Spatial and temporal zoning of hydrothermal alteration and mineralization in the Sossego iron oxide-copper-gold deposit, Carajas Mineral Province, Brazil: paragenesis and stable isotope constraints

摘要

The Sossego iron oxide-copper-gold deposit (245 Mt @ 1.1 percent Cu, 0.28 g/t Au) in the Carajas Mineral Province of Brazil consists of two major groups of orebodies (Pista--Sequeirinho--Baiano and Sossego--Curral) with distinct alteration assemblages that are separated from each other by a major high angle fault. The deposit is located along a regional WNW-ESE-striking shear zone that defines the contact between metavolcano-sedimentary units of the aprrox 2.76 Ga Itacaiunas Supergroup and tonalitic to trondhjemitic gneisses and migmatites of the aprrox 2.8 Ga Xingu Complex. The deposit is hosted by granite, grano-phyric granite, gabbro, and felsic metavolcanic rocks. The Pista--Sequeirinho--Baiano orebodies have undergone regional sodic (albite--hematite) alteration and later sodic--calcic (actinolite-rich) alteration associated with the formation of massive magnetite--(apatite) bodies. Both these alteration assemblages display ductile to ductile-brittle fabrics. They are cut by spatially restricted zones of potassic (biotite and potassium feldspar) alteration that grades outward to chlorite-rich assemblages. The Sossego-Curral orebodies contain weakly developed early albitic alteration and very poorly developed subsequent calcic-sodic alteration. These orebodies contain well-developed potassic alteration assemblages that were formed during brittle deformation that resulted in the formation of breccia bodies. Breccia matrix commonly displays coarse mineral infill suggestive of growth into open space. Sulfides in both groups of deposits were precipitated first with potassic alteration and more importantly with a later assemblage of calcite--quartz--epidote--chlorite. In the Sequeirinho ore-bodies, sulfides range from undeformed to deformed; sulfides in the Sossego-Curral orebodies are undeformed. Very late, weakly mineralized hydrolytic alteration is present in the Sossego/Currral orebodies. The sulfide assemblage is dominated by chalcopyrite with subsidiary signet, and millerite. Pyrrhotite and pyrite are minor constituents of ore in the Sequerinho orebodies while pyrite is relatively abundant in the Sossego-Curral bodies. Oxygen isotope partitioning between mineral pairs constrains temperatures in the deposit spatially and through time. In the Sequeirinho orebody, the early sodic-calcic alteration stage was characterized by temperatures exceeding 500 deg C and delta ~_(18)O_(H_2O) values for the alteration fluid of 6.9 +- 0.9 per thousand Temperature declines outward and upward from the zone of most intense alteration. Paragenetically later copper--gold mineralization displays markedly lower ternperatures (<300 deg C) and was characterized by the introduction of ~(18)O-depleted hydrothermal fluids -1.8 +- 3.4 per thousand . The calculated delta D_(H_2O) and delta~(18)O_(H_2O) values suggest that the fluids that formed the early calcic-sodic alteration assemblage were of formational/metamorphic or magmatic origin. The decrease of delta~(18)O_(H_2O) values through time may reflect influx of surficially derived waters during later alteration and mineralization events. Influx of such fluids could be related to episodic fluid overpressure, resulting in dilution and cooling of the metalliferous fluid, causing deposition of metals transported as metal chloride complexes.