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Fluid Inclusion Evidence from South Cornwall

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Fluid Inclusion Evidence from South Cornwall The role of metamorphic fluids in the development of the Cornubian orefield: fluid inclusion evidence from south Cornwall J. J. WILKINSON* Department of Geology, The University of Southampton, Highfield, Southampton SO9 5NH Abstract Veins developed during contact metamorphism associated with the emplacement of the Cornubian granite batholith contain both H20-rich and CO2-rich fluid inclusions. Microthermometric data indicate that unmixing of a low-COa, low-salinity fluid occurred at 400-200 ~ and 1000-500 bars to produce low-density CO2-rich vapour and saline aqueous fluids (8-42 wt. % NaC1 equivalent). Decrepitation- linked ICP analyses show that the cation composition of the brines is dominated by Na, K and Ca, but that significant amounts of Li, Sr, Ba, Fe, Mn, Zn and B are also present. Bulk volatile analyses confirm the dominance of CO2 over N2 and CH 4 in the vapour phase, with COz/N2 molar ratios of 15.3-28.7 and CO2/CH4 molar ratios of 66.9-292. The relative abundance of nitrogen suggests an aureole-derived 'organic' component is present. The source of the fluids is ambiguous as they are intermediate in composition between ideal 'mag- matic' and 'metamorphic' end-members. It is proposed that this is due to mixing of the two types of fluid in the contact aureole during granite intrusion. A model is suggested in which magmatic-meta- morphic circulation occurred synchronously with granite emplacement and subsequently evolved to a meteoric-dominated system with the bulk of the ore deposits forming in response to the influx of meteoric fluids. KEYWORDS: metamorphic fluids, fluid inclusions, Cornubian orefield, Cornwall. Introduction The coastline provides a well-exposed cross-sec- tion through Hercynian structures into the contact A s part of a detailed study of Hercynian orogenic aureole of the Cornubian batholith. In Cornwall, fluid evolution in southwest England, fluids the batholith was intruded into low-grade, domi- mobilized in the contact aureole of the Cornubian granite batholith have been investigated. The nantly pelitic metasediments of Devonian age, at composition of these fluids has been established shallow depths of approximately 4km (Willis- by a fluid inclusion study involving extensive Richards and Jackson, 1989). The currently microthermometric analysis and bulk volatile and exposed granite stocks represent apophyses of the decrepitate analyses. These data have allowed batholith and are compositionally and texturally constraints to be placed on the source and evolu- varied (Exley et al., 1983). The Tregonning gran- tion of fluids in the aureole rocks during granite ite, in the north of the study area (Fig. 1), is a emplacement, as well as providing evidence highly evolved Li- and F-rich topaz granite regarding the possible role of contact meta- (Stone, 1975), which was intruded by passive stop- morphic fluids in the genesis of the Sn-W and ing of the host slates at 280 + 4 Ma (Darbyshire polymetallic sulphide ore deposits. and Shepherd, 1987). Contact metamorphism and . deformation accompanied instrusion of the batholith, produc- Regional geology ing a zone of spotting, recumbent F 3 folds and The study area is situated on the west coast associated $3 crenulation cleavage (Rattey, 1980) of the Lizard Peninsula in south Cornwall (Fig. 1). extending up to 4 km southeast of the Tregonning *Present address: Dept of Geology, Royal School of granite (Fig. 1). The Tregonning granite postdates Mines, Prince Consort Road, London SW7 2BP. this deformation and produced its own zone of Mineralogical Magazine, June 1990, Vol. 54, pp. 219-230 (~) Copyright the Mineralogical Society 220 J. J. WILKINSON 5"30'W S'lS'W ~_~..~-- Hercynian thrust / ~ .... limit of Tregonning ) ~ ~ Normal fault aureole co.,wAa #--='-, 4~'--I limit of ......." ........ batholith ..... b + aureole TREGONNING ---Bou0 ....... i>, ,_._.1_..7-" ~ U- ( + GRANITE .... ,. ////i ~ ~ .......... .. + \ + + I I + ! ~ ~Lizard Point km MYLOR SLATE r FORMATION .......... f"~ 'ite i "J Megiliggar ................~<1/'r Rocks .,--" km thleven :" m o o GRAMSCATHO 0 1 2 o o o GROUP 0 0 ~/~" 0 0 0 0 MOUNT 'S 0 0 0 BAY 0 0 0 0 0 GunwaNo~ i o Fro. 1. Location of study area showing simplified geology and sample sites. contact metamorphism reaching hornblende was contemporaneous with the D1 deformation hornfels grade. Biotite, actinolite, epidote, albite, (Barnes and Andrews, 1981). Structural relation- andalusite and cordierite are developed at lower ships (Rattey, 1980) and K-At dating (Dodson grades. and Rex, 1971) both suggest that D1 significantly predated granite emplacement in this area. This Hereynian metamorphic fluids evidence precludes the involvement of M1 fluids Hercynian events in southwest England re- in the magmatic-hydrothermal system. sulted in the production of metamorphic fluids Jackson et al. (1982) considered that a meta- (i.e. fluids evolved as a result of devolatilization morphic source for the fluids responsible for reactions during prograde metamorphism) at two Sn-W and polymetallic sulphide mineralization distinct stages" (1) During ?early-mid Car- could not be ruled out on the basis of stable isotopic boniferous low-grade regional metamorphism data. However, they emphasized the consistency (M1); (2) During late Carboniferous-early Per- of the data with an early magmatic component mian contact metamorphism (M2), resulting from and a dominantly meteoric origin for later hydro- emplacement of the Cornubian granite batholith. thermal fluids, in agreement with Sheppard Previous studies have failed to constrain the (1977). On the basis of crush-leach studies of role of such fluids in the genesis of the Cornish inclusion fluids, Bottrell and Yardley (1988) and ore deposits. Primmer (1985) suggested that M1 Yardley et aL (1989) suggested that magmatic fluids in North Cornwall may have been involved fluids may be responsible for much of the metal in post-orogenic mineralization on the basis of transport and ore deposition in the system. stable isotopic compositions comparable to those The possible role of M2 fluids in the early stages estimated for hydrothermal fluids by Sheppard of fluid circulation and mineralization has been (1977). However, textural evidence indicates that largely ignored, or at best remained uncon- peak regional metamorphism in South Cornwall strained. Simple calculations of the relative pro- METAMORPHIC FLUIDS 1N CORNWALL 221 portions of 'magmatic' water (i.e. H20 released hosted veins suggests that fluid mobilization and by crystallization of a volatile-bearing melt) and contact metamorphism were contemporaneous. contact metamorphic H20 (produce by dehydra- Although the veins themselves are essentially tion reactions in the aureole) indicate fluid masses unmineralized, greenstone host rocks north of of the same order of magnitude. Despite the Porthleven do show the development of dissemi- uncertainties inherent in such calculations, the nated arsenopyrite, pyrite, sphalerite, chalco- amount of M2 fluid which is likely to have been pyrite and galena. This mineralization may be evolved is significant. Whether or, not this fluid temporally related to the mobilization of the became involved in the magmatic-hydrothermal aureole fluids and they may therefore play an im- system depends on a number of factors, including portant role in the generation of early ore-forming the relative timing and duration of fluid release, solutions. and the direction of flow. Constraining these Structural relationships indicate that the veins parameters is beyond the scope of the present formed late in the intrusion history of the Cornu- paper. Here, the composition of fluids mobilized bian batholith (290-280 Ma) since they crosscut in the aureole are examined directly in order to F3 folds and associated sub-horizontal crenulation assess the importance of different sources in their cleavage ($3) which formed during batholith generation. This information provides some con- intrusion (Rattey, 1980). The veins cannot have trol on the role of M2 fluids in subsequent formed later than the emplacement of the Tre- mineralization phenomena. gonning Granite at 280 + 4 Ma (Darbyshire and Shepherd, 1987), which occurred post-D3. Field relations Fluid inclusion study A study of veins developed in the contact aureole of the Tregonning Granite in southwest A variety of complex inclusion assemblages are Cornwall (Fig. 1) formed part of a wider in- present in the ten samples studied. The six main vestigation into Hercynian fluid evolution in the inclusion types observed are described below. The province. Field studies have revealed a series of subdivisions are not genetic, being based on their veins developed up to 4 km from exposed granite appearance at room temperature (e.g. Shepherd which formed contemporaneously with granite et al., 1985; Rankin and Alderton, 1985). emplacement. 1. 2-phase, aqueous liquid + vapour (F = 0.6- The veins are composed of greyish quartz with 0.95). a greasy lustre and are several centimetres to 2. 2-phase, vapour + aqueous liquid (F = 0.2- 30 cm thick, extending for up to several tens of 0.5). metres in outcrop. A variety of orientations are 3. Multiphase, aqueous liquid + vapour + one developed. Subvertical NNW-SSE-trending veins or more solid phases (F = 0.8-0.95). are abundant in the outer parts of the contact 4. (Apparently) monophase, vapour (F < 0.2). aureole, whereas closer to the granite, subvertical 5. 3-phase, CO 2 vapour-rich (F = 0.05-0.5). NNW-SSE-trending and NE-SW-trending, NW- 6. 2- or 3-phase, CO 2 liquid-rich (F = 0.05-0.2). or NE-dipping veins are observed. The veins are hosted by dark grey or grey-green pelites of the (Where F is the estimated volume fraction of the Mylor Slate Formation as well as by metadolerite aqueous phase.) sills, 'greenstones', which outcrop in the vicinity Inclusions are predominantly 10-20 txm in size, of Porthleven (Fig. 1). although some as large as 40 txm have been Thin-section examination reveals that the veins recorded. Inclusions of all types occur with nega- are composed of equant to elongate subhedral or tive crystal, rounded or irregular morphologies anhedral quartz crystals with rare fine white mica except type 3 inclusions which tend to have a flat- flakes.
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