Evolution of sulfide mineralization on Mars

Abstract

The presence of komatiitic igneous rocks on Mars, based on geochemical evidence from SNC meteorites and Viking X ray fluorescence analyses of the regolith, suggests that massive and disseminated iron sulfide mineralization occurs near the Martian surface. Analogies are drawn between possible ultramafic Fe‐Ni sulfides on Mars and terrestrial pyrrhotite‐pentlandite ore deposits associated with Archean komatiites formed during early crustal development on Earth. Partial melting of the mantle as a result of high radiogenic heat production then, extrusion of turbulent high‐temperature ultramafic lavas, segregation of immiscible FeS melts during cooling, gravitational settling and fractional crystallization of sulfide minerals in magma chambers or lava flows produced massive and disseminated sulfide mineralization associated with terrestrial komatiites. Comparable processes probably occurred on Mars where, on account of the inferred higher Fe/(Fe + Mg) ratio of the X ray mantle (estimated to contain ∼4.5 wt % S), iron‐rich basaltic magmas were produced by partial melting at depths and temperatures exceeding 165 km and 1400°C, respectively. Adiabatic diapiric emplacement of these iron‐rich, very low viscosity basaltic melts transported significant concentrations of dissolved sulfur as S2− and HS− from the mantle. Ensuing sulfide mineralization may have been either thinly disseminated within ultramafic lavas erupting over large areas of Mars or concentrated locally at the base of structural depressions. Cumulate ore deposits several meters thick may occur at the base of intrusions or in near‐surface magma chambers. The evidence for insignificant plate tectonic activity on Mars and minimal interactions of Martian mantle with crust, hydrosphere and atmosphere has restricted the evolution of sulfide ore deposits there. Thus terrestrial porphyry copper and molybdenum deposits, granite‐hosted mineralization, and related continental crust‐derived ores, including PbS‐ZnS deposits in sedimentary rocks, are unlikely to have formed on Mars. Ultramafic Fe‐Ni sulfides and perhaps iron‐rich sediments (gossans and abiotic banded iron formations) derived from chemical weathering of the basaltic crust, as well as cumulate chromites, are likely to be the only ore deposits present on Mars.