The rocks in the area are part of the Yukon-Tanana Terrane of the northern Cordillera; they consist largely of late Paleozoic to Devonian medium- to high-grade metamorphic rocks that have been intruded by Cretaceous and Tertiary granitic stocks and batholiths (Baknes, 2008; Wilson and others, 1998). The metamorphic rocks are structurally complex with an older, middle to early Paleozoic and/or late Proterozoic unit that consists of gneiss, schist, quartzite, and amphibolite and a younger, Mississippian and/or Devonian unit that consists largely of augen gneiss that locally forms domal structures. The metamorphic rocks are intruded by 55 Ma to 107 Ma, granitic plutons. The structure of the area is complex and difficult to determine because of poor exposure but several major thrust fault have been identified as well as a series of northeast-trending, northwest-dipping high-angle faults.
The dominant mineralization in the area and the target of most of the exploration are deposits related to Cretaceous intrusions, several classic examples being the Fort Knox Mine (FB115) near Fairbanks and the nearby Pogo Mine (BD033). These deposits are characterized by an association with moderately reduced, I- and S- type granitic intrusions and a lithophile, gold-bismuth-tungsten-arsenic-tellurium-molybdenum(-tin-copper-lead-antimony) suite of elements. The deposits vary from low-sulfide, sheeted or stockwork deposits in the intrusions to replacement deposits, veins, and stockworks in the country rock. The mineralization at the nearby Pogo deposit is the model used in exploring this and other prospects in the area. The mineralization at Pogo consists of thick gently-dipping quartz veins that grade laterally into pegmatite dikes related to 107 Ma syenite and monzonite intrusions. The quartz veins contain about 3 percent sulfides, mainly arsenopyrite, pyrite, pyrrhotite, loellingite, chalcopyrite, bismuthinite, native bismuth and native gold. The veins are associated with biotite alteration overprinted by sericite-dolomite-chlorite(-quartz alteration, that extends into the country rock for several meters.
Soon after the recognition of nearby Pogo as a major and economically attractive deposit in 1998, Rimfire Minerals Corporation in conjunction with several joint-venture partners began work in the area and identified this deposit. In 2008, they entered into a joint venture with Rubicon Minerals Corporation and commissioned a thorough technical report on their properties in the Goodpaster District (Baknes, 2008), which is the source of most of the information on this prospect.The rocks in the vicinity of the Eagle prospect are dominantly irregularly-shaped, multiphase granite to granodiorite intrusions surrounded by orthogneiss. The geology is poorly exposed but several lineaments roughly parallel soil-geochemical anomalies. Several samples with disseminated pyrite and trace chalcopyrite and one with molybdenite were identified on the surface and numerous samples were collected. Nine of the samples collected at the surface had significant metal values. These contained 113 to 8,080 parts per billion (ppb) gold, 0.8 to 3.05 parts per million (ppm) silver, 9 to 430 ppm arsenic, 0.11 to 2,600 ppm bismuth, 13 to 26 ppm copper, and less than 2 to 12 ppm antimony. Five holes were drilled in 2000 by Hyder Gold and another 9 holes were drilled in 2004 by AngloGold Ashanti (USA); these totaled 3,054 meters. The drilling intersected 26 stockwork zones with significant gold values that varied in thickness from 0.2 to 1.5 meters; the best of these intervals contained by 437 to 7,896 ppb gold, 0.1 to 4.0 ppm silver, 13 to 4,430 ppm arsenic, 1 to 100 ppm bismuth, and 1 to 77 ppm antimony. The stockwork zones, many of them lacking sulfides, consists of quartz, albite, and calcite, and are associated with variable sericitic and chlorite alteration.