Mineral Precipitation in the Quartz Reefs of the Bendigo Gold Deposit, Victoria, Australia

Gold precipitation at Bendigo postdates cleavage development and was initiated during folding, increased during a reverse faulting stage in bedding-parallel quartz veins, and culminated in quartz reefs associated with a strike-slip faulting event. Some gold was remobilized along carbonate-rich cataclasites associated with later brittle faults. Understanding the paragenesis of the sulfide and gangue minerals associated with the auriferous quartz reefs is critical to unraveling the cycle of ore genesis, from source to sink. In this study, a petrographic approach, closely tied to macroscopic structural observations, along with stable isotope and fluid inclusion analyses, places constraints on the relative timing of gold mineralization with respect to deformation. Early sulfide assemblages in bedding- and cleavage-parallel veins are dominated by pyrite-pyrrhotite-siderite, and are free of visible gold. Later assemblages in reactivated bedding-parallel veins and other fold-related veins are characterized by the presence of arsenopyrite-ankerite-gold. Fault-related veins and the massive quartz reefs are rich in sphalerite and galena with associated gold, but lack pyrrhotite. Small amounts of late-stage antimony-bearing minerals occur in many vein types and postdate the precipitation of gold. Decreasing temperature and increasing sulfur activity controlled successively younger sulfide assemblages in the quartz veins. Stable oxygen isotope data show that quartz from all vein types ( δ 18 O = 15.9–19.0) homogenized with the host rocks, whereas carbon ( δ 13 C =–14.0 to 4.1) and oxygen ( δ 18 O = 4.5–24.0) in carbonate have a wider range in values that is interpreted to be a function of decreasing temperature. This corresponds with early siderite, pyrrhotite, and anhedral pyrite, and with later ankerite and ferroan dolomite associated with arsenopyrite, galena, and euhedral pyrite that are crosscut by calcite veins. Fluid inclusions in quartz veins are predominantly composed of water with carbon dioxide, with smaller proportions of nitrogen and methane, particularly in the later strike-slip faults. The sporadic occurrence of methane suggests that there was an open fluid system with incoming fluids from an external source mixing with those in the host rocks. Although several recent studies have argued that synsedimentary preenrichment may be a significant factor determining the size and distribution of gold in the Bendigo orogenic gold deposits, we believe there is little direct or deposit-scale evidence for a relationship between original metal content in the host-rock metasedimentary rocks and the distribution of the gold mineralization in specific structural sites. It is suggested that deep-seated faults act as conduits for fluid flow and the source for the gold at Bendigo; the gold is externally derived from deeply sourced auriferous metamorphic fluids that have been focused into discrete structural sites.