We have revealed early productive disseminated pyrite-arsenopyrite mineralization with invisible gold in tectonic zones and late nest-veinlet gold-polysulfide mineralization with free gold in zones of silicified rocks at the Suzdal’ (Suzdal’skoe), Zherek, and Bol’shevik deposits in eastern Kazakhstan, localized within black shales. Two varieties of arsenopyrite differing in morphology, chemical composition, and gold contents have been established in them: acicular-prismatic and tabular. Gold was determined by a specially elaborated technique ensuring a detection limit of 30 ppm Au. Acicular-prismatic arsenopyrite is the main ore mineral of the early productive stage of mineralization; it has high gold contents (1400–5360 ppm) and a nonstoichiometric composition (S/As = 1.2) and is slightly depleted in Fe. The absence of correlation between the contents of the main arsenopyrite components and gold, and the strongly uneven distribution of gold among the mineral grains, and within a grain point to the presence of invisible gold, as elemental particles deposited together with arsenopyrite. Tabular arsenopyrite is abundant at the Suzdal’ deposit, where gold-polysulfide mineralization and argillization are widespread. It has low gold and high antimony contents and a stoichiometric composition. Visible gold usually grows over tabular arsenopyrite. The isotopic composition of sulfur of acicular-prismatic arsenopyrite and globular-crystalline pyrite, formed at the early mineralization stages, is characterized by δ34S = 0.0 to –3.3‰ and evidences a mantle source of sulfur with a partial borrowing of crustal sulfur. The tabular arsenopyrite and other sulfides of the second productive mineralization show a lighter isotopic composition of sulfur (δ34S = –7.7 to –10.2‰), which is due to sulfur fractionation under high oxygen fugacity at the late ore deposition stage. The coexistence of two sets of arsenopyrite of different morphologic varieties and compositions at the deposit points to a long ore deposition, the coexistence of mineralization formed at different stages, and the evolution of physicochemical parameters.
In Eastern Kazakhstan, Sb mineralization is the most widespread in the Irtysh and Bakyrchik ore districts of the West Kalba gold-bearing belt. It is spatially related to disseminated gold-sulfide ores at some deposits and is structurally and spatially isolated at others. Disseminated gold-sulfide mineralization is localized in Carboniferous carbonaceous–terrigenous carbonate rocks. It is marked off by zones of dynamic metamorphism and foliation and is characterized by the ribbon-like-lenticular morphology of ore deposits. Later Sb (predominantly, quartz–antimonite) mineralization is formed in an extension setting as brecciated/veined ores. In combination with gold-sulfide ores, Sb mineralization is more diverse. For example, microparageneses with berthierite, native Sb, aurostibite, ullmannite, jamesonite, and tetrahedrite coexist with pocket-vein quartz–carbonate–antimonite mineralization in the gold-sulfide ores of the Suzdal’skoe deposit. Also, Sb-containing minerals such as arsenopyrite and pyrite are observed. Two temperature regimes of mineralization are established here: 418–300 °C for gold-polysulfide mineralization and 280–200 °C for later Sb mineralization. The isotopic composition of antimonite sulfur at the Suzdal’skoe, Zherek, Zhanan, Bakyrchik, and Dal’ny I deposits shows close values within the interval δ34S of –3.8 to 2.5‰, suggesting its great-depth origin. No visible gold is found in the antimonite of the quartz–antimonite veins, but atomic-absorption analysis reveals few ppm or more gold. Point X-ray analysis indicates the possible presence of the so-called “invisible” gold. Microstructural observations, temporal relationships of the parageneses, and studies of gas–liquid inclusions at the Suzdal’skoe deposit permit assigning Sb mineralization to the second productive gold-polysulfide stage of the ore deposition. The late antimonite stage of mineralization is separated from the gold-polysulfide stage by 7 Myr long intramineralization tectonic shifts. Gold-polysulfide mineralization (248.3 ± 3.4 Ma) was synchronous with Triassic tectonomagmatic activity.
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