Summary:
The geological setting, hydrothermal alteration, styles of gold-silver mineralization, and close spatial and timing association with silica sinter deposition, indicate that Grassy Mountain is an example of the hot-springs subtype of low-sulfidation, epithermal precious-metals deposits. The Grassy Mountain deposit is characterized by stacked sinter terraces that demonstrate hydrothermal fluids vented at the paleosurface concurrent with lacustrine and intermittent fluvial sedimentation.
The Grassy Mountain gold–silver deposit is located largely within the silicic and potassic alteration, zones, beginning approximately 200 ft below the surface. The deposit has extents of 1,900 ft along a N60°E to N70°E axis, as much as 2,700 ft in a northwest-southeast direction, and as much as 1,240 ft vertically. The surface expression of the mineralization is indicated by weak to moderately strong silicification and iron-staining, accompanied by scattered, 1/8- to 1.0-inch wide creamy to light-gray chalcedonic veins that filled joints.
The deposit consists of a central higher-grade core with gold grades of >~0.03 oz/ton Au that is surrounded by a broad envelope of lower-grade mineralization. The central higher-grade core is almost 1,000 ft long on the N60°E to N70°E axis, by 450 ft in width and 450 ft in vertical extent, all of which is above the Kern Basin Tuff and below a distinctive sinter unit.
Central Higher-Grade Core Zone
Three distinct and overlapping types of gold–silver mineralization are recognized within the central core of the deposit. These are gold-bearing chalcedonic quartz ± adularia veins, disseminated mineralization in silicified siltstone and arkose, and gold and silver in bodies of clay matrix breccia.
Zones of high-grade mineralization are defined by the presence of chalcedonic quartz ± adularia veins. Mineralized quartz ± adularia vein types include single, banded, colloform, brecciated and calcitepseudomorphed veins. Colloform veins tend to carry the highest grades (>0.5 oz/ton Au), with visible gold to as much as 0.02 inches associated with argentite. Veins with relict bladed calcite texture also contain higher gold grades than the banded and single vein types. Gold mostly occurs as electrum along the vein margins or within microscopic voids. Some veins carry very little grade or are barren. At least some of the higher-grade zones of veins are thought to strike approximately N70°E.
Vein widths range from 1/16 to ~2.0 inches. Individually, such narrow veins are unlikely to have lateral or vertical extents of significance, but vein frequency can average one vein per foot in places. Vein swarms have strike lengths of 400 to 700 ft and vertical extents of 100 to 250 ft at elevations of 3,150 to 3,400 ft. Individual veins are too narrow to trace or correlate from hole to hole.
A steep southerly dip (70–85°) of the veins is inferred from vein intersection angles with drill core axes and bedding. Veins are mostly perpendicular to bedding, which generally dips 10–25° NNE within the deposit. Vein intersection angles of 10–25° to the core axis were mostly recorded in core holes GMC001 to GMC-008 angled at -50° at S20°E, compared with 25° to 50° intersection angles in holes GMC009 to GMC-011 angled -50° at N20°W. The N70°E strike of the veins is supported by: 1) surface mapping, 2) vein orientation perpendicular to bedding, 3) grade-thickness contouring, and 4) the overall trend in mineralization with grades in excess of ~0.03 oz/ton Au.
The veins crosscut the silicified sediments and have extremely sharp grade boundaries with the sediments. Vein frequency diminishes abruptly below an elevation of ~3,000 ft at the west–southwest limit of the higher-grade core to ~3,100 ft at the east-northeastern limit, and very few high-grade veins were encountered above the higher-grade core of the deposit
Within the higher-grade core, high gold grades are also present in silicified siltstone and arkose with no visible veins. In these cases, gold and silver are inferred to be very finely disseminated in a stratiform manner in the silicified rock. Fine-grained pyrite is commonly disseminated in the silicified siltstone and sandstone where oxidation has not occurred. Contacts between siltstone and arkose beds seem to be more favorable and carry higher gold grades. In places, beds of tuff and tuffaceous siltstone appear to be particularly favorable host for higher-grade mineralization that lacks associated veins.
The third style of gold–silver mineralization was referred to by Newmont and later operators as “clay matrix breccia”, bodies of which may be more prevalent in the lower portion of the higher- grade core of the deposit. These bodies are interpreted to extend at near-vertical angles up and down into the surrounding, low-grade gold-silver envelope. Clay matrix breccias are mainly of clast-supported types and contain sub-rounded to sub-angular, sand- to boulder-sized clasts of silicified and/or veined arkose and siltstone with minor amounts of clay and iron-oxide minerals between the clasts. In drill core, clay matrix breccia intervals are intersected over lengths of as much as several tens of feet, but their true thickness and exact orientations are poorly understood, in part because their margins are commonly irregular-to-gradational and not planar, except where structural fabrics related to fault movement are evident. In some cases, it is difficult to discern where clay matrix breccias end and similar fault-related breccias begin; it is possible the two are in some cases genetically related.
Lower-Grade Envelope
Lower-grade mineralization envelopes the higher-grade core and, farther from the core, extends outwards as stratiform, mineralized lenses parallel to bedding. There are very few visible chalcedonic veins; the gold and silver are inferred to be disseminated within the silicified arkose and siltstone units. Contacts between arkose, siltstone, and sinter appear to have been preferentially mineralized, and beds of tuff and tuffaceous siltstone also were favorable sites for mineralization. Low-grade mineralization is also present in numerous intervals of silica sinter, but not all sinter intervals are mineralized. Sinter-hosted mineralization may be disseminated, or within fractures where the sinter has been structurally disrupted.