Identifying what James meant in the word CARLIN.

Submicroscopic gold in sulphide minerals The study of submicroscopic gold in Carlin ores and in synthetic sulphides by Fleet and Mumin (1997) provides a number of insights into the behaviour of submicroscopic gold in sulphide minerals. Arsenopyrite synthesized by Fleet and Mumin (1997) contains a range of concentrations of submicroscopic gold up to a maximum of 3 wt.%, and almost all of these arsenopyrites are As-rich and S-deficient. Johan et al. (1989) report a maximum content of submicroscopic gold in natural arsenopyrite of 1.5 wt.%. Arsenian pyrite synthesized by Fleet and Mumin (1997) contains up to 9.3 wt.% As and a maximum of 6400 ppm of submicroscopic gold. However, at Carlin, the maximum amount of submicroscopic gold in arsenopyrite is ~700 ppm, while that in arsenian pyrite is ~3700 ppm (Fleet and Mumin, 1997). That is, arsenian pyrite at Carlin carries much higher levels of submicroscopic gold than arsenopyrite. In the ore deposits under investigation in this study, the submicroscopic gold content of arsenopyrite and pyrite is considerably less than reported for synthetic sulphides. The maximum average submicroscopic gold content of arsenopyrite is 276 ppm at Lancefield, and the maximum single SIMS analysis on one grain is 1069 ppm, also at Lancefield. This is broadly similar to levels measured in Carlin arsenopyrites (Fleet and Mumin, 1997). The maximum average submicroscopic gold content of pyrite is 43 ppm at Wiluna, and the maximum single SIMS analysis on one grain is 627 ppm, at Mickey Doolan. This is much less than reported in Carlin pyrites (Fleet and Mumin, 1997). In all cases, arsenopyrites in this study have higher average submicroscopic gold contents than co-existing pyrite. Arsenopyrite compositions from the Western Australian ores are almost all As-deficient with respect to stoichiometric FeAsS (As = 0.333 and S = 0.333 on Fig. 13). However, arsenopyrites carrying submicroscopic gold are more Asdeficient than those which do not carry gold. This is in marked contrast to the conclusions of Fleet and Mumin (1997) who found that arsenopyrites from Carlin were all As-rich compared to stoichiometric FeAsS. However, it is consistent with observations by Genkin et al. (1998) that gold-bearing arsenopyrites from Siberia are As-deficient. All of the arsenopyrites from Western Australian gold deposits are also very slightly Fe-deficient, consistent with observations from other localities (Cathelineau et al., 1989; Fleet and Mumin, 1997). The submicroscopic gold content of pyrite from Western Australian gold ores is directly related to their As content, which ranges up to a maximum of ~2.5 wt.% (Fig. 14). The gold content of pyrite crystals is directly related to As content (Fig. 14).  This is also consistent with observations from other deposits (Fleet et al., 1993; Fleet and Mumin, 1997). However, As contents of pyrite from Western Australian gold ores do not approach maximum levels of 9.3 wt.% recorded in synthetic pyrite (Fleet and Mumin, 1997). The reasons for the quite different patterns of incorporation of gold in Carlin-type epithermal ores (very low T) and Archaean-type mesothermal ores (sub-greenschist to amphibolite Ts) may reflect the different formation temperatures. Fleet and Mumin (1997) report a range of sulphide compositions between FeAsS and FeS2 for low-temperature sulphide minerals at Carlin, suggesting that, at low temperatures, sulphide compositions may be much more variable than at higher temperatures. Elements such as As and Au may be more easily incorporated into sulphide structures by adsorption onto high surface area, fine-grained sulphides, and this may explain the very high levels of arsenic and submicroscopic gold in lower-temperature Carlin-type ores. At higher temperatures (lower greenschist) the grain size of pyrite and arsenopyrite increases, their compositions approach stoichiometry and some submicroscopic gold is exsolved, leading to the lower gold concentrations found in sulphides of mesothermal ores. This is not to suggest that mesothermal ores are higher-temperature analogues of Carlin-type ores, but it could imply that initial deposition of at least some mesothermal ores proceeded via a lower temperature, finergrained precursor ore type. Several other observations concerning gold in epithermal and mesothermal ores can be explained if the above interpretation is correct. Firstly, a high proportion of gold in Carlin-type ores is contained in fine-grained sulphides in submicroscopic form, and relatively little is present as native gold. Secondly, pyrite in Carlin-type ores contains much more submicroscopic gold than arsenopyrite (Fleet and Mumin, 1997). In Wiluna-type ores (at the lower end of the temperature range for mesothermal ores), pyrite contains some submicroscopic gold, but much gold is present as inclusions in pyrite. This latter gold may have exsolved from finer-grained pyrite during recrystallization at relatively low temperatures. On the other hand, arsenopyrite at Wiluna still retains high levels of submicroscopic gold, supporting the contention that arsenopyrite is a more refractory sulphide. The mechanism of incorporation of submicroscopic gold into sulphides is still not well understood. Fleet and Mumin (1997) propose that gold is incorporated into sulphides by chemisorption onto As-rich, Fe-deficient surfaces of pyrite, marcasite and arsenopyrite, where it exists as a metastable solid solution. Simon et al. (1999a,b) conclude that gold is present in arsenian pyrite as Au0 microinclusions and Au1+ in the lattice, with Au0 concentrated in finer-grained, lower-temperature pyrite. Cabri et al. (2000) conclude that gold is present in arsenopyrite in two forms: covalent Au1+ and Au0 microparticles. The present study provides some support for the existence of two forms of submicroscopic gold in sulphides. The presence of colloidal gold in nonrefractory, higher-temperature ores from Western Australia (as indicated by SIMS depth profiles) possibly indicates the presence of Au0 microparticles, whereas non-colloidal gold in the lowertemperature ores may correspond to Au1+ in the lattices. Of the four gold deposits studied by Cabri et al. (2000), only the Olympiada deposit is hosted by amphibolite-facies rocks (Genkin et al., 1998); the other three (Sentachan, Sa˜o Bento and Sheba) are hosted by greenschist- or sub-greenschistfacies rocks (Genkin et al., 1998; Thorman et al., 2001; Wagen and Wiegard, 1986). Olympiada is the only deposit that contains Au0 microparticles. It is possible that submicroscopic gold is incorporated into arsenopyrite as covalent Au1+ at sub-greenschist/greenschist-facies temperatures, but is progressively expelled from the arsenopyrite lattice as Au0 microparticles at upper greenschist- to amphibolite-facies temperatures, which then aggregate and recrystallize as larger inclusions of native gold and stringers along microfractures in arsenopyrite, as observed in the Western Australian ores. The studies by Simon et al. (1999a,b) find both types of gold in Carlin-type deposits, although Au0 microparticles in arsenian pyrite are associated with lower-temperature, finer-grained ores, while Au1+ occurs in higher-temperature, coarsergrained pyrite. This appears to be the opposite of what has been observed for arsenopyrite, although the two deposit types (mesothermal and Carlin) are fundamentally different in form and origin. The present investigation indicates that submicroscopic gold appears to be exsolved much more rapidly, and at lower temperatures, from pyrite than from arsenopyrite in the Western Australian ores. With regard to the timing of gold mineralization, this investigation provides some evidence for pre-peak metamorphic deposition of gold at shallower crustal levels, followed by deeper burial and recrystallization and remobilization of gold at higher metamorphic temperatures. That is, native gold in some deposits that may appear to be texturally late, possibly could be remobilization of submicroscopic gold in pre-peak metamorphic ores.  From my MEMORY my recall is that Jim said "CARLIN STYLE" !