Summary:
The Spring Valley deposit is hosted within a porphyry intrusion and overlying felsic volcanic rocks.
The Spring Valley property is located in the Humboldt Range, a north-south oriented, up-thrown fault block (horst) bounded on the west by the Humboldt River valley and on the east by Buena Vista Valley. Quaternary alluvial deposits fill the intermontane basins and alluvial valleys.
The bedrock geology of the Humboldt range within 20 miles of the Spring Valley property consists of Triassic shales and carbonate rocks, a thick sequence of Permo-Triassic intermediate to felsic volcanic rocks, and a north-south trending belt of Tertiary volcanic rocks. Triassic leucogranite and Cretaceous granodiorite locally intrude the Permo-Triassic volcanic package.
The known Spring Valley mineral system is beneath an intermontane basin filled with postmineral Quaternary alluvial deposits, thereby masking the bedrock geology immediately overlying or containing the mineralization. At the scale of the Spring Valley property position, the bedrock units are distributed in blocks aligned approximately north-south. The bedrock geology is dominated by the Limerick Formation in the western one-third of the property, the Rochester Formation in the central and eastern half of the property, and the Natchez Pass Limestone in the extreme northeast corner of the property. At this scale, the geology is segmented by a number of faults: a relatively older north to northeast trending set including the West Spring Valley, Limerick and Black Ridge Faults; and, northwest trending, steeply dipping cross faults with oblique or lateral offsets that displace the older north to northeast trending faults. The West Spring Valley fault is interpreted as a steeply east dipping normal fault, whereas the Black Ridge and Limerick faults are interpreted as moderate to high angle normal faults with westerly dips. The Limerick fault may be listric in character, with flattening dip at depth. Eastwest and northeasterly faults are also mapped, but are not part of the predominant fabric on the property.
Breccia/Conglomerate
Underlying the WT Rhyolite is a breccia with large rounded to subangular clasts in a matrix of smaller rock fragments. It is largely clast-supported, and poorly sorted. Clasts include fragments of silicified limestone and a variety of intrusive and extrusive igneous rocks not seen elsewhere in Spring Valley, as well as local units. In one area, the breccia cuts upward through the WT rhyolite and part of the siltstone. Fragments of these rock types were observed deeper in the breccia. This feature was interpreted as a diatreme and was the focus of drilling in the early stages of the project. Adjacent to the pipe, breccia is conformably overlain by the WT rhyolite and is interpreted to be an eruption apron. The base of the breccia has been obliterated by intrusion of feldspar porphyry.
Intrusive rocks
The Rochester Rhyolite and Limerick Greenstone were intruded by a shallow, hypabyssal intrusion that underlies the volcanic rocks throughout most of Spring Valley. The intrusion has distinct feldspar phenocrysts in a fine-grained matrix and has been designated as the feldspar porphyry (FP). The top of the intrusion is very irregular and includes apophyses that form sills and dikes that extend into faults, and along contacts of the Limerick and Rochester rocks. The eastern margin of the intrusion formed a west-dipping dike along the Limerick fault between the Limerick Greenstone and Rochester Rhyolite. This dike is strongly mineralized.
Mineralization Style
Quartz veining, alteration, and gold mineralization at Spring Valley are irregularly distributed throughout the favorable host rock area. Large intervals of dense quartz veining and pervasive alteration are interspersed with unmineralized and less strongly altered country rock.
Gold has been observed in quartz veins and in adjacent alteration selvages as disseminated free gold. Free gold is likely deposited on fracture surfaces as well. Relatively coarse gold (30 to 90 microns) is common and can be observed as free gold liberated by drilling. Most quartz veinlets are in the ½ inch to 4 inch size range with associated alteration selvages of a few feet to tens of feet wide, varying to areas of dense quartz veining with pervasive alteration.
The quartz veins are translucent, intergrown, coarse quartz crystals with few if any open spaces or fissures. In combination with the relatively much larger alteration selvages, the character of these veins suggests a mesothermal or plutonic origin. Epithermal-style veins have not been observed at Spring Valley.
Quartz veins commonly contain pyrite (2-10%), less commonly galena and traces of sphalerite, magnetite and visible gold. From a limited amount of trace element data collected from drill samples, there are low levels (a few tens of parts per million above background) of anomalous lead, zinc, and arsenic associated with the gold mineralization.
Alteration
There are several distinct types of alteration at Spring Valley, as listed below:
1) Pervasive to fracture controlled quartz-sericite and quartz-sericite-pyrite alteration;
2) Strong pervasive to fracture controlled argillic alteration;
3) Very strong clay and clay filled breccia formation;
4) Pervasive to fracture controlled iron-carbonate alteration;
5) Pervasive to fracture-controlled hematite-quartz alteration.
Gold zones are most pronounced in the quartz-sericite-pyrite zones and in the pervasive argillic zones, although gold is found in every alteration type.
Other types of alteration include quartz-tourmaline, and potassic. Tourmaline occurs as disseminated crystals in sediments and the diatreme breccia and as quartz-tourmaline veins. While gold is found in some quartz-tourmaline veins, tourmaline generally is not correlated with the gold. Locally, the introduction or remobilization of potassium is seen by fresh overgrowths on feldspar or fine secondary biotite. This potassic alteration style may be much more extensive than currently understood due to the high potassium content of the Rochester Rhyolite which may mask the introduction of new potassium as an alteration product.
Carbonate alteration of the Limerick Formation greenstone rocks was observed locally adjacent to the Limerick fault. No direct correlation of carbonate alteration with gold mineralization was noted.
Geometry of Mineralization
The gold mineralization forms an irregularly-shaped cap encompassing at least the upper portions of the feldspar porphyry and significant volumes of the overlying or adjacent lithologies. The feldspar porphyry contact is very irregular. It was emplaced into a series of faults and irregular contacts, and may have been displaced by later fault movements. Overall the mineralization trends N20E to N30E and has the appearance of plunging 5-10 degrees to the north, though some of this plunge may be due to later fault-block subsidence.
Mineralization has been intercepted in drilling over a strike length of 7,500 feet and is open in both strike directions. Mineralization averages about 2,300 feet wide. The shallowest mineralization is found at the top of bedrock beneath 50 feet of alluvium. Deep core drilling has intersected gold mineralization as deep as 1,500 feet below the surface.
Quartz vein strike and dip directions recorded from oriented drill core show several distinct orientations. The most prominent orientation strikes N74E, dipping 60 degrees south. This principle vein orientation is oblique to the overall trend of the gold mineralization at Spring Valley.