Wall rock alterations and physicochemical conditions of the Sovetsk gold deposit formation (Enisei Ridge)

The mode of distribution of carbonaceous matter (CM) and specific character istics of the metasomatic zonation were studied, as well as physicochemical conditions of the ore deposition and wall rock alteration at the Sovetsk g old deposit. The deposit relates to the quartz vein low-sulfide type and is hosted by carbonaceous rock sequences ("black shales") within a wide shear zone. This shear zone spreads along the regional Ishimbin strike-slip deep fault. The Proterozoic Tatarsk-Ayakhtin granitic complex, intruding the or e bearing rocks, composes the core of the ore district. Granites compose a pluton roof, where Olimpiada and Veduga gold deposits occur, and in contact zones the Eldorado and Pereval'ninsk deposits are located. The Sovetsk dep osit is situated farther from the pluton contacts. The rocks of the ore-bea ring sequence ate metamorphosed in the chlorite-muscovite greenschists faci es. A specific feature of wall rock alteration is unclear lateral metasomat ic zonation with gradual transition between zones and alternating bands of strong and weak alterations. The main metasomatic minerals are muscovite an d chlorite. Quartz and carbonate are subordinate, and ilmenite, rutile, mon oclinic, and hexagonal pyrrhotites are accessories. Above ore bodies turmal ine is rather common, but inside and below ore bodies albite occurs. The di stribution and zonation of metasomatic parageneses allow us to call this ki nd of wall rock alteration as "disseminated metasomatism." The rock alterat ion developed sinchroneously with dynamic metamorphism in the strongly anis otropic sheared rocks. The absence or low intensity of acidic metasomatism is an important feature of the deposit. The temperature of metamorphism was 500-460 degrees C, of wall rock alteration-460-350 degrees C, and of ore d eposition (from early to late stages) from 380 to 150 degrees C. The dissem inated CM was metamorphosed to the low graphitic stage, and the condensed C M in veinlets corresponds to the late antracitic stage. The fugacity of oxy gen and sulfur in the fluids was estimated with ilmenite = rutile + pyrrhot ite equilibrium, composition of pyrrhotite and chlorite. The oxygen fugacit y was lower than that of the ilmenite-magnetite buffer and sulfur fugacity was lower than that of the pyrite-pyrhothite buffer. Nearer to the ore bodi es the sulfur fugaeity increases, and inside ore bodies it corresponds to t he pyrite + pyrrhotite paragenesis. The calculations of the oxygen fugacity with the gas composition in the system C-O-H suggests that the ratios CO2/ CH4/H2O are close to the QFM and ilmenite-magnetite buffers and similar to that received with some mineral fugometers. Hydrocarbons with 2-4 carbon at oms appear not to be in equilibrium with CO2. The variation intervals of pH , f(O2) and (before the ore deposition) of T are rather narrow. The pressur e under metamorphism exceeded 1.9 kbar, but it decreased to the beginning o f ore deposition to 0.7 kbar. This means that ores were deposited in an ext ension environment.