Summary:
The Berg deposit is a classic calc-alkaline copper-molybdenum porphyry deposit of Eocene age.
Two main intrusive bodies are exposed in the Property area. The largest consists of a north-trending, elongate body of quartz diorite (Unit QDR) that intrudes the contact between Hazelton Group and Skeena Group east of the mineralized area. The intrusion extends from 750 m north of the Berg Stock to over 6.5 km to the south. It ranges in width from 600 m on the property area to over 2 km at its southern extremity. Compositional and textural zonation of the quartz diorite is evident with a central core of pink quartz monzonite exposed 1.6 km south of the camp that grades outwards into quartz diorite and hornblende quartz diorite. Porphyritic phases are also present.
The other prominent intrusion in the deposit area is the Berg Stock, a multi-phase composite quartz monzonite stock that intrudes the Hazelton Group andesitic rocks. It is broadly cylindrical, approximately 600 to 750 m in diameter with typically sharp, subvertical contacts. Locally, these contacts are complex with brecciated xenoliths of andesitic rocks with diffuse clast boundaries. Panteleyev (1976, 1981) subdivided this composite stock into four main phases:
• A core of pre-mineral, very coarsely porphyritic quartz monzonite (Unit QMP).
• A pre-mineral coarse-grained plagioclase-biotite-quartz porphyry (Unit PBQP) that wraps around the northern flank of the QMP core.
• A northwest trending, pre-mineral medium-grained porphyritic quartz-plagioclase porphyry (Unit QPP) that extends to the west from the southern and western portion of the QMP core; and
• A narrow, subvertical and northeast-trending late-to post-mineral quartz-feldspar porphyry (Unit QFP) dyke or zone of dyking that cuts across each of the above phases and also cuts quartz diorite along trend and northeast of the stock.
The QMP unit is characterized by very coarse-grained plagioclase, quartz, biotite and commonly megacrystic orthoclase. The quartz, in particular, is distinctive and commonly comprising coarse resorbed crystals with sub-rounded and wormy boundaries and with poikilitic intergrowths of plagioclase. Feldspars and biotite are euhedral and minor hornblende is typically replaced by biotite.
The PBQP is a slightly finer-grained quartz monzonite than the QMP with a typically darker grey to brown matrix containing plagioclase, quartz, and biotite with rare orthoclase. Biotite is twice as abundant and typically finer-grained than in the QMP, comprising 2 mm books compared with the 4 - 6 mm books in unit QMP. Internal contacts and cross-cutting relationships within the Berg Stock between the PBQP and the QMP are poorly understood due to lack of drilling, but contacts with the andesitic country rocks appear to be largely subvertical.
Mineralization
Mineralization is related to hydrothermal activity associated with the intrusive granitic bodies of the Berg Stock and consists of porphyry style mineralization with minor late polymetallic veins. Such mineralization is extensive throughout the region, and similar examples of both vein and porphyry systems have been mined within several kilometres of the Property boundary.
Mineralization at the main Berg deposit is localized in and adjacent to the two Eocene intrusions in the area: quartz diorite, and quartz monzonite of the composite Berg Stock. Three phases of porphyry mineralization and their relative ages were identified during relogging in 2017 of 1,555 m of drill core collected from the 2011 campaign. The relogging program concluded that cross-cutting vein relationships and the distribution of alteration assemblages identified at least three porphyry intrusion events (QPP-P1, PBQP-P2, and QMP-P3) despite their similar mineralogy. QPP-P1 is strongly altered, with only relict phenocrysts and contains the highest vein density of all the porphyry bodies. PBQP-P2 is also altered, but to a lesser degree, and has lower vein density. QMP-P3 hosts the fewest veins and is the least altered of the intrusive bodies within the Berg composite stock and includes well preserved igneous biotite books. The Berg Stock is the prime control on copper-molybdenum mineralization at the Berg property as the deposit forms an annulus around the stock. Mineralization occurs in a highly fractured zone superimposed on hornfelsed Hazelton Group andesitic volcanic rocks, the adjacent quartz diorite intrusion, and, to a lesser degree, the Berg Stock (Harris and Stubens, 2008). Current known mineralization extends in a cylinder about the central core with a diameter of approximately 1300 m and depth of up to 900 m. It remains open at depth at to the east near surface.
Typical copper and molybdenum mineralization occur primarily in potassically-altered rocks related to the earlier phases of the Berg Stock (QPP-P1 and PBQP-P2), with most hypogene mineralization occurring in several generations of quartz-sulphide veins. The earliest veins appear to be the most copper- and molybdenum-rich. Associated alteration envelopes are either potassic or non-existent, implying equilibrium with the potassically-altered wall rocks. Later veins are typically poor in Cu ± Mo sulphides and are associated with phyllic and propylitic alteration assemblages. Calcite ± gypsum ± quartz-sphalerite-pyrite ± galena veins are a common late vein type and contain up to 1,020 g/t Ag. This argentiferous mineralization is particularly prevalent within the PBQP-P2 in the West Shell and to a lesser extent with PBQP-P2 in the North Shell.
A well-developed supergene enrichment blanket overprints and is developed above the hypogene mineralization and is subdivided into three mineralogically distinct zones: (1) supergene sulphide (chalcocite with lessor covellite, and digenite), (2) supergene oxide (malachite/azurite, cuprite, tenorite, and native copper) and (3) leached capping.
The presence or absence of these zones is determined by several factors including fracture intensity, abundance of hypogene sulphide and topography. Topography has the greatest effect on supergene profile development. In the ridge-top environments the supergene profile is complex, consisting of a strong leached and oxidized zone underlain by a thick but poorly enriched supergene sulphide zone. In the valley floor environments, where the water table is at or close to the surface, leaching is minimal and fresh hypogene minerals occur at surface. The most complex profile is developed on steep slopes, where highly variable water table levels and a high rate of ground water migration have coupled to produce a strongly enriched supergene sulphide zone overlain by a zone of supergene oxide. The supergene sulphide zone is dominant with supergene oxide and leached zones making up a small portion of the preserved supergene profile. The supergene sulphide zone overprints hypogene mineralization with the proportion of supergene sulphides relative to hypogene sulphides decreasing with depth.
The boundary between the supergene and underlying hypogene zones is marked by the last logged occurrence of chalcocite in drill core and can also correlate, but not consistently to the upper limit of gypsum fracture-filling. Supergene oxide mineralization is also strongly developed on the margins of, and commonly within, post-mineral andesite dykes where they transect the supergene zone. The buffering effect of the carbonate-bearing andesite dykes and the QFP dykes with the acidic cupriferous leachate appears to have resulted in the precipitation of the supergene oxide minerals, chiefly tenorite, malachite, and azurite. Supergene mineralization is less commonly present on the QFP dyke contacts.