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

Russian Academy of Sciences, Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Moscow, Russian Federation

Characterization of gold in four Siberian mesothermal gold deposits was done through a multidisciplinary investigation (including ore microscopy, INAA, SIMS, EPMA, and ¹⁹⁷ Au Moessbauer 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 pyrrhotite (or pyrite) as major constituents with minor scheelite, and sphalerite, chalcopyrite and native gold as minor constituents (with + or - 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 mu m 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 microprobe 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 percent for Olympiada, from <0.03 to 0.37 wt percent for Veduga, from <0.03 to 0.14 wt percent for Nezhdaninskoye, and from <0.03 to 0.26 wt percent for Sentachan. The gold concentration of arsenopyrite determined by SIMS varies as follows: Olympiada, from 0.07 to 2,298 ppm; Veduga, from 63.98 to 1,141.60 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 mu m in diameter. ¹⁹⁷ Au Moessbauer 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 the 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 ¹⁹⁷ Au Moessbauer 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 ores from these deposits are refractory, partly because 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 stoichiometric composition, but correlation of Au enrichment to Fe deficiency was not found. It is inferred from ¹⁹⁷ Au Moessbauer spectroscopy that substitution of Fe (super 3+) for Au (super 3+) is most likely. The temporal change in different modes of gold occurrence (i.e., 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 a 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.

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