Geology of the Bornite copper-zinc-cobalt carbonate-hosted deposit, southwestern Brooks Range, Alaska, The

Abstract

The Bornite Cu-Zn-(Co-Ge) deposit is located in the Cosmos Hills of the southern Brooks Range of northwest Alaska, approximately 260 km east of Kotzebue and 460 km north of Fairbanks. The deposit contains an indicated resource of 14.1 Mt of 1.08% Cu and an inferred resource of 165.2 Mt of 1.57% Cu. The deposit is hosted within late Silurian to middle Devonian limestones, argillaceous carbonate rocks, and highly graphic carbonate rocks as well as several carbonate debris flows. The carbonate rocks underwent an early stage of replacive alteration and a later stage of dolomite-calcite veining, dedolomitization, and sodium metasomatism that was temporally and spatially associated with copper mineralization. The hydrothermally altered rocks were metamorphosed to greenschist facies during the Jurassic-Cretaceous Brookian orogeny. Early replacive alteration utilized apparently permeable debris flow breccias as fluid conduits. Limestone in both the debris flow breccias and adjacent wallrock was converted to dolostone. Three major dolostone bodies were created at Bornite during this alteration stage. Each is zoned with respect to iron with an outer normal dolostone fringe that is gradational inwards with ferroan dolostone and in the basal dolostone body a siderite core. The later alteration stage produced normal dolomite-(calcite) veins and locally resulted in dedolomitization of dolostone wallrock. Sulfides often occur within dolomite-calcite veins and are zoned from pyrite and sphalerite along the upper fringes of the dolostone bodies, inward to chalcopyrite and bornite. Along the edges of the dolostone bodies stockwork dolomite-calcite veins and intense dedolomitization of dolostone wallrock are spatially associated with massive sulfides. The massive sulfides may be dominated by pyrite or chalcopyrite that replaced pyrite. The massive sulfide bodies and are zoned with increasingly copper-rich sulfide assemblages inward. The most copper-rich assemblages of bornite-chalcocite-digenite replaced dolostone wallrock and were associated with calcite precipitation. Tan-green phyllite beneath the dolostone bodies displays intense sodium alteration and pervasive dedolomitization that is zoned around high-grade copper orebodies within adjacent dolostone bodies. The timing of alteration and mineralization at Bornite is poorly understood. Published Re-Os dates of approximately 400 Ma and 384 Ma, from pyrite, chalcopyrite, and bornite at the deposit suggest that alteration and mineralization occurred in the Paleozoic relatively soon after sediment deposition. New textural evidence demonstrates generally simultaneous growth of copper sulfides phyllosilicate minerals, and albite that preserves a relict -Cretaceous deformational fabric, suggesting that copper mineralization occurred during or after metamorphism. Cymrite, a low temperature and pressure metamorphic barium aluminosilicate mineral, is intergrown with replacive dolostones, copper sulfides, and albite, also suggesting that alteration and mineralization at Bornite occurred during the Jurassic-Cretaceous metamorphic event.