Syn-metamorphic sulfidation of the Gamsberg zinc deposit, South Africa

The Mesoproterozoic Aggeneys-Gamsberg ore district, South Africa, is one of the world´s largest sulfidic base metal concentrations and well-known as a prime example of Broken Hill-type base metal deposits, traditionally interpreted as metamorphosed SEDEX deposits. Within this district, the Gamsberg deposit stands out for its huge size and strongly Zn-dominated ore ( >14 Mt contained Zn). New electron microprobe analyses and element abundance maps of sulfides and silicates point to fluid-driven sulfidation during retrograde metamorphism. Differences in the chemistry of sulfide inclusions within zoned garnet grains reflect different degrees of interaction of sulfides with high metal/sulfur-ratio with a sulfur-rich metamorphic fluid. Independent evidence of sulfidation during retrograde metamorphism comes from graphic-textured sulfide aggregates that previously have been interpreted as quenched sulfidic melts, replacement of pyrrhotite by pyrite along micro-fractures, and sulfides in phyllic alteration zones. Limited availability of fluid under retrograde conditions caused locally different degrees of segregation of Fe-rich sphalerite into Zn-rich sphalerite and pyrite, and thus considerable heterogeneity in sphalerite chemistry. The invoked sulfur-rich metamorphic fluids would have been able to sulfidize base metal-rich zones in the whole deposit and thus camouflage a potential pre-metamorphic oxidation. These findings support the recently established hypothesis of a pre-Klondikean weathering-induced oxidation event and challenge the traditional explanation of Broken Hill-type deposits as merely metamorphosed SEDEX deposits. Instead, we suggest that the massive sulfide deposits experienced a complex history, starting with initial SEDEX-type mineralization, followed by near-surface oxidation with spatial metal separation, and then sulfidation of this oxidized ore during medium- to high-grade metamorphism.

Is Time Spent Outside the Family Home a Risk Factor for Lead Exposure in Pre-School Children Living in Broken Hill?

Broken Hill is amongst a group of communities internationally that are at greater risk from lead due to active or historical lead industries. Current evidence suggests there is no safe level of lead for young children. This paper describes places outside the family home where young Broken Hill children spend time and considers the potential for this to contribute to lead risk. We interviewed 65 families of children 3 years old or younger and detailed the top five places children spent time at outside the family home. Exposure to private residences outside the family home was recorded for most (88%) young children. Nearly two thirds stayed there five or more hours per week. Most children went there on a weekly basis over many months (median, 12 months), increasing the likelihood of exposure to lead hazards. Further investigation of the lead hazard and risk behaviour of children at these residences would assist in developing guidelines for remediation of the lead hazard for all private residences in Broken Hill. This approach to elucidating the potential sources and pathways of lead and other heavy metal exposures for young children may have merit in other settings where comprehensive zonal remediation is not feasible or may not be warranted.