The Johnson River prospect occurs within the lower Jurassic Talkeetna Formation, an approximately 2,575 meter thick sequence of volcanic and volcaniclastic rocks (Determan and Hartsock, 1966). The volcanic section near the prospect is structurally bounded on the west by the Bruin Bay Fault, a major fault system traceable for over 500 kilometers. On the east, the volcanics are unconformably overlain by upper Jurassic marine sediments (Determan and Hartsock, 1965). The Talkeetna Formation is intruded by rocks of the Aleutian range -Talkeetna Mountain plutonic belt approximately 23 km. southwest of the prospect. These intrusives which consist dominantly of coarse-grained quartz diorites and quartz monzonites have been dated between 175 and 145 m.y. (Steefel, 1987).
At the Johnson River Prospect, the Talkeetna Formation has been divided into three major groups (Steefel, 1987). The lowest group consist of purple and green andesite flows and breccia which locally show well-developed pillows. The middle group that host the mineralization, consists of coarse-grain felsic breccias, reworked volcaniclastic rocks, crystal tuffs, and dacite flows. The upper group consists of polymictic andesitic breccias and conglomerates with minor andesitic flows. Informally, the middle group has been named the Johnson unit (Steefel, 1987). Numerous structures within the unit including turbidites and reverse graded bedding indicate a subaqueous origin.The mineralization and alteration at Johnson River can be subdivided into two stages which are mineralogically, chemically, and temporally distinct. Stage one is an early stage of nodular anhydrite and pyrite accompanied by magnesisum-chlorite, sericite, montmorillonite, and calcite. This stage is either geochemically barren or weakly anomalous in base metals and gold. (Steefel, 1987). Stage two which includes all the high-grade mineralization consists of quartz-sulfide mineralization accompanied by iron-chlorite, sericite, barite, carbonate, and vein-type anhydrite. Sulfide mineralization consists of early pale-colored sphalerite, later dark sphalerite, galena, chalcopyrite, pyrite, and native gold. The highest grade gold concentration is associated with chalcopyrite (Steefel, 1987). Overall the stage two, quartz-sulfide mineralization occurs in a fractured controlled, steeply plunging, pipe-like stockwork zone that measures 160 meters long by 50 meters wide and extends to a depth of at least 250 meters (Bill Ellis, personal communication, 1999).
|Alteration of deposit
||The alteration at Johnson River can be divide into two main events. An early stage of nodular anhydrite and fine-grained disseminated pyrite is accompanied by Mg-chlorite, sericite, montmorillonite and or calcite. This alteration is either barren or weakly anomalous in base metal mineralization. The later alteration consists of silicification and sulfidation accompanied by iron-chlorite, sericite, barite, carbonate, and vein anhydrite. This stage alteration includes all the mineralization at Johnson River (Steefel, 1987).