MINERALOGICAL MAGAZINE, DECEMBER 1983, VOL. 47, PP. 487-500

Sphalerite composition in relation to deposition and metamorphism of the Foss stratiform Ba-Zn-Pb deposit, Aberfeldy, Scotland

NORMAN R. MOLES Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW

ABSTRACT. is a common constituent of the deposit, which is the westerly of the two major mineralized rocks and host metasediments of the Foss deposits at Aberfeldy, in both of which sphalerite is baryte-base metal deposit, located near Aberfeldy in widespread and locally abundant. the central Scottish Highlands. Microprobe analyses of sphalerite show a wide range in minor element content (0-17 mol. ~o FeS, 0-3 mol. ~o MnS), and of contrasting composition are often found in the same rock. N 4~ _ ~==~S ~ This suggests that equilibrium domains in some rocks ~--'~-t-.---..~t ~o ~ ~ ~ --~-ff -- .~1(..~.""~'-- Pitloch ry were minute (< 1 ram), during regional metamorphism. -1- Pressures derived from the selective application of the / E,o,e. sphalerite geobarometer are consistent with other minera- _L~a%~ Forroclon ~ 56040 , f logical evidence of peak metamorphic pressures in the I Jlf HIIF toch range 7-10 kbar, at 540-580 ~ However, there is FOSS jrandt u ily~-~ considerable evidence of partial retrograde re-equilibration DEPOSIT of sphalerite, by continued buffering with + pyrrhotine and by outward diffusion of . Marginal "~- loY 0 Miles 3 depletion of Fe and Mn in sphalerite within coarse rocks is attributed to partitioning reactions following recrystallization. Sphalerite which has retained FIG. l. Map of the central Scottish Highlands showing its pre-metamorphic composition shows systematic varia- location of the Aberfeldy deposits. Outcrop of the Ben tions in composition through profiles of mineralized beds, Eagach Schist is shown by the stippled ornament. which may be related to depositional environments. Bimodal primary compositions can be explained by precipitation of sulphides under the contrasting The importance of sphalerite in geological studies chemical environments of hydrothermal vents and cooler, derives from its refractory nature, and its capacity exhaled brine layers on the sea floor. to record physical and chemical characteristics of the environment of deposition or metamorphism THE Aberfeldy stratiform baryte-base metal by changes in minor element composition (Barton, deposits, which have been described by Coats et al. 1970; Scott, 1983). The changing temperature and (1980, 1981), are situated in the central Scottish chemistry of mineralizing solutions has left a delicate Highlands (fig. 1). They are contained within steeply record in the growth-banded sphalerite of some inclined metasedimentary rocks of the Middle vein and Mississippi Valley type deposits (e.g. Dalradian (c. 600 Ma) which were subjected to McLimans et al., 1980). Sequential trends of polyphase deformation and garnet-grade meta- sphalerite composition have been described from morphism during the Grampian Orogeny (Bradbury the Kuroko deposits (Urabe, 1974), and from some et al., 1979; Siraprakash, 1982). The deposits have sedimentary exhalative deposits such as Sullivan many of the characteristics of the shale-hosted, (Campbell and Ethier, 1983). The Fe content of sedimentary exhalative class of stratiform sphalerite in equilibrium with pyrite and pyrrhotine deposits (Large, 1981). They are considered to has been calibrated as a geobarometer (Scott, 1973; have formed from metalliferous brines which were Lusk and Ford, 1981), which has been applied expelled into actively developing sedimentary basins to many metamorphosed sulphide deposits with during a period of increasing crustal instability variable success (Scott, 1976; Plimer, 1980; (Russell et al., 1981a; Laux et al., in prep.). This Hutchison and Scott, 1981). concerns the sulphide mineralogy of the Foss In this paper, the compositions and textural Copyrioht the Mineralogical Society 488 N.R. MOLES

Garnet hornblende I BEN calc. mica schist LAWERS

,e~-~oo~Oe0oo~?-~--~,K~, l 7--~J~~ ~xOC~.~'-~'*q,~" Calc. chlorite mica SCHIST 0% Uuo,~OoO*~* -- -- l ------:- ;~:r,~ - o~b 0 ~,o0"o~ schist ~~~.~, .....~.o0o.~ ['~ Graphitic barian ] muscovite schist BEN

quartzite SCHIST ~-- -=~JT~-I - w..,,, gr.0hit,c j Y'"-<~ +

- ---I .... '?'~ \\'~ St ratiform minaralisatlo~ .~.t- Fault ~'~'~ Craig na h-lolaire Anticline

~'~'~ Sr6n Mh6r Syncline

FtG. 2. Geological map of Foss deposit, Aberfeldy, based on maps by Sturt (1961) and Coats et al. (1981) and unpublished maps. IGS borehole sites located by numbers. Grid numbers refer to OS sheet 52. relationships of sphalerite in the Foss deposit are However, closely spaced drillholes have allowed described, and an attempt is made to distinguish the correlation of individual horizons (Laux et al., sphalerite compositions of primary origin from in prep. and unpublished work). Stratigraphic those modified by metamorphic processes. Con- correlation has been assisted by the recognition of sideration is given to the size of equilibrium domains thin (2 cm-2 m) metabasic layers of probable and the significance of diffusion processes affecting tuffaceous origin, which are closely associated with sphalerite during metamorphism. The application several of the mineralized horizons. of the sphalerite geobarometer is evaluated and Thickness and facies variations and the local results are compared with geothermometric and occurrence of synsedimentary breccias suggest a geobarometric data derived from calcite-, genetic relationship between the distribution of garnet-hornblende, and cymrite-celsian phase mineralization and penecontemporaneous faulting. equilibria. The range and spatial distribution Growth faults were active elsewhere in the Dalra- of pre-metamorphic sphalerite compositions are dian basin in the late Proterozoic-early Cambrian discussed and are tentatively related to depositional (Anderton, 1979) and are a characteristic feature processes in the mineralizing system. associated with sedimentary exhalative deposits (Large, 198i). These faults may have acted as conduits for mineralizing brines which were expelled Geology into the marine basin, possibly by means of seismic The Foss deposit, which has a strike-length of pumping (Sibson et al., 1975). It is envisaged about 2 km and an explored depth of 300 m, that baryte was deposited in relatively shallow, extends between the northern slopes of Meall oxygenated parts of the basin, while sulphides Tairneachan and Creagan Loch (fig. 2). The minera- accumulated on the downthrown sides of postulated lization occurs as multiple sheets or lenses within growth faults where pooling of the brines inhibited graphitic and calcareous mica schists of the Ben mixing with sea-water sulphate (Moles, 1982, and Eagach Schist formation, and the mineralized zone Laux et al., in prep.). As cross-cutting mineralization ranges from about 30 m to over 200 m in true (of pre-metamorphic origin) and alteration pipes thickness. It is terminated to the east by a N.-S. are absent from below the stratiform horizons at fault zone, and plunges to the west in the core of the the present level of exposure, the deposits may be Craig na h'Iolaire Anticline (Sturt, 1961). classified as distal (Large, 1981; Plimer, 1979). Although potentially a source of industrial baryte, much of the mineralization consists of carbonate, Sulphide mineralogy , and -silicate lithologies which are variably enriched in base-metal sulphides. Rapid In the Aberfeldy deposits, sphalerite is the second lateral facies variations within both mineralized most abundant sulphide after pyrite: horizons and metasediments, and localized struc- and pyrrhotine are less abundant, and tural attenuation associated with folding, are occurs in trace amounts. Although widespread in features of the deposit (Swenson et al., 1981). both the mineralized rocks and host metasediments, FOSS SPHALERITE COMPOSITION 489 sphalerite is seldom a major constituent. In sulphidic Fe content of the host sphalerite around these bands, the sphalerite content occasionally increases orientated inclusions suggests that this pyrrhotine to > 50 ~o over intervals of 1-20 cm. Sphalerite formed by exsolution from the sphalerite. Chalco- more frequently occurs as disseminated grains, or pyrite is more common as inclusions in sphalerite, as tenuous laminae which are parallel to the and has been observed in sphalerites of a wide range lithological layering. Fine sedimentary layering has of compositions. Chalcopyrite inclusions, which generally been preserved in lithologies which are are usually equant and can reach 30/~m in diameter, rich in quartz, barium feldspars, baryte, and may be randomly dispersed, orientated along , whereas micaceous rocks are usually crystallographic planes, or concentrated towards highly schistose. Remobilization of sulphides, along the centres of host sphalerite grains. Although high-angle planes or into veinlets and some chalcopyrite may have been exsolved from segregations, is generally minor and localized in sphalerite (Wiggins and Craig, 1980), the abundance extent, except in some mineralized carbonate rocks of inclusions (often about 5 ~o by volume) suggests where breccia textures have developed. that the chalcopyrite originated as a primary Sphalerite grains range in shape from equant and two-phase intergrowth which was recrystallized subhedral to highly irregular and embayed. Grain during metamorphism (Hutchison and Scott, 1981). size ranges from 0.03 to 3 mm corresponding to a Sphalerite analyses. Analyses were performed on similar range in the host rocks. Individual crystals a Cambridge Instruments Microscan Mk. 5 electron may reach 2 cm in veins and segregations. Small microprobe at Edinburgh University. The wavelength sphalerite grains are often encapsulated by pyrite, dispersive technique was employed, with an accelerating which often forms discrete rounded to euhedral voltage of 20 KeV, a probe current (measured at the cubic porphyroblasts. Sphalerite grains of differing Faraday Cage) of 30 nA, and an electron beam diameter of colour often occur in the same rock, and visible I #m. Metal standards were used for Zn, Fe, Mn, Cu, and zoning of sphalerite is sometimes observed in Cd, and a pyrite standard was used for sulphur. Data transmitted light. Microprobe analyses have con- reduction was performed by an on-line computer using firmed that these variations in colour correspond the ZAF correction procedures of Sweatman and Long (1969). Under the analytical conditions employed, to variations in the minor element contents of detection limits were 0.03 wt. ~ for iron and 0.02 wt. sphalerite. This allows a visual estimate to be made for , and precision for iron was usually better of the abundance of particular compositions (Fe- than ___1 ~ of the wt. ~ Fe present for sphalerites with rich or Fe-poor) in polished thin sections. A variety > 6 wt. ~ Fe (= 10 mol. ~ FeS). of textural and compositional relationships in Microprobe analyses were obtained from the polished sphalerite-bearing assemblages are illustrated in thin (c. 30/~m) sections used for transmitted and reflected figs. 3-8, and are described in detail below. light observations. The iron content of Foss sphalerite Irregular or rectangular-shaped grains of pyrrho- varies from 0.0 to 17.0 mol. ~ FeS, which corresponds tine are common in graphitic schist and dolomitic to a range in appearance from colourless, through yellow and orange, to dark red in transmitted light. The Mn celsian rocks, but pyrrhotine is rare in baryte content is low in most Fe-rich sphalerite (> 10 mol. rock. Pyrrhotine commonly coexists with sphalerite FeS) but ranges up to 2.7 mol. ~ MnS in Fe-poor and galena, and is often associated with minor sphalerite (see Coats et al., 1980). Fe-poor, Mn-rich chalcopyrite. The occurrence of pyrrhotine in sphalerite is grey in transmitted light. These relationships certain metabasic lithologies suggests that reactions between sphalerite composition and colour are similar to involving Fe-Mg silicates may have generated those noted by Page and Watson (1976). some pyrrhotine during metamorphism, but massive A small amount (0.1-0.3 wt. ~o) of Cd was detected in bands formed of pyrrhotine are undoubtedly many sphalerites, but Sb was below detection limits primary in origin. No evidence of pyrrhotine (c. 0.05 wt. ~) in most analyses. Cu was detected in trace amounts (up to 0.14 wt. ~) in some sphalerites, but replacing pyrite has been observed, but in many microscopicchalcopyrite inclusionsaccount for the higher rocks some or all of the pyrrhotine has been apparent Cu contents in some analyses of inclusion-rich pseudomorphed by pyrite and marcasite. The sphalerite. The metal : sulphur ratio of sphalerite confirms porous structure or 'bird's-eye'texture of secondary that it is stoichiometric (Zn,Fe,Mn)S,within the limits of pyrite distinguishes it from the pre-existing, well- analytical precision (+0.3 ~ of the amount present for crystallized pyrite. This replacement of pyrrhotine sulphur). is clearly post-tectonic and some has resulted from Textural and compositional relationships. Six supergene alteration. From the drillcore samples samples which illustrate a range in textural and examined, it is estimated that pyrite has replaced compositional relationships between sphalerite and roughly 20 ~o of the pyrrhotine originally present. associated phases, and which contain features Pyrrhotine occasionally occurs as small inclusions crucial to their interpretation, are now described in ferroan sphalerite, which may be crystallo- in some detail. Information on these samples is graphically orientated (fig. 7a). Depletion in the summarized in Table I and in figs. 3-8. 490 N.R. MOLES

TABLE I. Summary of textural and chemical features of sphalerite-bearin# rock samples described in the text

Sulphide No. of Range in Range in Fig. Sample* Lithology assemblage analyses mol. % FeS mol. ~ MnS Interpretation

3 705-9 Graphitic musc. py+po+sp, 22 10-14 0-0.I Buffered, in part qtz. dol. schist minor ga,cp retrogressive 4 80-30A Pyritic qtz. py + sp; py + po + 20 5-14 0-1.3 Primary (Fe-poor) + celsian rock sp, tetra, mar buffered (Fe-rich) 5 705-20 Sulphidic baryte py+sp, 30 3-15 0-0.2 Primary, bimodal rock minor ga 6 429-8 Qtz. musc. dol. py+sp; py+po+ 30 7 15 0-0.9 Primary, bimodal+ hyalophane sp, minor ga,cp locally buffered metachert 7 202-3 Muscoviticquartzite py+po+sp+ 38 11-17 0-0.l Retrogressively with sulphide ga + cp; retrograde buffered with lenses po, py + mar annealed cp intergrowth 8 705-21 Coarse sp. celsian sp; sp + py, 20 2-14 0-0.3 Diffusion zoned: dolomite rock minor ga loss of Fe + Mn to carbonate

* Samples from Creagan Loch (Foss East) except 429-8, from Meall Tairneachan (Foss West) (see fig. 2). All samples except for 80-30A are from drillcore. Abbreviations: py, pyrite; po, pyrrhotine; sp, sphalerite; ga, galena; cp, chalcopyrite; mar, marcasite.

Yellow | large grains, sphalerite cores 5 mE] [~] grain rims and Red small grains sphalerite ==4 ~kbandin dolomite '~ Pyrite -62 Pyrrhotine

~' Galena I i i t i i i 1 i I ,uar tz ~ : " " ' , lO 11 12 13 14 MOI% FeS Chatcopyrite

yellow coloured 1,4r" 4" grains + red coloured 1.21- + o grains 4- - 1,01- cjo :;ro~ ;,m nO' grain in contact r m with marcasite r " " "" )/~ ~pyri'hotirle + o I I I o 2 4 ; ; 10 12 14 MoI% FeS

FIGS. 3 and 4. (a) line drawings from polished thin sections (PTS). FIG. 3. Sample 705-9: (b) histogram of mol. % FeS for twenty-two sphalerite analyses. FIG. 4. Sample 80-30A: (b) plot of mol. ~ MnS vs. mol. ~ FeS for twenty sphalerite grains. FOSS SPHALERITE COMPOSITION 491

6 ~5 irains in barite matrix n=17)

trains encapsulated by 3 }yrite (n=13) ~2

Z

2 | 6 8 10 12 14 16 Mol% FeS

pyrrhotine + py + sp(red) "-1 -~ yellow coloured grains in muscovite ---- | 1 mm I red grains encapsulated by pyrite

cores~ of red grains at top and in o rims j central band

0.6 p grain in contact with pyrrhotine band of dolomite . small red sph~llerit~s \ ~=th yellow sp + py , encapsulated by pyrite ~ ~'~

o 0.2

3 7 8 9 10 11 12 13 14 15 Mol% FeS

FIGS. 5 and 6. (a) line drawings from PTS. FIG. 5. Sample 705-20: (b) histogram of mol. ~ FeS for 30 sphalerite grains. FIG. 6. Sample 429-8: (b) plot of mol. ~ MnS vs. mol. ~ FeS for thirty sphalerite grains

galena - iron-rich core iron depletion along sphalerite without inclusions; plane containing boundary mean spF~s=16.6 reel% pyrrhotine inclusions q FeS 16 . .

14 ..

12 .'" ~o

10 t i 1 q 1 ! , I t I a 100 200 300 400 500 600 700 ~1 ~rn

14 e Sp rim K 0.5ram I [• Mol % FeS 12 \, -.- "":: 6 10 8

6 0.3 MoI% o~176176~ o~ MnS 4 .o o o o D.2 O ~)i! o; 2 0.1 .'o~ dolomite ~. ~ .. 0 0 tO0 200 ~rn

FIGS 7 and 8 (a) line drawings from PTS. FIG. 7. Sample 202-3: (b) microprobe profile of FeS-content ofsphalerite along line a a'. FIG. 8. Sample 705-21: (b) microprobe profile of Fe (e) and manganese (o) contents of sphalerite along line b b'. 492 N. R. MOLES

Fig. 3a illustrates a sample of graphitic, calcareous a similar rock to the one illustrated, discrete metasediment in which sedimentary layering is sphalerite grains of contrasting colour and composi- expressed by variations in the proportions of tion are in mutual grain contact, but this occurrence muscovite to dolomite and quartz. Laminae of is exceptional. However, pyrite crystals often sphalerite+pyrite, with minor associated pyrrho- enclose several sphalerite grains of contrasting tine and galena, parallel this layering, but are partly composition. disrupted by small-scale folding and strain-slip A sample of laminated quartz-muscovite- cleavage oblique to the layering. The porous texture hyalophane metachert, which forms the hanging of some of the pyrite indicates that much of the wall to the upper baryte horizon in Foss West, is pyrrhotine coexisting with the sphalerite during illustrated in fig. 6a. Red-coloured sphalerite, metamorphism has been replaced by secondary containing 10.0-12.7 mol. ~ FeS, occurs as pyrite. All of the sphalerite is red in transmitted disseminated grains, some of which are in contact light, and microprobe analyses (fig. 3b) show a with pyrrhotine + pyrite, in crenulated muscovite- range in iron content from 10-14 mol. ~o, with rich rock at the top of the field of view. Sphalerite of a modal composition of 11.4 mol. ~ FeS. The similar composition, associated with pyrite and central parts of coarser crystals of sphalerite contain muscovite (but not with pyrrhotine), also forms less Fe than both the rims of these crystals and a thin band in hyalophane rock near the bottom of small grains, and sphalerite contained in a dolomitic the illustration. In the centre is another thin band of band is relatively Fe-rich. sphalerite which is orange-yellow in colour and The quartz celsian rock illustrated in fig. 4a contains 7-8 tool. ~o FeS. Disseminated yellow contains trace (< 1 ~) pyrrhotine in addition sphalerite of this composition occurs together with to disseminated pyrite and sphalerite. All three pyrite elsewhere in the sample where pyrrhotine is sulphides are occasionally in mutual contact, but absent. Small, red-coloured sphalerite grains are most of the sphalerite occurs as isolated grains and encapsulated in pyrite crystals associated with both aggregates with pyrite. Much of this sphalerite is red and yellow bands of sphalerite and elsewhere yellow-orange in colour, but grains which are in in the sample. These sphalerites are distinctive in mutual contact with pyrrhotine (+ pyrite) are red. that they contain appreciable manganese (up to Isolated grains of sphalerite (or of sphalerite+ 0.9 mol. ~ MnS), and up to 15 mol. ~o FeS (fig. 6b). pyrite) within a few millimetres of pyrrhotine are Strongly zoned sphalerite crystals are illustrated also red in colour. Two analysed grains of sphalerite in figs. 7 and 8, but on detailed examination these in mutual contact with pyrrhotine + pyrite contain are seen to have major differences. The large 11.2 and 10.8 mol. ~ FeS, and other red-coloured sphalerite crystal of fig. 7a is associated with coarse grains in the vicinity of pyrrhotine range from 9.9 to pyrite, pyrrhotine (partly replaced by porous pyrite) 13.0 mol. ~o FeS (fig. 4b). One grain containing and galena. Equant chalcopyrite blebs populate 14 mol. Y/o FeS adjoins marcasite of supergene dark red cores of the sphalerite crystal in which origin. Yellow-coloured sphalerites contain 5-7 the Fe content is 15.5-17.0 mol. ~ FeS. mol. ~o FeS, and in contrast to the red sphalerites, More transparent sphalerite containing 11.5-12.2 are variably enriched in manganese up to 1.3 mol. ~o FeS occurs at the margins of the crystal, mol. ~ MnS. and also along intersecting crystallographic planes The textures portrayed in fig. 5a are typical of containing planar inclusions of pyrrhotine and sulphide-rich baryte layers which commonly occur chalcopyrite. Usually pyrrhotine and chalcopyrite near to the stratigraphic base of the lower baryte form separate, parallel grains (typically 5 x 50/am in horizon in Foss East. Pyrrhotine is absent from size) in these Fe-depleted zones, but occasionally these rocks. Pyrite forms subhedral crystals up to chalcopyrite mantles pyrrhotine inclusions. 2 mm in diameter, which often enclose one or The highly irregular and embayed form of several grains of sphalerite. Encapsulated sphale- sphalerite shown in fig. 8a is typical of carbonate rites may exceptionally attain > 200 /~m in mineralization in which sedimentary layering has diameter but are usually much smaller. Sphalerite been obliterated by recrystallization during meta- surrounding the may be highly irregular in morphism. The sphalerite shows pronounced shape and intergrown with baryte, or (in other depletion in both Fe and Mn towards grain rims rocks) the sphalerite forms a continuous matrix and along internal cracks and cleavage planes. enclosing crystals of pyrite. A large proportion of Small grains (< 100/lm) are pale-coloured, whereas the sphalerite (typically > 90 ~o) is yellow in colour the central areas of larger grains or aggregates corresponding to a relatively low Fe content, but are orange-red and differ in Fe content by up to many of the encapsulated grains are red and 12 mol. ~ FeS from the rims (fig. 8b). In other Fe-rich (fig. 5b). A few isolated orange and red examples to the one illustrated, each sphalerite coloured grains also occur in the baryte matrix. In grain or aggregate is homogeneous in colour and FOSS SPHALERITE COMPOSITION 493 composition, but the Fe content appears to increase FeS activity (aF~s). Barton and Toulmin (1966) and in direct proportion to grain size. Scott and Barnes (1971) have shown that SpFeS is a Interpretation. Several general observations function only of pressure in the range 254-550 ~ which emerge from the foregoing detailed provided that the ares is buffered by coexisting descriptions are critical to the interpretation of pyrite + hexagonal 1C pyrrhotine. Under these textural and compositional relationships of conditions, sphalerite has a fixed FeS content of sphalerite in the Foss deposit. 20.8 mol. % at 1 bar, but with increasing pressure (1) Sphalerite which is in grain contact or in spFes decreases progressively. This compositional close association with pyrrhotine (or pyrite pseudo- change has been calibrated as a geobarometer to morphs of pyrrhotine) is invariably red-coloured 10 kbar by Scott (1973), Lusk and Ford (1978), and and ferroan (> 10 mol. ~ FeS). Such sphalerite is Hutchison and Scott (1981) (fig. 9). Above 550 ~ also usually Mn-poor. SpFes varies with both temperature and pressure, (2) Sphalerite occurring as isolated grains or but at high pressures the temperature-independent coexisting with pyrite, but not in the immediate field extends above 600 ~ vicinity ofpyrrhotine, has a wide range in composi- tions from Fe-poor to Fe-rich. Overall, Mn and 80(3 Fe contents are not systematically related, but I /I / F,s, in individual samples compositional grouping of ~o~ / # /~ laisso*c sphalerite may be based on variations in both Fe and Mn (e.g. figs. 4b and 6b). (3) Bands of massive sphalerite and individual disseminated grains are usually homogeneous, but ~o~ i~"~i~;i.i!;i:~;!iiii!i!i!iii.:ii~i!..i;!!i;: adjacent grains or bands may differ in composition. 400 o Sphalerite hosted by recrystallized carbonate rock is often strongly zoned, and zoned sphalerite also occurs in coarse sulphide rocks bearing evidence of .oo / Z retrograde alteration. The zonation is diffuse rather 100 sp+nCpo /? sp+mpoo Jp~C m o than abrupt or oscillatory, and is usually an I , , J ~ : D FeS, , * Pl JZnS outward decrease in Fe (and Mn) content. It is apparent from these observations that MOI~o FeS in sphalerite equilibrium domains have been minute in some rocks despite the effects of regional metamorphism. FIG. 9. T-X projection of the FeS-ZnS-S system showing the sphalerite + pyrrhotine + pyrite solvus at 1 bar (heavy At least some of the observed variation in sphalerite line), and at higher pressures (broken lines). Stippled composition within and between samples appears area is the temperature-independent field in which the to have been inherited from the time of deposition. sphalerite pyrite-hexagonal-pyrrhotine barometer can Several processes can be distinguished which have be applied. Modified from Scott and Barnes (1971), Scott erased or modified original sphalerite compositions (1973), Scott and Kissin (1973), and Lusk and Ford (1978). during metamorphism. These are: (a) buffering Box labelled a indicates estimated peak P, T conditions at of the Fe content of sphalerite coexisting with Abeffeldy, box labelled b indicates retrograde conditions pyrrhotine+pyrite; (b) homogenization of indivi- deduced from sample 202-3 (see text). Inset: isothermal dual grains or of massive bands of sphalerite by sections of the condensed system FeS-FeS2-ZnS at points a, b, and c. intracrystalline diffusion; (c) partitioning of Fe and Mn between sphalerite and carbonate or other sulphide phases; and (d) exsolution and retrograde Below 254 ~ (at 1 bar), 1C pyrrhotine undergoes alteration of sphalerite assemblages. structural and compositional changes, and mono- The background to the use of the sphalerite clinic 4C pyrrhotine becomes stable in the geobarometer and its application to the Foss assemblage pyrrhotine-pyrite-sphalerite. Low- deposit are considered first. After other processes temperature re-equilibration of sphalerite in this which have modified original sphalerite composi- assemblage results in a marked decrease in SpFeS. tions are considered, it is then possible to discuss Scott and Kissin (1973) and Groves et al. (1975) the significance of primary sphalerite compositions have found convincing evidence of a largely and their stratigraphic variation. temperature-independent monoclinic pyrrhotine- pyrite-sphalerite solvus corresponding to between Sphalerite geobarometry about 10.5 and 12.0 mol. ~ FeS in sphalerite (fig. 9, point c). The amount of FeS in solid solution in sphalerite Tests with magnetic colloid and XRD tracings (SpF~S) is a function of temperature, pressure, and have shown that in all samples from Foss that were 494 N.R. MOLES examined, the pyrrhotine is monoclinic. However, (b) The presence of chalcopyrite inclusions, which estimates of peak metamorphic conditions in the appear to promote retrograde reactions (see Boctor, Aberfeldy area are well in excess of 300 ~ (Table II) 1980; Hutchison and Scott, 1981). and therefore hexagonal 1C pyrrhotine would (c) The association with retrograde metamorphic have been stable during metamorphism. Before or supergene phases particularly marcasite and applying the sphalerite geobarometer, textural pyrite pseudomorphs ofpyrrhotine, and retrograde and geochemical evidence of retrograde alteration chlorite and , which indicate the passage of sphalerite compositions must be thoroughly of late metamorphic or meteoric fluids. examined. Equilibrium domains and retrograde diffusion. Retrograde re-equilibration. Evidence for low- Sulphide-rich rocks from the Foss deposit which temperature (< 250 ~ reactions involving contain major amounts of pyrrhotine (frequently sphalerite and monoclinic pyrrhotine include the associated with minor chalcopyrite) are seldom exsolution of pyrrhotine from ferroan sphalerite suitable for deriving peak pressure estimates because and the depletion in SpFeS adjacent to pyrite of the high incidence of retrograde re-equilibration and pyrrhotine. Both features are illustrated by of sulphides in such rocks. Disseminated sphalerite the sphalerite in fig. 7a. Re-equilibration of this in competent lithologies such as baryte rock and sphalerite crystal with monoclinic pyrrhotine+ quartz metachert, in which pyrite is common but pyrite, achieved in part by the exsolution of pyrrhotine is comparatively rare, has a greater pyrrhotine, has clearly not gone to completion and tendency to retain compositions generated by pro- the Fe-rich cores are relicts from buffering reactions grade buffering reactions. In such cases, sphalerites with coexisting hexagonal pyrrhotine + pyrite. in grain contact with pyrrhotine + pyrite are similar The Fe content of these relict cores allows an in colour and composition to nearby isolated estimation to be made of the confining pressure at sphalerite (+ pyrite) grains, but at a greater distance the point in the cooling history below which from the pyrrhotine, sphalerite grains differ in hexagonal pyrrhotine was no longer stable. Inserting composition and have apparently remained un- the mean of five analyses (16.6 mol. ~o FeS; fig. 7b) affected by buffering reactions during meta- into the equation of Hutchison and Scott (1981) morphism (e.g. figs. 4 and 6). These observations gives a pressure of 3.1 +0.3 kbar. The hexagonal suggest than an interstitial fluid phase in the pyrrhotine-monoclinic pyrrhotine transition, and immediate vicinity of pyrrhotine+pyrite was therefore the horizontal limb of the solvus in the buffered with respect to arcs, and that this fluid Zn-Fe S system (fig. 9), may be displaced to higher provided the transport medium for enrichment in temperatures due to the effect of confining pressure. SpFes. This has not been determined, but an upper limit The rock volume, or equilibrium domain, within of 300 ~ at 3.4 kbar can be interpolated from which disseminated sphalerite has been buffered by the experimental data of Scott (1973). Therefore it interstitial fluid in equilibrium with one or more may be postulated that, at some time during uplift, grains of pyrrhotine (+ pyrite) ranges from less than the rock mass enclosing the Foss deposit passed 1 mm 3 (e.g. sample 80-30A) to at least the size of a through the P-T area of 3.1+0.3 kbar and 260- hand specimen, and is possibly on the scale of 300 ~ (fig. 10). metres in schistose lithologies. In well-banded A record of metamorphic conditions during rocks, buffered equilibrium domains may be planar uplift has been preserved in this sphalerite crystal in shape, particularly where banding is defined by (fig. 7a) because of its large size and incomplete variations in mica content (e.g. sample 429-8). re-equilibration at low temperatures. Of critical In other rocks, the shape of buffered equili- importance is the ability to distinguish between brium domains relate to cross-cutting veinlets. small, unzoned sphalerites which have been buffered Characteristically, equilibrium volumes are sharply by monoclinic pyrrhotine + pyrite at low tempera- defined and grains as little as 30 #m apart, at the tures (< 250 ~ and sphalerite which (although boundary of a fluid-buffered domain, may differ currently in the same assemblage), has retained the greatly in colour and composition. equilibrium composition generated by buffering Two independent processes appear to have with hexagonal pyrrhotine + pyrite at about 9 kbar, affected sphalerite compositions subsequent to since in both cases the sphalerite will contain about equilibration during climactic metamorphism. 11 mol. ~ FeS (fig. 9). Features which allow the These are: (1) Partial down-pressure re-equilibration recognition of sphalerites of this composition which of sphalerite (particularly if in grain contact with have been involved in low-temperature reactions, pyrrhotine), resulting in an increase in SpFeS (e.g. are: sample 705-9). (2) Outward diffusion and depletion (a) Zoning or heterogeneity, and the presence of in SpFeS in small (< 100 #m diameter) grains and in exsolved pyrrhotine. the rims of larger grains. This depletion is typically FOSS SPHALERITE COMPOSITION 495 in the order of 0.5-2.0 mol. % FeS, but may exceed Aluminosilicate-bearingrocks have not been found 10 mol. % FeS in carbonate-hosted sphalerite in the vicinity of the deposit, but kyanite is recorded (e.g. sample 705-21). Outward diffusion of FeS in by Sivaprakash (1982) from Grandtully (about sphalerite has previously been invoked by Barton 6 km to the SE), and by Wells and Richardson and Skinner (1979) to explain anomalous high- (1979) from Schiehallion (about 10 km to the W.; see pressure estimates from sphalerite immediately fig. 1). Recent recalculations of the plagioclase- adjacent to pyrrhotine. In the Foss deposit, outward garnet-kyanite-quartz equilibrium (Newton and diffusion of FeS in sphalerite (to varying extents) Haselton, 1981) suggest that the pressures derived seems to be widespread, and is not restricted to by these authors may be several (1-3) kilobars too mutual grain boundaries with pyrrhotine. high. As these processes have opposing effects on Metamorphic temperatures derived from calcite- sphalerite composition, the application of the dolomite geothermometry by Sivaprakash (1982) sphalerite geobarometer using the mean of a number and from unpublished data, range from 550 of analyses of sphalerites which show evidence of to 420 ~ which suggests that retrograde re- having been buffered by pyrite + hexagonal pyrrho- equilibration of carbonates is common. The tine, need not necessarily yield peak metamorphic temperature range 540-580 ~ derived from garnet- pressures. The centres of relatively large (> 100/~m) biotite and garnet-hornblende geothermometry, is sphalerite grains within fluid-buffered equilibrium regarded as representative of peak metamorphic domains (but not always in contact with pyrrhotine) temperatures. Temperatures in this range are also are taken as the most suitable compositions for indicated by 334S fractionations between some geobarometry. These compositions (taken from coexisting sulphide and sulphide-baryte pairs 705-9, 80-30A, 429-8, and other samples) range (Willan and Coleman, 1981, and unpublished from 10.4 to 12.7 mol. % FeS, from which pressures work). of 9.7-6.9 kbar are calculated using the equation of Further petrological work has supported the Hutchison and Scott (1981) (fig. 10). suggestion by Fortey and Beddoe-Stephens (1982) Comparison of silicate and sphalerite #eobaro- that much of the celsian occurring at Aberfeldy was meters. Estimates of peak metamorphic pressures in formed by the post-deformational replacement of the Aberfeldy area from several silicate geo- the platy mineral, cymrite, by the reaction: barometers (Table II), compare favourably with pressures in excess of 7 kbar suggested by the BaA12Si2Os.H20 = BaA12Si20 s +H20. selective application of the sphalerite geobarometer. cymrite celsian fluid

TABLE II. Estimates of metamorphic temperatures and pressures in the Aberfeldy area

Author Area Mineral equilibria T ~ P kbar

Wells and Richardson (1979) Schiehallion (Dalradian) Plag-gt-ky-qtz / Garnet_biotite a J 560+ 50 12 + 1.5 Wells (1979) Schiehallion (Moine) Hb-plag-qtz-gt-ep / Garnet-biotitea ~ 550 + 50 11.3 + 2.0 Gt-hornblende J Sivaprakash (1982) Aberfeldy-Grandtully Plag-gt-ky-qtz 10_ 1 Ep-gt-plag-qtz 8-10 Garnet-biotitea 500-580 Calcite-dolomite b 420-550 Moles (1983) and unpublished Foss deposit, Abeffeldy Gt-hornblendec 540-580 data Calcite-dolomite b 480-530 Cymrite-celsian d > 7.0* Sph-py-po 8.5 __+1.5 / 6.8+0.3 Willan (1980, 1981) Aberfeldy deposits Sph-py-po [ 6.1_ 0.1 Willan and Coleman (1981) Aberfeldy deposits 634S of sulphide 550__ 50 and baryte pairs

* at T = 540 ~ (see text). Calibrations used: a Ferry and Spear (1978); b Bickle and Powell (1977); c Graham and Powell (in prep.); a Nitsch (1980). 496 N. R. MOLES

Cymrite relics have been found in many celsian- detailed textural relationships of these sphalerites bearing rocks, and the presence of aligned inclusions are not described. of quartz, rutile, and sulphides in coarse celsian rock suggests the replacement of a folded cymrite Primary sphalerite compositions fabric. The reaction has been calibrated experi- mentally by Nitsch (1980) and further experimental Depositional growth zoning in sphalerite, charac- work is in progress (Moles, 1983) to ascertain the terized by abrupt and often oscillatory variations in displacement of the reaction curve by substitution minor element content, has been observed in of (K + Si) for (Ba + AI) in both cymrite and celsian several non-metamorphosed stratabound base- (cf. Reinecke, 1982). Using the pure end-member metal deposits, such as McArthur River (Croxford calibration of Nitsch (fig. 10), a minimum pres- and Jephcott, 1972) and Bleiberg, Austria (Schroll sure of 7 kbar at T = 540 ~ is indicated by the et al., 1983), and in Mississippi Valley type deposits reaction. (McLimans et al., 1980). However, sphalerite zona- tion of this type is absent at Aberfeldy, and it is inferred that depositional growth zoning, if 10 -10 originally present in the sphalerite, has been erased by recrystallization and intracrystaUine diffusion during prograde metamorphism (Barton, 1970). 9 ~ -11 The presence of encapsulated sphalerites of con- trasting composition may provide evidence for 8 -12 the homogenization of the matrix sphalerite in massive sulphide rocks, in rocks containing dis- o seminated sphalerite with a bimodal distribution of compositions, some sphalerite grains of inter- ! mediate composition (fig. 5b) may have formed by the coalescence of several grains of end- s _'Z member composition. Coalescence during grain / t -15 coarsening may account for the scarcity of mutual 4 // grain contacts between sphalerites of contrasting / -16 composition. 3 -17 Partitioning of minor elements between sphalerite and coexisting phases during metamorphism must i i i | I i i i i I , i i i I i i i i 20( 300 400 500 600 also be considered as a mechanism by which T, oC primary sphalerite compositions may have been FIG. 10. The equilibrium curve for the reaction, cymrite = modified. Marginal depletion in the Fe and Mn celsian + HzO, from Nitsch (1980). Mol. ~ FeS scale refers contents of sphalerite in a matrix of recrystallized to sphalerite buffered by pyrite + hexagonal pyrrhotine carbonate, has been described above (fig. 8). Catho- (fig. 9). Boxes labelled a and b correspond to those in fig. 9 doluminescence studies have revealed striking and represent estimates of peak and retrograde P, T oscillatory zonation patterns, which correspond conditions in the Foss deposit. to variations in Fe and Mn concentrations deter- mined on the microprobe, in coarsely recrystallized dolomite and calcite. The association of zoned The widespread replacement of cymrite in the sphalerite with recrystallized carbonate suggests Aberfeldy deposits suggests that lithostatic pressures that Fe and Mn have diffused out of the sphalerite were decreasing and/or temperatures were in- and entered the carbonate matrix during meta- creasing in the area after the development morphism. Preferential partitioning of zinc from the of tectonic fabrics (Moles, 1983). Annealing and matrix into the sphalerite may also have occurred, continued re-equilibration of sphalerites buffered but the zinc content of carbonates is below by pyrrhotine+pyrite during this period may detection limit on the microprobe. Diffusion profiles account for some of the spread in pressures derived in sphalerite have often been frozen in (fig. 8b), but by sphalerite geobarometry. Previously published in some carbonate rocks individual sphalerite results from the application of the sphalerite geo- grains appear to have homogenized after marginal barometer at Aberfeldy (Willan, 1980, 1981, Table II) depletion in Fe and Mn, resulting in a positive appear to be underestimates. Willan used a small correlation between grain size and minor element number of sphalerite grains stated to be in mutual content. contact with pyrite + pyrrhotine and lacking evi- Encapsulated sphalerites are often enriched in dence of retrograde alteration. Unfortunately, the Mn in comparison with sphalerites of similar Fe FOSS SPHALERITE COMPOSITION 497

content in the matrix. This may be of primary formation. Many authors have sought to define the origin, or could be due to a preferential partitioning nature of ore-forming fluids and the physico- of manganese into sphalerite from the surrounding chemical environment of deposition of submarine- pyrite during metamorphism. Campbell and Ethier exhalative deposits (e.g. Finlow-Bates, 1980; Large, (1983) have demonstrated a positive correlation 1981; Russell et al., 1981b). The chloride-complex between the manganese contents of coexisting models of Sato (1972, 1973) and Large (1977) for sphalerite, pyrite and pyrrhotine in a laminated ore solutions forming massive sulphide deposits in sulphide rock from the Sullivan deposit. In the Foss volcanic terrains, are generally accepted as broadly deposit, the manganese contents of iron sulphides applicable to sedimentary-exhalative deposits. are very low (Willan, 1980), and the grain size The progressive change in temperature, total of encapsulated sphalerites does not appear to sulphur concentration, and fugacity in the correlate with the degree of manganese enrichment. brines which formed the Kuroko deposits has been Therefore, pyrite-sphalerite partitioning of Mn described by Sato (1977). Decrease in the Fe (and Fe) has probably not affected primary content of sphalerite with stratigraphic height in sphalerite compositions. Buffering of spFes by fluids the Kuroko deposits (Urabe, 1974) is ascribed in equilibrium with pyrite +pyrrhotine appears to largely to a decline in temperature (typically by result in the depletion of SpM~S (e.g. sample 80-30A, 50 ~ from the stockwork to the top of the fig. 4), but the reason for this is not clear. stratiform ore). However, this trend may also be In summary, the following criteria allow the related to an increase in total sulphur concentration selection of primary sphalerite compositions which and oxygen fugacity in the brines (Finlow-Bates, are least likely to have been modified during 1980), which also favours the formation of baryte. metamorphism: The following scenario is envisaged during (1) The absence of adjoining or nearby pyrrhotine formation of the ore-bearing stratiform lenses of the (or retrograde marcasite and pyrite pseudomorphs Foss deposit. During the expulsion of relatively hot of pyrrhotine). (> 200 ~ saline, weakly acidic, and sulphur- (2) The occurrence of sphalerite as small dis- poor metalliferous brines at exhalative vents, fine- seminated grains rather than as a continuous grained and Fe-rich sphalerite (or ; cf. Styrt matrix. et al., 1981; Scott, 1983) was precipitated and (3) The absence of zoning, or of a strong positive carried upwards in the buoyant brine plume correlation between grain size and minor element together with sulphides of iron and (Solomon content (particularly if the matrix is recrystallized and Walshe, 1979). The brine then spread laterally carbonate rock). as it entrained sea-water (Turner and Gustafson, (4) The occurrence of matrix sphalerite of similar 1978) and became cooler, less saline, more oxidized composition to sphalerite grains encapsulated in and enriched in sulphur (derived by bacteriogenic pyrite. reduction from sea-water sulphate; Willan and These criteria are most frequently satisfied in Coleman, 1981). Sphalerite with a lower Fe content baryte rocks containing disseminated sulphides. precipitated from the bottom-hugging or buoyant However, primary sphalerites have been recognized brine layer, together with baryte, pyrite, and (with in other mineralized lithologies such as quartz increasing fo) . Alternatively, Fe-poor celsian cherts (e.g. samples 80-30A and 429-8) sphalerite may have crystallized within the uncon- and also in some metasediments, notably thick solidated ore as reduced sulphur became available (> 1 m) beds of graphitic dolomite rock containing (cf. McArthur River ore: Croxford, 1968). sphalerite laminae. In this model, the Fe-rich sphalerite fraction is Primary sphalerites and ore formation. Within regarded as allochthonous whereas the Fe-poor many samples, a bimodal distribution of sphalerite sphalerite crystallized in situ. If this is the case, compositions is observed, the Fe-poor fraction one might expect to find a lateral variation in the Fe usually predominating (fig. 5). Primary Fe-rich content of the Fe-poor sphalerite fraction with sphalerites are commonly finer-grained, more increasing distance from the exhalative centre, but equant in shape and more often encapsulated in less variation in the Fe-rich fraction. However, pyrite than the Fe-poor sphalerite in the same rock. a simple pattern in the distribution of primary The sum of these textural features suggest that the sphalerite compositions has not emerged from a Fe-rich sphalerite may have been precipitated study of the lower mineralized horizon in Foss East earlier than both the Fe-poor sphalerite and much (fig. 2) using over 50 samples. Variation in primary of the pyrite. sphalerite compositions is greater between samples To account for the origin of a range of primary taken through drill hole intersections of this horizon, sphalerite compositions within the same rock, it is than from east to west in the area. necessary to speculate on the mechanisms of ore Stratigraphic variation in sphalerite compositions 498 N. R. MOLES

Strati - inclined DDH Creagan Loch mineralized horizons and wedging out of the graphic Depth 708 aboveheight 8~ (m) intervening metasediments (Laux et al., in prep.). This trend in sphalerite compositions may reflect footwall(m) 6 I33.232,2 a decrease in sulphur concentration due to the 31.0 4 29.9 restriction of bacterial activity in a relatively m shallow, oxidizing environment during deposition T 2 27~028.5 m of the baryte. Oxidizing conditions are also 24,3 O suggested by the absence of carbonaceous matter in way Mol% FeS the intervening and overlying metasediments in this up DDH 105 area. Frenich Burn Primary sphalerites have been described from several other stratabound base-metal deposits, but ::2:::::::::::::1 analytical data from unmetamorphosed deposits is ::;:::::::'7'::I tg.1 scarce. Croxford (1968), Croxford and Jephcott m (1972), and Oehler and Logan (1977) describe 4t5.416.3 iiii)]ii!i~i!i~l bedded, disseminated and colloform sphalerites in 14.6 __r black shales from the McArthur River deposit, 2 14.0 ::3:::;:~:':':':">:.Z.>.v...I r Northern Territory, Australia. The sphalerite varies O 12.3 from colourless to red-brown in appearance, corres- o ~ ~. ~ ~, 'lb't'2't', MOI% FeS ponding to a range in iron content from 1.4 to I 11.2 mol. ~o FeS. Fe-rich compositions occur late in Graphitic quartz ~ Baryte rock the paragenesis in concentrically zoned colloform muscovite schist BE and vein sphalerite. A similar increase in SpFcS with Calc mica schist ~ Celsian rocks paragenetic sequence has been observed by Schroll Metebasite ~ Quartz metachert et al. (1983) in zoned colloform sphalerite at Fq (marker bed) l:::::l Bleiberg, Austria. m sphalerite buffered by py+po during metamorphism Pyrrhotine is absent from the McArthur River r iron- rich fraction deposit, but is a major constituent of the low-grade FIG. 11. Primary sphalefite compositions in samples from metamorphosed deposit of Hilton, Queensland. At two drillhole intersections of the lower mineralised horizon Hilton, pyrrhotine and chalcopyrite are associated (M3) in Foss East, located near IGS boreholes 1 and 3 with Fe-rich sphalerite in the upper part of the ore (see fig. 2). horizon (Mathias et al., 1973). Elsewhere in the deposit, sphalerite ranging in colour from pale yellow to dark brown occurs as disseminations can be discerned locally (fig. 11). Sphalerite becomes and late-stage veinlets. The association of Fe-rich progressively Fe-poor towards the hanging wall sphalerite with pyrrhotine in metamorphosed near the eastern limit of the deposit in Foss East sulphide deposits containing a wide range of (near IGS borehole 3), where a lense of baryte spbalerite compositions, has also been observed at overlies thick sulphidic cherts deposited on the Gamsberg, South West Africa (Rozendaal, 1978, downthrown side of a probable synsedimentary and Stumpfl, 1979) and at Balmat, New York (Doe, fault (Laux et al., in prep.). This stratigraphic trend, 1962, and Scott, 1976). The preservation of primary which is similar to that observed in the Kuroko sphalerite compositions in some rocks, while in deposits, may be due to an increase in fo~ and others sphalerite has been buffered by coexisting decrease in temperature and salinity as the ponded pyrite+pyrrhotine, is one feature which these brine mixed with the overlying sea-water. This deposits have in common with Foss. hypothesis is supported by sulphur isotope studies which show a marked decrease in ($34Sof baryte Conclusions and pyrite near the stratigraphic hanging wall (Willan and Coleman, 1981; Moles, in prep.). Disillusionment with the sphalerite-pyrite- In contrast to the trend described above, the pyrrhotine barometer has been expressed by several Fe content of primary sphalerite increases strati- authors in recent years (e.g. Stumpfl, 1979; Plimer, graphically upwards through the footwall chert 1980) and, indeed, 'sphalerite geobarometry does and the overlying baryte in the vicinity of the not appear to provide a panacea for estimating Frenich Burn headwater (figs. 2 and 11). Sphalerite pressures in metamorphic rocks' (Brown et al., is absent from the upper part of the relatively thick 1978). This is due to the extensive resetting of baryte unit in this area (Coats et al., 1981; IGS sulphide compositions at lower pressures in many borehole 1), which formed by the stacking of five metamorphosed orebodies. The present study FOSS SPHALERITE COMPOSITION 499 amplifies the need for detailed textural studies Thanks are expressed to C. M. Graham, K. R. Gill, S. J. before applying the geobarometer, but also illu- Laux, A. J. Hall, and R. C. R. Willan for critically strates the variety of other geological informa- reviewing the manuscript, and to the secretarial staff of the tion, pertaining to metamorphism and to the Grant Institute of Geology, Edinburgh University, for environment of deposition, which can be derived assistance with typing. N. J. Fortey (IGS, London) kindly loaned a number of polished thin sections. Analytical from sphalerite studies. work at the Edinburgh Microprobe Unit was made In many granoblastic baryte, quartz and feldspar possible by support from the Natural Environment rocks in the Foss deposit, the variable FeS and Research Council, and P. Hill is thanked for his assistance. MnS contents of disseminated sphalerite grains appear to have remained essentially unaltered through metamorphism to garnet-grade conditions. REFERENCES Very localized buffering of sphalerite adjoining Anderton, R. (1979) In The Caledonides of the British pyrrhotine grains is evidence of small (1 cm-1 ram) Isles--reviewed (A. L. Harris et al., eds.), Geol. Soc. equilibrium domains in these lithologies. In contrast, Lond. 483-8. sphalerite in the enclosing mica schists has under- Barton, P. B., Jr. (1970) Mineral. Soc. Am. Spec. Pap. 3, gone widespread buffering by fluids in equilibrium 187-98. with pyrite + pyrrhotine, which suggests that large --and Skinner, B. J. (1979) In Geochemistry of Hydro- thermal Ore Deposits, 2nd edn (H. L. Barnes, ed.), Wiley, equilibrium domains are associated with penetrative New York. 278-403. deformation. In rocks which lack evidence of --and Toulmin, P. (1966) Econ. Geol. 61, 815-49. retrograde alteration, the occurrence of buffered Bickle, M. J., and Powell, R. (1977) Contrib. Mineral. sphalerites containing 10.4-12.7 mol. ~ FeS indi- Petrol. 59, 281-92. cates that peak metamorphic pressures of 7-10 Boctor, N. Z. (1980) Am. Mineral. 65, 1031-7. kbar were attained in the Aberfeldy area. Similar Bradbury, H. J., Harris, A. L., and Smith, R. A. (1979) In pressures are also indicated by silicate geobaro- The Caledonides of the British Isles--reviewed (A. L. meters including the cymrite-celsian reaction Harris et al., eds.), Geol. Soc. Lond. 213-20. (Moles, 1983). Continued buffering of sphalerite Brown, P. E., Essene, E. J., and Kelly, W. C. (1978) Am. Mineral. 63, 250-7. coexisting with pyrrhotine (+pyrite) to low Campbell, F. A., and Ethier, V. G. (1983) Mineral. pressures may be used in charting the P-T path in Deposita, 18, 39-55. the Aberfeldy area during cooling and uplift. Coats, J. S., Smith, C. G., Fortey, N. J., Gallagher, M. J., Depositional growth zoning, if originally present May, F., and McCourt, W. J. (1980) Trans. Inst. Minin9 in sphalerite of the Foss deposit, has been erased by Metall. B, 89, 110-22. intracrystalline diffusion during metamorphism. et al. (1981) Inst. Geol. Sci. Miner. Reconn. Rep. Partitioning of Fe and Mn between sphalerite and No. 40, 116 pp. enclosing phases has produced diffuse zonation in Croxford, N. J. W. (1968) Proc. Aust. lnst. M ininO Metall. sphalerite in recrystallized carbonate rocks. 226, 97-108. and Jephcott, S. (1972) Ibid. 243, 1-26. Primary sphalerite compositions vary in vertical Doe, B. R. (1962) Geol. Soc. Am. Bull. 73, 833-54. transects through mineralized horizons, which Ferry, J. M., and Spear, F. S. (1978) Contrib. Mineral. suggests that mineral chemistry relates to deposi- Petrol. 66, 113-17. tional processes. Bimodal sphalerite compositions Einlow-Bates, T. (1980) Geol. Jahrb. D40, 131-68. occur in many mineralized rocks: the Fe-rich Fortey, N. J., and Beddoe-Stephens, B. (1982) Mineral. sphalerites are usually subordinate in quantity and Ma9. 46, 63-72. grain size to the Fe-poor sphalerite, and are often Groves, D. I., Binns, R. A., Bassett, F. M., and McQueen, encapsulated by pyrite. The Fe-rich fraction is K. G. (1975) Econ. Geol. 70, 391-6. thought to have precipitated from the metalliferous Hutchison, M. N., and Scott, S. D. (1981) Ibid. 76, 143-53. Large, D. E. (1981) In Handbook of stratabound and brines during their expulsion from hydrothermal stratiform ore deposits 9 (K. H. Wolf, ed.), 469-508. vents, and to have been carried in suspension in the Elsevier, Amsterdam. laterally spreading brines. The Fe-poor sphalerite Large, R. R. (1977) Econ. Geol. 72, 549-72. subsequently precipitated from cooler, bottom- Lusk, J., and Ford, C. E. (1978) Am. Mineral. 63, 516-19. hugging brine layers, or within the ore sediment as McLimans, R. K., Barnes, H. L., and Ohmoto, H. (1980) reduced sulphur became available. Econ. Geol. 75, 351-61. Mathias, B. U., Morris, D., and Russell, R. E. (1973) Bull. Acknowledgements. A research studentship from the Bur. Mineral. Resour. Australas. 141, 33 58. Department of Education, Northern Ireland, is gratefully Moles, N. R. (1982) Mineral. Soc. Bull. 57, December acknowledged. Dresser are thanked for financial 1982. support during fieldwork, and for permission to sample (1983) Geol. Soe. Lond., Newsletter, September 1983. drillcore obtained by the company. The project has Newton, R. C., and Haselton, H. T. (1981) In Thermo- benefitted from the guidance and co-operation of company dynamics of Minerals and Melts (R. C. Newton, staff. A. Navrotsky, and B. J. Wood, eds.), Springer-Verlag. 500 N. R. MOLES

Nitsch, K.-H. (1980) Fortschr. Mineral. 58, 98-100. Solomon, M., and Walshe, J. L (1979) Econ. Geol. 74, Oehler, J. H., and Logan, R. G. (1977) Econ. Geol. 72, 797-813. 1393 409. Stumpfl, E. F. (1979) Mineral. Deposita, 14, 207-17. Page, D. C., and Watson, M. D. (1976) Ibid. 71, 306-27. Sturt, B. A. (1961) J. geol. Soc. Lond. 117, 131-56. Plimer, I. R. (1979) Mineral. Deposita, 14, 207 18. Styrt, M. M., Brackmann, A. J., Holland, H. D., Clark, --(1980) Ibid. 15, 237-41. B. C., Pisutha-Arnond, V., Eldridge, C. S., and Ohmoto, Reinecke, T. (1982) Contrib. Mineral. Petrol. 79, 333-6. H. (198l) Earth Planet. Sci. Lett. 53, 382-90. Rozendaal, A. (1978) In Mineralisation in metamorphic Sweatman, T. R., and Long, J. V. P. (1969) J. Petrol. 10, terrains (W. J. Verwoerd, ed.), Geol. Soc. S. Afr. Spec. 332 79. Publ. No. 4, 235-65. Swenson, D. H., Laux, S. J., Burns, A. R., Perley, P. C., and Russell, M. J., Willan, R. C. R., Anderton, R., Hall, A. J., Boast, A. M. (1981) Trans. Inst. Mining Metall. B, Nicholson, K., and Smythe, D. K. (1981a) In Correla- 90, 57. tion of Caledonian stratabound sulphides, symposium Turner, J. S., and Gustafson, L. B. (1978) Econ, Geol. 73, volume (A. J. Hall and M. J. Gallagher, eds.), Glasgow 1082-100. 1-2 May 1981, 24-9. Urabe, T. (1974) In Geology of Kuroko Deposits. (S. --Solomon, M., and Walshe, J. L. (1981b) Mineral. Ishihara et al., eds.), Min. Geol., Tokyo, Spec. Issue no. 6, Dep. 16, 113-27. 377-84. Sato, T. (1972) Min. Geol. Tokyo, 22, 31-42. Wells, P. R. A. (1979) J. geol. Soc. Lond. 136, 663-71. --(1973) Geochem. J., Tokyo, 7, 245-70. --and Richardson S. W. (1979) In The Caledonides of --(1977) In Volcanic processes in . Geol. Soc. the British Isles--reviewed (A. L. Harris et al., eds.), Lond., Special Pub. 7, 153 61. Geol. Soc. Lond. 339-44. Schroll, E., Schulz, O., and Pak, E. (1983) Mineral. Wiggins, L. B., and Craig, J. R. (1980) Econ. Geol. 759 Deposita, 18, 17-25. 742 51. Scott, S. D. (1973) Econ. Geol. 68, 466 74. Willan, R. C. R. (1980) Nor. geol. Unders. 3110, 241-58. --(1976) Am. Mineral. 619 661-70. --(1981) In Correlation of Caledonian stratabound --(1983) Mineral. Mag. 47, 427 35. sulphides, excursion guide (A. J. Hall and M. J. Gallagher, --and Barnes, H. L. (1971) Econ. Geol. 66, 653-69. eds.), Glasgow 1-2 May 1981. ---and Kissin, S. A. (1973) Ibid. 68, 475-79. --and Coleman, M. C. (1981) Inst. Geol. Sci. Stable Sibson, R. H., Moore, J. M., and Rankin, A. H. (1975) Isotope Rep. No. 60. J. geol. Soc. Lond. 131, 653 9. Sivaprakash, C. (1982) Scott. J. Geol. 18, 109-24. [Revised manuscript received 12 July 1983]