Sulfide oxidation as a process for the formation of copper rich magmatic sulfides
2013 (English)In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 48, no 1, 115-127 p.Article in journal (Refereed) Published
Typical magmatic sulfides are dominated by pyrrhotite and pentlandite with minor chalcopyrite, and the bulk atomic Cu/Fe ratio of these sulfides is typically less than unity. However, there are rare magmatic sulfide occurrences that are dominated by Cu-rich sulfides (e.g., bornite, digenite, and chalcopyrite, sometimes coexisting with metallic Cu) with atomic Cu/Fe as high as 5. Typically, these types of sulfide assemblages occur in the upper parts of moderately to highly fractionated layered mafic-ultramafic intrusions, a well-known example being the Pd/Au reef in the Upper Middle Zone of the Skaergaard intrusion. Processes proposed to explain why these sulfides are so unusually rich in Cu include fractional crystallization of Fe/(Ni) monosulfide and infiltration of postmagmatic Cu-rich fluids. In this contribution, we explore and experimentally evaluate a third possibility: that Cu-rich magmatic sulfides may be the result of magmatic oxidation. FeS-dominated Ni/Cu-bearing sulfides were equilibrated at variable oxygen fugacities in both open and closed system. Our results show that the Cu/Fe ratio of the sulfide melt increases as a function of oxygen fugacity due to the preferential conversion of FeS into FeO and FeO1.5, and the resistance of Cu2S to being converted into an oxide component even at oxygen fugacities characteristic of the sulfide/sulfate transition (above FMQ + 1). This phenomenon will lead to an increase in the metal/S ratio of a sulfide liquid and will also depress its liquidus temperature. As such, any modeling of the sulfide liquid line of descent in magmatic sulfide complexes needs to address this issue.
Place, publisher, year, edition, pages
2013. Vol. 48, no 1, 115-127 p.
Cu/Fe, Fractional crystallization, Oxidation, Sulfide melt
IdentifiersURN: urn:nbn:se:su:diva-88311DOI: 10.1007/s00126-012-0420-9ISI: 000313813400005OAI: oai:DiVA.org:su-88311DiVA: diva2:611861