Geochemical Results of a Hydrothermally Altered Area at Baker Creek, Blaine County, Idaho: Usgs Bulletin 2064-Gg

The area immediately east of Baker Creek, Blaine County, Idaho, is underlain by a thick section of mafic to intermediate lava flows of the Eocene Challis Volcanic Group. Widespread propylitic alteration surrounds a zone of argillic alteration and an inner core of phyllic alteration. Silicified breccia is present along an east-trending fault within the zone of phyllic alteration. As part of a reconnaissance geochemical survey, soils and plants were sampled. Several species of plants (Douglas-fir [ Pseudotsuga menziesii ], mountain big sagebrush [ Artemisia tridentata ssp. vaseyana ], and elk sedge [ Carex geyerii ]) were collected from 10 upland localities and stream sediments, panned concentrates, and aquatic mosses were collected from 16 drainage basin localities all of which were generally within the area of alteration. Geochemical results yielded anomalous concentrations of molybenum, zinc, silver, and lead in at least half of the seven different sample media and of gold, thallium, arsenic, antimony, manganese, boron, cadmium, bismuth, copper, and beryllium in from one to four of the various media. Part of this suite of elements? silver, gold, arsenic, antimony, thallium, and manganese? suggests that the mineralization in the area is epithermal. Barite and pyrite (commonly botryoidal-framboidal) are widespread throughout the area sampled. Visible gold and pyromorphite (a secondary lead mineral) were identified in only one small drainage basin, but high levels of gold were detected in aquatic mosses over a larger area. Data from the upland and stream sampling indicate two possible mineralized areas. The first mineralized area was identified by a grab sample from an outcrop of quartz stockwork that contained 50 ppb Au, 1.5 ppm Ag, and 50 ppm Mo. Although the soil and plant species that were sampled in the area indicated mineralized bedrock, the Douglas-fir samples were the best indicators of the silver anomaly. The second possible mineralized area centers on the fault-controlled silicified breccia that is most likely the source of anomalous silver and molybdenum levels identified in the soils; silver, molybdenum, and manganese in stream sediments; thallium in Douglas-fir; bismuth and silver in concentrates; and gold, silver, arsenic, antimony, and molybdenum and lead in aquatic mosses. An interpretation of regional aeromagnetic data delineated the subsurface extent of shallow, steeply dipping magnetic sources inferred to be shallower parts of an Eocene batholith thought to underlie much of the Baker Creek area. The Eocene intrusive event(s) may have served as the heat source(s) that caused the hydrothermal alteration. Examination of core from a 1,530-ft-deep (466 m) hole drilled in 1982 confirmed a bedrock source for the anomalous silver and base-metal suite at the quartz stockwork location, and indicated subeconomic levels of molybdenum.