Mineralogical, textural and metal residence studies of primary, recrystallized and remobilized ores of the Greens Creek Deposit / Минералогические, текстурные и геохимические исследования образования, рекристаллизации и ремобилизации руд месторождения ГК

The Greens Creek deposit is a 24.2-million-ton polymetallic massive sulfide with a diverse base- and preciousmetal-rich mineralogy, which has been subjected to regional lower greenschist facies metamorphism. Roughly 30 percent of the ores retain primary mineralogy and mineral textures as well as gross original ore stratigraphy. Ore lithologies fall into two groups: massive sulfide ores (greater than 50 percent sulfides) and semimassive or disseminated sulfide gangue-rich “white” ores (less than 50 percent sulfides). There are two types of massive ore: massive pyritic and massive base-metalrich ore. The white ores are of three types: white carbonate and white siliceous ores (common), and white baritic ore. All the ore types can be further subdivided on the basis of modification by veins, breccias, and gouge or rubble zones produced during faulting or folding. Veining due to metamorphic remobilization and recrystallization can result in enrichment of free gold and a variety of silver-sulfosalt minerals. The ore stratigraphy at Greens Creek is characterized by proximal copper-arsenic-gold-enriched massive pyritic ores centered over white siliceous ores and silicified footwall. Laterally, the white siliceous ores grade outward into white carbonate and barite ores. White ores are overlain by massive pyritic ores that change upward and outward toward lower copper-gold grades.

Proximal ores transition to increasingly higher grade zinclead-silver-(gold)-rich, massive, fine-grained, base-metal-rich ores toward the argillite hanging wall and the margins of the deposit. Distal ore commonly is characterized by carbonateand barite-rich white ores against footwall phyllites, which grade into massive, fine-grained, base-metal-rich ores toward the hanging wall. This progression of ore types is the most common throughout the mine. Primary mineral textures are characterized by framboidal, colloform, dendritic, and “spongy” pyrite intergrown with base-metal sulfides and sulfosalts. Primary assemblages also include sphalerite, galena, tetrahedrite, chalcopyrite, free gold, and a variety of lead-antimony-arsenic (-mercury-thallium) sulfosalts. The abundance of polyframboidal, colloform, and nodular pyrite textures coupled with their δ34S-depleted isotopic signature provides strong evidence that the main stage of massive sulfide mineralization at Greens Creek occurred primarily during early diagenesis, synchronous with accumulation of the hanging-wall sediments. The early development of framboid-derived, atoll-shaped textures in the ores may indicate that a zone-refinement process occurred in the presence of colloidal base-metal-sulfide gels. Metamorphic recrystallization produced advanced atoll-shaped structures in the ores and resulted in much coarser textures and the formation and(or) remobilization of secondary, precious-metal-enriched minerals. Secondary minerals are present as matrix to pyrite euhedra and in late fractures and veinlets. Secondary mineralogy includes chalcopyrite, sphalerite (low iron), galena, free gold, electrum, tetrahedrite (antimony-rich), pyrargyrite, and many other sulfosalt minerals. Metamorphism resulted in visible cleaning and coarsening of the ore mineralogy and caused local trace-element redistribution and upgrading of the ores.  <...>