Abstract

Different assemblages of argentian pentlandite occur in intermediate-grade metamorphic rocks from the pilot hole of the German Continental Deep Drilling Program (KTB). Assemblage 1 consists of small inclusions in chalcopyrite within quartz veins. Argentian pentlandite is also intergrown with chalcopyrite, pyrrhotite, pentlandite and pyrite (assemblage 2) or cobaltian pentlandite, chalcopyrite, pyrrhotite and sphalerite or chalcopyrite and galena (assemblage 3); assemblages 2 and 3 occur in paragneissic rocks. The Ag content of argentian pentlandite ranges from 10.3 to 12.6 wt.%. The exsolution origin of argentian pentlandite from chalcopyrite, as well as inversion twins and negative delta 34 S values, of -6.7 per mil in chalcopyrite from assemblage 1, point to mineralization during retrograde metamorphism at conditions of elevated pressure and temperature. Pressures of 2-3 kbar and temperatures of about 400 degrees C are in agreement with the modeling of the retrograde P-T-t path for the KTB rocks. Temperature and sulfur fugacity estimates based on sphalerite and chlorite compositions suggest conditions of formation in assemblages 2 and 3 at or slightly below 260 degrees C and at a value of a(S 2 ) less than 10 (super -13) . Bulk-rock delta 34 S values of -11 per mil are consistent with sulfur from a sedimentary source. Argentian pentlandite and associated sulfides are attributed to hydrothermal mineralization in which sulfur and the metals are derived from sedimentary host-rocks.

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