Geologic description
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, 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 at the ER-Ogo-Fire prospect are Paleozoic augen gneiss interlayered with quartz-feldspar-biotite paragneiss, and Cretaceous granitic intrusions. Felsic dikes that vary in texture from aplite to pegmatite are common. Scattered occurrences of pyrrhotite and pyrite occur in the vicinity of the prospect and an occurrence of pyrrhotite, pyrite, and chalcopyrite is at the contact between quartz-feldspar and biotite-feldspar schist. Quartz veins are common at the surface but few contain sulfides. Four samples of quartz veins collected at the surface (one of which has jamesonite) contained 15 to 1,521 parts per billion (ppb) gold, less than 0.2 to 0.9 parts per million (ppm) silver, 484 to 2,833 ppm arsenic, up to 0.76 ppm bismuth, 7 to 286 ppm lead, and 13 to 68 ppm antimony. Nine holes were drilled by Anglo-Gold Ashanti (USA) in 2003 and 2004. The drilling cut numerous intercepts up to 3.6 meters long of quartz veins and breccia with arsenopyrite, pyrite, and pyrrhotite. The quartz veins and breccia appear to occur along faulted contacts and are often associated with extensive quartz-sericite-pyrite alteration. There is also post-mineralization, silica-dolomite(-pyrite) alteration associated with re-brecciation of the mineralization. The best of the mineralized intercepts contained 9.0 to 5,310 ppb gold, 0.1 to more than 1 ppm silver, 1,535 to more than 1 percent arsenic, 0.06 to 11.80 ppm bismuth, and 19 to more than 1,000 ppm stibnite. |
