Abstract

Darasun, Teremkyn, and Talatui are neighboring deposits in the Darasun gold district of Transbaikalia (Russia) that are hosted by Middle-Late Jurassic subduction-related porphyry intrusions and dikes. Darasun consists of vertically extensive, steeply dipping Au mineralized veins and zones, spatially related to a K-rich granodiorite-porphyry intrusion. Within the deposit are also pipelike bodies of tourmaline sulfide hydrothermal breccias. Teremkyn consists of gently and steeply dipping veins and zones, always associated with felsic dikes comagmatic with the Darasun intrusion. Talatui consists of variably shaped mineralized zones systematically associated with dikes of different compositions and with the host Middle-Late Jurassic granite porphyry. A large proportion of both veins and mineralized zones in these deposits consists of sulfide and oxide minerals, with pyrite and arsenopyrite mostly present at Darasun and Teremkyn and pyrite, magnetite, and hematite present at Talaui. The three deposits host also a large number of sulfosalt, bismuth, oxide, and telluride minerals. Gold of variable purity occurs as free nuggets or hosted by opaque phases. The gangue mineralogy is similar in the three deposits and includes commonly quartz, tourmaline, and calcite.

Microthermometry and microchemical analyses of fluid inclusions with petrographic and compositional data on ore minerals constrain the stages of ore deposition in the district. At room temperature, the entrapped fluids are of four compositional types: (1) multiphase, with a vapor bubble, liquid, and one or more solids; (2) liquid-vapor aqueous; (3a) vapor-rich with a small proportion of liquid and rare solids, and also (3b) vapor-rich with a small proportion of aqueous liquid and carbonic liquid. Types (1) and (3) or (2) and (3) coexist within the same assemblages and are present mostly in paragenetically early quartz, whereas the type (2) fluid within later quartz, sphalerite, and calcite shows consistent phase proportions. At Darasun and Teremkyn, type (2) fluid is the main ore fluid, but all fluid types are associated with gold deposition at Talatui.

Microthermometric measurements from all fluid types show a range of phase transitions that vary systematically at the deposit scale. At Darasun and Teremkyn, coexisting types (1) and (3) and types (2) and (3) homogenize into the vapor and liquid states, respectively, within the same 290° to 460°C range. The corresponding bulk salinities vary between 0.7 and 44.8 wt percent NaCl equiv. In contrast, at Talatui coexisting types (1) and (3) and types (2) and (3) homogenize into the vapor and liquid phases and show a distribution of Th(total) and bulk salinities between about 300° and 610°C and 0.9 and 48 wt % NaCl equiv, respectively. Mass spectrometric and chromatographic data from bulk samples show that the main-stage ore fluid in the three deposits varies compositionally according to location within the district. In detail, Cl, Na, K, Cs, Br, Sr, Rb, Mn, and Tl are relatively enriched in the Talatui fluid, whereas CO2, CH4, HCO, Cu, Pb, Sb, Fe, Hg, and REE are enriched at Darasun and Teremkyn or show no systematic variations. Au concentrations vary within the 0.01-to 3-g/t range at Darasun and Teremkyn, whereas at Talatui values do not exceed 0.4 g/t.

The salinities and homogenization temperatures of the main-stage ore fluids types (1), (2), and (3) are typical of the intrusion-related class of gold deposits. This suggests a dominant magmatic component of the ore fluid during the first stages of mineral precipitation and a magmatic derivation of gold itself in the district. Phase separation was active during the early stages of ore deposition and took place between about 600° to 400°C at Talatui and 450° to 300°C at Teremkyn and Darasun at pressures of 10 to 160 MPa (i.e., 1- to 2.5-km depth). This explains the systematic distribution of ore minerals and fluid chemistry within and around the orebodies. Gold precipitation took place within the three deposits during both early-stage phase separation (Talatui and Darasun) and main-stage ore formation. Microchemical data of fluid inclusions show that the ore fluid was characterized by Au/Cu ratios between 15 and 35 (units: g/t vs. wt %). This range is five orders of magnitude higher than that determined for porphyry Cu deposits (~1·10−4, Ulrich et al., 1999) and compares well with the range of Au/Cu ratios measured in the most fractionated melt hosting the Timbarra intrusion-related deposit of Australia (20–200, Mustard et al., 2006). Hence, similar to porphyry Cu systems, the Au/Cu ratios of the ore fluid at Darasun could have been controlled by the fundamental Au/Cu ratio of the magmatic source rock.

The petrographic evidence for an association between gold and galena and the evidence for consistently high concentrations of Pb, Zn, and Fe in the fluid inclusions suggest that a fraction of the fluid that deposited the orebodies did not always reach saturation in gold and other ore minerals during the early magmatic-hydrothermal stage. We speculate that a fraction of the ore fluid was transported through the orebodies and dispersed at lower temperatures into the outer fringes of the deposits, where they formed the documented galena-sphalerite halo around the Darasun stock.

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