Newly located occurrences of lode gold near Table Mountain, Circle Quadrangle, Alaska

Abstract

Most of the goldbearing samples are clustered in two areas.The highest concentrations of gold (2.6-140 ppm) occur, just west of Table Mountain, in black biotite schist and in quartz veins adjacent to a fault zone that is intruded by a hypabyssal felsic dike. Five kilometers to the northeast, where a small granite pluton crops out, gold was detected in lesser amounts ( 0.05-0.2 ppm) in country rocks adjacent to the granite, in the granite adjacent to dikes, and in felsic dikes. The occurrences have a simple mineralogy: pyrrhotite, arsenopyrite, minor chalcopyrite, and rare enargite and sphalerite. They formed from low-saline fluids ( 10 weight percent NaCl equivalent) whose sulfur fugacities were within the stability field of pyrrhotite. Based upon fluid-inclusion data, we infer that the occurrences formed at moderate temperatures from fluids that initially boiled, over the range 370 to 320 °C, and then gradually cooled. Field relations and estimated depths of formation of contact metamorphic effects and quartz veins indicate that the occurrences formed in an active tectonic environment.