Journal of the Geological Society, London, Vol. 157, 2000, pp. 589–600. Printed in Great Britain.

Post-magmatic hydrothermal circulation and the origin of base metal mineralization,Cornwall, UK

S. A. GLEESON1,3, J. J. WILKINSON1, H. F. SHAW1 & R. J. HERRINGTON2

1T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road,London SW7 2BP, UK

2Department of Mineralogy, Natural History Museum, London SW7 5BD, UK3Present address: School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK

(e-mail: sarah@earth.leeds.ac.uk)

Abstract: A detailed fluid-inclusion study of post-Variscan quartz�carbonate�sulphide�anhydrite�fluorite veins hosted by Palaeozoic basement (Porthleven, Menheniot, Cornwall) andPermo-Triassic sediments (Western Approaches Basin) has identified two broad fluid types. A high salinity(20–27 wt% NaCl), low temperature (80–150�C) brine is present in base metal mineralized veins hosted byPalaeozoic rocks onshore and in sulphate-bearing veins within the Permo-Triassic offshore succession.Microthermometric data indicate that saline fluids found in the Permo-Triassic basinal sequences havesimilar compositions to the base metal mineralizing fluids and confirms that such basins are likely sourcesfor the mineralizing fluids. A second, hotter (200�C) more dilute (0–5 wt%) fluid has been identified infault-hosted east–west-trending veins in the Porthleven area. It is likely that the temperatures of both thedilute and saline fluids have been affected by the local high heat producing Cornubian granite batholith.There is also limited evidence for fluid mixing of these two fluids which may have been responsible for basemetal precipitation.

Keywords: Cornwall, base metals, fluid inclusions, brines.

The Cornubian peninsula of SW England is a classic area forthe study of granite related Sn–W deposits. Much of themineralization occurs in polymetallic veins or lodes whichtrend dominantly east–west (Fig. 1). Also present are a seriesof generally north–south-trending Pb–Zn-bearing veins. Pre-vious fluid inclusion studies on the latter group of veins haveshown that the chemistry of the Pb–Zn mineralizing fluids iscomparable with that of evolved oil-field brines and theoff-shore Permo-Triassic basins in the Western Approaches tothe English Channel (Fig. 2) have been suggested as potentialsources for these fluids (e.g. Wilkinson 1990; Scrivener et al.1994). Here, we report the results of a detailed fluid-inclusionstudy on base metal mineralized and unmineralized veins fromsouth Cornwall and, for the first time, characterize fluids foundin fractures in the putative source basins for the mineralizingbrines. These data provide good evidence for a basinal sourcefor low temperature base metal mineralizing brines in SWEngland.

Background geology

Cornubian PeninsulaVariscan compression in SW England during the Carbonifer-ous resulted in regional low-grade metamorphism, whichreached at maximum the prehnite–pumpellyite facies, and theformation of a series of thrust nappes into which a majorgranite batholith with surface expression of five major exposedplutons was emplaced. These granites and their country rocksare host to the classic granite related Sn–W hydrothermaldeposits of Cornwall and Devon. Mineralization styles in theprovince include Sn-, W-, Cu-, Fe- and As-bearing skarns,mineralized pegmatites, breccia pipes, greisens, sheeted vein

systems and stockworks (e.g. Dines 1956; Hawkes 1974;Dunham et al. 1978; Jackson et al. 1982, 1989; Scrivener et al.1987); however, most of the metal production came fromswarms of large, east–west-trending veins or ‘lodes’ (Fig. 1)(Jackson et al. 1989). Along with Sn- and W-bearing lodesthere is a suite of sulphide-bearing veins such as those found atGeevor mine and in the Kit Hill–Gunnislake district whichformed from moderately hot (200–280�C), variably saline(3–15 wt% NaCl equiv.) fluids (e.g. Jackson et al. 1982;Scrivener et al. 1986, 1987).

Base metal mineralized crosscoursesA suite of distinct north–south- to NW–SE-trending lodes,known locally as ‘crosscourses’ or ‘guides’, are found through-out the Cornubian Peninsula (Fig. 1). These veins cross-cut themain east–west-lodes with dextral oblique-slip offsets in places,and are thus interpreted as post-dating the granite-relatedmineralization events (Dines 1956). These structures arecommonly clay filled but, where mineralized, they may bedivided into six general vein types: Ba-, Pb-, Sb-, U-, Fe- andAu-bearing veins with quartz, carbonate, jasper, fluorite andhematite as gangue minerals (Dines 1956; Jackson et al. 1989).Early workers concluded that the crosscourses must representan outer, cooler, final stage of the granite-related mineraliza-tion (Dines 1956). Subsequently, Sawkins (1966) and Alderton(1978) found from fluid-inclusion studies that these base metalmineralizing fluids were low-temperature, high-salinity solu-tions, similar to those recorded in the northern Pennineorefield and from the Mississippi Valley-type deposits of thecentral USA. Shepherd & Scrivener (1987) suggested that thefluids were basinal brines that had been expelled from basins to

589

the north and south of the structural high formed by theCornubian Peninsula (e.g. Fig. 2).

Crosscourse galenas have been dated by Pb–Pb and U–Pband yielded an age of 280�20 Ma (Moorbath 1962). Uraniumminerals from a variety of localities in Devon and Cornwallappear to define three periods of hydrothermal activity at290 Ma, c. 225 Ma and 50–60 Ma (Pockley 1964; Darnley et al.1965). Scrivener et al. (1994) dated inclusion fluids in quartzusing a Rb–Sr technique, and suggested a Triassic age of236�3 Ma for a series of samples of crosscourse materialfrom the Tamar Valley area.

Geology of the study area

Stratigraphy and structureThe main study area is situated on the west coast of the LizardPeninsula in southwest Cornwall (Fig. 3). The Devonianmetasediments in this region have been divided into twolithofacies (Holder & Leveridge 1986): the PorthscathoFormation and the Late Devonian Mylor Slate Formation(Holder & Leveridge 1986; Floyd & Leveridge 1987). In southCornwall these rocks underwent two phases of compressionin the Lower Carboniferous (Dodson & Rex 1971; Rattey1980; Wilkinson & Knight 1989; Alexander & Shail 1995)followed by a period of extension in the late Carboniferous(?Stephanian) which is thought to predate batholith intrusionand may even have controlled its emplacement (Alexander &Shail 1995). These structures are postdated by moderate- tohigh-angle extensional faults, which frequently host late mag-matic felsic dykes and the granite lode mineralization andthe high-angle NW–SE to north–south-striking subverticalcrosscourses (Shail & Wilkinson 1994).

Evolution of the Western Approaches BasinIn the Late Carboniferous, extensional basins, following thestructural grain of the Variscan, formed on either side of theCornubian massif. These basins are cut by NW–SE-trendingtear faults (Coward 1995) that link with a series of strike-slip faults that transect the Cornubian high such as the

Sticklepath–Lustleigh Fault Zone. The three, off-shore, domi-nantly Permo-Triassic basins, the Plymouth Bay, St Mary’sand Melville Basins, have collectively been named the WesternApproaches Basin (e.g. Hillis & Chapman 1992).

In the Western Approaches Basin the basement predomi-nantly consists of Palaeozoic clastic sediments and limestones,similar to those which crop out on the Cornubian High (Evans1990). The post-Variscan successions are represented by 6000–10 000 m of ?Carboniferous–Triassic red bed sequences. Basal?upper Carboniferous–Lower Permian conglomerates, in partassociated with volcanic extrusives, are overlain by mud-richred beds, and thick, Upper Permian sandstones and shales(Evans 1990; Ziegler 1987). These sequences are truncated by aregional unconformity. In the Early or Mid-Triassic, renewedextension and clastic deposition occurred (Chapman 1989),followed in the Late Triassic by an evaporitic playa-type facieswhich is known to contain thick halite sequences in theMelville Basin. Thermal subsidence continued until the EarlyJurassic (Evans 1990). Renewed rifting in the Jurassic allowedthe establishment of open marine conditions. In the EarlyAptian, a major phase of block faulting and wrench-inducedinversion affected the Western Approaches Basin. Followingthis, the basin became inactive and started to subside locallydue to cooling and sediment loading. This subsidence, alongwith a global rise in sea-level, resulted in the regional deposi-tion of Late Cretaceous chalk and Palaeogene shallow-marinesediments.

Field relationships and sample petrography

Porthleven CrosscoursesThree examples of base metal mineralized crosscourses arefound at Porthleven (Fig. 3). The contact of all the veins withthe host lithologies varies from sharp and planar to irregularand brecciated, with wall-rock fragments and stringers oftenincorporated in the vein. All veins dominantly consist ofalternate precipitation of siderite and quartz; however, in thePorthleven Lode (Fig. 3), much of the carbonate has beenpseudomorphed by goethite and minor hematite. Commonly,the first quartz growth is microcrystalline, followed by thegrowth of large, inclusion-free, prismatic crystals that contain

Fig. 1. Distribution of granite andmineralized veins in SW England,including some of the main areas ofcrosscourse mineralization. 1, St Ivesdistrict (U, Bi–Co–Ni–Ag–As);2, Parranzabuloe district (Pb, Zn, Fe);3, Burngallow district (Fe, U,Ni–Co–Ag–As); 4, Wadebride district(Sb, Pb, Ag, Fe); 5, Menheniot district(Pb, Ag, F, Sb); 6, Tamar Valley district(Pb, Ag, F, Zn, Sb); 7, TeignValley/Bridford district (Pb, Ag, Ba,F, Zn).

590 S. A. GLEESON ET AL.

sporadic growth zones defined by abundant fluid inclusions,fine-grained potassium micas and hematite flakes. Grey orbrown/yellow chalcedony occurs infrequently in thin bands.Early arsenopyrite, chalcopyrite, sphalerite and galena havebeen identified. Pyrite occurs rarely in the crosscoursesin cross-cutting fractures and as fine-grained rims onsiderite. Host-rock alteration is minor and is dominated bysilicification.

Quartz samples are dominated by two-phase (liquid andvapour) aqueous inclusions (type 1) with a degree of fill of0.9–0.98 (Table 1) with a modal size of c. 25 �m. Mostinclusions are subhedral, elongated in the direction of crystalgrowth, are commonly found in arrays defining growth zonesand are unambiguously primary in origin.

Menheniot crosscoursesThe Menheniot region (Fig. 1) is host to a group of north–south-trending Pb-bearing lodes. Samples of crosscoursematerial were collected from the Wheal Mary Ann dump tofacilitate a brief comparative study with the crosscourse min-eralization at Porthleven. Menheniot samples are characterizedby multiple phases of prismatic quartz and chalcedony withtextures very similar to the Porthleven crosscourses. Breccia-tion and silicification of the country rock is common. Yellowor green fluorite occurs late in the paragenesis and is associatedwith galena, chalcopyrite and minor sphalerite. Pyrite andminor marcasite are occasionally present in late veinlets, oftencross-cutting quartz and fluorite.

Quartz samples from Menheniot contain abundantmonophase aqueous inclusions (type 2) and minor two phaseinclusions (type 1). Most inclusions range in size from3–30 �m, are found in growth zone arrays and are consideredto be primary in origin. Fluorite contains some relatively large(10–40 �m) primary inclusions in growth zones. However, the

majority of inclusions are smaller (2–20 �m), occur in micro-fractures and have been classified as pseudosecondary orsecondary inclusions.

Porthleven east–west veinsQuartz veining is also associated with a series of east–west-trending late- to post-Variscan normal faults (Fig. 3). Theseveins are variable in thickness (mm to 50 cm) and exhibitbrittle deformation of the wall-rock. In regions of maximumdilation, drusy quartz crystals of up to 6 cm in length aredeveloped indicating open cavity growth. The sulphide para-genesis of these veins is dominated by fine-grained dissemi-nated pyrite. Minor galena occurs late in the paragenesis and isfound filling interstitial gaps in quartz.

Fluid inclusions in these veins are dominated by type 1primary inclusions. Type 1 inclusions also occur in annealedmicrofractures and are clearly of secondary origin. Someprimary inclusions have very irregular shapes and are sur-rounded by smaller monophase inclusions and may havevariable degrees of fill. These inclusions are interpreted ashaving been formed by necking-down and/or leakage. In somegrowth zones, monophase vapour inclusions (type 3) are thedominant inclusion type. In a small number of zones, however,type 3 and liquid and vapour (type 1) inclusions with variabledegrees of fill are trapped together. This association suggests

Fig. 2. Potential source basins for base metal mineralizing fluids inSW England and location of off-shore wells 87/12-1A and 86/18-1.

Fig. 3. Map of the Porthleven area showing the major lithologiesand the vein sets sampled (after Wilkinson et al. 1995). PL,Porthleven Lode; WR, Wheal Rose; WP, Wheal Penrose.

BASE METAL MINERALIZATION, CORNWALL, UK 591

heterogeneous trapping has taken place within these structuresimplying that periodic boiling has occurred (Shepherd et al.1985).

Western Approaches BasinDrillcore samples were obtained from two commercial explo-ration wells, 87/12-1A and 86/18-1, which lie approximately70 km south of the Cornubian peninsula (Fig. 2).

Lithologies. The oldest rocks sampled in both wells have beendated as ?Carboniferous/Devonian and represent the Palaeo-zoic basement (Fig. 4). This succession comprises a stronglysilicified shale in 86/18-1 and a sandstone/siltstone sequencewith angular and very corroded grains of quartz, lithic frag-ments, albite and K-feldspar in 87/12-1A. In the latter well, thesandstone is unconformably overlain by Middle Devonianskeletal limestones. The Lower Permian successions in bothwells consist of dark red and light grey mudrocks and marlswith sub-angular, corroded, clasts of quartz, albite, mica andlithic fragments in a fine-grained, iron stained, dolomiticmatrix. The Upper Permian/Lower Triassic is represented byinterbedded red quartzo-feldspathic siltstones, claystones and

sandstones cemented by ferroan dolomite and anhydrite.Authigenic dolomite, illite and some minor kaolinite have alsobeen identified in these sequences.The youngest rocks sampledare of Triassic age (87/12-1A) and are predominantly red,medium-grained, well-sorted quartzo-feldspathic sandstones(with some minor interbedded siltstones). A patchy, corrosive,dolomite cement was found associated with a late, sub-vertical,dolomite-filled fracture. Kaolinite is also occasionally presentas a poorly developed cementing phase.

Petrography of veins in the Palaeozoic basement. In 87/12-1Athe Middle Devonian biosparite is cut by an undeformed,1.5 cm wide, ferroan dolomite�siderite-filled fracture. Theferroan dolomite is generally cloudy making it difficult to seefluid inclusions clearly. When inclusions can be seen they arecommonly small (<6 �m), either monophase aqueous (type 2)or are clearly necked. A small number of type 1 inclusions havebeen observed; these are elongated in the direction of growthand appear to be primary in nature.

Petrography of veins in the red bed succession (intra-basinveins). The two red-bed sequences in 87/12-1A have veiningassociated with them. At approximately 1697 m the sandstone

Table 1. Summary of microthermometric data

Vein type Sample no.Inclusion

type*Inclusion

age Min.† nTh

(mean)Max. T

correctionTmi

(mean)Tmh

(mean)Salinity‡

(wt%)NaCl(wt%)

CaCl2(wt%)

Ca/Na(molar)

Porthleven Xcourse SP92017/1 1 Primary Qtz 41 129.5 +5 �23.9 5.5 26.8 23.0 3.8 0.09Porthleven Xcourse SP92017/2 1 Primary Qtz 40 129.0 +5 �24.2 5.0 26.9 22.6 4.3 0.11Porthleven Xcourse SP92017/3 1 Primary Qtz 40 136.1 +5 �25.0 4.9 27.1 20.9 6.2 0.17Porthleven Xcourse SP92017/4 1 Primary Qtz 42 130.6 +5 �26.0 5.0 27.0 19.4 7.6 0.22Porthleven Xcourse SP92018/1 1 Primary Qtz 46 130.4 +5 �23.5 3.3 26.6 23.4 3.2 0.08Porthleven Xcourse SP92018/2 1 Primary Qtz 47 133.1 +5 �24.1 3.5 26.9 21.9 5.0 0.13Porthleven Xcourse SP92018/3 1 Primary Qtz 27 127.9 +5 �26.2 4.5 27.4 19.5 7.9 0.23Porthleven Xcourse SP92018/4 1 Primary Qtz 22 126.7 +5 �23.8 5.2 26.8 22.4 4.4 0.11Porthleven Xcourse SP92018/5 1 Primary Qtz 27 126.5 +5 �25.2 4.5 27.1 20.6 6.5 0.18Porthleven Xcourse J131/1 1 Primary Qtz 36 126.9 +5 �24.1 2.8 26.9 21.9 5.0 0.13Porthleven Xcourse SP93L2 1 Primary Qtz 51 129.9 +5 �24.3 7.1 26.9 23.9 3.0 0.07Porthleven Xcourse VL1 1 Primary Qtz 37 128.4 +5 �24.0 3.7 26.9 22.3 4.6 0.12Porthleven Xcourse 1 Sec. Qtz 193.5 �0.2 0.4Porthleven Xcourse 1 Sec. Qtz 127.0 �33.4 4.7 29.0 10.2 18.8 1.06Porthleven E–W SP92001 1,3 Primary Qtz 18 236.5 0 �4.0 6.4Porthleven E–W SP92006/1 1,3 Primary Qtz 26 224.5 0 �0.4 0.7Porthleven E–W SP92006/2 1,3 Primary Qtz 18 236.7 0 �0.6 1.1Porthleven E–W SP94003 1 Primary Qtz 41 207.9 0 �0.6 1.1Porthleven E–W SP93003 1 Primary Qtz 33 222.5 0 �0.6 1.1Porthleven E–W SP93004 1 Primary Qtz 21 194.7 0 �0.2 0.4Porthleven E–W 1 Sec. Qtz 143.8 �22.2 3.7 26.1 23.8 2.3 0.06Menheniot Xcourse SM92001 1,2 Primary Qtz 8 98.3 +20 �20.3 6.6 26.4 25.9 0.5 0.01Menheniot Xcourse SM92006 1,2 Primary Qtz 15 91.4 +20 �26.4 5.8 26.6 18.1 8.5 0.27Menheniot Xcourse SM92004 1,2 Psec/Sec. Qtz 4 151.2 +20 �23.1 5.2 26.4 25.9 0.5 0.01Menheniot Xcourse SM92005 1,2 Primary Qtz 13 93.8 +20 �23.1 5.2 26.4 23.8 2.6 0.06Menheniot Xcourse SM92005 1 Primary Fl 40 110.3 +20 �25.1 1.8 27.1 22.0 5.1 0.13Menheniot Xcourse SM92005 1 P/Psec. Fl 138.0 +20 �24.1 �23.9 22.0 16.7 5.3 0.18Menheniot Xcourse SM92001 1 P/Psec. Fl 43 148.6 +20 �19.5 �23.7 21.0 15.5 5.5 0.20Menheniot Xcourse 1 Sec. Qtz 100.0 �35.7 29.8 10.7 19.1 1.0287/12-1A 1697 m 1,2 Primary Qtz 21 108.4 c.+60 �23.9 2.3 27.5 23.1 4.4 0.1187/12-1A 1697 m 1,2 Primary Anh 15 66.9 c.+60 �20.7 4.1 26.4 25.9 0.5 0.0187/12-1A 1697 m 1 Sec. Qtz 113.2 �34.9 29.4 10.3 19.1 1.0787/12-1A 1932 m 1,2 Primary Fdol 8 96.8 c.+40 �20.8 4.3 26.4 24.8 1.6 0.04

All temperatures are given in �C.*Inclusion types include monophase liquid (type 2), monophase vapour (type 3) and 2 phase liquid and vapour (type 1).†Host minerals studied are quartz (qtz), fluorite (fl), anhydrite (anh) and ferroan dolomite (fdol).‡Salinity for Ca-rich fluids is wt% NaCl+CaCl2 equivalent as calculated from the phase diagram of Oakes et al. (1990). For NaCl dominated fluidssalinity is wt% NaCl equivalent calculated using the equation of Bodnar (1993).

592 S. A. GLEESON ET AL.

is fractured and a vug containing euhedral quartz overgrownby bladed anhydrite with minor barite is preserved. Drusytextures suggest that mineral growth took place in an openfluid-filled fracture.

Inclusions in euhedral quartz in this vein are dominated byirregular, often flattened, monophase (type 2) inclusions. In-clusions vary in size from 3 to 10 �m, and are commonly foundin growth zones, although some were also observed in cross-cutting fractures. A small number of type 1 (liquid and vapour)inclusions are also present. These inclusions are often larger(10–20 �m), and have relatively consistent degrees of fill(F=0.85–0.95), although they are often surrounded by groupsof small type 2 inclusions, which may suggest these inclusionshave undergone some necking down.

Fluid inclusions in anhydrite are dominantly of type 2 withnegative crystal morphologies. Where type 1 inclusions areobserved, they occur in clusters with variable degrees of fill andare commonly surrounded by small monophase inclusions;thus they have been interpreted as having necked down.

Microthermometric techniqueFluid inclusions in 150–200 �m thick doubly polished quartz waferswere analysed using a Linkam� THMG600 heating-freezing stage,employing standard laboratory procedures (Shepherd et al. 1985).

Stage calibration was carried out at �56.6, +0.0, +30.8 and 293�Cusing synthetic H2O–CO2 fluid inclusion standards. Estimatedaccuracy is �0.2�C in the range �60 to +200�C and replicatemeasurements on the standards gave a precision of �0.1�C.

Results

Porthleven crosscoursesModal eutectic melting (Tfm) in type 1, 2 and 3 inclusionsoccurs in the range �57.0 to �56.0�C (Fig. 5a), comparablewith the ternary eutectic reported by Borisenko (1977) for thesystem NaCl–CaCl2–H2O.

Equilibrium ice melting (Tmi) occurs over a wide range oftemperatures but indicates the presence of three main popu-lations of inclusions (Fig. 5b). The bulk of the measurements(96%) range from �21 to �26�C with a strong mode at�23.1�C. In these inclusions the final phase to melt is a salthydrate, probably hydrohalite, which has melting tempera-tures (Tmh) in the range �3.9 to +12.6�C (Fig. 5c). Hydro-halite is present at higher temperatures than those predicted byphase diagrams. There is no change in appearance in thehydrate at temperatures above 0.0�C and there is no evidenceto suggest a phase change to halite at these temperatures. Aseries of experiments were carried out where inclusions with asingle crystal of hydrohalite were heated to just below thehydrate melting point and left for up to 8 hours without anychange in the size and appearance of the crystal. Once themelting point is reached, the hydrate melts reasonably rapidlyand this behaviour is reproducible. We do not believe, there-fore, that the hydrate is metastable. The high hydrate meltingtemperatures may be due to solid solution effects resultingfrom the presence of significant K and Ca in the fluids. MolarCa/Na ratios calculated using ice and hydrate melting tempera-tures (using the ternary phase diagram of Oakes et al. 1990)range from 0.7 to 0.23 (Fig. 6).

The second Tmi population (3% of the data) comprisesinclusions with comparable eutectic and hydrohalite melting tothe first group but shows lower ice melting temperatures in therange �36.1 to �27.0�C. This population of inclusions ismore calcic with Ca/Na molar ratios of approx. 1.1 (Fig. 6).

The third fluid inclusion group is represented by only threedata (Fig. 5b), all of which have eutectic melting at c. �21.3�Ccomparable with the NaCl–H2O eutectic temperature (Hallet al. 1988). Ice melts in these inclusions at c. �0.3�C andhydrohalite was not observed.

All fluid inclusions from these veins homogenize to theliquid phase at temperatures between 84.9 and 216.2�C with amodal Th of 132�C (Fig. 5d). Some of the measurements athigher temperatures are probably due to the inadvertentanalysis of necked or leaked inclusions. Nonetheless, the diluteinclusions have Th values at the upper end of this range(197.1–216.2�C).

Menheniot crosscoursesEquilibrium ice melting in quartz occurs over a wide rangebetween �36.8 and �0.2�C, with the bulk of the databetween �23.0 and �25.0�C (Fig. 7a). The same three datagroups occur as in the crosscourse samples from Porthleven: adominant population of saline NaCl–CaCl2–H2O inclusions(Fig. 7a); a more saline and calcic fluid (Fig. 6) present in across-cutting fracture; and also a dilute fluid. Hydrohalitemelting temperatures for quartz hosted type 1 inclusions

Fig. 4. Sedimentary logs of the two off-shore wells sampled showingthe location of the vein material sampled. Depth values given aredepth below sea level.

BASE METAL MINERALIZATION, CORNWALL, UK 593

(5.5–10.1�C) are also comparable with the Porthleven datawith the exception of two values from SP92005 of c. �22�C(Fig. 7b).

A histogram of Th data (Fig. 7c) shows the bulk of the type1 inclusions homogenize to the liquid phase in the range63.2–125.5�C, with a mode in the interval 80–100�C. The lowsalinity inclusions homogenize to liquid at values above 190�C.

Primary fluid inclusions in fluorite have ice and hydratemelting temperatures in the range �28.7 to �23.5�C and�4.3 to 8.4�C respectively. Pseudosecondary/secondary, type1, trails have a comparable range in ice melting temperatures(�27.6 to �19.3�C). However, hydrate melting occurs just

before ice, thus, these fluids are less saline than the primaryinclusions (Fig. 7b, Table 1).

Primary inclusions in fluorite have higher Th values (96.7–130.3�C) than those observed in quartz but are significantlylower than those found in pseudosecondary/secondary inclu-sion trails, which have values in the range 132.8–154.9�C(Fig. 7d).

Porthleven east–west veinsA histogram of first melting temperatures for type 1 and 3inclusions (Fig. 8a) shows two populations. The bulk of the

Fig. 5. Histograms of (a) eutecticmelting; (b) last ice melting; (c) hydratemelting and (d) homogenizationtemperatures for type 1 and 3 inclusionsfrom the Porthleven crosscourses.

Fig. 6. Histogram of molar Ca/Na ratiosas calculated from ice and hydratemelting temperatures using the ternaryphase diagram of Oakes et al. (1990).

594 S. A. GLEESON ET AL.

data correspond to the NaCl–H2O eutectic of Hall et al.(1988). However, a second population of inclusions, found assecondary trails in one sample, displays first melting tempera-tures comparable with the NaCl–CaCl2–H2O eutectic(Fig. 8a).

Ice melting temperatures indicate the presence of threepopulations of inclusions in these samples (Fig. 8b). The bulkof the data (74%) are in the range �1.6 C to �0.1�C. Theintermediate Tmi population is centred on �4.4�C and mayrepresent intermediate compositions between the low salinitygroup and a smaller (13%) third population of inclusionswith ice melting at c. �22.5�C. Hydrohalite melting was

observed only in this third group of inclusions with meltingtemperatures ranging from 2.0 to 5.9�C (Fig. 8c).

Homogenization temperature data show a prominent modeat 210–240�C for type 1 inclusions (Fig. 8d). The large spreadin data for these veins is probably partly due to the inadvertentmeasurement of necked inclusions.

Veins in Palaeozoic basementLate ferroan dolomite veins are dominated by type 2 inclusionsand contain few type 1 inclusions. The limited data availableindicate the inclusions are dominated by NaCl–CaCl2 salts andhave ice melting temperatures similar to the crosscourse min-eralizing fluids (Tmi= �21.9�0.2�C; Table 1). Hydrate melt-ing occurs at c. �0.6�C in two inclusions and at c. +9.2�C inthe other suggesting the Ca/Na molar ration of this fluid isapproximately 0.4 (Fig. 6). Homogenization temperaturesrange from 90.2 to 111.2�C (Fig. 9a).

Intra-basin veinsQuartz and anhydrite in red bed hosted veins from 87/12-1Aare dominated by type 2, monophase liquid inclusions. Of thetype 1 and 3 inclusions which have been identified andanalysed, many have metastable ice melting, but must be ofhigh salinity as the bubble commonly pops back coincidentwith instantaneous melting of metastable ice at c. �18�C.Equilibrium ice melting temperatures in quartz vary from�35.1 to �21.0�C, with a mode at �24.0�C, and fromc. �20 to �21�C in anhydrite (Fig. 9b). A salt hydrate meltsabove 0.0�C in both groups. The molar Ca/Na ratios of thequartz-hosted inclusions is lower than the crosscourse fluids(0.11), and, the ratio of the anhydrite fluid inclusions is thelowest encountered in this study at 0.01 (Fig. 6). Like thePorthleven crosscourse fluids, however, the more saline fluidfound in secondary trails is also significantly more calcic(molar Ca/Na�1.1).

Inclusions hosted by anhydrite appear to have lower hom-ogenization temperatures than those found in quartz. Tem-peratures vary from 46.5 to 94.1�C, with a mode at around60�C (Fig. 9c). It is likely that at least some of the higher valuesrecorded for these inclusions are due to the inclusion of neckedor leaked inclusions in the data. Most of the inclusionsanalysed in quartz were relatively large (up to 20 �m) and hadconsistent degrees of fill. However, they are commonly sur-rounded by groups of small monophase inclusions suggestingthat necking-down may have occurred. The Th data have anunusual distribution with a separate lower temperature popu-lation, which coincides with the anhydrite homogenizationdata. Based on textural observations, the bulk of the data athigher temperatures are thought to be anomalous and due tonecking down, with the lower population in quartz andco-existing anhydrite considered to be representative of thetrue trapping conditions (Fig. 9c).

Pressure correction of homogenization dataThe presence of type 1 inclusions with variable degrees of fill,together with monophase vapour inclusions in east–west veins,is probably due to heterogeneous trapping of a boiling fluidphase. If this is the case then the homogenization temperature(Th) equals trapping temperature (Tt), and this temperaturecan be used to estimate the depth of trapping by assuming that

Fig. 7. Histograms of (a) last ice melting, (b) hydrate melting and (c)homogenization temperatures for type 1 and 3 inclusions from theMenheniot crosscourses.

BASE METAL MINERALIZATION, CORNWALL, UK 595

fluid pressure is approximately equal to hydrostatic pressure.Using the diagram of Haas (1978), a NaCl solution of salinity<5 wt%, at c. 220�C would boil at a maximum depth of 256 m(assuming the fractures are freely connected to the surface). Ifthe fluids are boiling at pressures greater than hydrostatic, it islikely that the depth of trapping will be even lower than this.The addition of small quantities of CO2, not visible bymicrothermometry, would increase this depth estimate by lessthan 100 m (calculated after Henley 1984).

The occurrence of secondary brine inclusions in the east–west veins and a small number of dilute secondary inclusionsin the crosscourses suggests that these structures were activeover the same time interval and thus were likely to haveformed at the same maximum depth of 300 m. The presence ofchalcedony in epithermal veins is often interpreted as beingindicative of boiling in hydrothermal systems (Fournier 1985).Chalcedony is observed in the crosscourse veins; however,given the depth constraint above, the saline brines are unlikelyto be boiling at the time of trapping. This means a pressurecorrection is necessary for accurate estimation for trappingtemperatures. The temperature correction for a 27 wt% equiv.NaCl brine at 300 m hydrostatic depth is less than 5�C (Zhang& Frantz 1987).

For the Menheniot crosscourses, a maximum estimate oftrapping temperature can be derived from the assumption thatthe depth of trapping was less than or equal to the maximumdepth of emplacement of the Bodmin granite (4 km; Jacksonet al. 1989; Charoy 1986), the veins currently exposed being ator around the level of the roof zone of the pluton. Assumingthe fluid is hydrostatically pressured, this provides an upperlimit for the trapping pressure of c. 400 bar which correspondsto a maximum pressure correction of c. 20�C (Zhang & Frantz1987).

The estimation of burial depth and, therefore, pressurecorrection of the intra-basin and Palaeozoic-hosted veins off-shore is difficult in the absence of any time constraints on veinformation. However, a general constraint can be placed on thedata using the model presented for well 86/18-1 by Jeans et al.(1994). In this paper it is suggested that the Western Ap-proaches Basin has had about 2500 m of sediments removedsince Permian times. Taking into account present-day sampledepth, this suggests the maximum depth of formation of thequartz anhydrite intra-basin vein is c. 4200 m and of thePalaeozoic-hosted dolomite vein is c. 4500 m. If a geothermalgradient of 30�C km�1 (e.g. Carter et al. 1995) is taken,assuming that the fluids were in thermal equilibrium with the

Fig. 8. Histograms of (a) eutectic melting, (b) last ice melting, (c) hydrate melting and (d) homogenization temperatures for type 1 and 3inclusions from Porthleven east–west veins.

596 S. A. GLEESON ET AL.

host rocks and a surface temperature of 10�C, then thesedepths indicate maximum trapping pressures of c. 450 bar (andtemperatures of 136�C) for the intra-basin fluids and 490 bar(145�C) for the Palaeozoic-hosted veins. Minimum estimates oftrapping temperatures are provided by homogenization tem-peratures considered to be reliable, i.e. c. 50–95�C for quartz/anhydrite veins (1970 m) with no evidence of necking down,and c. 90–111�C (2000 m) for Palaeozoic-hosted dolomiteveins. If a temperature correction for 2500 m of sediment isapplied to the homogenization temperatures than the calcu-lated trapping temperatures (125–170�C for veins at 1970 mand 165–186�C for 2000 m veins) are significantly higher thanthe predicted temperatures given above. This suggests that theveins may not have formed at the maximum burial depth, but

at shallower depths either during burial prior to or during alater inversion event.

DiscussionA Th–Tmi plot illustrates the relationships between the fluidspresent in the veins studied (Fig. 10). The Porthleven andMenheniot quartz data are dominated by saline brines withquite a wide range in salinity. The data sets from both localitiesoverlap but, in general, there is a trend towards ice melting athigher temperatures (lower salinities) in the Menheniot data(Fig. 10). Even if the maximum pressure correction of 20�C isapplied to the Menheniot data set, the bulk of the temperaturedata are still lower than the Porthleven samples suggestingthat the Porthleven hydrothermal system was hotter. Primaryinclusions in Menheniot fluorite have salinities and homogen-ization temperatures similar to the quartz. However, thepseudosecondary/secondary trails, are less saline, more calcicand are significantly higher temperature than the earlier quartzand fluorite.

The Palaeozoic-hosted dolomite and intra-basinal anhydriteveins have similar salinities and Na/Ca ratios to the bulk ofMenheniot primary quartz–fluorite fluids. However, the intra-basin quartz-hosted fluids have a wide range of ice meltingtemperatures and have salinities and Na/Ca ratios which aremore typical of the Porthleven crosscourse fluids. Both sets ofcrosscourses have a late, higher salinity, more calcic fluidinclusion population which is also seen as secondary trails inthe intra-basin quartz vein studied. Microthermometric sal-inity determinations suggest, therefore, that the same chemis-try fluids are observed in the Western Approaches Basin as inthe crosscourses onshore and thus, we believe that these basinsare indeed the source of the mineralizing fluids. This interpret-ation is also supported by oxygen and hydrogen isotopicanalyses on crosscourse, east–west and off-shore veins studied(Gleeson et al. 1999).

However, although the chemical composition of the fluids inthe sedimentary basin are comparable with the base metalmineralizing fluids the homogenization temperatures of thebasinal fluids are in general lower than those of the cross-courses. Figure 11 suggests that a pressure correction ofbetween 300 bar (2.8 km hydrostatic with a 23�C km�1

Fig. 9. Histograms of (a) homogenization temperatures from thePaleozoic-hosted dolomite vein, (b) last ice melting and (c)homogenization temperatures for type 1 and 3 inclusions fromintra-basin vein quartz and anhydrite.

Fig. 10. Summary Th–Tmi plot of fluid inclusion data from all veinsets.

BASE METAL MINERALIZATION, CORNWALL, UK 597

gradient) and 430 bar (4.0 km hydrostatic with 30�C km�1) isnecessary for the intra-basin anhydrite and quartz fluid inclu-sion data to be consistent with the maximum inferred tempera-tures of the Porthleven crosscourse fluids. Similarly, themaximum crosscourse temperatures can be reached with apressure correction of between 180 bar (or 1.7 km hydrostaticwith a 30�C km�1 geothermal gradient) and 240 bar (or2.2 km hydrostatic with a 23�C km�1 geothermal gradient)applied to the Palaeozoic-hosted fluid-inclusion data. As dis-cussed above, none of these veins are likely to have beentrapped at the maximum depth of burial of the basin and thedepths required to attain the temperatures of the crosscoursemineralization are therefore unrealistically high. In order toform the Menheniot veins the Palaeozic-hosted fluids wouldneed to be cooled and the intra-basin anhydrite veins wouldneed pressure corrections on the order of between 120 bar(1.1 km hydrostatic with a 30�C km�1 geothermal gradient)and 170 bar (or 1.6 km hydrostatic with a 23�C km�1 geother-mal gradient). The correction is even greater for the primaryand later fluorite mineralizing fluids (Fig. 11). This suggeststhat if the mineralizing fluids were sourced in the basin, anexternal heat source is necessary to reach the temperaturesobserved in the crosscourses.

The spatial association of the crosscourses with large strike-slip faults which may have been active during basin develop-ment (Coward & Trudgill 1989) suggests that fluid flow out ofthe basin may have been ultimately fault controlled. Openspace, banded textures in the crosscourse veins also suggestthat fluid influx into dilatant faults was episodic and is thuslikely to be related to discrete events. The driving mechanismfor fluid flow remains unknown; however, it is conceivable thatmovements on these structures initiated fluid expulsion fromthe basin, possibly due to the rupture of overpressured com-partments. The brines are thought to have migrated laterallythrough the strike-slip fault zones in Palaeozoic ‘basement’rocks into the topographic high of the Cornubian peninsula,where we believe to they were drawn into convective circula-tion driven by the high heat producing Cornubian batholith.Indeed, to this day, thermal waters (<50�C) are still beingproduced in some of the granite stocks (Edmunds 1986).

The east–west veins at Porthleven, although in the sameorientation as the granite-related lodes, have some fundamen-tal differences. The Porthleven veins are only weakly mineral-ized and homogenization temperatures for these veins are atthe lower end of the temperature range for primary inclusions

measured in the lodes. The range of salinities observed in thelodes are likewise absent in the Porthleven veins. The lowertemperatures and evidence for boiling may suggest that thePorthleven veins were formed either at shallower levels inthe crust or post-date the main granite lodes. However, thetemperatures are still sufficiently high that a heat source mustbe invoked to allow for relatively hot fluids to be found atshallow depths. The low salinities and isotopic characteristics(Gleeson 1996) of the east–west fluids suggest that these fluidsdid not interact with the granite batholith significantly, how-ever, convection of these fluids may have been driven by agranite related thermal anomaly as suggested by Jackson et al.(1989).

The occurrence of secondary brine inclusions in the east–west veins and the presence of a small number a small numberof secondary low salinity inclusions in the Menheniot andPorthleven quartz suggests that these structures were activeover the same time interval (Fig. 10). There is, also, fluidinclusion evidence for the infiltration of saline fluids intoeast–west trending structures (Shepherd & Scrivener 1987;Christoula 1993), which suggests that there may have beenearly influxes of basinal brines during late- to post-mineralization granite-driven convection of meteoric waters(Sheppard 1977). Mixing of these two fluids could result in thetrend to hotter, less calcic, more dilute fluids recorded by theMenheniot fluorites and the spread of salinities reported byWilkinson (1990). However, the fluid inclusion populationsobserved are dominated by end-member compositionsindicating that mixing was extremely inefficient and/or a rareoccurrence.

ConclusionsMicrothermometric studies show base metal mineralizingfluids from Porthleven and Menheniot crosscourses to beNaCl–CaCl2–H2O brines (c. 24–27 wt% NaCl+CaCl2 equiv.)although the Menheniot samples show a trend towards moredilute and less calcic compositions. Fluid inclusions fromfracture-fills in Palaeozoic and Mesozoic rocks from theWestern Approaches Basin are also dominated by salineNaCl–CaCl2–H2O brines with similar bulk salinities. Micro-thermometric data indicate that the crosscourse fluids have asimilar composition to the saline fluids found in the Permo-Triassic basinal sequence. Fluids moved laterally from the

Fig. 11. Isochores calculated frommicrothermometric data using theequations of Zhang & Frantz (1987) forPorthleven and Menheniot mineralizingbrines and the off-shore intra-basin andPalaeozoic-hosted veins. Vertical linesindicate the range of pressure correctionsnecessary for the off-shore fluids to reachthe temperatures found in the Porthlevenbase metal veins.

598 S. A. GLEESON ET AL.

basin into the Palaeozoic horst along repeatedly reactivedstructures where they were heated by the Cornubian batholithto produce the observed temperatures of the base metalmineralizing fluids.

Data from the low salinity east–west veins suggest thatheterogeneous trapping of a boiling fluid may have occurred inthese structures, implying that the veins may have beenemplaced at depths of less than c. 250 m. There is limitedevidence for fluid mixing between the dilute east–west fluidsand the crosscourse mineralizing fluids that may have played arole in the precipitation of base metal sulphides.

We would like to thank BP and BGS for access to core material andlogs. S.G. was supported in the course of this work by ShellInternational Oil Company. J.J.W. was supported by a lectureshipfunded by Minorco Services UK. We would also like toacknowledge the thorough review of an early version of thismanuscript by F. Stuart and the reviews of R.C. Srivener and ananonymous referee.

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Received 12 February 1999; revised typescript accepted 19 September 1999.Scientific editing by Hugh Rollinson.

600 S. A. GLEESON ET AL.