Gold-silver mineralization in metagabbroids of the Bazhenov ophiolite massif (Middle Urals)

Abstract

The relevance of the work is due to the need to study the conditions for the occurrence and distribution of precious metals in ophiolite massifs, which are often associated with gold placers. Purpose of work: Study of gold-silver mineralization from metagabbroids of the Bazhenov ophiolite massif (Middle Urals). Research methodology. The chemical composition of native gold and silver was obtained from polished thin sections of metagabbroids at the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences (Еkaterinburg) on a CAMECA SX 100 electron probe microanalyzer with five wave spectrometers and using standard samples of reference minerals. Results. Native gold in metagabbroids composes poor and scattered dissemination. The metal forms isometric grains, no larger than 10 microns in size. According to microprobe analysis, native gold is quite fine and contains small impurities of silver (no more than 4 wt. %). Both gold and silver occur in sulfidized metagabbroids. Large ”gold pieces”, up to 50–60 microns in size, are noted in the aggregate of chalcopyrite and pyrite. According to microprobe profiling, they have a complex composition: gold passes into electrum, then into kustelite, and then into silver. Of the impurities, copper (up to 1.6 wt. %), iron (up to 1.1 wt. %) and mercury (up to 0.5 wt. %) are noted. Conclusions. In the metagabbroids of the Bazhenov massif, gold-silver mineralization was established in the form of native gold and silver, as well as their alloys – electrum and kustelite. The formation of gold and silver in metagabbroids is metamorphogenic in nature and no connection with acidic fluids has been established. Noble metals and their alloys crystallized at temperatures below 350°C, which corresponds to the bottom of the greenschist facies.