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1 Camborne School of Mines, University of Exeter, Redruth, Cornwall TR15 3SE, UK
2 Laboratoire de Géochimie et de Mineralogie, CP160/02 - Université Libre de Bruxelles, 50 Avenue Roosevelt, B-1050 Bruxelles, Belgium
3 Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
* E-mail:
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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KEYWORDS: platinum-group minerals, magmatic sulphide, gersdorffite, appinite, Talnotry, Scotland
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Introduction |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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Geology |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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). Each fault-bounded sequence youngs to the northwest but the overall younging direction is to the southeast (e.g. Stone et al., 1987). The distribution of lithologies and the regional structure is interpreted to represent an accretionary prism (Leggett et al., 1979; Leggett, 1987) or a southward migrating foreland basin (Stone et al., 1987). Late syntectonic to post-tectonic (e.g. Rock et al., 1986) lamprophyre dyke swarms occur throughout, largely following the regional strike. Several large post-tectonic granites (41034 to 39232 Ma; Thirlwall, 1988; Halliday et al., 1980) were emplaced within the sedimentary rocks and gave rise to substantial metamorphic aureoles that locally exceed 5 km in width (e.g. Cook, 1976).
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). The ore body is lenticular, reaching a maximum thickness of 4 m and extending laterally for 20 m (Wilson and Flett, 1921). Much of the mineralized zone was removed during limited trial workings at the end of the 19th century (Wilson and Flett, 1921). These workings comprise a small upper chamber and a larger lower chamber within which a shallow shaft was sunk. Approximately 100 tons of rock was extracted at which point the mineralization was effectively exhausted (Wilson and Flett, 1921) but little was processed and most was dumped in a large waste pile at the entrance to the workings. Two small adits were also driven along strike and there is a small opencast excavation where the ore body crops out at the surface. A small amount of ore was dumped immediately south of the open-cast excavation but no mineralized lithologies are present in the spoil tips from the adits suggesting that the ore body was not reached. VLF and magnetic geophysical investigations also indicate that the ore body is probably limited to that delineated by the trial workings (Parker, 1977). The area is now afforested and the exposure is relatively poor but a detailed (unpublished) description of the mineralization was carried out soon after mining had ceased and was summarized by Stanley et al.(1987) on which Fig. 3 is largely based. The main body of mineralization lies at the lower contact of (and is restricted to) the diorite body and comprises a 3 m thick zone of pyrrhotite with minor pentlandite and chalcopyrite (Fig. 3). Within this zone Ni reaches 1.4 wt.% whilst Pt concentrations reach 1.8 ppm and total precious metal values exceed 2.8 ppm (Stanley et al., 1987). Cobaltian gersdorffite is slightly enriched in both Pt and Rh (0.3 wt.% and 0.9 wt.%, respectively) but no discrete PGM were identified (Stanley et al., 1987). A well-defined but irregular contact separates the pyrrhotite-rich mineralization from the overlying 1 m thick zone which is dominated by stringers and veins of niccolite and gersdorffite with subordinate chalcopyrite and pyrrhotite. Very elevated Ni (7.3 wt.%) and PGE (up to 49 ppm Pt) concentrations have been reported from within this zone (Stanley et al., 1987) but again, despite the elevated PGE concentrations, no discrete PGM were identified leading Stanley et al.(1987) to suggest that the PGE were in solid solution within niccolite and gersdorffite.
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). The Glen of the Bar mineralization occurs primarily within metasediments but lies within a few metres of the smaller of the two diorite intrusions. On the southern bank of the Palnure Burn the mineralization occurs within a fault zone at the margin of the Cairnsmore of Fleet granite. No in situ exposure is visible at either locality but mineralized specimens are common in the associated waste dumps. There has long been debate over whether the Talnotry ore body is hydrothermal (e.g. Gregory, 1928; Cook, 1976) or magmatic (Wilson and Flett, 1921; Stanley et al., 1987) in origin. Both Gregory (1928) and Cook (1976) cited the presence of hydrothermal arsenopyrite veins nearby as evidence that the deposit is hydrothermal in origin and assumed that the arsenopyrite mineralization was co-genetic with the copper-nickel mineralization. However, the two mineralization styles are very different both chemically and texturally, and the overall similarity of the lower levels of the Talnotry ore body to the magmatic sulphides at Sudbury, particularly those in the southern range (e.g. Cabri and Laflamme, 1976), strongly suggests that the Talnotry ore body has a magmatic origin (Stanley et al., 1987).
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Analytical techniques |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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Terminology |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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Petrography |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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) with the lower lithologies containing relatively elevated MgO, FeO and CaO whilst the upper, more leucocratic lithologies have relatively enriched SiO2, Al2O3 and Na2O.
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Apatite is abundant in all samples and euhedral zircon crystals <100 µm long are common throughout, often with small (<10 µm) inclusions of uraninite and thorianite. Discrete grains of uraninite, thorianite, monazite and baryte also occur, especially in the more leucocratic fractions. All samples contain small, widely disseminated, sulphides most commonly pyrite although pyrrhotite, chalcopyrite, pentlandite, galena and sphalerite are present in the more mafic lithologies together with small, subhedral and often corroded grains of gersdorffite.
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Ore mineralogy |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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) although bismuthinite, Bi telluride (?tellurobismuthite), Pb telluride (?altaite) and galena are present throughout (Fig. 4a). In addition to the three mineral assemblages hosted by the diorite, the metasediment-hosted arsenopyrite mineralization from the Glen of the Bar is also described below.
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). All of the gersdorffite grains are very cobalt-rich in composition (typically Co >13 wt.%) and straddle the gersdorffite–cobaltite boundary (Fig. 5). Subhedral grains of Ag telluride (?hessite) and Bi telluride (?tellurobismuthite) are common within, or concentrated at the margins of the gersdorffite (Fig. 4b). One small grain of an unidentified Re-Bi-Pb sulphide mineral was located, possibly similar to other Re-rich phases described by Jedwab and Fletcher (1991).
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), or entirely replaced, suggesting that the gersdoffite is forming alteration rims. Conversely, gersdorffite overgrowths enclose and preserve earlier euhedral gersdorffite grains (Fig. 4d). The gersdorffite overgrowths have a restricted chemistry which is characterized by very low Co (typically <1 wt.%) and variable Fe contents (<1 to >8 wt.%) whereas the earlier euhedral grains typically have a uniform Fe content (mean = 6.5 wt.%) and Co contents ranging from Co-rich (17.5 wt.%) through to Co-poor (below detection; Fig. 5). Both morphological forms of gersdorffite are optically isotropic. Some small euhedral grains of niccolite remain unaltered and in some cases contain small euhedral inclusions of early euhedral gersdorffite (Fig. 4e) suggesting that niccolite and euhedral gersdorffite are contemporaneous. On weathered surfaces, niccolite is oxidized to the Ni-As-oxide annabergite (Fig. 4d).
Chalcopyrite-gersdorffite
The chalcopyrite-gersdorffite mineralization comprises thin stringers of chalcopyrite and gersdorffite that cross-cut both the niccolite-gersdorffite and the upper levels of the pyrrhotite-chalcopyrite assemblages and locally extend into the largely unmineralized meladiorite above. Chalcopyrite (75%) and gersdorffite (25%) are present with minor pyrrhotite and niccolite. In common with the niccolite-gersdorffite assemblage, the gersdorffite occurs as large euhedral grains (variable Co, uniform Fe content) and as overgrowths (variable Fe, uniformly low Co; Fig. 5) not only around earlier euhedral gersdorffite grains but also chalcopyrite (Fig. 4f).
Arsenopyrite mineralization
Arsenopyrite occurs as brecciated stringers and rare euhedral grains, often in association with quartz. Galena commonly forms tiny inclusions within individual arsenopyrite grains and contains significant Ag. Chalcopyrite and pyrite are also present but are rare; the pyrite often contains numerous bismuthinite inclusions.
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Platinum-group mineralogy |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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).
Pyrrhotite-chalcopyrite
The pyrrhotite-chalcopyrite assemblage contains the most abundant and diverse PGM assemblage and >100 individual PGM grains have been located in 19 polished sections. The grains are widely distributed and relatively sparse; a single 4x2 cm polished thin section rarely contains more than 10 PGM grains. Although the PGM assemblage is quite diverse, almost 90% of the grains are sperrylite (PtAs2), irarsite (IrAsS) or electrum (50%, 26% and 13%, respectively). The remaining 11% comprises a variety of Pd- bearing phases including merenskyite (PdTe2), michenerite (PdBiTe) and froodite (PdBi2) (Table 3).
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). It is also commonly enclosed within sulphides usually as large (>10 µm) anhedral grains (Fig. 6c) but also, more rarely as euhedral crystals (Fig. 6d). Furthermore, large (>10 µm) anhedral grains occur entirely enclosed by silicates such as hornblende (Fig. 6e). Irarsite (commonly Pt- and Rh- bearing) only occurs as small (typically <5 µm) anhedral blebs at the centre of euhedral cobaltian gersdorffite. Electrum is also relatively restricted in its occurrence being most commonly enclosed by chalcopyrite although in some samples it is enclosed within silicates. The Pd-rich phases always occur within silicates (Fig. 6f).
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), that are substantially enriched with respect to Rh (up to 2.9 wt.%) and to a lesser extent Ir (up to 1 wt.%); Pt and Pd are below detection limits (~0.25 wt.%). Very commonly, irarsite or euhedral sperrylite grains occur at the centre of the concentric zones (e.g. Fig. 6a) whilst anhedral sperrylite inclusions are rarely coincident with the zoning (Fig. 6b). The chemistry of the PGE- and PGM-bearing cobaltite-gersdorffite grains is remarkably restricted (Fig. 5) but there is no correlation between PGE content and Ni, Co or Fe content suggesting that the PGE substitute for one or more of these elements.
Niccolite-gersdorffite
Despite the previously identified extremely high PGE values (Stanley et al., 1987), PGM are relatively rare within the niccolite-gersdorffite mineralization; 63 grains have been located in 17 polished thin sections. A relatively restricted assemblage is present and is dominated by electrum (63%), Pd-Bi-Te phases (21%) and sperrylite (13%) with rare hollingworthite (RhAsS) and irarsite. Electrum occurs in a variety of mineralogical associations but most commonly as blebs and veins within gersdorffite often in association with thin, cross-cutting chalcopyrite veins (Fig. 6g) but may also occur enclosed within silicates. The Pd-bearing phases typically occur at the boundary between niccolite and gersdorffite (Fig. 6h) but also occur enclosed within amphibole and biotite. Sperrylite occurs sporadically throughout, enclosed within sulphides and silicates but, as with the other PGM, never enclosed within niccolite. The PGE values within gersdorffite are all below detection limits.
Chalcopyrite-gersdorffite
The diffuse nature of the chalcopyrite-gersdorffite mineralization and the fact that it cross-cuts both the pyrrhotite-chalcopyrite and niccolite-gersdorffite assemblages, means that it is difficult to identify the PGM assemblage associated with this stage of mineralization. Sperrylite and electrum both occur, but only the electrum is unambiguously associated with the chalcopyrite-gersdorffite assemblage where it occurs as small blebs and veins within chalcopyrite. The PGE concentrations within gersdorffite are below detection limits.
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Sulphur isotopes |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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(Table 4). There is no significant difference between the pyrrhotite-chalcopyrite, the niccolite-gersdorffite or the chalcopyrite-gersdorffite assemblages. The two analyses of the metasediment-hosted arsenopyrite mineralization have values (–1.5 and –1.8) that are indistinguishable from the analyses from the mineralized diorite. These results are similar to the S isotopic ratios in magmatic sulphide mineralization associated with mafic lithologies (typically 0; Hoefs, 1997). Similarly, although granitic S isotopic ratios are slightly more varied, they also straddle 0(e.g. Hoefs, 1997). Furthermore, marine sedimentary rocks can have a very wide range of S isotopic values from very negative to strongly positive (–56 to +40; e.g. Rollinson, 1993). The results are therefore ambiguous as they could suggest a common S source (and presumably As; cf. Stanley et al., 1987), two separate S sources with coincidentally similar isotopic ratios, or that the isotopic signature has been reset, possibly during contact metamorphism following the emplacement of the Cairnsmore of Fleet granite.
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Discussion |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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). The chemistry of the early euhedral cobaltite-gersdorffite within both the pyrrhotite-chalcopyrite and niccolite-gersdorffite assemblages therefore indicates a temperature in excess of 550°C (Fig. 6). This is unusually high for an externally driven hydrothermal system, even given the proximity of the Cairnsmore of Fleet granite and is therefore more consistent with magmatic processes. A largely magmatic origin is therefore a more plausible model but, although the Fe-Cu-Ni-S-PGE system is fairly well constrained by experimental studies (e.g. Fleet et al., 1993; Li et al., 1996; Ballhaus et al., 2001), little is known of the effects of significant As concentrations; this may explain the presence of the niccolite-gersdorffite assemblage. The pyrrhotite-chalcopyrite assemblage displays compositions and textures typical of monosulphide solid solution (MSS) crystallization. In metal-rich, S-under-saturated systems (or S-saturated systems at high T), Ni acts incompatibly and preferentially partitions into the sulphide liquid (Li et al., 1996) whilst Co, Ir, Ru, Os and Rh preferentially partition into the MSS. Therefore, during MSS fractionation and crystallization, the residual sulphide liquid becomes increasingly rich in Ni, Cu, Pt, Pd and Au (e.g. Li et al., 1996) and in this case As (cf. Fleet et al., 1993). Following MSS crystallization, it is proposed that a NiAs liquid fractionated from the sulphide liquid and crystallized to form the niccolite-gersdorffite assemblage leaving the residual sulphide liquid enriched in Cu (and presumably volatiles). This eventually crystallized as an intermediate solid solution (ISS) to form the chalcopyrite-gersdorffite assemblage which, possibly due to the high volatile content and low viscosity, locally reacted with and cross-cut the pre-existing ore mineralization resulting in the development of gersdorffite alteration rims around niccolite and the formation of electrum-bearing chalcopyrite veins and disseminations. Minor late-stage hydrothermal redistribution of Cu and Au may also have occurred but the lack of PGE-oxides and vein-hosted PGM suggests that if this occurred it was limited in extent.
This model is broadly in accord with the distribution of the PGE. As noted by Fleet et al.(1993) and Peregoedova and Ohnenstetter (2002), Pt phases commonly crystallize directly from the sulphide liquid at an early stage (prior to MSS crystallization) to give PtFe alloy in Fe-rich systems or PtS in Ni-Cu-rich systems. Significantly, experimental data show that there is a strong correlation between the distribution of Pt and As in quenched sulphide liquids (Fleet et al., 1993) suggesting that in As-rich systems, sperrylite may preferentially crystallize directly from the sulphide liquid (cf. Skinner et al., 1976). This is in accord with the presence of large scattered sperrylite grains enclosed within amphibole, pyrrhotite and gersdorffite in the pyrrhotite-chalcopyrite assemblage and suggests that crystallization of sperrylite preceded MSS fractionation and crystallization. Similarly, anhedral sperrylite grains within cobaltian gersdorffite are rarely coincident with Rh-rich zones suggesting that these grains are inclusions and that the cobaltian gersdorffite and anhedral sperrylite are approximately contemporaneous. In contrast, Rh-rich zones are concentric to irarsite and euhedral sperrylite cores in cobaltian gersdorffite which is consistent with the exsolution of Ir and Pt (probably on cooling) indicating that these elements also partition preferentially into sulpharsenide phases at an early stage, again possibly before MSS fractionation and crystallization. The partitioning of Ir, Os, Ru and Rh into MSS and early sulpharsenides results in a sulphide liquid depleted in these elements, hence the lack of detectable amounts of PGE within the euhedral gersdorffite of the niccolite-gersdorffite assemblage.
The proposed paragenetic sequence of events in the main ore body is shown schematically in Fig. 7 and outlined below:
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Implications of the PGE mineralization
This study shows that although PGM and PGE-enriched gersdorffite are relatively common within the Talnotry ore body, they are not abundant enough to account for the strongly enriched whole-rock PGE concentrations (especially in the niccolite-gersdorffite assemblage). Therefore, as proposed by Stanley et al.(1987), the sulphides, sulpharsenides and arsenides are probably enriched with respect to PGE (albeit below the 0.25 wt.% detection limit of the SEM).
One notable feature of the mineralization is that despite the presence of irarsite grains and Rh-rich zones within cobaltite-gersdorffite, no platarsite (PtAsS) is present, only sperrylite. Platarsite, irarsite, hollingworthite and cobaltite are isostructural and complete solid solution exists between hollingworthite and cobaltite-gersdorffite (e.g. Gervilla et al., 1997; Szentpéteri et al., 2002). The exsolution of Pt from the cobaltite structure should result in platarsite rather than sperrylite (which has a pyrite structure). Therefore, the presence of euhedral sperrylite at the core of cobaltite-gersdorffite rather than platarsite is unexpected and possibly indicates that platarsite may not be stable under as wide a range of conditions as sperrylite.
Elevated PGE values within the euhedral cobaltian gersdorffite have previously been described (Stanley et al., 1987), with maximum Rh and Pt values of 0.9 wt.% and 0.3 wt.%, respectively. However, this study shows that the PGE concentrations within gersdorffite can be substantially higher with Rh contents reaching almost 3 wt.% and Ir contents exceeding 0.9 wt.%. This is in common with numerous other PGE deposits where Rh, Pd and to a lesser extent Ir, are concentrated in cobaltite-gersdorffite (e.g. Cabri, 1992; Gervilla et al., 1997; Barkov et al., 2002). The exsolved grains of sperrylite and blebs of irarsite within gersdorffite suggest that the solubility of Pt and Ir in the cobaltite-gersdorffite lattice decreases with temperature whilst the high Rh concentrations and absence of exsolved hollingworthite blebs are indicative of extensive solid solution over a range of temperatures. Notably, all of the gersdorffite grains that contain either discrete PGM, or have elevated PGE contents, have a restricted range of Ni and Co contents, straddling the cobaltite-gersdorffite boundary (Fig. 5). This is a feature common to many examples of PGE-rich cobaltite-gersdorffite (e.g. Gervilla et al., 1997; Barkov et al., 1996; Szentpéteri et al., 2002) suggesting that the solubility of Rh, Ir and Pt is closely related to the Co:Ni ratio of cobaltite-gersdorffite.
Although the PGE mineralization is probably magmatic in origin, the presence of such elevated PGE concentrations in such a small intrusion is rather unusual, as most PGE deposits require a large volume of magma from which to scavenge and concentrate the PGE. This suggests that the Talnotry diorite was considerably larger, or a large flux of magma has passed through the intrusion (cf. Maier et al., 2001), or that the magma was anomalously enriched with respect to PGE. As there is no significant aureole associated with the diorite, it is unlikely that the intrusion was substantially larger or that a large flux of magma has passed through it. Hence it seems most likely that the magma was PGE-rich, possibly through assimilation of PGE-enriched lithologies or melting of a PGE-rich source region.
Notably, the overall timing, chemistry, mineralogy and range of the host lithologies exposed at Talnotry (e.g. Table 1) bear a remarkable similarity to the appinite suite of intrusive rocks (cf. Fowler and Henney, 1996). Appinites are primarily plutonic vogesites and spessartites (LeMaitre, 2002) together with more leucocratic lithologies that occur as abundant intrusions (commonly coeval and cogenetic with lamprophyre swarms; Rock, 1991) associated with, but pre-dating, the post-tectonic Caledonian granites throughout Scotland (Platten, 1991). They are typically very heterogeneous, often with more mafic marginal lithologies (Platten, 1991) and commonly containing diffuse leucocratic inclusions (Barnes et al., 1986). The Talnotry diorite is essentially a plutonic spessartite and displays all of these features. Despite these similarities, the Talnotry diorite has never been formally recognized as an appinite. In fact, the Southern Uplands granites are among the few Caledonian post-tectonic granites in Scotland that do not have well-documented appinites associated with them although Barnes et al.(1986) suggested that many of the diorite dykes in the region could be reclassified as appinites. As lamprophyres may carry PGE in addition to Au (Rock and Groves, 1988), by association appinites may also carry these elements. Indeed, rare disseminated PGM are present in sulphide-poor appinites from the Scottish Highlands (MRP, unpublished data) despite low whole-rock PGE and Au contents (Pt up to 36 ppb; Henney, pers. comm., 2003). Conversely, very enriched Au values (>500 ppb) were obtained from calc-alkaline lamprophyres from the Southern Uplands (Rock and Groves, 1988) suggesting that the mantle source beneath the Southern Uplands may be anomalously enriched with respect to Au and PGE (cf. Rock and Groves, 1988). Thus, assuming the diorite intrusion at Talnotry is an appinite, it is possible that the high PGE content may be due to melting of an enriched mantle source region coupled with efficient concentration of the PGE by magmatic sulphides. However, a variety of other mechanisms such as assimilation of a PGE-rich body cannot be discounted.
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Acknowledgements |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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[Manuscript received 16 June 2003: revised 10 October 2003]
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References |
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TOPABSTRACTIntroductionGeologyAnalytical techniquesTerminologyPetrographyOre mineralogyPlatinum-group mineralogySulphur isotopesDiscussionAcknowledgementsReferences |
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