Au-Cu-Ag mineralization in rodingites and nephritoids of the Agardag ultramafic massif (southern Tuva, Russia)

Abstract

Gold-bearing albite-amphibole-pyroxene rodingites of the Agardag ultramafic massif (southern Tuva, Russia) are confined to the E-W striking serpentinite crush zone. A zone of gold-bearing nephritoids is localized at the contact of rodingites with serpentinites. Optical and scanning electron microscopy, electron probe microanalysis, and fluorescent, chemical, ICP MS, and X-ray phase analyses were applied to study Au-Cu-Ag mineralization in the serpentinites, rodingites, and nephritoids. Copper sulfides, chalcocite and digenite, are present in the serpentinites, whereas gold and silver minerals are absent. Copper impurity is found in antigorite, Cr-spinel, and magnetite (up to 0.1-0.3 wt.%) as well as parkerite (up to 1.2 wt.%) and millerite (up to 7.9 wt.%). A wide variety of native gold and copper minerals has been identified in the rodingites: (1) cuproauride and tetra-auricupride free of or containing silver impurities (0.1 to 1.2 wt.%); (2) electrum of composition Ag0.50-0.49Au0.50-0.51 (650-660%c) intergrown with AuCu, sometimes as exsolution structures; (3) electrum of composition Ag0.70-0.64Au0.30-0.36 (440-510%c), with inclusions of AuCu and copper sulfides (geerite and yarrowite); (4) high-fineness gold (750-990%c) as veinlets in electrum; and (5) native copper. The composition of copper sulfides varies from chalcocite to covellite. Submicron inclusions of hessite Ag2Te were found in chalcocite. The amount of copper, gold, and silver minerals in the nephritoids is much less than that in the rodingites. The nephritoids contain chalcocite, electrum of composition Ag0.64-0.63Au0.36-0.37 (530-540%c), cuproauride, and tetra-auricupride. The detected hypergene minerals are auricuzite, apachite, brochantite, high-fineness gold, native copper, and cuprite. The sequence of mineral formation in the Agardag ore occurrence has been established on the basis of mineral structures and mineral relations in the rodingites and nephritoids. It is proved that Au-Cu-Ag mineralization formed with the participation of Au- and Ag-bearing chloride-free low-sulfur carbon dioxide alkaline fluids in reducing conditions.