New insights into gold enrichment process during the growth of chalcopyrite-lined conduits within a modern hydrothermal chimney from PACMANUS Basin

Hu, Si-Yu1; Barnes, Steve1; Pages, Anais2; Verrall, Michael1; Parr, Joanna3; Quadir, Zakaria4; Binns, Ray3; Schoneveld, Louise1

1CSIRO Mineral Resources, Kensington, Western Australia, 6151, Australia, 2Department of Water and Environmental Regulation, Joondalup, Western Australia, 6027, Australia, 3CSIRO Mineral Recourses, Lindfield, New South Wales, 2070, Australia, 4Microscopy and Microanalysis Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA 6102, Australia

Seafloor hydrothermal systems are modern analogous of ancient volcanogenic massive sulfide deposits. The hydrothermal chimneys above the seafloor from back-arc basins are important hosts for metals, such as Cu, Zn, Pb, Ag and Au. Although the general growth history of chimneys has been well acknowledged, recent studies have revealed that the fine-scale mineralogy formed from variable physicochemical conditions can be highly complex. Knowledge of detailed mineralogy and formation process in complex chimney structure helps us better understand the spatial distribution and enrichment mechanisms of precious metals. This study utilized a novel combination of scanning electron microscopy (SEM)-based electron backscattered diffraction (EBSD) and synchrotron x-ray fluorescence microscopy (SXFM) to investigate the mechanism of native gold precipitation during the growth of multiple chalcopyrite-lined conduits as part of a modern chalcopyrite-sphalerite chimney. A thin tubular conduit of fine-grained (< 1 µm) sphalerite was initially precipitated under supersaturated conditions when hot hydrothermal vent fluids mixed with surrounding low temperature fluids within an already formed chimney structure. Accretionary growth of chalcopyrite onto this substrate thickened the chimney walls by bi-directional growth inward and outward from the original sphalerite tube wall. A group of similar conduits, but with slightly different mineral assemblages, is interpreted to continue to form in the vicinity of the main conduit during the further fluid mixing process. Four distinct gold-sulfide associations were developed during the growth process, including associated to triangular tennantite in coarse chalcopyrite, thin sphalerite layer, euhedral pyrite, and in cavities of chalcopyrite. The gold is thought to precipitate from various mechanisms, including fluid mixing, sphalerite replacement by chalcopyrite, and the dissolution and re-precipitation of chalcopyrite. A previously unobserved paragenesis of gold nanoparticles occurs as chains at the boundary of early sphalerite and chalcopyrite, distinct from gold observed in massive sphalerite as identified in previous studies. These observations provide baseline data in a well-preserved modern system for studies of enrichment mechanisms of native gold in hydrothermal chimneys. Furthermore, this study provides significant implies that 1) native gold is closely associated to chalcopyrite-line conduits but not necessarily occurs along with tennantite, Bi-rich minerals and bornite as reported before; 2) the broad spectrum of gold occurrence in chalcopyrite-line conduits is likely to be determined by the mixing process between hot hydrothermal fluids with various surrounding fluids.


Biography

I’m a research scientist in CSIRO-Mineral Resources and interested in utilizing a combination of advanced analytical techniques to understand the ore-forming processes through multiple scales. I’m particularly enthusiastic about the modern seafloor hydrothermal systems and the life behaviors in such extreme environments.

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