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    View of Newmont’s Phoenix Mine from GSV’s Virgin Fault mineral zone

Lewis Project Geology

The Lewis Gold Project is strategically located within the Battle Mountain Trend of Nevada. Mining commenced in the vicinity of the Lewis Gold Project in the 1860s with the exploitation of gold-silver lodes by surface and underground methods.  Newmont reports that the Phoenix deposit, immediately south of the Lewis Gold Project, is a skarn-hosted polymetallic massive sulfide replacement deposit.

Several mineral deposit types are present on the Lewis property include intrusion related, sediment-hosted, precious metal skarn and silicified fault/vein systems. Gold Standard Ventures recognizes more than eight prospective mineralized zones on the Lewis property. The best understood Au-Ag systems to date include the Virgin, Buena Vista, Meagher, White-Shiloh and Trinity structural zones, which are open along strike and at depth for approximately 4.8 km to the north. The highest Au-Ag grades occur in northerly-trending segments of the mineralized faults and in north-trending fault splays. The involvement of Northwest- and northeast-striking cross-faults are important in localizing high-grade mineralization within the northerly trending target zones.

The property contains units from the Cambrian to the Miocene, with many features containing evidence of multiple orogeny’s and periods of sedimentation. The Ordovician Valmy Formation, consisting of deformed layers of interbedded chert, shale, quartzite and greenstones, underlies the project and forms the base of the Roberts Mountain Allochthon. The Devonian Scott Canyon Formation lies unconformably above the Valmy Formation and consists of interlayered chert, carbonaceous shale, greenstone and limestone.  The Cambrian Harmony Formation overlies the Scott Canyon Formation and Valmy Formation on the Dewitt thrust an is transitional between eastern and western facies rock types. It consists of sandstones, arkose and limestone derived from Pre-Cambrian granites and metamorphic terrains. This unit was thrust over eastern faces units during the Antler Orogeny as part of the Roberts Mountain Allochthon. The Pennsylvanian and Permian Antler Sequence is an overlap assemblage which was deposited after the Antler Orogeny, resting unconformable on the Scott Canyon, Valmy and Harmony Formations.  The sequence is divided into 3 units:

  • The lower Battle Formation is a matrix supported boulder/cobble conglomerate composed of angular to sub-rounded poorly sorted clast of chert and sandstone in a sandy to calcareous matrix and is a principle host for many of the deposits in the Phoenix area (East Deposit, Tomboy-Minnie, Upper Fortitude).
  • The Antler Peak Limestone lies disconformable on the Battle Conglomerate and consist of medium bedded limestone with shale and conglomerate interbeds and was the most important host rock in the Fortitude Pit.
  • The Edna Mountain Formation lies on an erosional unconformity above the Antler Peak Limestone and consists of debris flow pebble-conglomerates with interbedded siltstones, calcareous sandstones and shales and is a minor host of Lower Fortitude Skarn deposit.

Pennsylvanian-Permian Havallah Sequence in the project area is composed of the Pumpernickel and Havallah Formations, consisting of thinly bedded sandstones, shales limestones, cherts conglomerates and greenstone and are thrust over the Antler Sequence as part of the Golconda Allochthon during the Golconda Orogeny.  The above sequence was intruded by several Eocene Granodiorite Intrusives dated at 38Ma and responsible for Au, Ag, and Cu mineralization throughout the district as Porphyry copper deposits, skarns and epithermal veins (Copper Canyon Deposit, Fortitude, etc.). Post Eocene volcanic units unconformably overly all other units.

Several N, NE, NW and E striking features control mineralisation to varying degrees in different deposits. The N-S striking Virgin and Copper Canyon fault zones run throughout the project and control mineralisation in the Fortitude Pit. These faults dip down to the west at 55 and 65 degrees and have offsets of 275 and 460 meters, respectively. The Virgin fault zone can be up to 189 meters wide where it intersects NE striking faults, whilst the Copper Canyon fault is up to 137 meters wide. North of the Fortitude Pit, the Copper Canyon Fault bends to an NNW strike and is known as the Buena Vista Fault.

Major displacement on these faults likely occurred pre-stock emplacement as displacement of the Copper Canyon stock is minimal. Some post mineralisation movement has occurred, offsetting the stock by a few 10s of metres.  The Virgin Fault also hosts a granodiorite porphyry dike that extends to the north off the main stock.  Faults on the same N-S trend within the area include the Hayden, NEX and Monitor faults to the east of the Fortitude pit.

The downthrown Antler Sequence block between the two faults hosts most of Au mineralisation in the Virgin target area, with the Virgin Fault zone itself containing the highest grades. The smaller N-S Echo fault, found between the Virgin and Copper Canyon Faults contains high grades in its footwall, constrained to the east by the Virgin Fault. Steep NE striking, NW dipping faults are also important for mineralisation in the area and include the F1 and F2 faults in the fortitude pit. Gold is mostly hosted in their hanging walls and in ‘ore shoots’ where they intersect major N-S fault zones. These faults are known as F-Faults in the hanging wall of the Virgin Fault and L or Link faults in the footwall to the east. Faults striking N to NE, like the L faults in the Phoenix deposit, are found in the footwall of the Virgin Fault and also control mineralisation in the Virgin deposit.

Reference:
Dawson, Robert, 2018, Characterisation and paragenesis of metamorphic and hydrothermal alteration mineral assemblages within the Galena area of the Copper Canyon – Battle Mountain Mining District, Lander County, Nevada, USA: Master of Science in Mining Geology dissertation, University of Exeter, UK.


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