Forms of Gold and Some Typomorphic Characteristics of Native Gold of the Pavlik Orogenic Deposit (Magadan Oblast)

Abstract

Abstract—The ratios of vein and ore minerals bearing native gold, the shape and size native gold particles, their surface features, variations in chemical composition, and the presence and distribution of impurities and inclusions in them, allow sufficiently substantiated assumptions about ore-forming settings and composition, phase state, and certain other parameters of ore-forming fluids that precipitated native gold. If these data are insufficient for a full-fledged judgment on the ore formation processes, they can at least be used to curtail various genetic speculations. Certain typomorphic features of native gold were studied from one of the largest orogenic gold deposits in northeastern Russia. New data on the grain size distriubtion and chemical composition of native gold and gold contents in sulfides of the Pavlik deposit were obtained. Optical and SEM images were used to determine the ore texture, morphology, and internal structure of native gold and auriferous pyrite and arsenopyrite. ICP-MS, EDS, EDX were used to assess the composition of gold grains, EPMA and LA-ICP-MS were used to qualitatively and quantitatively characterize gold-bearing grains of pyrite and arsenopyrite. It was established that microscopic (finely dispersed (>0.0005–0.01), dusty (>0.01–0.05), extremely fine (>0.05–0.1)) (peak size at 0.075 mm), visible (very fine (>0.1 –0.25)), and fine (>0.25–1.00)) (size peaks at 0.25, 0.55 and 0.85 mm) native gold in association with sphalerite, pyrrhotite, pyrite, arsenopyrite, quartz, carbonates, albite, and sericite is widespread at the Pavlik deposit. Gold particles with an average size of 0.3 mm predominate by weight. Comparison of grain size distribution data with the technological properties of ores and the results of mining in the Omchak ore–placer district made it possible to establish that the share of dusty and fine-dispersed size classes accounts for at least 15–20% of the gold mass at the deposit. ICP-MS analysis of a bulk sample of a monofraction of native gold, which included native gold grains from different parts of the deposit, showed that the composition of the monofraction includes Fe, Hg, As, Zn, Cu, and Pb impurities. The average gold fineness was 806 with additional peaks of 775, 855, and 985 (EPMA determinations). The heterogeneity of native gold is due to the redistribution of silver during late deformation events and, to a lesser extent, removal of silver from gold in the ore oxidation zone. Inclusions in gold are represented by microvoids, disseminated fragments of host metasomatites, and crystals or crystal fragments of potassium feldspar, albite, ankerite–dolomite, and arsenopyrite. The low gold content in pyrite and arsenopyrite, with a high (up to 287 g/t) gold content in arsenic-bearing pyrite and ratios of molar amounts of gold and arsenic in arsenopyrite, may indicate that gold is bound in the crystal structure of sulfides. The relationship between native gold and vein and ore minerals, the features of native gold, and the regularities of gold distribution in pyrite and arsenopyrite do not contradict the hypothesis that native gold was deposited at a single mineralization stage in a different substrate: (1) in the intergranular space (quartz–carbonate metasomatites), (2) in the interstitial space (in arsenopyrite and carbonates), (3) in microfissures in the host minerals. Obviously, the deposition of native gold is an independent process occurring against a background of metamorphogenic (quartz–carbonate veins) and magmatic–metamorphogenic (disseminated pyrite and arsenopyrite) mineralization. Comparison of native gold from ore and placer objects of the Omchak ore–placer district has culminated in methods for using the typomorphic features of native gold to search for placer sources. It seems that various inclusions in native gold with a porous surface are promising for identifying old sources and reconstructing its crystallization conditions.