The Effect of Co-Crystallising Sulphides and Precipitation Mechanisms on Sphalerite Geochemistry: A Case Study from the Hilton Zn-Pb (Ag) Deposit, Australia

Abstract

High-tech metals including Ge, Ga and In are often sourced as by-products from a range of ore minerals, including sphalerite from Zn-Pb deposits. The Hilton Zn-Pb (Ag) deposit in the Mount Isa Inlier, Queensland, contains six textural varieties of sphalerite that have formed through a diverse range of processes with variable co-crystallising sulphides. This textural complexity provides a unique opportunity to examine the effects of co-crystallising sulphides and chemical remobilisation on the trace element geochemistry of sphalerite. Early sphalerite (sph-1) is stratabound and coeval with pyrrhotite, pyrite and galena. Disseminated sphalerite (sph-2) occurs as isolated fine-grained laths rarely associated with co-crystallising sulphides and represents an alteration selvage accompanying the precipitation of early stratabound sphalerite (sph-1). Sphalerite (sph-3) occurs in early ferroan-dolomite veins and formed from the chemical remobilisation of stratabound sphalerite (sph-1) during brittle fracturing and interstitial fluid flow. This generation of veins terminate at the interface, and occurs within clasts of the paragenetically later sphalerite-dominated breccias (sph-4). Regions of high-grade Cu (>2%) mineralisation contain a late generation of sphalerite (sph-5), which formed from the recrystallisation of breccia-type sphalerite (sph-4) during the infiltration of a paragenetically late Cu- and Pb-rich fluid. Late ferroan-dolomite veins crosscut all previous stages of mineralisation and also contain chemically remobilised sphalerite (sph-6). Major and trace elements including Fe, Co, In, Sn, Sb, Ag and Tl are depleted in sphalerite associated with abundant co-crystallised neighbouring sulphides (e.g., pyrite, pyrrhotite, galena and chalcopyrite) relative to sphalerite associated with minor to no co-crystallising sulphides. This depletion is attributed to the incorporation of the trace elements into the competing sulphide minerals. Chemically remobilised sphalerite is enriched in Zn, Cd, Ge, Ga and Sn, and depleted in Fe, Tl, Co, Bi and occasionally Ag, Sb and Mn relative to the primary minerals. This is attributed to the higher mobility of Zn, Ge, Ga and Sn relative to Fe and Co during the chemical remobilisation process, coupled with the effect of co-crystallising with galena and ferroan-dolomite. Results from this study indicate that the consideration of co-crystallising sulphides and post-depositional processes are important in understanding the trace element composition of sphalerite on both a microscopic and deposit-scale, and has implications for a range of Zn-Pb deposits worldwide.

Ore genetic implications of pyrite and sphalerite compositions fromRajpura‐Daribaand ZawarmalaPb‐Zndeposits, Northwestern India

“…Overlap in sphalerite compositions may indicate complex host rock lithologies of the deposits as compared to the broad lithological classification used in our PCA. For example, the Hilton deposit in Australia is dominantly hosted in carbonaceous shale, however the host rock also comprises of dolomites (Cave, Lilly, & Hong, 2020b). On the other hand, sphalerite from quartz–mica‐feldspar gneiss and metapelitic gneiss do not show any distinct clusters in PC1‐PC2 score plot (Figure 9k), which can be attributed to insufficient data of sphalerite from these lithologies (Appendix B).…”

LA-ICP-MS Trace Element Geochemistry of Sphalerite and Metallogenic Constraints: A Case Study from Nanmushu Zn–Pb Deposit in the Mayuan District, Shaanxi Province, China

“…Blocks of two standards (one NIST 610 and one GSE-2G) and one MASS-1 sulfide standard analyses were followed by five to eight unknown samples. For sphalerite, the following 18 isotopes were measured (with their respective dwell times in milliseconds listed in parentheses): 34 S, 55 Mn, 57 Fe, 59 Co, 60 Ni, 65 Cu, 66 Zn, 71 Ga, 74 Ge, 75 As, 77 Se, 107 Ag, 111 Cd, 118 Sn, 121 Sb, 126 Hg, 205 Tl, and 208 Pb, corresponding to a total dwell time of 180 ms. The MASS-1 sulfide standard [37] was used as the primary standard for calibrating 34 S, 55 Mn, 57 Fe, 59 Co, 60 Ni, 65 Cu, 66 Zn, 71 Ga, 74 Ge, 75 As, 77 Se, 107 Ag, 111 Cd, 118 Sn, 121 Sb, 126 Hg, 205 Tl, and 208 Pb.…”

Pyrite chemistry records a multistage ore forming system at the Proterozoic George Fisher massive sulfide Zn-Pb-Ag deposit, Mount Isa, Australia

“…The main ore stage sulfide minerals in the George Fisher, Hilton, and Mount Isa massive sulfide deposits are pyrite, sphalerite, galena, chalcopyrite, and pyrrhotite (e.g., Valenta, 1994;Chapman, 2004;Cave et al, 2020a;Cave et al, 2020b). At the George Fisher deposit it is generally accepted that sulfide mineralization formed epigenetically and during multiple stages (Chapman 1999;Murphy, 2004;Rieger et al, 2020a), although the volumetric importance of individual ore stages (syn-diagenetic vs. syn-deformation) and the relationship of Zn-Pb and Cu-Zn-Pb mineralization are not clear.…”

“…Indeed, it has been suggested that Mount Isa Cu was sourced from mafic volcanic rocks in the sequences below the deposits (Eastern Creek Volcanics; e.g., Heinrich et al, 1989;Hannan et al, 1993). Sphalerite and chalcopyrite associated with Cu-mineralization at Mount Isa and Hilton is also relatively enriched in Co (Cave et al, 2020a;Cave et al, 2020b), which may indicate that fractionation effects of Co among pyrite and other sulfide phases were relatively minor and that Cu-mineralization at George Fisher formed from relatively Co-rich fluids coeval with Mount Isa and Hilton Cu.…”