The mineralogy and paragenetic sequence of minerals present in Log-26 ore field of the Topolninskoe gold area (Gorny Altai)

Log-26 ore field is located in the Topolninskoe area which is situated in Gorny Altai within the Ural-Mongolian mountain belt. Par-agenetic and textural studies have established seven productive stages of mineralization of ores of Log-26 field. Early prograde skarn and late retrograde metasomatites are widespread in the area of Log-26 with increase in fault zones. Metasomatites of epidote-actinolite and chlorite-epidote facies form caps in the periphery of skarn bodies. The skarns are composed of clinopyroxene, garnet, calcic amphiboles (ferroactinolite, actinolite and magnesiohornblende), wollastonite, plagioclase, potash feldspar, epidote (clinozoisite, epidote), scapolite, chlorite and calcite. Garnets are grossular-andradite with 0.03 to 4.72 mol% piralspite, whereas pyroxenes are hedenbergitic to diopsidic in composition. Fine sulfide mineralization is concentrated in hydrothermally altered rocks. The research has shown that ores are formed in three mineral stages: arsenopyrite-molybdenite-pyrite, polymetallic and telluride-sulfotelluride-sulfide. For the first time, such minerals as rucklidgeite, tsumoite, hessite, gersdorffite, ullmannite and cobaltite (telluride-sulfotelluride-sulfide stage) have been detected in this area. Late stages are represented by quartz-carbonate cross-cutting veinlets and hypergene carbonates (limonite, malachite, azyrite). Native gold and electrum prevail in molybdenite, bornite and are associated with telluride minerals as fine-grained inclusions; grain size varies from 5 to 20 ^m. As a result of this study the paragenetic scheme was made. The chemical composition and crys-tallochemical formulas were determined for all silicates and opague minerals. The results obtained are in concordance with the data received from similar ore fields (Sinyukhinskoe, Maysko-Lebedskoe). These fields show similar series of hydrothermally altered rocks such as early prograde skarns and late retrograde alterations of propilitic, beresite and argillisite facies. Sulfide mineralization forms gold-magnetite-sulfide, gold-polymetallic with tellurides and quartz-carbonate assemblages. The referred facts evidence the relation of the examined Log-26 ore filed to the group of perspective gold-bearing skarn formations.

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