Physical-chemical modeling of the ore-forming fluid of the Sukhoi Log deposit (Irkutsk region, Russia)

The main hypotheses on the origin of Sukhoy Log deposits are referred to as (a) postmagmatic-hydrothermal and metamorphic-hydrothermal. The former states that the inflow of gold is caused by the emplacement of post-metamorphic intrusions being the granitoids of the Konstantinovsky stock and hypothetic basic rock complexes of the Ugakhan pluton occurring at depth. The latter is based on the supposition that the ore-forming solutions generated at regional metamorphism of metalliferous carbonaceous sequences, and intrusions were of minor importance depending on the temperature regime only. The physical-chemical modeling (PCM) is currently used for investigating both global geochemical processes, i.e. formation of zo-nation in the weathering crust, phenomenon of mass transfer in the upper mantle of the Earth, and local ones, i.e. formation of mineral associations under metamorphism conditions, construction of physical-chemical model for the hydrothermae of Kipyaschii Lake. Besides, PCM is applied for constructing models disclosing formation of the deposits of gold, molybdenum-tungsten and barite-pyrite ores. Authors Shapkin (1998), Karpov (2001), Kashik (1978) and Chudnenko (2019) reported the results acquired by these studies. In addition, PCM was employed to check the metamorphic-hydrothermal hypothesis on the Sukhoy Log gold ore deposits. Modeling proceeded through three stages. (1) sedimentation, when sediments with siderite-chalcophyle elements accumulate; (2) early collision linked with catagenic alteration of sediments and (3) collision, when the ore bodies were derived. The goal of modeling was to verify or to reject the assumption on possible formation of ore-forming fluid from host rocks. In the computations, we used the composition of host rocks, e.g. metapelites of the Khomolkho formation within the Baikal-Patom upland and mineralized shales of supra-ore zone. The reference sample SLG-1 was referred to as the composition of ore. The composition of gaseous-liquid inclusions in the minerals of sulfide-quartz veinlets was taken as the standard for ore-forming fluid. The model involved 25 independent components: Ag, Al, As, Au, C, Cl, Cu, Ca, F, Fe, H, K, Mg, Mo, N, Na, Ni, O, P, Pb, S, Si, Ti and Zn, 1894 mineral phases, 27 gases and 504 components of aqueous solution. The computations performed with the "Selector" program identified, that in the simulated ore-bearing fluid the Au content comparable with the composition of inclusions was reached at 237 bar and 350 °C. This fluid is in equilibrium with the solid phase of Au fitting the ore gold from mineralized shales. Consequently, the host black shale rocks represent the main source for gold. The mineralization originated from the mineralized shales within the ore body and surrounding mineralized shales.

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