The geochemical behavior of metalloids and their effect on the highly siderophile elements during the crystallization of a magmatic sulfide liquid in relation to the formation of Ni-Cu-PGE magmatic sulfide deposits

Abstract

The highly siderophile elements (HSE), which include Re, Au and the platinum-group elements (PGE: Ru, Rh, Pd, Os, Ir, Pt) are of scientific and economic interest because they are tracers of geological processes (e.g., planetary differentiation) or have industrial or investment value. In most ore deposits, HSE are usually mined as by-products of Ni-Cu mineralization, such as those of the Sudbury district (Canada). Although at high temperature HSE are expected to be in lattice of sulfides they are usually found as discrete phases or platinum-group minerals (PGM). Many of PGM have metalloids (As, Se, Sb, Te and Bi) as essential constituents but, despite this relationship, the role of metalloids on HSE mineralization is still unclear. This dissertation documents the results of three experimental studies undertaken to investigate the effect of metalloids on the geochemistry of HSE. The first study examined whether the metalloids can mobilize HSE in an anhydrous environment. The results show that metalloids do not transport the HSE but also that Au, Pd and Pt can be efficiently fractionated from the other HSE and transported away from a sulfide assemblage through a relatively low-porosity, low-permeability medium in absence of hydrothermal fluids. The second study investigated the solubility of metalloids in monosulfide solid solution (mss) and intermediate solid solution (iss), to asses the conditions required for the formation of immiscible metalloid liquids and their effect on HSE partitioning. The results show that the metalloids form two separate phases, one that is As-rich and strongly sequestered Pt from the sulfides and another that is Bi-Te rich and affected mostly Pd and Au. The results also show that the other HSE (Ru, Rh, Re, Os, Ir) are largely unaffected by the presence of metalloids. In addition the results show that mss can dissolve significant amounts of metalloids (hundreds of ppm) before metalloid V saturation can occur. The third study constrained the geochemical behavior of metalloids and HSE in a pyrite-bearing sulfide system. The results show that Co-Ni-rich and HSE bearing pyrite can form during cooling of a sulfide melt and also that the HSE partition roughly equally between mss and pyrite, whereas Pd that partitions preferentially into mss and Au partitions preferentially into iss. Moreover, pyrite showed compositional zonation of many elements similar to that documented in natural pyrite samples that are also Co-rich and HSE-bearing. Thus, identification of such pyrite in natural assemblages is consistent with a magmatic origin.