A deposit model is used as an empirical basis for understanding how an ore deposit formed. There are many types of geological models that are used, and exploration and mining of an ore deposit can help to understand exactly how a deposit formed. The Thor deposit belongs to a unique collection of silver deposits that are extremely high-grade, and have characteristic small tonnages.
In 2019, Taranis made considerable progress understanding the surface geology of the Thor deposit, and this has led to the outline of a new deposit model. The deposit is now known to be associated with a large fault corridor called the Thor Fault Zone ("TFZ"). Within this corridor is found all of the precious and base metal mineralization.
Structural and Lithological Control on Mineralization
Mineralization within the TFZ is spatially related to the contact between the underlying Sharon Creek Formation and the overlying Broadview Formation. Because the location of this contact has been folded in a northwest trending direction, geological mapping is critical in locating this contact. Where this contact between the Sharon Creek/Broadview Formation comes into contact with the Thor Fault Zone, the lowest part of the Broadview Formation is mineralized. This contact is preserved in the rocks found to the east of the TFZ at Thor, and to the west rocks of the non-receptive Sharon Creek Formation are generally found.
Genesis of the Mineralization
Hydrothermal fluids have moved within the Thor Fault Zone from an unknown source and deposited at the Sharon Creek/Broadview Contact. Sharon Creek phyllites are prone to ductile deformation and are not receptive host rocks. In contrast, the Broadview Formation is prone to brittle deformation, and is a good receptive host rock subject to extensive quartz vein stockwork development and emplacement of base and precious metals.
The hydrothermal fluids that were responsible for mineralization also caused significant alteration of the Broadview Formation rocks into what is now called the ‘Green Tuff’. Although little is known about the chemistry of the hydrothermal fluids, the alteration is known as ammonium-illite alteration and is generally sericitic in nature. The location of this rock unit indicates proximity to mineralization. It is interesting to note that ammonium-illite alteration is one of the major alteration types in Carlin Deposits in Nevada, including some of the largest – Betze Post and Jarret Canyon, but it is not described in many other deposits!
In general, the deposit model can be described as fault-related, but it also incorporates an epithermal type of mineralization that has seen the fluids move along the Thor Fault Zone. In general, the Thor deposit appears to be base metal rich at the lowest parts of the deposit, and gold-rich at the top. In addition to this, the gold-rich areas such as SIF exhibit “vuggy” textures characteristic of epithermal deposits.