Sulfide Petrology and Geochemistry in Pyroxenites
The geochemistry of sulfide in pyroxenite is virtually unknown.
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Base metal sulfides (BMS) are a minor yet ubiquitous phase in mantle rocks. These minerals play a crucial role in controlling the abundance and behavior of chalcophile and siderophile elements, which are critical and strategic metals. To date, most research has focused on the geochemistry of sulfides in peridotites (e.g., Alard et al., 2000; 2011; Lorand & Alard, 2001; Lorand et al., 2008), the predominant rock type in the mantle. In contrast, sulfides within pyroxenites have been seldom studied (Guo et al., 1999), and their geochemical characteristics remain largely unknown.
| Figure 1 - a: An impressive pyroxenite outcrop in Cabo-Ortegal massif (Nothern Spain, 7th Lherzolite Conference 2024). b: Projection in the Fe-(Ni+Co)-S system of bulk sulfide grain compositions in pyroxenites; purple shade denotes population density (data from Montanini et al., 2025 and O. Alard unpublished). The compositional field of monosulfide solid solution (MSS) is reported in orange for temperatures between 900ºC (light shade) and 1100ºC (dark shade); solid horizontal line denotes the range of pentlandite (pn) composition commonly reported in peridotites. Po, pyrrhotite. Phase relationships in the S-Fe Ni+Co system are after Kullerud et al. (1969) and Craig (1973). | c and d, are microphotography in reflected light and in BSE, respectively of a sulfide associated with the kelyphite rim of garnet in Ronda pyroxenites (C. Marchesi and O. Alard unpublished data) |
Although pyroxenites are much less abundant than peridotites, their contribution to mantle melts is likely significant, as they are more fusible compared to the more refractory peridotites. Further they may represent a pre-concentratrion stage essential to transfer metal-rich melt from the mantle lithosphere to the crust.
References:
Kullerud, G., Yund, R. A. Mohr, G., 1969. Phase relations in the Cu-Fe-S, Cu-Ni-S system. In. Wilson, H.D.B. (ed) Magmatic Ore Deposits, Economic Geology Monography, 4, 323-343.
Craig, J. R. 1973. Pyrite-pentlandite assemblages and other low temperature relations in the Fe-Ni-S system. Amer. J. Science 273A, 496-510.
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