Trace element distributions and partitioning trends in hydrothermal base metal sulphide ores comprising sphalerite, galena, chalcopyrite and tetrahedrite-tennantite

Abstract

This study addresses trace element concentrations and distributions in hydrothermal base metal sulphide (BMS) ores using samples from a wide variety of ore deposits and conditions of ore formation. The ranges of trace elements that can be incorporated into natural sphalerite, galena, chalcopyrite and tetrahedrite-tennantite are determined, as are the preferred equilibrium trace element partitioning trends among these sulphides. The previously documented coupled substitution Ag⁺+(Bi, Sb)³⁺↔2Pb²⁺ in galena is confirmed, yet should also be modified to include Cu⁺ and Tl⁺. However, when Bi and/or Sb are present at concentrations above ~2000 ppm, incorporation likely includes the creation of site vacancies. Thallium is always principally hosted in galena when BMS assemblages including sphalerite and chalcopyrite are mapped with LA-ICP-MS. Trace element mapping also reveals oscillatory and sector compositional zoning of various elements in galena for the first time. It is inferred that the partitioning of certain minerals between galena and sphalerite pairs is both predictable and systematic. This systematic partitioning is explored and it is shown that the primary factors controlling the preferred BMS hosts of almost all trace elements in sphalerite-galenachalcopyrite assemblages are element oxidation state, ionic radii of the substituting elements, element availability and the maximum trace element budget that a given sulphide structure can accommodate. In contrast, it is revealed that temperature, pressure, redox conditions at time of crystallization and metal source, do not significantly affect the preferred BMS host of almost all trace elements. The only exceptions to this recognized in the study are the critical metals Ga, In and Sn in assemblages recrystallized at high metamorphic grades. Observed partitioning patterns can be used to assess whether a particular BMS assemblage cocrystallized. Compared to sphalerite and galena, trace element concentrations in chalcopyrite are typically quite low (tens to hundreds of ppm). Nevertheless, it is shown that chalcopyrite can host a wide range of trace elements, and the concentrations of such elements generally increase in chalcopyrite in the absence of other co-crystallizing sulphides. Importantly, chalcopyrite is generally a poor host for most elements considered harmful or unwanted in the smelting of Cu (except for Se and Hg on occasions), which suggests it is rarely a significant contributor to the presence of such elements in copper concentrates. The concentrations of Zn and Cd in chalcopyrite show systematic variation that depends, at least in part, on the temperature of BMS crystallization. The Cd:Zn ratios in coexisting chalcopyrite and sphalerite may be used to assess if the physiochemical conditions remained constant during BMS crystallization. Since minerals of the tetrahedrite isotypic series are also common components in base metal ores, investigation into the trace element chemistry of tetrahedrite-tennantite is relevant to understanding the controls on trace element partitioning in such ores. It is shown that tetrahedrite-tennantite will always be the primary host of Ag, Fe, Cu, Zn, As and Sb, and will be the secondary host of Cd, Hg and Bi in co-crystallizing BMS assemblages. Conversely, tetrahedrite-tennantite is a poor host for the critical metals Ga, In and Sn, all of which will prefer to partition to co-crystallizing BMS.