ISSN 1028-334X, Doklady Earth Sciences, 2014, Vol. 459, Part 1, pp. 1381–1386. © Pleiades Publishing, Ltd., 2014. Original Russian Text © B.R. Tagirov, Yu.P. Dikov, M.I. Buleev, E.V. Koval’chuk, D.A. Chareev, M.A. Kokh, S.E. Borisovskii, V.D. Abramova, N.N. Baranova, M.I. Garas’ko, V.A. Kovalenker, N.S. Bortnikov, 2014, published in Doklady Akademii Nauk, 2014, Vol. 459, No. 1, pp. 90–95. 1381 The gold in ores of hydrothermal deposits either occurs in the form of a native mineral—the natural solid solution—or is dispersed in sulfides as an iso- morphous admixture, nano-sized inclusions of gold, or its compounds with chalcogenides and semimetals (S, Se, Te, As, Sb, and Bi). The dispersed (“invisible”) form of gold is much more difficult to recover in the course of ore treatment, and most of it goes to waste. Therefore, this type of gold is called “refractory”. Despite the importance of the problem, a structural and chemical state of “invisible” gold is unknown for most sulfides. For our studies, gold-containing covel- lite CuS (cr) —a mineral occurring under both hypoge- nous and hypergenous conditions—was selected as the subject. Unlike other minerals of the Cu–Fe–S system, covellite does not form solid solutions with iron, distinguished by this feature from the minerals of the digenite Cu 2– x S–bornite Сu 5 FeS 4 –chalcopyrite CuFeS 2 group. The stability limit of covellite is 507°С; upwards of this temperature, it is decomposed into digenite and liquid sulfur [1]. Covellite is crystallized in hexagonal syngony and, despite the simple compo- sition, shows the complicate layered structure includ- ing two types of Cu atoms in tetrahedral and triangular coordinations and two types of S atoms in the forms of a simple sulfide ion and paired disulfide groups [2]. It is suggested that covellite is a significant concentrator of gold in porphyry copper and epithermal systems [3]. The present study aimed to determine the conditions of gold accumulation and its speciation in covellite using synthetic phases. The experiments on syntheses were carried out within the field of thermodynamical stability of covel- lite in two ways: by a hydrothermal procedure and using the eutectic chloride melts. The hydrothermal synthesis was realized at 450 and 475°С under 1 kbar. We used titanium autoclaves; in the upper parts of them, powdered copper or a mixture of powdered syn- thetic Fe sulfide (troilite, FeS) and copper was placed into a container near which a strip of gold foil was fixed. The autoclaves were filled either with distilled water or with solutions of sulfuric acid, and weighed samples of elemental sulfur were added into them. In the course of the experiment, sulfur interacted with the weighed solid phase and dissolved in the fluid. Then, the dissolved sulfur was disproportionated by the reaction 2S + 4H 2 O = H 2 SO 4(solution) + H 2 S (solution) + 4H 2(gas) (1) “Invisible” Gold in Covellite (CuS): Synthesis and Studies by EPMA, LA–ICP–MS, and XPS Techniques B. R. Tagirov a , Yu. P. Dikov a , M. I. Buleev a , E. V. Koval’chuk a , D. A. Chareev c , M. A. Kokh a , S. E. Borisovskii a , V. D. Abramova a , N. N. Baranova b , M. I. Garas’ko a , V. A. Kovalenker a , and Academician N. S. Bortnikov a Received July 2, 2014 Abstract—Samples of covellite CuS(cr) containing up to 0.3 wt % of gold in “invisible” form were obtained by means of hydrothermal synthesis (450–475°C, 1 kbar) and synthesis in eutectic chloride melts (495°C). The studies using EPMA and LA-ICP–MS techniques showed that gold was uniformly dispersed within the volume of covellite grains and Au content increased with increasing of sulfur fugacity. The XPS-analysis of the samples obtained showed no chemical shift of the Au-4f 7/2 line compared to Au and Au 2 S, whereas a pos- itive chemical shift of this line was revealed for mixed Au–Ag sulfides and tellurides. The obtained data allow one to assume that gold does not occur in covellite as an isomorphous admixture (solid solution) but forms nanoparticles of Au or gold sulfide. DOI: 10.1134/S1028334X14110087 a Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow, Russia b Vernadskii Institute for Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia c Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Russia e-mail: tagir@igem.ru GEOLOGY