ANOMALOUS OSMIUM ISOTOPIC RATIOS IN SEDIMENTARY SULFIDES FROM THE VIRGINIA FORMATION PRODUCED AS A RESULT OF INTERACTION WITH MAGMATIC FLUIDS DERIVED FROM THE DULUTH COMPLEX, MIDCONTINENT RIFT SYSTEM

Metapelitic rocks of the ~1.85 Ga Virginia Formation, which occur as footwall rocks to the ~1.1 Ga Duluth Complex, contain pyrite layers and lenses that are characterized by Os isotopic ratios that are similar to those of chondritic, uncontaminated sulfide assemblages found in the complex. The origin of these anomalous Os isotopic characteristics must be attributed to exchange and mixing with a fluid, perhaps derived from magma of the Duluth Complex. Layer-parallel veins in the Virginia Formation are comprised of quartz, calcite, ankerite, biotite, muscovite and apatite, along with sulfide assemblages containing either pyrite and chalcopyrite or pyrrhotite, chalcopyrite and cubanite (±pentlandite). The latter assemblage is similar to that found in gabbroic rocks of the Duluth Complex. Fluid inclusions in quartz from the veins represent two distinct populations, CO₂- and H₂O-rich. The CO₂-bearing inclusions homogenize at temperatures between 340 and 400°C, whereas the H₂O-rich inclusions homogenize between 260 and 300°C. Freezing temperatures of the H₂O-rich population indicate salinities that vary from 11 to 20 equivalent weight percent sodium chloride. Pressure correction of the homogenization temperatures suggest that quartz crystallized between 500 and 600°C. LA-ICP-MS analyses of CO₂-bearing inclusions show variable compositions, but general enrichment in Mg, Fe, Zn, Cu, La, and B. The Mg and Fe contents are consistent with derivation of fluid during crystallization of late, interstitial magma of the Duluth Complex. d¹⁸O values of quartz in the veins range from 7 to 9 per mil and indicate d¹⁸O(H₂O) values between 5 and 6 per mil which are consistent with a magmatic origin. Apatite in the veins contains up to 3.9 wt% fluorine and biotite near sulfide contains up to 1 wt%. Fluorine may have been important as a transport agent for metals in the veins. Data from the veins are consistent with the premise that fluids having chondritic Os isotopic ratios, derived from magmas of the Duluth Complex, have been responsible for the development of the anomalous osmium isotopic ratios exhibited by sedimentary pyrite in the Virginia Formation.