Mobilization and Fractionation of Magmatic Sulfide: Emplacement and Deformation of the Munali Ni-(Cu-Platinum Group Element) Deposit, Zambia

Magmatic Ni-Cu-platinum group element (PGE) deposits are commonly located in tectonically active regions that typically undergo significant deformation and metamorphism and subsequent reworking of sulfide. The Munali Ni deposit is hosted by a dynamic intrusive mafic-ultramafic system situated within the Zambezi belt in southern Zambia. The deposit comprises Fe-Ni–dominant magmatic sulfides, present as a number of lenticular massive sulfide bodies that display a variety of magmatic and metamorphic sulfide textures. The sulfide lenses are uniformly deficient in iridium subgroup PGEs (IPGEs), Au, and Cu, with unusual but characteristically high bulk Ni/Cu ratios (~10) and a consistent precious metal mineral assemblage dominated by Pd and Pt tellurides. On a centimeter to meter scale, Cu tenors and Ni/Cu ratios are extremely variable (Ni/Cu between 0.1 and 71.5), while Ni and Pd tenors are consistent, indicative of the high mobility and variable concentrations of Cu sulfide within the deposit. Sulfur isotope signatures of the ore sulfides (δ34S ~6‰; Δ33S ~0‰) indicate a local crustal S contaminant from host marbles yet display S/Se ratios suggestive of a postmagmatic overprint. The consistent geochemical similarities of the bulk sulfide throughout the complex and the absence of primary silicate-sulfide textures suggest that the Munali ores were not sourced from a parental magma directly represented by units within the complex. Instead, it is suggested that the sulfide liquid was introduced from elsewhere in the magmatic system during the later stages of the emplacement of the complex. Fractional crystallization of the sulfide liquid during emplacement resulted in the primary segregation of a Cu-rich residual liquid that migrated away from the bulk of the Fe-Ni sulfide, accounting for the high bulk Ni/Cu ratio, with the potential for the accumulation of a separate and thus far undiscovered Cu orebody. In addition, intense deformation during the Pan-African orogeny and interaction with hydrothermal fluids have locally overprinted some of the primary magmatic textures, resulting in localized sulfide mobilization and the extreme variations of Ni/Cu ratio between sulfide samples. Munali therefore represents a complex dynamic deposit showcasing a variety of mechanisms for sulfide fractionation of an Ni-Cu-PGE orebody by both syn- and postmagmatic processes.</p

Mobilization and Fractionation of Magmatic Sulfide: Emplacement and Deformation of the Munali Ni-(Cu-Platinum Group Element) Deposit, Zambia

Magmatic Ni-Cu-platinum group element (PGE) deposits are commonly located in tectonically active regions that typically undergo significant deformation and metamorphism and subsequent reworking of sulfide. The Munali Ni deposit is hosted by a dynamic intrusive mafic-ultramafic system situated within the Zambezi belt in southern Zambia. The deposit comprises Fe-Ni–dominant magmatic sulfides, present as a number of lenticular massive sulfide bodies that display a variety of magmatic and metamorphic sulfide textures. The sulfide lenses are uniformly deficient in iridium subgroup PGEs (IPGEs), Au, and Cu, with unusual but characteristically high bulk Ni/Cu ratios (~10) and a consistent precious metal mineral assemblage dominated by Pd and Pt tellurides. On a centimeter to meter scale, Cu tenors and Ni/Cu ratios are extremely variable (Ni/Cu between 0.1 and 71.5), while Ni and Pd tenors are consistent, indicative of the high mobility and variable concentrations of Cu sulfide within the deposit. Sulfur isotope signatures of the ore sulfides (δ34S ~6‰; Δ33S ~0‰) indicate a local crustal S contaminant from host marbles yet display S/Se ratios suggestive of a postmagmatic overprint. The consistent geochemical similarities of the bulk sulfide throughout the complex and the absence of primary silicate-sulfide textures suggest that the Munali ores were not sourced from a parental magma directly represented by units within the complex. Instead, it is suggested that the sulfide liquid was introduced from elsewhere in the magmatic system during the later stages of the emplacement of the complex. Fractional crystallization of the sulfide liquid during emplacement resulted in the primary segregation of a Cu-rich residual liquid that migrated away from the bulk of the Fe-Ni sulfide, accounting for the high bulk Ni/Cu ratio, with the potential for the accumulation of a separate and thus far undiscovered Cu orebody. In addition, intense deformation during the Pan-African orogeny and interaction with hydrothermal fluids have locally overprinted some of the primary magmatic textures, resulting in localized sulfide mobilization and the extreme variations of Ni/Cu ratio between sulfide samples. Munali therefore represents a complex dynamic deposit showcasing a variety of mechanisms for sulfide fractionation of an Ni-Cu-PGE orebody by both syn- and postmagmatic processes.</p

Thermodynamic and hydrochemical controls on CH4 in a coal seam gas and overlying alluvial aquifer: new insights into CH4 origins

Using a comprehensive data set (dissolved CH(4), δ(13)C-CH(4), δ(2)H-CH(4), δ(13)C-DIC, δ(37)Cl, δ(2)H-H(2)O, δ(18)O-H(2)O, Na, K, Ca, Mg, HCO(3), Cl, Br, SO(4), NO(3) and DO), in combination with a novel application of isometric log ratios, this study describes hydrochemical and thermodynamic controls on dissolved CH(4) from a coal seam gas reservoir and an alluvial aquifer in the Condamine catchment, eastern Surat/north-western Clarence-Moreton basins, Australia. δ(13)C-CH(4) data in the gas reservoir (−58‰ to −49‰) and shallow coal measures underlying the alluvium (−80‰ to −65‰) are distinct. CO(2) reduction is the dominant methanogenic pathway in all aquifers, and it is controlled by SO(4) concentrations and competition for reactants such as H(2). At isolated, brackish sites in the shallow coal measures and alluvium, highly depleted δ(2)H-CH(4) (<310‰) indicate acetoclastic methanogenesis where SO(4) concentrations inhibit CO(2) reduction. Evidence of CH(4) migration from the deep gas reservoir (200–500 m) to the shallow coal measures (<200 m) or the alluvium was not observed. The study demonstrates the importance of understanding CH(4) at different depth profiles within and between aquifers. Further research, including culturing studies of microbial consortia, will improve our understanding of the occurrence of CH(4) within and between aquifers in these basins

Earliest seadfloor hydrothermal systems on Earth- Comparison with modern analogues

Recent developments in multiple sulfur isotope analysis of sulfide and sulfate minerals provide a new tool for investigation of ore-forming processes and sources of sulfur in Archean hydrothermal systems, with important implications for the Archean sulfur cycle, the origin and impact of various microbial metabolisms and the chemistry of surface waters. In the current study we show that most of the sulfides and sulfates in the 3.49 Ga Dresser Formation and 3.24 Ga Panorama Zn-Cu field of Western Australia have non zero ΔS values that indicate variable proportions of seawater sulfate and elemental sulfur of UV-photolysis origin were incorporated into the deposits. Our results show that the multiple sulfur isotope systematics of the Dresser Formation sulfides and sulfates mainly reflect mixing between mass independently fractionated sulfur reservoirs with positive and negative ΔS. Pyrite occurring with barite is depleted in S relative to the host barite that has been interpreted as evidence for microbial sulfate reduction. We note, however, that the reported quadruple sulfur isotope systematics of pyrite-barite pairs are equally permissive of a thermochemical origin for this pyrite, which is consistent with inferred formation temperatures for the chert-barite units in excess of 100°C. The variably positive ΔS anomalies of the Panorama VHMS deposits, disequilibrium relations among sulfides and sulfates and general trend of increasing sulfide ΔS with stratigraphic height in individual ore systems most likely reflects temperature evolution and fluid mixing through the life of the hydrothermal system. The absence of sulfides with significant negative ΔS anomalies suggests that volcanic sulfur, not seawater sulfate, was the dominant sulfur source for the Panorama mineral system. The data presented here require Paleoarchean seawater to be at least locally sulfate bearing