Mineral Precipitation in the Quartz Reefs of the Bendigo Gold Deposit, Victoria, Australia
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Abstract:
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.Keywords:
Arsenopyrite
Paragenesis
Sulfide Minerals
Gangue
Overprinting
Polymetallic sulfide lenses occur in Ordovician-Silurian sequences of clastic and volcanic rocks of the western Cape Breton Highlands, Nova Scotia. The rocks have been affected by thrusting and metamorphism during late Silurian to Devonian Acadian deformation. The sulfides occur as highly sheared and recrystallized lenses concordant to bedding. Mineral compositions of equilibrium sphalerite - arsenopyrite - pyrrhotite - pyrite intergrowths provide a sliding-scale indicator of P-T-f(S 2 ) variations during metamorphism. The arsenopyrite contains between 33 to 29 atom % As, a range reflecting recrystallization within a changing thermal regime, at temperatures between 510 degrees and 300 degrees C. Pyrrhotite with up to 47.3 atom % Fe is recorded; however, most of the pyrrhotite is relatively sulfur-rich, likely owing to low-temperature inversion or partial oxidation. The dominant composition of sphalerite, between 13-14 mol % FeS, indicates intermediate-to high-pressure conditions. Pressure estimates of 5.5 to 6.9 kilobars are consistent with upper-greenschist metamorphic assemblages found in the surrounding rocks. Decreasing fugacity of sulfur accompanied retrogression in the sulfide lenses as a consequence of the buffering capacity of the coexisting pyrite and pyrrhotite.
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A basis for the interpretation of Fe-As-S mineral assemblages in ores has been provided through systematic collection of physical and chemical data in the synthetic system Fe-As-S. Most interesting is the temperature limitation of 491 degrees C. for coexistence of the commonly observed mineral pair pyrite-arsenopyrite. The As:S ratio in arsenopyrite and/or loellingite may add another to the group of geothermometry techniques now available for ore studies. The observation that Au diffuses rapidly through fine-grained arsenopyrite at temperatures above 600 degrees C. and confining pressures up to 2,070 bars provides some insight into the relations of Au and arsenopyrite in many ores. Equilibrium phase relations in the Fe-As-S system were determined at 600 degrees C., and changes in assemblages were studied in the 400 degrees to 800 degrees C. temperature range. At 600 degrees C. a very narrow liquid field lies along the As-S side of the ternary system between 100 and 22.8 + or - 0.2 weight percent S. Tie-lines connect various parts of this liquid field to pyrite, to pyrrhotite, and to arsenopyrite. At this temperature there are also tie-lines between pyrrhotite-arsenopyrite, arsenopyrite-As, arsenopyrite-toellingite, pyrrhotite-loellingite, pyrrhotite-FeAs, and FeS-Fe 2 As. At temperatures above 600 degrees C. synthetic arsenopyrite has the approximate composition FeAs (sub 1.1) S (sub 0.9) . Compositions that are S rich relative to ideal FeAsS become stable at lower temperatures and under high confining pressures. Changes in the phase assemblages at various temperatures are governed by the reactions: pyrite + arsenopyrite pyrrhotite + liquid or vapor, arsenopyrite + As loellingite + liquid or vapor, and arsenopyrite pyrrhotite + loellingite + liquid or vapor. The invariant temperature at which both liquid and vapor are present in these assemblages are 491 degrees + or - 12 degrees C., 688 degrees + or - 3 degrees C., and 702 degrees + or - 3DGC., respectively. The first reaction was investigated at confining pressures up to 2,070 bars, at which pressure pyrite and arsenopyrite can coexist up to 528 degrees + or - 10 degrees C.
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