W 20 Wall Rock Alterations and Physicochemical Conditions of the Sovetsk Gold Deposit Formation (Enisei Ridge) [Text] / O. V. Rusinova, V. L. Rusinov, S. S. Abramov, S. V. Kuznetsova, Yu. V. Vasyuta // Geology of Ore Deposits. - 1999. - Vol. 41, № 4. - P276-294. - Bibliogr. : p. 294 (24 ref.) Рубрики: ХИМИЧЕСКИЕ НАУКИ Кл.слова (ненормированные): WALL ROCK ALTERATIONS -- PHYSICOCHEMICAL CONDITIONS -- DEPOSIT FORMATION -- GOLD Аннотация: The mode of distribution of carbonaceous matter (CM) and specific characteristics of the metasomatic zonation were studied, as well as physicochemical conditions of the ore deposition and wall rock alteration at the Sovetsk gold 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 ore 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 deposit is situated farther from the pluton contacts. The rocks of the ore-bearing sequence are metamorphosed in the chlorite-muscovite greenschists facies. A specific feature of wall rock alteration is unclear lateral metasomatic zonation with gradual transition between zones and alternating bands of strong and weak alterations. The main metasomatic minerals are muscovite and chlorite. Quartz and carbonate are subordinate, and ilmenite, rutile, monoclinic, and hexagonal pyrrhotites are accessories. Above ore bodies turmaline is rather common, but inside and below ore bodies albite occurs. The distribution and zonation of metasomatic parageneses allow us to call this kind of wall rock alteration as "disseminated metasomatism." The rock alteration developed sinchroneously with dynamic metamorphism in the strongly anisotropic sheared rocks. The absence or low intensity of acidic metasomatism is an important feature of the deposit. The temperature of metamorphism was 500-460°C, of wall rock alteration - 460-350°C, and of ore deposition (from early to late stages) from 380 to 150°C. The disseminated CM was metamorphosed to the low graphitic stage, and the condensed CM in veinlets corresponds to the late antracitic stage. The fugacity of oxygen and sulfur in the fluids was estimated with ilmenite = rutile + pyrrhotite equilibrium, composition of pyrrhotite and chlorite. The oxygen fugacity 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 bodies the sulfur fugaeity increases, and inside ore bodies it corresponds to the 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 atoms appear not to be in equilibrium with CO2. The variation intervals of pH, fo2 and (before the ore deposition) of T are rather narrow. The pressure under metamorphism exceeded 1.9 kbar, but it decreased to the beginning of ore deposition to 0.7 kbar. This means that ores were deposited in an extension environment. |