Involvement of variably-sourced fluids during the formation and later overprinting of Paleoproterozoic Au-Cu mineralization: Insights gained from a fluid inclusion assemblage approach

Abstract

Au-Cu mineralized quartz veins of the Reef Deposit, Wisconsin, USA, originally formed prior to or early in the Paleoproterozoic Penokean orogeny in central North America as either the root zone of a gold-rich VMS deposit, or in an orogenic gold setting. Nearly 400 m.y. later, magmatic hydrothermal fluids associated with a continental scale anorogenic magmatic event remobilized mineralization within the veins. And still later, during the Paleozoic, remobilization and perhaps upgrading of Au and other metals occurred in response to circulation of fluids associated with Mississippi Valley-type deposits found in overlying supracrustal rocks. Fluid constraints on the formation of the Reef Deposit and later 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.Presumably primary fluid inclusions in Au- and Cu-bearing quartz-sulfide veins that formed during the Paleoproterozoic event show H2O-CO2-CH4 ± NaCl compositions based on microthermometry and laser Raman spectroscopy. Possibly primary H2O-NaCl and secondary H2O-CO2-CH4 -NaCl FIA formed prior to regional deformation and are recognized as discrete relict and neonate clusters. The intersection of isochores from the highest density neonate inclusions of either fluid composition indicates re-equilibration at lower amphibolite conditions during regional deformation. Quartz hosted secondary fluid inclusion assemblages characterized by H2O-CH4 and CO2-CH4 compositions formed at high temperatures and have not been re-equilibrated and are interpreted to post-date regional metamorphism. These FIA occur along healed fractures with chalcopyrite mineralization. These inclusions are likely associated with fluid circulation driven by emplacement of the adjacent Wolf River Batholith (ca. 1.45 Ga), that altered pyrite to pyrrhotite and remobilized or emplaced chalcopyrite at temperatures as high as 700 °C. This reduced fluid was in equilibrium with locally observed graphitic sediments. Secondary lower temperature (<50–210 °C) calcite and quartz hosted H2O-NaCl inclusions show variable homogenization temperatures and salinities. H2O-NaCl inclusions 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 ~24 wt% NaCl equiv. Lower temperature H2O-NaCl inclusions are most likely related to the Paleozoic-aged MVT fluid overprint that remobilized and possibly emplaced gold mineralization.