Summary:
Deposit Type
The geological setting, alteration and characteristics of the gold mineralization defined at Sandman all provide strong evidence that Sandman is a low-sulfidation epithermal style of deposit that formed close to the surface.
Sandman is located in north-central Nevada, which contains a number of vein-type deposits of middle Miocene age which have similar geology consistent with low sulfidation, epithermal gold-silver deposits. The epithermal deposits in north-central Nevada formed at shallow depth, and some are associated with sinter deposits which reflect hotspring activity at the paleosurface.
Key features at Sandman that support this interpretation include:
• The presence of quartz adularia veins which are in common association with the higher-grade gold mineralization.
• Zones of advanced argillic alteration (opaline silica-quartz-kaolinite-alunite) mainly above the quartz-adularia zone, which extend along some favourable stratigraphic horizons or contacts away from the large structures. These are likely pre mineral phase.
Although the genetic model for the Sandman deposits and surrounding gold-silver deposits may be similar there are often significant variations within each deposit type due to variations in the structural framework and host rock porosity compositions.
The presence of gold-silver hosted within a pipe-like hydrothermal breccia, structurally controlled gold mineralization associated with quartz-adularia veining and dispersed lower grade gold mineralization along stratigraphic horizons which are more favourable for fluid flow (typically horizontal orientation), all exist at least in part within the Sandman gold deposits.
The dominant target types which exist at Sandman for future exploration are largely one of three main deposit types:
1. Extensive shallow and low-grade gold mineralization, typically forming within 100m of the paleo water-table, which more favourable to the more porous host rocks such as basalt, conglomerate and altered diatreme breccia. This deposit type has most prominently been identified at the North Hill deposit to date.
2. Higher grade Au-Ag mineralisation in quartz-adularia small cm side veinlets hosted within feeder structures underneath the broader shallow stratigraphically controlled and lower-grade gold mineralization. This style is evidenced in the Silica Ridge and Southeast Pediment deposits, where potentially larger more continuous zones may exist at greater depths and laterally along the currently defined major structures.
3. Higher-grade dyke-hosted gold mineralization and/or hydrothermal breccia’s, both of which appear to have a strong influence from cross-cutting structures which trend due east. This style of gold mineralization is observed at both Abel Knoll and Silica Ridge.
Gold Mineralization
North Hill Gold mineralization is associated with both high-angle faults and the generally low-angle contacts between the various basalt flows and the interbedded fluvial conglomerates and tuffaceous rocks. Prior to the 2006 drilling program, East and West Hill mineral deposits were identified.
The West Hill area was a focus of drilling in 2006. This drilling expanded both gold deposits leaving a 200-foot gap between the two mineralized areas associated with the NE structural trend. The deepest significant gold mineralization at North Hill occurs at a vertical depth of 300 feet.
Silica Ridge
Narrow anastomosing structural zones of gold mineralization are unique to the SR deposit. The primary host units at SR are fluvial siltstone, sandstone, conglomerate, and epiclastic tuff of the Comforter Basin. Mineralization is primarily focused along the main NNE and NW fault sets over a known strike length of 850 meters.
High gold values are usually located in fault breccias, especially where coincident with conglomerate beds. Strong quartz-adularia flooding of conglomerate and sandstone matrix in mineralised samples shows fine grained subhedral and anhedral pyrite.
Southeast Pediment
The higher-grade gold mineralization at Southeast Pediment is structurally controlled within highly fractured and brecciated zones, which exhibits pre- and post-mineralization movement. Supergene enrichment may also in part be responsible for this mineralization. The assessment of the degree of supergene influence is complicated by the primary mineralized fault also influencing the redox boundary.
Approximate dimensions of Southeast Pediment are 305 meters in length by 30 to 60 meters in width. Mineralization occurs as finely disseminated gold or electrum in fine-grained quartz–adularia, and as rare coarse-grained visible gold grains up to 1.0 mm. Coarse gold is observed on fractures, in very narrow clear quartz veinlets, in late-stage breccia matrix, and in rare, very localized quartz banding.
Abel Knoll
There are two sources of gold mineralization in this deposit: (a) western area is hosted in and around a small diatreme breccia body, possibly related to mafic magmatism; and (b) an area to the east-northeast of the diatreme where mineralization appears to be related to altered andesite, and generally extends parallel to the strike of Tertiary rocks.