The distribution of trace elements in iron sulfides and associated chlorine-bearing silicates

Abstract

Pyrrhotite and pyrite commonly contain minor quantities of cobalt, nickel, arsenic and other trace elements, which reflect major effects of source lithology and fluid chemistry, and subordinate effects relating to local processes. Electron probe micro-analyses (EPMA) of silicates from a suite of fifteen metasomatic Fe-oxide and affiliated ore samples show enrichments of chlorine in biopyriboles associated with Co-bearing iron sulfides and arsenides. Biotite from several localities contains from 1.5 to over 5.5 wt.% Cl. Amphiboles from several localities are also Cl-enriched, with up to 2.7 wt.% Cl. Amphibole from the Osborne Au-Cu deposit in Queensland also contains up to 1.5 wt.% scandium. Otherwise, mild enrichments of both scandium and vanadium are widespread in the silicates from many of these samples, particularly in epidote (up to 550 ppm Sc and up to 1860 ppm V), amphibole (to 260 ppm Sc) and diopside (to 0.57 wt.% V from one locality). A more extensive study was undertaken to determine the trace element distribution in iron sulfides. Over 1100 EPMA and 150 SIMS analyses of pyrite, pyrrhotite, and other sulfides were collected, representing over 100 localities and a dozen major ore-forming environments. These results were combined with data from the literature to assess overall Co, Ni, As, Se, Pd and Au distributions in iron sulfides across a range of geologic processes and lithologies. Iron sulfides from mafic-hosted magmatic oxide and sulfide systems are enriched in Co and Ni. In contrast to magmatic systems, iron sulfides from hydrothermal systems dominated by magmatic-derived aqueous fluids tend to be low in overall trace elements, regardless of the host lithology. Iron sulfides from hydrothermal systems dominated by external fluids show a wide variation in trace element content. In deposit types formed by dilute fluids (e.g. epithermal Au), Co and Ni tend to be low in pyrite whereas As may be quite high. Where high salinity fluids are implicated (as in metasomatic Fe-oxide systems), As and Ni contents in pyrite are variable but Co is typically enriched. Iron sulfide compositions also reflect source lithologies. Among diverse volcanogenic massive sulfide (VMS) deposits, pyrite from the more mafic Cyprus and Besshi types show notable enrichments of Co whereas Kuroko deposits rarely show any enrichment. Hence, source lithology and fluid chemistry play major roles in the minor element make-up of the iron sulfides across a range of deposit types. A parallel investigation was undertaken to assess the role of thermodynamics on the trace elements contents of iron sulfides. From a synthesis and evaluation of published experimental results, phase relationships in the Co-S-O-(Si) system were calculated and contrasted to the equivalent Fe system. The boundaries between the sulfides and oxides in the Co-S-O system are shifted to higher oxygen fugacities relative to the phase boundaries in the Fe-S-O system. In addition to the Fe-Co-S-O-(Si) system, phase relations in the Fe-(Ni,Co)-As-S system have been evaluated. Arsenic is implicated in a coupled substitution mechanism [CoAs]₁[FeS]₋₁ observed in some pyrite. The observed substitution behavior may be a reflection of the limiting assemblages present.