Igneous rocks and structures of the Nixon Fork Mine, Alaska, and their relations to ores

Abstract

The Nixon Fork Mine is a high-grade Cu-Au skarn deposit located near the western contact of the 5 square km Late Cretaceous Mystery pluton with marble, 7 km southeast of the Iditarod-Nixon Fork (I-NF) fault. This fault strikes at ~060�, and can be traced for ~400 km, with a minimum dextral displacement of ~90 km. Close proximity suggests that the Nixon Fork deposit should have been affected by the I-NF fault. In order to assess the structural evolution, I analyzed the orientations of geologic structures. I transcribed 1172 structures from previous mapping (to assess structures) and converted 186 maps into Vulcan CAD software (to create a three-dimensional model). I also acquired ������Ar/�_��_Ar dates for eleven representative potassium-bearing minerals and rocks. I identified six different episodes of deformation, including intrusion of felsic dikes, intrusion of mafic dikes, two episodes likely related to the I-NF fault, and two other poorly constrained episodes. ������Ar/�_��_Ar dates show the skarn is significantly younger than the Mystery Creek pluton, indicating it was likely sourced from an unexposed pluton. The main skarn pipe can be approximated as a line oriented ~210�/65�, which is approximately the intersection of planes defined by felsic dikes and major veins.