977). The isotopic compositions of the fluids are calculated here by applying the following to the mineral isotopic data: (a) temperature data derived from fluid inclusion homogenization studies on comparable material from Cornwall and Brittany (Sawkins I966 , Charoy & Weisbrod i975, Jackson and others, unpublished work), (b) the mineral H,O isotopic fractionation factors (Taylor i974, Suzuoki & Epstein I976 ) and (c) chemical compositions of the micas (Cundy et al. I96O, Hall I97i ). The calculated field for the hydrothermal fluids is given on Fig. 5. A range of temperatures is used because different samples formed at different temperatures and the precise temperature of formation of the sample is not known. Also, most veins were active over a period of time and formed over a range of temperatures

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(Jackson and others, unpublished work). This field, however, has quite a restricted range of D/H ratios, noting that the micas come from a variety of locations, both in the granite and sediments. The deuterium-rich properties of these hydrothermal fluids clearly distinguishes them from both primary magmatic waters and the more 180-rich Oornubian magmatic waters. The absence of a dominant magmatic water component in these solutions during phyllic alteration confirms earlier interpretations (Sheppard et al. z 97 I). The hydrothermal fluids responsible for phyllic alteration in Cornubia are the most D-rich solutions so far encountered which are associated with batholithic granites (Sheppard z977, fig. 9). The D/H ratios are similar to ocean water values from the late Palaeozoic to the present insofar as there is no compelling evidence for any major change in the isotopic composition of the oceans during this period. The small change from the present SMOW value to about 8D ------7, ,~180 = --I ~oo that has been proposed by Shackleton & Kennett (z975) for pre- Miocene seas, due primarily to the growth of the Antarctic ice cap, is well within our range of D/H values. Meteoric waters which precipitate at low to middle

' /,~.ow , , ,. ' , - /. f" " ---. • ,<.o~i.,. - / = Hydrothermat "'-. .~l ~ "-., & Dickite ~,q~/ ~..., Waters- ser icltes -.. _~"/~,,.# .,. --... (2ooo-4oooc) ; i A Calculated H20" ,~,.'-/~/ ".. --...... _%],

7,'t- .... 7-._.,oo 0 • 7-< / ~ -\ ~-~ -40 / I Curve for HzO in ...... ~'~ ) av f Present .... ;,=k.: ...... Cornubian I \ i to Meteoric ¢~u.,,u.ka.otinite, u,,,SP w$ ,~,, Primary i ~J Geevor Waters Magmat ic I • ~7

-60 Magmaticl-Waters ,l sPs / ' '~ China • ~. HzO ~ Cloy:~ ~"

-80 - Bohemia / rk,~ i I I I I / I -5 0 5 10 15 20 8'80(%o) FIG. 5. Plot of SD versus 8zsO for calculated isotopic composition of the waters that would be in equilibrium with the unaltered granites at magmatic temperatures, and the alteration minerals at the stated temperatures. Also shown are the kaolinite from the Geevor mine (Jackson and others, unpublished work) and kaolinites and a dickite from Bohemia (Savin & Epstein z97o).

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latitudes also have D/H ratios within the range calculated for the hydrothermal solutions. Metamorphic waters may be similarly D-rich (Taylor I974, Sheppard I977) and are a potential source for hydrothermal fluids during some of the earlier periods of activity and generation of phyllic alteration (Jackson and others, unpublished manuscript). However, if these waters were an important constituent of the hydrothermal fluids, the necessary metamorphic dehydration reactions must have occurred at depths well below the current level of exposure, because of the very low metamorphic grade of the country rocks. In the absence of more data, meteoric waters rather than either metamorphic or connate or formation waters are considered to be the dominant source of the hydrothermal fluids during greisenization. The sedimentary country rocks had been regionally metamorphosed, albeit to very low grade at the present level of exposure, prior to the emplacement of the batholith. The bulk of any connate or formation water that was originally present was probably lost during the regional metamorphism. A meteoric water source is also consistent with palaeogeographi- cal evidence. Immediately following the emplacement of the Cornubian batholith, SW England was a land surface (Edmonds et aI. 1969). The late Palaeozoic continental sediments indicate that the climate was hot. Palaeomagnetic data places England within I o o of the equator during the late Palaeozoic with a move to 3o °N by the middle Mesozoic (Briden et al. I974). The high D/H ratios of the proposed meteoric waters are consistent with these data. These arguments and the conse- quences of the evidence for rejuvenation of hydrothermal activity after the main Hercynian mineralization event are discussed in more detail by Jackson and others (unpublished manuscript). The relatively high-lsO character of the hydrothermal fluids reflects that the oxygen isotope composition of the rocks, granites and country rocks, through which the fluids have migrated have had a major influence on the 180/leO ratios in the fluids. In this meteoric-hydrothermal system water/rock ratios were relatively low. (C) KAOLINIZATION There is a dramatic contrast between the isotopic composition of the china clay kaolinites and the alteration micas (Fig. 4). Similarly the calculated isotopic composition of waters in equilibrium with these kaolinites, using SPS kaolinite as an example, are distinctly different from the hydrothermal waters associated with the alteration micas (Fig. 5). None of the china clays could possibly have formed from the types of hydrothermal fluids responsible for vein controlled phyllic alteration unless every kaolinite from the china clay deposits has undergone drastic post- depositional isotopic exchange to a similar extent and with complete loss of any earlier 'memory'. Assuming no major post-formational exchange the constraints placed by the very restricted field for the kaolinites and its location by the kaolinite weathering line make a hydrothermal origin seem unlikely. For a formation temperature I oo °C, the data require either that a magmatic-hydrothermal event followed a

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widespread meteoric-hydrothermal process, which was responsible for phyllic alteration, or that meteoric-hydrothermal waters underwent a major 1sO-shift prior to the kaolinization event, because the kaolinites are enriched in 1sO relative to the feldspar precursor. Additionally if meteoric waters are involved, the decrease in 8D since phyllic alteration requires an increase in latitude and elevation or a decrease in mean annual air temperature (see below). The data strongly support a low temperature supergene or weathering origin. Post-depositional isotopic exchange. As kaolinite is such a fine-grained mineral, usually less than five or so microns (Grim I968), is there any evidence that the china clays attained their present isotopic composition through major post- depositional hydrogen and/or oxygen isotope exchange ? Kaolinites of weathering origin are approximately ~ 27 per rail enriched in 1sO and ~ -- 3 ° per mil in D relative to water (Savin & Epstein 197 o, Lawrence & Taylor 197 I). Thus meteoric waters had 8D ~ --3o, 8180 ~ --5 during kaolinization in the St Austell and Land's End granites. This is very similar to modern meteoric waters in SW England (Fig. 4). However, this does not constitute evidence for re-equilibration with present meteoric waters. Modern waters are isotopically within the range observed in tropical to temperate climates where kaolinization is an active weathering process. Hydrogen isotope exchange has been demonstrated in laboratory kaolinite- water isotopic exchange experiments conducted at temperatures ~> I oo°C and for run lengths of up to nearly 2 years (Sheppard & Taylor, unpublished data, O'Neil & Kharaka I976 ). However, at ioo°C only 5% exchange of hydrogen occurred with immeasurable oxygen isotope exchange. Because of this marked contrast in the hydrogen and oxygen isotope exchange rates, under their experimental conditions, O'Neil & Kharaka proposed that hydrogen exchanged by a mechanism of proton exchange which is independent of the slower 180 exchange process. The china clay data indicate that if low temperature re-equilibration occurred then both the hydrogen and the oxygen isotopes exchanged, and essentially to completion. Although the china clay field is very restricted (Fig. 4) there is a 6%0 variation in the D/H ratio. This is just outside the analytical uncertainties. However, the presence of up to a few per cent of hydrous mineral impurity (e.g. sericite) that is known to be present in many of these samples could be responsible for part of this variation. The most D-depleted clays (e.g. SA-9) come from localities within 2o m of the present surface. But not all near-surface kaolinites are depleted in D relative to kaolinites occurring deeper (e.g. LE-3). Thus, any post-depositional exchange process requires that china clays behaved similarly, isotopically, whether they come from depths of 125 m or from the surface. The range could also be a normal consequence of the range of D/H ratios expected in meteoric waters coming from a region the size of SW England during the period of supergene kaolinization. No convincing argument for major post-depositional exchange is known. However, there are a number of observations which forcefully argue against any extensive post-formational isotopic exchange. (i) Recently, kaolinite has been separated and analysed from an intensely

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argillized granite sheet, in kaolinized shale, which is adjacent to a major ore- bearing vein occurring at a depth of about 4oo m from the surface at the Geevor mine, Land's End granite (Jackson and others, unpublished work). This kaolinite, labelled Geevor, is plotted on Fig. 5. It lies between the china clay and alteration mica fields. Isotopically this is a hydrothermal kaolinite which formed from a slightly more D-depleted fluid than the sericites. This is consistent with the evidence for polyphase hydrothermal activity over a considerable period of time at this locality (Jackson and others, unpublished work). In contrast to the Geevor sample, the shales adjacent to the china clay deposits are not kaolinized. Thus certainly not all kaolinites have undergone major isotopic re-equilibration. The field evidence suggests that there may be more than one type of kaolinization process. (2) Kaolinites from the altered Hercynian granites in the Karlovy Vary area of Bohemia, Czechoslovakia are given on Fig. 5 (Savin & Epstein 1970). They have been interpreted as being of weathering origin (Konta 1969). A dickite associated with Sn-W vein mineralization from Horni Slavkov, Bohemia is also plotted on Fig. 5 (Savin & Epstein 197o ). The geological situation of the Bohemian samples has many similarities with those from SW England. The distinct isotopic difference between the hydrothermal dickite and supergene kaolinite is preserved. (3) The Tertiary kaolinite-rich sediments composing part of the Bovey Forma- tion are derived, at least in part, from the Dartmoor region (Edwards 1976 ). After making allowance for the detrital quartz and sericite (illite) content, these two detrital kaolinites are isotopically indistinguishable from the in situ kaolinites of the St Austell and Land's End granites. Even though all these kaolinites are probably of common origin, they are preserved in very different geological and hydrological environments. Post-formational hydrogen and oxygen isotope re- equilibration of all these kaolinites is implausible. These data favour a Tertiary or earlier age for the formation of the residual kaolinites. (4) Isotopic data on kaolinites of Mesozoic and Cenozoic age from a wide variety of near-surface environments, including porphyry copper deposits, indicate that they have preserved their original 180/leO and D/H ratios (Sheppard et al. 1969, Savin & Epstein 197o, Lawrence & Taylor i971, Sheppard & Gustafson 1976, Lombardi & Sheppard 1976 ). Many of these kaolinites are out of equilibrium with modern meteoric waters. Also, some of these kaolinites occurring at or very near the surface formed in a hydrothermal environment. They have not re- equilibrated with modern meteoric waters. Although every sample must be judged on its own merits, post-formational hydrogen and oxygen isotope exchange of kaolinite with ground water does not appear to be an important process for Mesozoic and younger kaolinites. There are at present too few data on Palaeozoic and older clays to extend this generalization. These four arguments, when considered together, essentially discount any possi- bility that the china clay kaolinites were originally formed in a hydrothermal environment and subsequently underwent exchange during weathering processes. Thus, we conclude that the kaolinites of the china clay deposits are of weathering or low-temperature supergene origin, forming at about 2o°C or a little lower.

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(D) LOCALIZATION OF KAOLINIZATION Pervasive kaolinization of the Hercynian granites is a widespread process in SW England, France and Czechoslovakia (Bristow 1969, Damiani & Trautmann 1969, Kuzvart I969). The deep and intense kaolinization that forms the commercial china clay deposits is more localized. In SW England, intense kaolinization is particularly well developed in the St Austell granite. More than half of the exposed area of granite has been intensely altered. Most of the china clay deposits are within the biotite-free lithium mica granites that compose the western half of the complex (Fig. 2). In the St Austell granite, it is the lithium mica granite which has been most extensively affected by late-magmatic and post-magmatic alteration processes (Exley 1959). There is a broad correlation between the occurrence of kaolinite and veins (Fig. 2). Although there are many examples of veins with or without greisen haloes which cut unkaolinized granite (e.g. greisen veins on St Michael's Mount), intense and deep kaolinization is invariably restricted to granite which is frequently cut by veins. Thus, as there is no isotopic evidence for a hydrothermal kandite mineral as a precursor to the kaolinite, it is proposed that the late-magmatic processes and/or the post-magmatic vein systems were critical in 'preparing the ground' for subsequent deep-weathering processes and generation of kaolinite.

E) CONDITIONS DURING KAOLINIZATION Kaolinization of granite is a process which involves hydrogen-metasomatism. During weathering mildly acid conditions are sufficient (Garrels & Christ I965). For example, carbon dioxide produced during the bacterial decay of organic matter in soils can generate in water the necessary hydrogen ions for a reaction of the type: 2NaA1Si308 + 2CO~ + 3H~O -~ AI~Si2Os(OH)4 + 4SIO2 + 2HCO-8 + 2Na +. This process requires a certain minimum rate of flow of water to leach out the alkali and alkaline earth ions and to prevent or reduce the formation of montmorillonite. The calculated isotopic composition of the supergene solutions that were active during kaolinization in SW England and Bohemia are like those observed today in warm temperate to tropical climates with mean annual air temperatures of about I2°C or a little higher (Dansgaard 1964). Suitable climates probably existed, for example, during the Cretaceous and Tertiary (Edmonds et al. 1969). The kaolinites from Bohemia formed during weathering in the Cretaceous and Tertiary (Konta 1969). If the weathering processes that produced the china clay deposits on the Hercynian granites of Europe were essentially contemporaneous then the slightly lower D/H ratios of the Bohemian kaolinites implies, quite reasonably, that Bohemia was at a slightly different latitude, elevation and/or distance from the sea from SW England.

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6. Conclusions Kaolinite from the china clay deposits in the Cornubian granites and the sedimentary kaolinitic ball clay deposits is, from the isotopic data, of weathering origin. Although there is evidence for hydrothermal kaolinite in Cornwall and Brit- tany from isotopic and inclusion studies (Jackson and others, unpublished work, Charoy & Weisbrod I975, pers. com.), its occurrence is probably quite restricted. In the china clay deposits, the distribution of alteration products between veins and unaltered granite is generally unlike that observed in hypogene hydrothermal wall rock alteration (Meyer & Hemley 1967). The types of silicification, alunitization and chalcedony veins characteristically associated with kaolinization in an acid, hot spring system are absent in SW England, although minor chalcedony veins have been recorded. Also, dickite, a typical mineral in advanced argillic hydrothermal assemblages, has not been recorded from SW England. No convincing argument is known for a hydrothermal kandite mineral as a precursor to the china clay kaolinite. However, most (all ?) granite that was intensely kaolinized during weathering processes had been affected by earlier postmagmatic alteration processes. The local occurrence in the St Austell granite of intensely kaolinized granite beneath unkaolinized metasediments at Carpalla (Collins 1909) and unkaolinized granite sheet (i 3 m thick) at Gunheath (Walker in Konta I97O) have been cited as critical evidence for hydrothermal kaolinization. The lack of intense kaolinization in these rocks could also reflect the importance of (i) the rock type, its mineralogy, permeability and nature and occurrence of veins, and (2) the temperature and chemistry of the solutions. At Geevor, the shale adjacent to the hydrothermally kaolinized granite is kaolinized (Jackson and others, unpublished work). During deep and intense weathering processes there is no reason why the migrating ground waters cannot have a significant lateral component. Large commercial deposits of residual clays have in the past been generally considered to be of hydrothermal origin (e.g. Grim 1968). The weathering origin of the major high quality kaolin deposits in the Hercynian granites of SW England, Bohemia and by analogy Britanny indicates that this generalization requires re- assessment. Commercial kaolinite deposits of hydrothermal origin rarely produce high grade kaolinite because of the intimate presence of other clays and fine grained minerals like alunite and quartz which are undesirable constituents for many applications and difficult to separate from the kaolinite. There are several important implications of a weathering origin for the over- whelming bulk of kaolinization in SW England. For the alteration of feldspar to kaolinite, the ratio of the activities of the base cations to H ÷ decreases with increase in temperature up to about 3oo°C, the upper stability limit of kaolinite (Hemley 1959). The lack of evidence for widespread hydrothermal kaolinization implies that the pH of the fluids was generally too high during the main stages of hydrothermal activity where phyllic alteration was dominant. Kaolinization at temperatures of about 150 °C or a little higher as deduced from fluid inclusion studies (Charoy & Weisbrod 1975, Jackson and others, unpublished work) does not de- mand particularly low pH conditions.

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Wall-rock alteration patterns and chemical variations across the alteration zones have often been studied in searches for guides to ore. However, the interpretation of such data is by no means straightforward if part of the alteration is hydrothermal and part is of weathering origin. Reconnaissance data on the Cornubian batholith have outlined the widespread high-180 character of the granites (HH Group). Insofar as batholithic granites can be generated during the regional metamorphism of continental crust (e.g. Wyllie et al. I976), the results imply that melting, assimilation and/or exchange with argillaceous rich metasediments, the commonest source of 1sO-rich rocks, occurred at depth. Late-magmatic processes were undoubtedly complex in Cornubia and the isotopic composition of the magmas may have been locally modified by exchange and assimilation processes at or near their present level. However, magmatic- and meteoric-hydrothermal solutions were isotopically distinct and meteoric-hydrothermal fluids were dominant during the vein- controlled greisenization processes. Thus, post-magmatic mineralization and the cooling history of the batholith were influenced by the nature of the interaction of the plutons with the convecting ground water system that was established in the surrounding country rocks. Hot springs of meteoric origin (Alderton & Sheppard, unpublished work) are still locally active in SW England. High-XSO HH Group granites of batholithic proportions may be more common than has previously been appreciated.

ACKNOWLEDGEMENTS. I am grateful to N. J. Jackson, R. L. Nielsen, I. Pringle, C. M. Rice, B. D. Rayrnent and I. R. Wilson for supplying some of the samples, to the management of English Clays Lovering Pochin and Co. Ltd, for granting access to their properties, to S. Epstein and H. P. Taylor for use of laboratory facilities at the initial stage of this study and to J. Borthwick for assistance in the laboratory. This study has benefited from discussions and correspondence with C. M. Bristow, C. Gronow, H. P. Taylor and I. R. Wilson. The Isotope Geology Unit is supported by the Scottish Universities and N.E.R.C.

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Received 7 October I976.

Read at Burlington House, 28 April I976. Meeting entitled 'Use of stable isotopes in geology'.

SIMON M. F. SHEPPARD, Isotope Geology Unit, Scottish Universities Research Reactor Centre, East Kilbrlde, Glasgow. Present address: Centre de Recherches Pdtrographiques et Gdochimiques, Case OfficioUe No. I, 54500 Vandoeuvrc-l~s-Nancy, France.

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