Structural Evolution of the Rock Creek Gold Deposit, Seward Peninsula, Alaska

Abstract

Sheeted quartz veins at Rock Creek contain an indicated gold resource of 404,000 ounces (oz) at a grade of 1.31 grams per ton (g/t). The deposit, located on the Seward Peninsula, 12 km north of Nome, Alaska, occurs in variably graphitic greenschist-facies schist, fine-grained quartzite, and dirty marble of the lower Paleozoic to upper Proterozoic Nome Group. The veins at Rock Creek, as at many prospects in the southwestern Seward Peninsula, are hosted by the most carbonaceous part of the Nome Group. These strata underwent complex polyphase deformations that were progressively less penetrative through time. They include four distinct styles of folding, followed by an episode of thrust faulting. The Mid-Cretaceous dilational vein system formed during the onset of D4 deformation along the northeast-trending Albion fault. The overall shape of the vein system in relationship to controlling faults shows that a preexisting structural fabric may have influenced its formation; tension veins lie along but slightly oblique to the northeast-striking Albion fault in a left-stepping array. Younger “Albion” veins parallel and lie within the Albion fault zone. The highest ore tenor and greatest concentration of veins occur near the intersections of northeast and northwest structures. The veins diminish in frequency along strike across the northwest-trending structures. The vein system occurs primarily in the hanging wall of a curviplanar dextral thrust fault that splays from the high-angle Albion fault and becomes gently dipping in its upper reaches. Vein concentration diminishes abruptly below this curviplanar transpressional structure. Small stratigraphic offsets show that the vein system was not displaced by the structure but rather formed preferentially above it. The orientations and sequencing of structural elements show that the tension vein system formed during a period of highly focused dilation. The Albion fault zone subsequently propagated through the maximum zone of dilation as it evolved to a throughgoing dextral strike-slip fault system. Vein chemistry changed from primarily Au-As (tension veins) to Sb-Au-Ag-As-Pb (Albion-style veins) contemporaneously with a 10° counterclockwise migration of vein orientation. These changes mark a period of highly focused extension immediately prior to the onset of throughgoing transpression along the Albion fault, which resulted in a collapse of the hydrothermal system. The left-stepping geometry of tension veins and later evolution of the overall structural system to compression suggest that it formed within a restraining bend along the dextral strike-slip fault system. The general shape of the vein system, the positions and orientations of controlling structures, and their vectors and styles of displacement define a sidewall ripout that flanks the Albion fault zone.