Carbonate replacement and thallium enrichment: ultra-high-resolution trace element mapping, and the origin of Proterozoic sediment-hosted Zn-Pb deposits

There is ongoing debate on the origin of many sediment-hosted Zn-Pb-Ag deposits: the syndepositional 'sedex' model versus the diagenetic replacement model. Unequivocal evidence for exhalative ore is lacking, often underpinned simply by the presence of laminated ore textures. An alternative carbonate replacement diagenetic-epigenetic genetic model for some deposits is also not universally accepted. In this study on a supergiant sediment-hosted Zn-Pb-Ag deposit with contrasting interpretations as either a sedex or carbonate replacement system (McArthur River [HYC], north Australia), we investigate this issue using ground-breaking technology that allows us to quantitatively characterize ore material from the sub metre to nanometre at ultra-high resolution. We show the wide spatial distribution of ore textures in a large context. Furthermore, we are able to, for the first time, show the spatial and mineralogical distribution of trace elements such as thallium that were previously impossible to measure at this scale. Our data show that base metal sulfide mineralization occurs entirely as a product of carbonate replacement in both finely laminated and nodular carbonate samples. Thallium enrichment is associated exclusively with late stage diagenetic (but pre-ore) pyrite overgrowths. Finally, with fluid chemistry modelling, these findings strongly imply a deep subsurface diagenetic-epigenetic model and preclude a syngenetic 'sedex' genetic model.