The role of halogens with sulfide melting at Broken Hill, New South Wales, Australia

Abstract

This study reports the discovery of a suite of primary lead bromine-substituted chlorides and bromine-iodine substituted oxy-hydroxychlorides which have subsequently exsolved from a sulfogen melt. The implications are that halogens have substituted into sulfide ores at peak metamorphic conditions, during partial melting and exsolved.They provide a direct view of the chemical evolution of the Broken Hill partially melted ore system and may be used as an indicator of coeval melting. The presence of halogens and low melting point chalcophile elements (LMCEs) in these ores suggest that they were introduced together and are a protolith feature. It appears that halogens in the Broken Hill ore deposit were associated with the ores from the outset. Partial melting, melt residue fractionation, enrichment and subsequent mobilisation is predicted. Lead halide compounds have been found in other styles of mineralisation such as Cannington Pb-Zn-Ag ore deposit in NW Queensland and the magmatic Merensky Reef, Bushveld layered complex, South Africa. The highest halogen concentrations recorded in the Broken Hill ore deposit are also confined to the coarse-textured pegmatitic galena. This suggests, that the galena host and a suite of lead halides, represent a bulk composition or final fractionate of a sulfide melt preserved in the post-metamorphic ore deposit at Broken Hill. The halogens have affectively lowered the melting point of the ores at Broken Hill, allowing exsolution of lead halide exsolution lamellae and halide-bearing compounds. A high halogen solubility in galena at regional metamorphic temperatures of ~800{u00B0}C, is a prerequisite for a lowered eutectic. Observed eutectic intergrowth textures, globular textures and low dihedral angle relationships in the ores provide cogent evidence of melting and likely coexisted with an immiscible silicate melt at Broken Hill. Halogen fractionation and partitioning between silicates and sulfides is modeled. Remobilization of massive sulfide orebodies may result from the process of in situ partial melting and liquidus undercooling. Indeed, the lode pegmatite and sulfide ore lithologies at Broken Hill are considered coeval. Halogens are also recorded in the ores of the regional Thackaringa-type deposits of the Consols and Junction Mine lodes. This style of mineralisation is modelled as melt derived. Geochemical and isotopic links are made between the Broken Hill ore deposit and the Thackaringa-type deposits through mobilisation of (LMCEs) and halogens, via F{u2084} fault conduits. The economic implications for partial melting of both styles of mineralisation are profound. Previous experimental work by Mavrogenes et al. (200I), Frost et at. (2002), Kalinowski (2002), Sparks and Mavrogenes (2005) and Wykes and Mavrogenes (2005) provide evidence that partial melting of sulfides occurred. Experimental work in this thesis demonstrates that the solubility of galena in a lead sulfide-halide melt ' sulfogen melt' increases with temperature at a constant pressure of 5 kbar. The behaviour of PbCI{u2082}-PbI{u2082}-PbBr{u2082}-PbS mixtures under high Pressure Temperature define an identifiable eutectic, cotectic relationships as low as 200{u00B0}C, which are well below peak metamorphism conditions of 800{u00B0}C and 5 kbar at Broken Hill. This demonstrates that it is possible for halogen-bearing ore to partially melt and to persist to temperatures well below peak metamorphic conditions. One single experiment performed herein has determined significant melting point depression of pure galena. A general relationship between temperature increase and dissolution of galena into melt above the solidus is established. Melting the PbCl{u2082}-PbI{u2082}-PbBr{u2082}-PbS mixture experimentally suggests a general increase in grain size and in increase in halogen concentration within coarse-grained pegmatitic galena at Broken Hill. The coarse-grained pegmatitic galena is modeled as a product of fractionated and evolved high temperature melts. Thus sulfide melts will accumulate in areas of highest temperatures. A correlation between morphology, temperature and halogen content is established. The Pressure Temperature conditions which formed solid solutions of lead halide and galena phenocrysts in both natural and experimental systems are equivalent. Therefore metamorphic sulfide melting at Broken Hill is fundamentally and theoretically proven to have taken place. Show more