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 prospect 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 area of the Boundary deposit are iron-stained gneiss intruded by granitic dikes and sills that are generally aplite but locally grade into pegmatite. The gneiss shows variable sericite-quartz-pyrite/pyrrhotite alteration. The rocks are poorly exposed and the only recognizable structures are lineaments that probably represent faults. Several shows of mineralization occur in an area about 2 kilometers in diameter that roughly coincides with an arsenic-bismuth-antimony-gold soil anomaly of about 1.5 x 2.5 km in size. The Boundary stockworks is a 100 by 300 meters talus zone with scattered, vuggy quartz veinlets that contain traces of arsenopyrite and bismuthinite. The Daydreamer showing is an area of about 25 meters by 75 meters in size with quartz boulders that contain finely disseminated blebs of arsenopyrite and rare pyrite and bismuthinite. The Topgun showing is an area about 10 meters in diameter with mineralization similar to the Daydreamer showing. The Pebble showing is 15 to 20 meters in size with weathered quartz pebbles and fine-grained quartz breccia. Eighteen samples collected at the surface from these showing contained significant metal values: 20 to 24,853 parts per billion (ppb) gold), 0.6 to 33.2 parts per million (ppm) silver, 2 ppm to more than 1 percent arsenic, 12 to 1,215 ppm bismuth, 8 to 168 ppm copper, and 6 to 1,055 ppm lead. Five holes that totaled 1,446 meters were drilled at the Boundary prospect in 2000 and 2001. Three of the holes intercepted mineralization 0.1 to 4.2 meters long; the metal values in these intercepts varied from 1,110 to 35,240 ppm gold, 1 ppm to more than 1 percent arsenic, and 14 to 1,250 ppm bismuth.