• journal_title：The Canadian Mineralogist
• Contributor：Kirsten Drüppel ; Thomas Wagner ; Adrian J. Boyce
• Publisher：Mineralogical Association of Canada
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.44.4.877
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：44
• issue：4
• firstpage：877
• section：Articles

The Mesoproterozoic (ca. 1140–1120 Ma) ferrocarbonatite dykes of Swartbooisdrif, northwestern Namibia, transect the anorthositic rocks of the Kunene Intrusive Complex. The ferrocarbonatite is mainly composed of ankerite and magnetite, and is highly contaminated by fragmented and fenitized wallrock anorthosite (albite, biotite and sodalite), with Fe–Cu–Ni sulfides (assemblage: pyrite, chalcopyrite, millerite, polydymite, fletcherite) being widespread minor components. In order to constrain the compositional evolution and to reconstruct the T–f(O₂) conditions during progressive crystallization and subsolidus re-equilibration of the ferrocarbonatites, a detailed study of the mineral chemistry and sulfur isotope signatures of the sulfide mineralization has been carried out. Owing to the finely intergrown nature of the sulfide assemblages, sulfur isotope analysis has been performed using an in situ laser-microprobe technique. The δ³⁴S values of early carbonatite-hosted pyrite range from 3.8 to 5.1‰. Similar δ³⁴S values were obtained for metasomatically formed pyrite from the fenitized anorthosites (+3.3 to +3.4‰). Distinctly lower, negative δ³⁴S values have been determined for pyrite (−2.4 to −2.1‰) and chalcopyrite (−3.3 to 0.0‰) in late sulfide–oxide veins, with the lowest values obtained for sulfides from samples that contain additional secondary barite. This overall isotopic trend is interpreted in terms of changing proportions of oxidized and reduced sulfur species in the carbonatite-derived fluids, and is supported by calculations of phase equilibria in the system Cu–Fe–S–O–H and modeling of isotopic mass-balance. During the early stage of mineralization, the oxygen fugacity was presumably low, with reduced sulfur species (H₂S, HS⁻) dominant in the fluids, promoting the precipitation of pyrite and the metasomatic formation of nearly pure, SO₄-free sodalite. A subsequent increase in f(O₂) led to a systematic increase in the ∑SO₄/∑H₂S ratio of the fluid. This resulted in a depletion of ³⁴S in the residual reduced sulfur, which is manifested by the negative δ³⁴S values of the late sulfide mineralization.