Geology and origins of the peak gold-copper-silver skarn deposit, Tok, Alaska

Abstract

The distal Peak Au-Cu skarn deposit was discovered in 2012 on Tetlin native lands 15 miles southeast of Tok, Alaska. The deposit contains anomalous Au-Cu-Bi-Ag-Mo-Fe-Te-As-Pb-Co-W-Cd-Ni. At the time of drilling deposit style, mineralization and origin were not understood. Upper Paleozoic and older metamorphic rocks had been previously mapped in the area; however, no igneous or metasomatic rocks had been recognized. To better understand the deposit I created a 1:25,000 scale geologic map of the 25 km² exploration area; employed approximately 682,500 assays for metal ratios; described textures and minerals in 138 polished thin sections; employed approximately 3,000 meters of new core logging to produce a cross-section and long-section; acquired microprobe compositions of skarn amphibole, pyroxene and garnet; analyzed 122 rocks by X-ray fluorescence analysis; had Pb isotopic ratios determined for 3 galena samples; and had 5 rocks dated by ⁴⁰Ar/³⁹Ar and U-Pb techniques. Based upon this data, I define Peak as a distal amphibole-rich Cu-Au skarn. The skarn's plutonic source remains undiscovered, and is likely NE of, and well below the deposit. Based radiometric dating and trace element analyses of plutonic and volcanic rocks, the immediate area experienced intrusive and extrusive activity at 70-75 Ma in a volcanic arc setting.