OXYGEN AND CARBON ISOTOPIC ANALYSES OF THE CARBONATE-HOSTED BORNITE CU-CO DEPOSIT, NW ALASKA

The Bornite Cu-Co deposit is situated within the Cosmos Hills and the Ambler Mining District in the western Brooks Range in Northwestern Alaska. This remote area is 60km north of the Arctic Circle, and has been actively explored since the discovery of outcropping malachite and azurite in the early 1950’s. Trilogy Metals Inc. began exploring within the Ambler Mining District in 2004, expanding the known extent of the deposit. Mineralization in the Cosmos Hills is constrained to the Bornite Sequence which consists of two variably altered carbonate packages known as the Upper and Lower Reefs, both dated to the Late Silurian or Ordovician based on recent fossil analyses. Interlayered within these carbonate packages are phyllite layers with black shale protoliths, and there are stratigraphically traceable phyllite units that separate the reefs. Each reef has been affected by multiple overprinting dolomitization with variable Fe-metasomatism. The ore-forming event appears to be locally accompanied by Na-metasomatism.

The focus of this research is to expand on a previous d18O and d13C isotopic study (Connor, 2015, unpub. M.S. thesis, Col. School Mines). This project is the most in-depth analysis of the d18O and d13C isotope values to date, with over 200 total samples collected from drill cores during the summer of 2018. We intend to identify deposit-scale isotopic trends to delineate alteration envelopes and to correlate them with brecciation and mineralization.

Preliminary analyses have been focused on two drill holes that intercept both Reefs, RC11-0181 and RC11-0182. The cores were continuously sampled with 10m spacing where possible, and samples were prepared at Washington State University for stable isotope analyses. Preliminary results show decreasing and increasing trends in d18O exhibited by Upper Reef carbonates. The zone containing the massive sulfides is a relative midpoint in d18O values when considering the sequence as a whole. There are saw-tooth patterns and excursions within the trends that need further examination, but we can say at this time that the patterns in d18O values do not simply correlate with depth or with Cu%. These trends are the focus of ongoing research, and will illuminate more about the nature of the mineralizing fluid, as well as the relationship between alteration and stratigraphy.