Katsuo Kase

Compositional variations in manganese carbonate micronodules from the Lower Proterozoic Nsuta deposit, Ghana: product of authigenic precipitation or post-formational diagenesis?

Abstract Detailed petrographic and microprobe studies of micronodules identified in the manganese carbonate orebody and adjacent Mn-rich rocks at the ∼2.2 Ga Nsuta deposit, Ghana, indicate distinct lithologic control on nodule structure and chemical composition. Nodules in the Mn-rich rocks adjacent to the manganese carbonate orebody conform with the well-banded nature of the rock and are distinctly zoned into alternating bands compositionally dominated by Mg kutnahorite and Mg–Ca rhodochrosite. Individual nodules are partially or completely cemented by thin dark silica and carbonates of Ca–Mg±Mn solid solution. Nodules in the orebody, on the contrary, are generally massive, homogeneous, consist predominantly of rhodochrosite and are only partially cemented by dark silica. Based on the stratigraphic relationship of the nodule-bearing rocks to the manganese carbonate orebody and the variations in texture and composition observed in the nodules, it would appear that the Mn carbonate nodules precipitated diagenetically in response to changing pore water chemistry and/or metal (Mn and Ca) supply. The overall mineralogical and geochemical features indicate, in addition, that the nodules may have preserved original carbonate compositions, both metastable and stable, during burial diagenesis and low-grade regional metamorphism.

Ore mineralogy and sulfur isotope study of the massive sulfide deposit of Filon Norte, Tharsis Mine, Spain

The volcanogenic massive sulfide deposit of Filon Norte at Tharsis is hosted by carbonaceous black slate and connected only partly with stockwork veins. The massive ores are usually composed of fine-grained pyrite with subordinate amounts of sphalerite, chalcopyrite, galena and arsenopyrite. Monoclinic pyrrhotite sometimes occurs in massive pyritic ores in the apparently middle and upper horizons of the orebody, and siderite-rich ores are interstratified with compact pyritic ores in the apparently lower horizons. From the occurrence of monoclinic pyrrhotite, together with the FeS contents of sphalerite mostly ranging from 11 to 16 mol %, it is inferred that the sulfide minerals of the massive orebody were precipitated in euxinic muds on the sea-floor at temperatures below 250°C. The negatively shifted, highly variable δ34S values of the massive ores and their close similarity to those of the underlying black slates strongly suggest that the sulfide sulfur of the massive orebody and the slates is cognate and biogenic.