A multidisciplinary study of invisible gold in arsenopyrite from four mesothermal gold deposits in Siberia, Russian Federation

Abstract

Characterization of gold in four Siberian mesothermal gold deposits was done through a multidisciplinary investigation (including ore microscopy, INAA, SIMS, EPMA, and 197Au Mossbauer spectroscopy). The deposits are Olympiada and Veduga in the Enisei Mountain Range and Nezhdaninskoye and Sentachan in the Verkhoyansk-Kolyma area, in which vein and disseminated orebodies are hosted by sedimentary sequences metamorphosed to amphibolite (Olympiada) and greenschist facies. Ore minerals include arsenopyrite and yrrhotite (or pyrite) as a major constituents with minor scheelite, and sphalerite, chalcopyrite, and native gold as minor constituets (with ± aurostibite, galena, gudmundite, tetrahedrite, stibnite, and berthierite). Some of the gold mineralization is metallurgically refractory in all four deposits, largely due to the presence of 'invisible' gold in arsenopyrite, and to a lesser extent, in pyrite. Several generations of arsenopyrite are recognized in the auriferous zones of the Veduga, Nezhdaninskoye, and Sentachan deposits. Textural observations indicate late deposition of native gold with respect to arsenopyrite. No evidence was found of arsenopyrite recrystallization and the crystallization of native gold is considered to result from an evolution of fluids rather than redeposition of gold. Wide variations in the gold contents of arsenopyrite occur within deposits, within samples, and within individual grains. INNA analyses of individual arsenopyrite grains (30-70 μm in size) show that the gold concentration ranges from <0.2 to 762 ppm for Olympiada from 6.5 to 393 ppm for Veduga, from 2.0 to 938 ppm for Nezhdaninskoye, and from 71 to 292 for Sentachan. EPMA and X-ray distribution maps using the electron mircoprobe reveal a heterogeneous gold distribution within individual arsenopyrite grains, with gold-rich areas usually depleted in arsenic and gold. The Au contents range from <0.03 to 0.47 wt % for Olympiada, from <0.03 to 0.37 wt % for Veduga, from <0.03 to 0.14 wt % for Nezhdaninskoye, and from < 0.03 to 0.26 wt % for Sentachan. The gold concentration of arsenopyrite determined by SIMS varies as follows: Olympiada, from 0.07 to 2298 ppm; Veduga, from 63.98 to 1141.60 ppm; Nezhdaninskoye, from 1.17 to 386.64 ppm; and Sentachan, from 26.7 to 424.5 ppm. Pyrite from Olympiada contains from <0.03 to 0.53 ppm Au, and from Nezhdaninskoye, from <0.23 to 12.97 ppm Au. SIMS ion imaging confirmed the gold heterogeneity and the depth profiles which occasionally detected invisible gold occurring as micro inclusions, ranging from <0.02 to 0.075 μm in diameter. 197Au Mossbauer spectra of arsenopyrite concentrates from the Veduga, Nezhdaninskoye, and Sentachan deposits exhibit mainly a component with isomer shifts between 3.3 and 3.6 mm/s which is interpreted as strong evidence of presence of chemically bound gold in the arsenopyrite crystal structure. The spectra of samples from the Veduga deposit also contain minor amounts of native gold, for which negative isomer shifts near -1.2 mm/s are expected. Metallic gold dominates in the 197Au Mossbauer spectra of samples from the Olympiada deposit, but varying amounts of chemically bound gold are also visible in the spectra, some attributed to aurostibite. It is concluded that the cores from these deposits are refractory, partly becasue the gold occurs in arsenopyrite as chemically bound gold as well as submicroscopic metallic particles probably as small as 2 nm. This study also shows that Au-bearing arsenopyrite is usually depleted in As with respect to its toichiometric composition, but correlation of Au enrichment to Fe deficiency was not found. It is inferred from 197Au Mossbauer spectroscopy that substitution of Fe 3+ for Au 3+ is most likely. The temporalchanges in different modes of gold occurrence (ie, as native gold or invisibly in sulfides) in the mineralized zones and veins is interpreted to result from an evolution of fluids rather than to be recrystallization phenomenon. Dissemination of Au-bearing sulfides within the altered halos infers that a pervasive fluid-rock interaction was an effective mechanism for deposition of minerals and gold. Absorption of gold complexes onto sulfide surfaces is also suggested to have played an important role in the deposition of native gold during the latest episodes in the formation of the deposits.