Turbidite-Hosted Gold Deposits in the Bendigo-Ballarat and Melbourne Zones, Australia. II. Nature of Ore Fluids

Abstract

Fluids in mesothermal, turbidite-hosted, gold vein deposits in central Victoria, Australia are characterized by temperatures ranging from 180 to 310° C and salinities of less than 10 wt% NaCl equivalent. From west to east there is a general decrease in temperatures, averaging near 300° C in the western Bendigo-Ballarat Zone (BBZ), 270° C in the eastern BBZ, and 230° C in the Melbourne Zone (MZ). Pre-mineralization veins generally are lower in temperature than those hosting mineralization, whereas post-mineralization veins have the lowest average temperatures. Salinities are low, <10 wt% NaCl equivalent; those in deposits in the MZ are slightly lower than those in the BBZ. Salinities are variable in a given deposit or even a single vein period, but there is a weak trend of dilution toward late mineralization and post-mineralization stages. Some pre-mineralization veins have higher salinities (to 9 wt%) than mineralized veins.Fluids in the BBZ have C-O-H compositions, whereas those in the MZ are C-O-H-N. In the BBZ, the CH 4 :CO 2 ratio in the gas phase of fluid inclusions varied from 74:26 to 1:99. In the MZ, similar values occurred, as well as high N 2 , to 69% of the gas phase. Minor hydrocarbons (such as C 3 H 6 ) occur, suggesting low values of H. For most of the deposits there is a trend of increasing concentration of CH 4 and CO 2 during mineralization. Average CH 4 :CO 2 ratios for the western BBZ (Maxwells and Cambrian deposits) are 0.56 and 0.33, and for the eastern BBZ (Wattle Gully), 0.23.One or more periods of immiscibility occurred, especially in BBZ deposits during mineralization, but not in pre-or post-mineralization veins. Average pressures present during mineralization were as follows: western BBZ (Maxwells deposit and New Cambrian deposit), 1200 and 1400 bars; eastern BBZ (Wattle Gully deposit), 800 bars; and MZ (Brunswick and Bailieston deposits), 300 and 900 bars. Oxygen fugacities between the HM and QFM buffers varied from 10 -43

Windows of metamorphic sulfur liberation in the crust: Implications for gold deposit genesis

“…Reaction (6) may be responsible for the regional siderite spotting seen in many terranes. Methane has been observed in fluid inclusions in many gold deposits, including Archean deposits, and in some cases much of this is likely to be locally derived (such as in deposits situated amongst carbonaceous units; e.g., Gao and Kwak, 1995). In other localities the observation of methane requires an external source.…”

Turbidite-Hosted Gold Deposits in the Bendigo-Ballarat and Melbourne Zones, Australia. I. Geology, Mineralization, Stable Isotopes, and Implications for Exploration

“…18). At Nagambie, the quartz has δ 18 O values of 20.5 to 22.9%o (Gao et al, 1995). The δ 18 O range of veins in the Walhalla-Woods Point dike swarm (after Green et al, 1982;Wilson and Golding, 1988) are shown for comparison.…”

“…At Maxwells, the range in δ 18 O values is only 0.7%o for laminated vein quartz (17.2) and recrystallized quartz (17.9), whereas at the New Cambrian Mine, the comparable values are 16.7 and 17.5%o. The only quartz samples with δ 18 O values outside these ranges are from stage 2 and stage 4 (19.1 and 14.8%o quartz, respectively) in the Wattle Gully mine, which pre-and post-date a fluid immiscibility event (see Gao and Kwak, 1995). However, the low δ 18 O value (14.8%o) cannot be attributed to immiscibility because: (1) phase separation of vapor H 2 O will result in the depletion of δ 18 O in the vapor and enrichment of 18 O in quartz, and (2) although CO 2 is a volatile substance that strongly partitions 18 O (Bottinga, 1968), there is little fractionation of 18 O / 16 O between CO 2 in solution and CO 2 vapor.…”

“…These data suggest that there is no regular variation trend, although the sam ples were selected from various types of quartz at different mining levels. Hydrogen isotopic compositions were measured in the fluid inclu sion waters of the Nagambie deposit in the MZ (Gao et al, 1995). The δD values of fluid inclusions range from -99 to -88%o and are lighter than the δD values of the BBZ fluids.…”

“…Detailed fluid-inclusion data are presented in an accom panying paper (Gao and Kwak, 1995). One of the objectives of this paper is to discern varia tions, if any, across: (a) the Muckleford Fault, which separates the BBZ into two parts; and (b) the BBZ and MZ zones, separated by the Heathcote Fault Zone.…”