Orogenic-type copper-gold-arsenic-(bismuth) mineralization at Flatschach (Eastern Alps), Austria

• Published in: Mineralogy and petrology 2015-10, Vol.109 (5), p.531-553
• Main Authors: Raith, Johann G., Leitner, Thomas, Paar, Werner H.
• Format: Article
• Language: English
• Subjects: Arsenic; Bismuth; Calcite; Carbonates; Chalcopyrite; Copper; Cretaceous; CUPRITE; Dolomite; Earth and Environmental Science; Earth Sciences; Enargite; Fineness; Geochemistry; Geology; Gold; GOLD (PURE); Hydroxides; Inorganic Chemistry; Low temperature; Metamorphic rocks; Metamorphism; Mineralization; Mineralogy; Minerals; Miocene; Original Paper; Pyrite; Sulfides; Veins; Veins (geology)
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-015-0391-5

Structurally controlled Cu-Au mineralization in the historic Flatschach mining district (Styria, Austria) occurs in a NE–SW to NNE–WSW oriented vein system as multiple steep-dipping calcite-(dolomite)-quartz veins in amphibolite facies metamorphic rocks (banded gneisses/amphibolites, orthogneisses, metagranitoids) of the poly-metamorphosed Austroalpine Silvretta-Seckau nappe. Vein formation postdated ductile deformation events and Eoalpine (Late Cretaceous) peak metamorphism but predated Early to Middle Miocene sediment deposition in the Fohnsdorf pull-apart basin; coal-bearing sediments cover the metamorphic basement plus the mineralized veins at the northern edge of the basin. Three gold-bearing ore stages consist of a stage 1 primary hydrothermal (mesothermal?) ore assemblage dominated by chalcopyrite, pyrite and arsenopyrite. Associated minor minerals include alloclasite, enargite, bornite, sphalerite, galena, bismuth and matildite. Gold in this stage is spatially associated with chalcopyrite occurring as inclusions, along re-healed micro-fractures or along grain boundaries of chalcopyrite with pyrite or arsenopyrite. Sericite-carbonate alteration is developed around the veins. Stage 2 ore minerals formed by the replacement of stage 1 sulfides and include digenite, anilite, “blue-remaining covellite” (spionkopite, yarrowite), bismuth, and the rare copper arsenides domeykite and koutekite. Gold in stage 2 is angular to rounded in shape and occurs primarily in the carbonate (calcite, Fe-dolomite) gangue and less commonly together with digenite, domeykite/koutekite and bismuth. Stage 3 is a strongly oxidized assemblage that includes hematite, cuprite, and various secondary Cu- and Fe-hydroxides and -carbonates. It formed during supergene weathering. Stage 1 and 2 gold consists mostly of electrum (gold fineness 640–860; mean = 725; n  = 46), and rare near pure gold (fineness 930–940; n  = 6). Gold in stage 3 is Ag-rich electrum (fineness 350–490, n  = 12), and has a high Hg content (up to 11 mass %). The Cu-Au deposits in the Flatschach area show similarities with meso- to epizonal orogenic lode gold deposits regarding the geological setting, the structural control of mineralization, the type of alteration, the early (stage 1) sulfide assemblage and composition of gold. Unique about the Flatschach district is the lower-temperature overprint of copper arsenides (domeykite and koutekite) and copper sulfides (djurleite, yarrowite/spionkopite) on earlier forme