Summary:
Deposit Type
VMS deposits are the product of hydrothermal vents on the sea floor that form syngenetically with active volcanism and/or plutonism. They form at or just below the sea floor as a product of the discharge of high temperature, seawater-dominated hydrothermal fluid. There are six main elements typically present and are considered essential for the formation of VMS hydrothermal systems and their associated base metal deposits.
This deposit type is typically an accumulation of massive to semi-massive sulphides that are syngenetic, stratabound, and in part strataform. They usually consist of two parts: a concordant massive sulphide lens and an underlying discordant vein-type sulphide stringer or stock-work zone that is within a footwall alteration zone. In some cases, the stringer zone extends into the hangingwall as well, which appears to be the case at Picket Mountain. This continuation of the stringer zone into the hangingwall may indicate the continuation of the hydrothermal system and could represent additional exploration opportunities.
VMS deposits typically form in a diverse spectrum of volcanic-sedimentary environments that range from those dominated by flow, volcaniclastic, and or sedimentary rock types. Any of the three end members may be dominant, but what is characteristic for exploration purposes are the overall characteristics listed above.
Mineralisation
The mineral zone at Pickett Mountain is a volcanogenic massive sulphide deposit that strikes at approximately 057°. It has been traced by drilling approximately 900m along strike and to 750 vertical metres below surface. It consists of 4 primary lenses and several minor lenses that likely reflect the original formation of the mineralisation. It is stratabound and is hosted primarily by an intermediate to felsic lapilli tuff to volcanic breccia unit (Scully, 1988).
Primary minerals of economic interest are chalcopyrite, galena, and sphalerite intercalated with variable amounts of pyrite. Accessory minerals include tetrahedrite and minor arsenopyrite. There are two primary lenses of massive sulphide that have been discovered to date that have been subdivided into four lenses (W1, W2, E1, and E2). These vary from 0.5m to about 25m in horizontal width and with the highest base metal grades situated at or near the base of the massive sulphide lenses. The high-grade Cu-Pb-Zn sulphides are typically finely laminated and are overlain and in sharp contact with massive pyrite (Scully, 1988).
The high-grade sulphides typically include 45% to 60% pyrite, 15% sphalerite, 3% galena, and4% chalcopyrite. There are also minor amounts of tetrahedrite, tennantite, arsenopyrite, magnetite, and barite. Laminations are typically 2 mm to 5 cm in thickness and are compositionally defined (Scully, 1988).
The West Lens is the most prominent massive sulphide lens discovered to date having been traced by drilling over a 300m strike length and to a vertical depth of 825m. Notably, it also is the highest grade of all lenses based on current and historic drilling and the most intense footwall alteration. The West Lens, especially along its eastern edge, includes fold repetitions and structurally stacked mineralisation in the hanging wall (previously interpreted as W2 Lens). Re-logging, which is in progress, has been completed and lithogeochemical data are supporting the updated interpretation. It is also likely that additional holes will need to be drilled to finalise the updated interpretation. As well, the West Lens lies directly on either felsic volcanic in the upper part of the zone and sedimentary rocks, in the lower part of the zone. This suggests that, the massive sulphides were deposited, both on top of the felsic volcanic rocks and in local, likely structurally-controlled sub-basins, with local structures, likely controlling mineralising hydrothermal fluid flow.
The East Lens (E1 and E2 Lenses) can be traced over a strike-length of close to 550m and to a maximum vertical depth of about 550m below the surface with the bulk of the zone above 400m. The QFP unit occurring below the East Lens, may have modified the orientation of the lens resulting in a locally shallower dip and may have propagated a fault or dislocation between the East and West Lenses. In addition, the QFP and a massive feldspar quartz porphyry unit appears to partially truncate the East Lens at its eastern extremity, occurring as a felsic dome intruding the upper rhyolite. The continuation of the favourable stratigraphy hosting the West and East Lenses, beyond the eastern limits of this intrusive cut-off, is unknown.
The Footwall Zone (FWZ) occurs at the contact between the QFP and/or sediments and FWBX felsic units about 150m north of the trend of the West and East Lenses. First identified in the 2018 drill results, about 700m below the surface, it now appears that some of the historical drilling also intersected this zone. Notable intercepts from the 2018 and 2019 drill programs include 10.0% Zn, 5.0% Pb, 1.1% Cu, 396.9 g/t Ag, and 0.4 g/t Au (24.7% ZnEq) over 7.10m (PM-18-31) as well as 4.4% Zn, 2.2% Pb, 0.5% Cu, 62.7 g/t Ag, and 0.3 g/t Au (9.0% ZnEq) across 9.1m (PM-19-31a). The FWZ is generally narrow but can locally contain very high-grades. In a few drill holes, the FWZ consists of discrete zones of semi-massive to massive sulphide (as seen in PM-18-31 and PM-19-31A), while in others, it occurs as heavy disseminations, stringers, and bands of sulphide mineralisation. Stringer mineralisation along this time break, some 170m from the discovery hole, suggests potential for additional massive sulphide mineralisation between the discovery hole and the stringer zone. The FWZ is open both up and down plunge, and to the east, as depicted in the footwall longitudinal section. Further drilling to test extensions is warranted. This drill testing can be cost-effectively accomplished by extending a couple of the previous holes.