Summary:
Deposit Type
Based upon the styles of alteration, the nature of the veins, the alteration and vein mineralogy, and the geologic setting, the silver and gold mineralization at the Tonopah West project is best interpreted in the context of the volcanic-hosted, intermediate- to low-sulfidation type of epithermal model [e.g., Heald et al., 1987; Ashley et al. 1990; John et al., 2018]. This model has its origins in the De Lamar - Silver City district, where it was first developed by Lindgren [1900].
The host rock setting of mineralization at the Tonopah West project is similar to the simple model with the Sandgrass Andesite through the Mizpah Andesite occupying the stratigraphic position of the volcano-sedimentary rocks shown below, shortly prior to the eruption of the Fraction Tuff.
Mineralization
Mineralization at the Tonopah West property is exclusively hosted in hydrothermal quartz veins and quartz-cemented breccias that do not crop out at the surface. Drilling discussed in Section 10 and reports from historical underground workings indicate the principal host rocks include the West End Rhyolite, and to a lesser extent, the Mizpah Andesite. Mineralized quartz veins range from a few centimetres to several metres in thickness. Overall, the veins average 4.3 metres in width based on the geologic modelling. Thicker vein zones tend to be characterized by sub-parallel quartz fissure veins as mapped in the Victor mine area by Nolan [1930] where the Victor vein was over 20 metres wide and 165 metres in length.
Vein mineralogy is characterized by quartz centerlines with local adularia, pyrite, and parallel bands of fine-grained black sulfide and/or sulfosalt minerals. The zones of fine-grained black sulfide and/or sulfosalt minerals typically occur at the vein margins or in millimetre-scale veinlets parallel to the larger veins and are inferred to contain the silver and gold. Related quartz-cemented breccias contain pyrite and fine-grained black sulfide and/or sulfosalt minerals in the matrix.
Although petrographic data have not yet been obtained, the presence of polybasite, pyrargyrite, acanthite, freibergite/tennantite and possibly naumannite are inferred based on sample geochemistry. In places, subsequent stages of quartz veins have crosscut and overprinted the black-matrix quartzcemented brecciated zones. Argillic and propylitic alteration of the wall rock is observed proximal to mineralized veins.
Three groups of mineralized veins have been defined that comprise the four areas of estimated mineral resources and mineralized material in the Tonopah West property: the Denver-Paymaster-BermudaMerten vein group, which Blackrock refers to as the “DPB” vein group, the Victor vein, and the NW Step Out vein group. The DPB vein group is located approximately 1 kilometre west of the town of Tonopah and was historically accessed by the westernmost underground mining workings in the Tonopah district. Because of the higher-grade nature of the gold and silver mineralization identified in the Bermuda vein, the vein was estimated separately from the Denver, Paymaster and Merten vein sets. The Victor vein historically was accessed by workings more proximal to the central Tonopah mining district. The NW Step Out vein is located approximately one kilometre northwest of DPB area. This vein area is a bona fide new discovery as no previous work, records or reports of historical exploration or mining are known to exist.
Veins in the Tonopah West property appear to parallel the structural margin of the Fraction caldera along the caldera’s southern boundary. At Victor, the veins strike east-northeast and rotate to an eastwest to west-northwest alignment in the DPB area. On the western side of the DPB area, the veins change to a northwest orientation toward the NW Step Out area. All the veins dip north at various angles toward the interior of the caldera. Dip angles of some veins, such as the Merten vein, are low to moderate (approximately 30° to 40°) while other veins, such as Denver, Bermuda, Paymaster, and Victor veins, dip more steeply (approximately 60° to 75°).
Victor Vein
The Victor vein was accessed via the historical Victor shaft in the northeast part of the Tonopah West property. The Victor vein (see Figure 10-3) includes relatively high-grade silver and gold mineralization within several adjacent sheeted veins occurring along, and sub-parallel with, the Pittsburgh-Monarch fault. The Victor vein dips approximately 70° to the north and possesses multiple mineralized splays and sub-parallel veins. Higher-grades range in thickness from about 0.5 metres to a maximum thickness of 24 metres [Carpenter et al., 1953] along the footwall of the Pittsburg-Monarch fault.
As of the Effective Date of this report, the known extent of the Victor vein is approximately 750 metres in an east-west direction. From the surface, drilling has encountered mineralized veins from 400 metres to a depth of approximately 800 metres. The Victor veins are open below the depth of Blackrock’s drilling.
Denver-Paymaster-Bermuda
Vein Groups Major veins in the DPB group include, from south to north: the Merten vein, the Bermuda vein, the Paymaster vein, and the Denver vein. All veins in the DPB area dip to the north at angles ranging from approximately 30° to approximately 75°. The veins have a presently known vertical extent of approximately 500 metres.
Mineralized material in the DPB area consists of parallel sets of veins and stockwork veins in three dominant dip orientations. These include: a package of shallow- to moderately-dipping veins (approximately 30° to 45°) following the Merten veins in the southern part of the DPB area; a package of high-angle veins dipping at approximately 75° following the Bermuda vein in the center of the DPB area; and a package of moderately-dipping veins at angles of approximately 60° following the Paymaster and Denver veins in the northern part of the area. The steeper-dipping vein sets paralleling the Bermuda, Paymaster, and Denver veins in the central and northern portions of the group were the target of historic underground development, but no mining, and generally contain higher-grade mineralization than the shallow-dipping vein sets to the south.
The DPB veins are open below the depth of Blackrock’s drilling. These veins are also open to the east toward the Victor area and to the northwest toward the NW Step Out vein zone.
Northwest Step Out Vein
Group As of the Effective Date of this report, there are two vein sets identified in the NW Step Out area with only limited drilling. The veins strike northwest and dip moderately to the northeast at approximately 30 to 45°. The shallower-dipping vein set is host to the high-grade gold and silver. The steeper-dipping vein set contains low-grade mineralization. The NW Step Out veins are open to the northwest, southeast toward DPB, and are open at depth.