On the Stability of Sulfides, Oxides, and Native Metals in Serpentinite

Topologic relations indicate that the lowest relative oxygenfugacity attained in magnetite-bearing serpentinites occursin the presence of the assemblage serpentine-olivine-brucite.This is consistent with the observation that Ni-Fe alloys arepreferentially associated with this assemblage, both in progrademetaserpentinites, where the stable serpentine mineral is antigorite,and in retrograde serpentinites, where either antigorite, lizardite,or chrysotile may be present. The presence of the Ni-Fe alloyindicates that the assemblage serpentine-olivine-brucite-magnetitein prograde metaserpentinites equilibrated with an oxygen fugacityfour to five log units below the FMQ buffer whereas in retrogradeenvironments it equilibrated at oxygen fugacities as low assix or seven log units below FMQ. In carbonate-bearing metaperidotitesthe oxygen fugacity is buffered by the assemblage Fe-Mg carbonate-Feoxide and this buffering allows the oxygen fugacity to attainvalues above those of the HM buffer for rocks with relativelyhigh XCo2, such as those containing the assemblage talc magnesite.A considerable gradient in fO₂ may therefore be present acrossa serpentinite body from a partially serpentinized core to acarbonatized margin. This gradient is reflected in the compositionof the Fe and Ni sulfides. Sulfur-rich sulfides, such as milleriteor vaesite and pyrite, occur in carbonate-rich peridotites whilethe S-poor sulfide heazlewoodite is found in carbonate-freeserpentinites, and native Ni-Fe alloys, commonly without anyassociated sulfide, are preferentially found in the highly reduced,partially serpentinized peridotites. Under proper conditions the strong gradient info can lead tomobilization of metals or sulfides. Josephinite nodules, forexample, are postulated to have formed during high-temperature,reversible serpentinization. In such a situation, the locationof the serpentinization front and its associated environmentof extreme reduction will be thermally fixed, forming a sitefor deposition of native metals. The Ca in solution will bedeposited along with the metals as diopside and andradite, silicatescommon in josephinite. The tendency for sulfides in metaperidotiteto show evidence of mobilization in bodies metamorphosed toupper greenschist facies or higher may be explained by the factthat above 400 ?C the carbonate-bearing portion of an ultramaficbody may be associated with a fluid dominated by SO₂. Underthese conditions, sulfides may be dissolved and transportedto more-reducing areas of the body, such as the carbonate-freeserpentinite or the contact with the country rock, where theywould be reprecipitated.