A MULTIDISCIPLINARY STUDY OF INVISIBLE GOLD IN ARSENOPYRITE FROM 4 MESOTHERMAL COLD DEPOSITS IN SIBERIA, RUSSIAN-FEDERATION

Characterization of gold in four Siberian mesothermal gold deposits wa s done through a multidisciplinary investigation (including ore micros copy, INAA, SIMS, EPMA, and Au-197 Mossbauer spectroscopy). The deposi ts are Olympiada and Veduga in the Enisei Mountain Range and Nezhdanin skoye and Sentachan in the Verkhovansk-Kolyma area, in which vein and disseminated orebodies are hosted by sedimentary sequences metamorphos ed to amphibolite (Olympiada) and greenschist facies. Ore minerals inc lude arsenopyrite and pyrrhotite (or pyrite) as major constituents wit h minor scheelite, and sphalerite, chalcopyrite and native gold as min or constituents (with +/- aurostibite, galena, gudmundite, tetrahedrit e, stibnite, and berthierite). Some of the gold mineralization is meta llurgically refractory in all four deposits, largely due to the presen ce of ''invisible'' gold in arsenopyrite, and to a lesser extent, in p yrite. Several generations of arsenopyrite are recognized in the aurif erous zones of the Veduga, Nezhdaninskoye, and Sentachan deposits. Tex tural observations indicate late deposition of native gold with respec t to arsenopyrite. No evidence was found of arsenopyrite recrystalliza tion and the crystallization of native gold is considered to result fr om an evolution of fluids rather than redeposition of gold. Wide varia tions in the gold contents of arsenopyrite occur within deposits, with in samples, and within individual grains. INNA. analyses of individual arsenopyrite grains (30-70 mu m in size) show that the gold concentra tion ranges from <0.2 to 762 ppm for Olympiada, from 6.5 to 393 ppm fo r Veduga, from 2.0 to 938 ppm for Nezhdaninskoye, and from 71 to 292 f or Sentachan. EPMA and X-ray distribution maps using the electron micr oprobe reveal a heterogeneous gold distribution within individual arse nopyrite grains, with gold-rich areas usually depleted in arsenic and gold.The Au contents range from <0.03 to 0.47 wt percent Tor Olympiada , from <0.03 to 0.37 wt percent for Veduga, from <0.03 to 0.14 wt perc ent for Nezhdaninskoye, and from <0.03 to 0.26 wt percent for Sentacha n. The gold concentration of arsenopyrite determined by SIMS varies as follows: Olympiada, from 0.07 to 2,298 ppm; Veduga, from 63.98 to 1,1 41.60 ppm; Nezhdaninskoye, from 1.17 to 386.64 ppm; and Sentachan, fro m 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, r anging from <0.02 to 0.075 mu m in diameter. Au-197 Mossbauer spectra of arsenopyrite concentrates from the Veduga, Nezhdaninskoye, and Sent achan deposits exhibit mainly a component with isomer shifts between 3 .3 and 3.6 mm/s which is interpreted as strong evidence of the presenc e 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 ex pected. Metallic gold dominates in the Au-197 Mossbauer spectra of sam ples from the Olympiada deposit, but varying amounts of chemically bou nd gold are also visible in the spectra, some attributed to aurostibit e. It is concluded that the ores from these deposits are refractory, p artly because the gold occurs in arsenopyrite as chemically bound gold as well as submicroscopic metallic particles probably as small as 2 n m. This study also shows that Au-bearing arsenopyrite is usually deple ted in As with respect to its stoichiometric composition, but correlat ion of Au enrichment to Fe deficiency was not found. It is inferred fr om Au-197 Mossbauer spectroscopy that substitution of Fe3+ for Au3+ is most likely. The temporal change in different modes of gold occurrenc e (i.e., as native gold or invisibly in sulfides) in the mineralized z ones and veins is interpreted to result from an evolution of fluids ra ther than to be a recrystallization phenomenon. Dissemination of Au-be aring 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.