Mineralogy and Mineral Chemistry of Polyphase Au-Ag-As-Cu-Bi Mineralization at Rotgülden, Austria

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Abstract

Rotgülden is the largest known Au-Ag-As-Cu-Bi deposit in the eastern Tauern Window and a special type of gold mineralization within the Hohe Tauern area. It is situated within the Silbereck Series and Brennkogel Formation partly forming massive replacement ores linked to a large-scale Alpine fold structure. It is bound to dolomite marbles (Silbereck Series) as well as calcareous and black shales (Brennkogel Formation) and comprises of five types of mineralization: (1) Pyrite-rich stockwork ore in dolomite marble in the footwall of the main ore body, (2) Pyrrhotite-rich massive ore in calcareous shale, (3) Arsenopyrite-rich massive ore in calcareous-black shale beneath the major fold zone, (4) Chalcopyrite-rich massive ore within black shale beneath the fold zone, and (5) Disseminated ore in calcareous shale in the hanging-wall of the main ore body. The ore minerals in these ore types have formed during different paragenetic stages: Stage I) Main sulfide stage with pyrrhotite, pyrite, and arsenopyrite (375° - 425°C), Stage II) Gold and sulfosalt stages (Stage II-a and II-b) with primarily gold, sulfosalts, and chalcopyrite formed under prolonged, retrograde metamorphic conditions, Stage III) Low temperature stage affecting gold, bismuth, Type 2 sulfosalts, and pyrrhotite and forming silver minerals within micro-cracks. Arsenopyrite and chlorite geothermometry and sphalerite barometry have been applied to constrain the formation temperatures of mineralization. Arsenopyrite thermometry yielded temperatures of 310° to 505°C for Stage I and II. Chlorite geothermometry revealed highly variable temperatures between 275° and 655°C; the high temperatures are geologically meaningless. Pressures obtained from sphalerite barometry yielded 1.4 to 1.7 kbar. Gold occurs in different mineral associations, but it is commonly intergrown with chalcopyrite and sulfosalts. It is normally rich in silver (40 - 70 at.% Ag), but higher gold contents (>70 at.% Au) were found in the cores of some grains. Gold shows a chemical zonation, which can be interpreted as three growth stages and the deceasing Au/Ag ratios reflect an increase of Ag-activity during cooling. The gold mineralization is associated with Ag-Bi-Pb-sulfosalts and chalcopyrite. Three types of sulfosalt associations are distinguished: Type 1 sulfosalts are present in symplectitic intergrowths, Type 2 are prismatic-shaped sulfosalts, and Type 3 sulfosalts are present in low-temperature decomposition and reaction textures. The sulfosalts studied can be classified as members of the galena-matildite solid solution, lillianite and pavonite homologous series, and Ag-Cu-rich sulfosalts.

Details

Translated title of the contributionMineralogie und Mineralchemie der mehrphasigen Au-Ag-As-Cu-Bi Vererzung in der Goldlagerstätte Rotgülden, Österreich
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
Supervisors/Advisors
Award date23 Mar 2018
Publication statusPublished - 2018