Geology of the Ruby Creek copper deposit, southwestern Brooks Range, Alaska

At Ruby Creek, Alaska, a 1-km-diameter body of hydrothermal dolostone cuts through over 500 m of Middle to Late Devonian carbonate sediments and contains in excess of 100 million tons of 1.2 percent copper. It is one of several similarly mineralized dolostone bodies in the Cosmos Hills which occur near the boundary between a carbonate platform and a shale basin. A complex alteration sequence is recognized in the Ruby Creek body which shows distinct variations with depth. The alteration sequence is separated into two major stages: an early period of dolomitization with minor base metal mineralization (A and B dolostones) and a later period of fracture-controlled copper mineralization (C assemblage). Clasts of A dolostone in sedimentary breccias indicate alteration began during early diagenesis.Early alteration resulted in progressive dolomitization of the host limestone and produced a zonal arrangement of low iron dolostone (A dolostone) around a ferroan dolostone (B dolostone) to siderite core. Siderite-bearing assemblages are either adjacent to, or replace, argillaceous lithologies. Alteration in footwall shale (now phyllite) is not recognized but may have been structurally displaced. Sulfide mineralogy also exhibits spatial and temporal variations. Sulfides are sparse in the A and upper, low iron B dolostones. Pyrite is a common accessory with later ferroan B dolostone and is commonly associated with minor sphalerite, barite, and barium silicates. In the core of the system, pyrrhotite, chlorite, and barium silicates are present with the siderite-bearing assemblage. A zone of discontinuous massive to semimassive iron sulfide separates the siderite-bearing zone from overlying ferroan B dolostones. Late assemblages of massive pyrite, often cobaltiferous, with minor sphalerite, occur in the middle and upper portions of the system adjacent to unaltered argillaceous beds. Conversion of dolomite to calcite accompanied pyrite deposition in these zones. Sulfur isotope studies suggest that temperatures in the core of the system during the early alteration stage exceeded 300 degrees C. Reflectance studies of organic material contained within the various dolostones indicate that the alteration was produced by several fluid pulses, at least one of which carried hydrocarbon.Dolomite veins (C veins) associated with the second major stage of mineralization show a distinct zonation. Veins are most abundant just above the massive iron sulfide layers separating ferroan dolostones from siderite-bearing assemblages. Vein density decreases toward the periphery of the system. Mineralogy also displays distinct spatial variations. Deep in the system the veins contain dolomite with sparse quartz, in the middle of the system dolomite, chalcopyrite with minor calcite, and pyrite are present. On the edge of the system the veins contain dolomite and calcite with minor sphalerite, pyrite, and fluorite. Massive copper sulfides are, in large part, restricted to former zones of massive pyrite. Chalcopyrite, replacing earlier pyrite, is the dominant copper-iron sulfide but is in turn replaced by bornite and finally chalcocite-digenite. Bornite- and chalcocite-rich assemblages display dolomite-destructive alteration and concurrent calcite precipitation. These copper-rich zones contain carrollitc apparently formed from the breakdown of cobaltiferous pyrite. Fluid inclusion studies indicate decreasing temperatures in the veins outward from the core of the system. Deep veins yield temperatures of up to 215 degrees C whereas veins on the periphery of the system give temperatures of approximately 125 degrees C. Zones of massive copper sulfides formed at temperatures between 100 degrees and 120 degrees C. The copper mineralization predates a Jurassic-Cretaceous period of deformation and metamorphism and appears to be a continuation of A and B dolostone alteration.