Summary:
The Pine Grove Property is located within the Walker-Lane mineral trend. According to Stone (2008), the style of mineralization encountered at the Pine Grove project most closely resembles the “Shear Zone” subtype of the “Plutonic-Related Au Quartz Veins and Veinlets L02” deposit type as described by Lefebure and Hart (2005).
In particular, the gold mineralization at the Pine Grove project has the following features in common with the “Plutonic-Related Au Quartz Veins and Veinlets L02” deposit type:
- Commonly found in tectonic settings of continental margin sedimentary assemblages where intruded by plutons behind margin arcs. Typically developed late in the orogeny or post-collisional settings.
- Host rocks are equigranular granodiorite with associated, highly differentiated, porphyritic dikes.
Mineralization can be divided into intrusion-related, epizonal, and shear veins. Intrusion related mineralization typically occurs in widespread sheeted vein arrays parallel to the major structural trends. Veins are commonly just hairline fractures to a few centimeters wide and hosted by extensional shears. Veins contain native gold, pyrite, chalcopyrite, and pyrrhotite. Gangue consists of quartz, and sulfides comprise less than 3 percent of the veins. Epizonal mineralization is typically less focused and may be disseminated or occur as replacements. The shear-vein style of mineralization may occur in fault zones outside of the pluton.
Alteration consists of biotite, albite, and sericite, and is spatially restricted to the mineralized zone.
Veins occur close to the associated granite dikes.
Mineralization within the quartz vein and stockwork zones occurs in relatively small tonnage but at relatively higher (2.042 opt) grades. Epizonal deposits have gold grades of 0.058 to 0.146 opt. Combined, these two styles of mineralization can form deposits of ten to hundreds of millions of tons.
Known gold mineralization at the Pine Grove project is found at the Wheeler and the Wilson mines. The two areas show similar alteration and mineralization characteristics but differ in their structural signatures due to differing locations relative to the Pine Grove fault. Gold is found in transitional quartz veins and in thin, crosscutting pyritechalcopyrite stockwork veinlets; the transitional quartz veins occurred between prograde potassic and albitic alteration and retrograde sericite-pyrite-quartz alteration (Jackson, 1996). Dilles (1990) reports that sulfide mineralization is also disseminated.
Wheeler Mine
The Wheeler mine is situated in a fault-bounded block of granodiorite adjacent to the hanging wall of the Pine Grove fault at the contact with the Tertiary conglomerate and above the granite porphyry and other dikes that intrude the granodiorite. Princehouse and Dilles (1996) noted that the hydrothermal alteration and mineralization are spatially and temporally associated with the granite porphyry dikes. Gold and copper mineralization within the sheared block of granodiorite is exposed in outcrop, roadcuts, and underground workings, where it occurs with quartz veining and minor stockwork sulfide veinlets.
Structure
The approximately 330 ft-wide block of mineralized granodiorite is confined on the east by the hanging-wall structure of the Pine Grove fault and on the west by a parallel fault that was termed the Stonehouse fault. The Wheeler fault (Tetra Tech incorrectly identifies the Pine Grove Fault as the Wheeler Fault) dips about 30° to the east, and the Stonehouse fault dips roughly 70° to the east. The block of mineralized granodiorite between the faults is strongly sheared and brecciated, with textures ranging from early, shallow-dipping, brittle-ductile smearing of foliated biotite to more steeply-dipping brittle, cataclastic breccia and gouge zones that parallel the Pine Grove fault.
Post-mineral shearing has disrupted the internal structure at the Wheeler mine veins system such that sizable volumes of gold-bearing gouge are typically encountered. This shearing has disrupted the veins and produced zones of crushed and pulverized material containing tiny blebs of silica that were probably once portions of discrete veins.
Alteration
The mineralization is accompanied by strong hydrothermal alteration that post-dated the metamorphic foliation (Jackson, 1996). In general, the alteration increases in intensity to the northeast, reaching a maximum at the contact between the granodiorite with the hanging wall Morgan Ranch conglomerate.
Hydrothermal alteration consists of early, prograde, high-temperature potassic alteration (biotization and potassium feldspar replacement), followed by an albititc (sic) alteration event, then a transitional chlorite-actinolite event that hosts the gold mineralization. The chlorite-actinolite alteration is confined to the mineralized block between the Stonehouse and Pine Grove faults. Mineralization was followed by retrograde quartz-sericite-pyrite alteration. The alteration events are telescoped and overlap each other, and for the most part are restricted to the mineralized block of granodiorite.
Wilson Mine
At the Wilson Mine the Pine Grove fault is not present but a footwall splay that separates mineralized granodiorite with overlying Tertiary rhyolite strikes in a northwest to west-northwest direction and dips 15° to 35° to the northeast. The mineralized granodiorite at Wilson is separated by several tabular dikes of granite porphyry, rhyolite porphyry and dacite.