Summary:
Deposit Type
The zinc-copper-gold-silver bearing sulfide mineralization identified on the Back Forty Property exhibits typical characteristics of VMS mineralization.
Mineralization
Mineralization at the Back Forty Deposit consists of four discrete zones:
1. Zn or Cu-rich massive sulfide (±Pb), which may contain significant amounts of Au and Ag;
2. Stockwork stringer and peripheral sulfide, which can be Au, Zn, and Cu-bearing (±Pb/Ag);
3. Precious metal-only, low-sulfide mineralization; and
4. Oxide-rich, precious metal-bearing gossan
At the Back Forty Deposit, MASU refers to rocks composed of at least 80% sulfide and semi-massive sulfide is composed of 40% to 80% sulfide. Both also contain variable amounts of Zn, Cu, and Ag that occur most commonly as sulfides and Au is mostly found as finely divided native metal or as a natural amalgam. Sulfide stringer (“SFST”) mineralization consists of 10% to 40% pyrite in veins with variable amounts of Cu, Ag, and Au that predominantly penetrates RCTF. Oxides and hydroxides of iron form a crudely bedded gossan (“GOSS”) above MASU, where it is exposed at the bedrock surface. GOSS contains variable amounts of Au, Ag, and Cu and accessory minerals including chlorite and calcite. Certain intervals are characterized by significant amounts of finely laminated hematite and magnetite and may represent an exhalative iron formation deposit.
Massive Sulfide Mineralization
VMS-style mineralization has been identified within at least two stratigraphic levels within the felsic sequence at the Back Forty Deposit. The rhyolitic rocks hosting the Back Forty sulfide mineralization are indistinguishable with respect to appearance; however, the two main rhyolites (rhyolites 1 and 2) have distinctive geochemical signatures that can be observed through aluminum-titanium and zirconium-titanium ratios. The Main Zone massive sulfide, which accounts for most of the massive sulfide mineralization, lies at the stratigraphic boundary of these two rhyolite units. Rhyolite 1 lies stratigraphically below this sulfide horizon (footwall) while Rhyolite 2 lies above the horizon (hanging wall). Another massive sulfide horizon, the Tuff Zone, is located at or near the upper contact of Rhyolite 2 and the lower contact of an overlying package of tuffaceous and siliceous sediments. A possible third massive sulfide horizon, the Deep Zone, may represent a lower mineralized zone.
Main Zone Massive Sulfide
The Main Zone is composed of three separate massive sulfide bodies: East, Hinge, and South Limb Zones, that form a plunging antiform and are considered the same horizon. Rhyolite 1 (footwall) hosts the Main Zone along and stratigraphically below its contact with Rhyolite 2 (hanging wall). This massive sulfide hosted strata-bound Main Zone is locally enveloped by stockwork and semi-massive sulfide-types mineralization. Pervasive sericite and disseminated pyrite alteration as well as variable silicification are abundant and extend outward for an undetermined distance. This zone extends along strike for over 450 m in a west-southwest direction, is up to 100 m wide, and subcrops at its eastern end under a thin (less than 10 m) glacial overburden and local Paleozoic sandstone.
Pinwheel Zone Massive Sulfide
The Pinwheel Zone is located in the northwest portion of the Back Forty Deposit. It is found structurally along the gently north-dipping northern limb of the antiform and is truncated to the south by the E-W fault. Limited geochemical data suggests that this unit is in fact located along the contact between Rhyolite 1 and Rhyolite 2 and is therefore likely the equivalent to the Main Zone massive sulfide and represents a ‘faulted-up’ portion of the north limb of this important massive sulfide horizon. Massive sulfide mineralization along strike of the Pinwheel Zone has been traced for roughly 700 m to the west-southwest where the gentle north-dip of the unit steepens.
Deep Zone Massive Sulfide
The Deep Zone is located north of one of the QFP dykes, juxtaposed against the South Limb Horizon. Recent geological and geochemical data interpretation suggests that the Deep Zone may be the down-dip continuation of the South Limb (Main Zone), where it has been folded and rotated.
The Deep Zone is relatively enriched in copper compared to the three zones of the Main Zone (East, Hinge, and South Limb) and suggests that a more copper-rich portion of this VMS system may occur at depth.
Tuff Zone Massive Sulfide
The Tuff Zone massive sulfide occurs at the south edge of the Project. Stratigraphic and structural data suggest this zone is located at a higher level in the volcanic sequence. In cross sections and three-dimensional models, the zone appears to have a bowl-shaped geometry possibly reminiscent of a small relict depositional basin or local graben structure.
Stockwork and Peripheral Sulfide Mineralization
Widespread and pervasive sulfide mineralization occurs throughout the host rocks and peripheral to the massive sulfide bodies at the Deposit in the form of stockwork-stringer sulfides, massive to semi-massive discontinuous sulfide lenses, and disseminated sulfides. Geochemical data suggests that there are at least two distinct zones of peripheral sulfide mineralization that formed stratigraphically below each of the massive sulfide horizons and can be differentiated from each other by their relative location in the stratigraphy and relative metal content. Sulfide mineralization that occurs within Rhyolite 1 appears to have a mineralogical affinity with the Main Zone and Deep Zone massive sulfides and consist of predominantly pyrite (commonly gold-bearing), minor to moderate sphalerite with variable amounts of chalcopyrite.
Copper Mineralization Associated with Sulfide Mineralization
Drilling has shown that the VMS mineralization of the Main and Tuff Zones is generally Copper-poor. Copper mineralization; however, appears to be more prevalent in the lower stratigraphic horizons of the Back Forty Deposit. Copper/Zinc ratios show a marked decrease that occurs at the Rhyolite 1-Rhyolite 2 contact indicating that fluids responsible for the Main Zone massive sulfide formation and underlying peripheral sulfide mineralization (stringers, disseminations, etc. of Rhyolite 1) were of higher temperature and carried more copper compared to those fluids associated with sulfide mineralization in the stratigraphically higher sulfide zones (Tuff Zone and Rhyolite 2 peripheral sulfides).
Precious Metal-rich Low Sulfide Mineralization
In addition to gold values in both the massive and stockwork-stringer sulfide zones, significant gold and silver mineralization is present in surrounding low sulphur hosted-rocks. This mineralization typically contains less than 10% sulfide, although it may contain more locally. There are three main zones in this style of mineralization which include different host rock depending on the location. The typical host rocks include tuffaceous sediments and underlying rhyolite tuff (90 Zone), sheared rhyolite (NS Zone), and QFP (PM Zone and NS Zone).
Gossan (Supergene) Mineralization
Near surface iron-oxide and precious metal-rich gossans cap the East Zone and the Pinwheel Zone. Both are consistently enriched in gold and in the case of the Pinwheel Gossan, silver and copper.
The East Gossan caps the East Zone Massive Sulfide. It subcrops under glacial overburden and at its eastern extent is capped by Paleozoic sandstone. The East Gossan is generally 3 m to 5 m thick, hematitic, and gold-rich.
Mineralization Encountered at Depth
Mineralization has been encountered in two separate stratigraphic horizons which have tentatively been called the ‘Upper Deep’ and ‘Lower Deep’ zones. Drill hole LK-479, drilled to the southwest of known mineralization and to a total depth of 911.0 m, encountered each of the two zones and represents the ‘discovery’ hole for each.