Grassy Mountain is the largest of twelve recognized epithermal hot spring precious metal deposits of the Lake Owyhee volcanic field. The Lake Owyhee volcanic field occurs at the intersection of three tectonic provinces: the buried cratonic margin, the northern Basin and Range, and the Snake River Plain. During the mid-Miocene caldera volcanism occurred in the shallow crust throughout the region, predominately ash-flow sheets and rhyolite tuff cones. The resulting regional stratigraphic section is a thick sequence of mid-Miocene volcanic rocks and coeval-to-Pliocene age non-marine lacustrine, volcaniclastic, and fluvial sedimentary rocks.
Bedrock outcrops in the vicinity of the Grassy Mountain property are typically composed of olivine-rich basalt and siltstones, sandstones, and conglomerates of the late Miocene Grassy Mountain Formation. These rocks are locally covered with relatively thin, unconsolidated alluvial and colluvial deposits. Erosion-resistant basalts cap local topographic highs. Arkosic sandstones have been encountered at the surface and at depth, but have not been correlated across the project area, in part due to lateral discontinuity associated with sedimentary facies changes and structural offset.
The basal unit to the Grassy Mountain Formation is the Kern Basin Tuff, a non-welded, pumiceous, crystal tuff which displays cross beds and local surge structures. The Kern Basin Tuff ranges in thickness from 300 ft on the south bluffs of Grassy Mountain, to 1,500 ft in a drill hole beneath the Grassy Mountain Project area.
The Kern Basin Tuff is overlain by a series of fluvial, lacustrine, and tuffaceous sediments. Most of the sedimentary units in the project area are silicified and strongly indurated. These sedimentary units include granitic clast conglomerate, arkosic sandstone, fine grained sandstone, siltstone, and tuffaceous siltstone/mudstone. The sedimentary facies of the Grassy Mountain Formation reportedly range from 300 to over 1,000 ft thick, and provide the host rocks of the Grassy Mountain mineral resource.
Several siliceous terraces are interbedded with the silicified sediments of the Grassy Mountain Formation. Terrace construction was apparently episodic and intermittently inundated by fluvial/lacustrine sediments and ash, resulting in an interbedded sequence of siltstone, tuffaceous siltstone, sandstone, conglomerate, and sinter terrace deposits. Proximal deposits are angular, inhomogeneous, clast-supported breccias of sandstone, siltstone, and sinter with indistinct clast boundaries in a sulfidic mud-textured matrix.
Grassy Mountain is a prominent, 150 ft. high, silicified and iron-stained knob. Bedding is approximately horizontal at the hilltop, and dips at 10° to 25° to the north-northeast on the northern and eastern flanks of the hill. The bedding dip steepens to 30° to 40° on the west side of the hill due to drag folding in the footwall of the N20W striking Antelope Fault. The southwest slope of Grassy Mountain is covered by silicified arkose landslide debris.
Grassy Mountain is a horst block which has been raised 50 to 200 ft. in a region of complex block faulting and rotation. Faulting at Grassy Mountain is dominated by post-mineral N30W to N10E striking normal faults developed during Basin and Range extension. On the northeast side of the deposit, these faults progressively down-drop mineralization beneath post-mineral cover. These offsets are suggested by interpreted offsets of a prominent white sinter bed in drill holes as well as intersections with fault gouge. The N70E striking Grassy Mountain Fault shows minor vertical offset of only 10 to 40 ft.
The surface expression of the Grassy Mountain system is indicated by weak to moderately strong silicification and iron staining with scattered chalcedonic veins/veinlets. Approximate dimensions of the Grassy Mountain deposit are 1600 ft. long by 1000 ft. wide by 600 ft. thick. The deposit has a general N70 E elongation and a 15° bedding plane dip to the north-northeast as a result of faulting and fault block rotation. There is an envelope of lower grade mineralization at depths of 200 to 800 ft. which contains a higher-grade zone of mineralization between 500 and 750 ft. below the surface. The well-defined base of higher grade mineralization from about 700 to 750 ft. in depth suggests a strong pressure-temperature control on gold deposition. Sinters and breccias parallel the paleosurface present at the time of mineralization. Fractures created a stockwork pattern generally found below the sinter, though some vein extensions may extend to the surface. The stockwork is surrounded by silicified sediments.
Mineralized quartz-adularia stockwork and vein types include single, banded, colliform, brecciated, and calcite-pseudomorphed veins. Visible gold (0.5 mm) has been found within the stockwork portions of the boiling horizon. The gold mostly occurs as electrum along the fracture margins or within microscopic voids. A brassy color is imparted due to the high silver content. The average silver to gold ratio at Grassy Mountain is 2.5:1.
Silicification occurs both pervasively as silica flooding and as cross-cutting veins, veinlets and stockworks. The silicified envelope has plan dimensions of 3000 ft (N-S) by 2500 ft (E-W). Silicification is surrounded by barren, relatively unaltered, clay-rich (20-40% montmorillonite), tuffaceous siltstone and arkose with minor disseminated diagenetic pyrite. Many of the sinters occur as sheets instead of mounds, which suggests that they may be related to vents along faults rather than point sources. Potassic alteration occurs as adularia flooding with destruction of biotite. The adularia is extremely fine-grained and is identified microscopically or by cobaltinitrite staining. Sulfate phases identified by XRD include jarosite and alunite in several mineralized samples. Clast-supported breccias contain sub-rounded to sub-angular sand to boulder-sized clasts of silicified arkose and siltstone in a jarosite-sericite clay matrix.