ASSORTED ROLES OF DIVERSE VARIABLY-SOURCED FLUIDS DURING THE PROTRACTED FORMATION HISTORY OF A AU-CU DEPOSIT: A FLUID INCLUSION ASSEMBLAGE PERSPECTIVE

Au-Cu mineralized quartz veins of the Reef Deposit originally formed prior to or early in the Paleoproterozoic Penokean orogeny in central North America. Nearly 400 m.y. later, magmatic hydrothermal fluids associated with a continental scale anorogenic magmatic event remobilized mineralization. And still later, during the Permian, remobilization and perhaps upgrading of Au and other metals occurred, consistent with fluids associated with nearby Mississippi Valley-type deposits. Fluid constraints on the development of the Reef Deposit from initial formation and subsequent overprinting over a 1.5 b.y. timeframe are examined using the Fluid Inclusion Assemblage (FIA) approach. Distinctive assemblages are examined on individual merit, each offering individual stories to aid in our understanding of the protracted development of the Reef Deposit.

Primary fluid inclusions in primary Au and Cu bearing quartz-sulfide vein growth show H2O-CO2-CH4-NaCl compositions based on microthermometry and laser Raman spectroscopy. Possibly primary H2O-NaCl and secondary H2O-CO2-CH4-NaCl FIA originally formed prior to regional deformation are recognized by discrete relict and neonate clusters. Intersection of either fluid composition’s highest density neonate inclusion isochores indicates re-equilibration at lower amphibolite conditions. Quartz hosted secondary fluid inclusion assemblages of H2O-CH4 and CO2-CH4 compositions have not been re-equilibrated and therefore post-date regional metamorphism. Chalcopyrite mineralization on the same plane as these FIA indicates fluid movement driven by emplacement of the adjacent Wolf River Batholith (ca. 1.45 Ga) altered pyrite to pyrrhotite and remobilized or emplaced chalcopyrite at temperatures as high as 700 °C by a reduced fluid. Secondary lower temperature (<50-210 °C) calcite and quartz hosted H2O-NaCl inclusion assemblages, most likely related to Permian aged MVT fluid overprint, show variable homogenization temperatures and salinities. H2O-NaCl inclusion assemblages in quartz homogenize at 78-210 °C, with lower salinities of 3.0-13.9 wt. % NaCl equivalent, and are observed to crosscut CH4 bearing FIA. H2O-NaCl inclusion assemblages in calcite have maximum homogenization temperatures of 103 °C and salinities of 23.6 to 24.6 wt. % NaCl equiv.