Summary:
The deposits that comprise the Kensington Operations are considered to be examples of mesothermal vein-style, or orogenic-style gold deposits.
Mineralization of economic importance is restricted to veins within the Jualin Diorite. Minor goldanomalous quartz veins have been reported in Wrangellia Terrane metabasalt and in the metasedimentary rocks of the Treadwell Formation.
Discrete Veins
Discrete veins are hosted in shear zones that typically trend to the north or northwest, and are steeply easterly or northeasterly dipping. East dipping fault-fill veins are the dominant structures in the Kensington deposit.
The quartz–pyrite–carbonate–sericite ± chlorite veins were developed along the plane of the shear zones, but also underwent continued deformation after emplacement.
Gold hosting shear zones have a northeast side up (reverse)–right lateral shear sense, typical of other shear zones in the deposit area. If this shear sense is coeval with vein formation, it may provide predictable potential ore shoot control. Evidence from previously-mined (e.g., Raven) and drill-tested veins suggests that shallow to moderate south–southwest plunging ore shoots occur at jogs and steps in the shear vein system, which is compatible with a dominantly reverse sense.
Discrete-style veins are observed in the Raven, Kensington Zone 41, Elmira, Eureka, Comet, and Jualin deposit areas.
Extension Veins
Mineralized zones of this style are defined by increases in concentration and abundance of quartzcarbonate-chlorite veins, and later quartz–iron-carbonate veining.
Extensional-style veins are recognized in the main Kensington deposit, the Eureka area and in the Elmira zone to the east. Most of the veins in the Kensington system are the extensional veins, which occur approximately orthogonal to the discrete fault-fill veins. These extensional veins are higher density and lower grade, and mostly occur between stacked fault-fill veins.
Horizontal Veins Horizontal to sub-horizontal thin (<1 ft) vein occurrences are found throughout the Kensington system. These veins or ‘flats’ are locally continuous for the entire width of the economic zone, are often very high-grade and locally host lenses composed of almost 100% pyrite.
Mineralization
Vein mineralization is characterized by gold and gold-silver telluride minerals with minor associated native gold. Most of the gold is contained in calaverite (AuTe2), which occurs in association with native gold as inclusions in and interstitial to pyrite grains and in microfractures in pyrite. Trace amounts of petzite (Ag3AuTe2), coloradoite (HgTe) and altaite (PbTe) have also been noted. Minor amounts of chalcopyrite are also present along with trace amounts of bornite, molybdenite, sphalerite, galena, and pyrrhotite. The auriferous pyrite typically occurs in small to large blebs or clots within the quartz and quartz–carbonate veins.
Deposit descriptions are provided for the deposits with mineral resource estimates, namely the Kensington, Eureka, Raven, Jualin and Elmira deposits.
Kensington Deposit
The Kensington deposit consists of both discrete shear veins and a network of extensional veins. It is about 3,500 ft long, 2,100 ft wide, ranges in thickness from 1–100ft, and is drill tested to about -600 ft depth.
The Kensington vein system is a north-trending, steeply east-dipping network of quartz extension veins and shear veins. The overall veining style is semi-brittle, defined by both vein development (an overall brittle structural style), and the development of sigmoidal folds to vein sets and foliation (ductile style), which accommodate ductile displacement and shortening across the zone.
Mineralization occurs primarily as disseminated pyrite or pyrite seams and blebs, that range from 1–6 inches in thickness, and are contained within discrete shear veins or stacked networks of sheeted extensional veins. Minor amounts of chalcopyrite occur within the deposit as well.
Eureka Deposit
The Eureka deposit consists of both discrete shear veins and a network of extensional veins. It is about 950 ft long, 900 ft wide, ranges in thickness from 1–30ft, and is drill tested to about - 1,350 ft depth. The Eureka deposit sits immediately in the footwall of the Kensington deposit and has many similar characteristics to the Kensington deposit.
The Eureka vein system is a north-trending, steeply east-dipping network of quartz extension veins and shear veins. The overall veining style is semi-brittle, defined by both vein development (an overall brittle structural style), and the development of sigmoidal folds to vein sets and foliation (ductile style), which accommodate ductile displacement and shortening across the zone. The parallel, hosting, structure of the Eureka deposit commonly contains gouge, <1 inch thick, on both the footwall and hanging wall.
Mineralization occurs primarily as disseminated pyrite or pyrite seams and blebs, 1–6 inches thick, which are contained within discrete shear veins or stacked networks of sheeted extensional veins. Minor amounts of chalcopyrite occur within the deposit.
Raven Deposit
The Raven vein is located about 2,000 ft west of the main Kensington deposit. The Raven deposit is about 2,200 ft long, 1,500 ft wide, ranges in thickness from 1–20 ft, and is drill tested to about -250 ft depth. It consists of one main, economic vein with associated splays that vary in grade.
The Raven vein is a north-trending, moderately east-dipping shear vein hosted within a subparallel, tightly foliated, ductile, shear zone. Vein thickness and orientation is controlled by the prevalent C–S fabric (metamorphic fabric formed by the intersection of shear surfaces within rocks affected by dynamic metamorphism) of the shear with blow-outs occurring along jogs in the system.
The shear vein contains pods, lenses, and shear bands of pyrite, petzite, calaverite, hessite, chalcopyrite, galena and native gold. Mineralization occurs primarily as disseminated pyrite or pyrite seams and blebs, 1–6 inches thick, contained within the hosting discrete shear vein.
Jualin Deposit
The Jualin deposit is about 675 ft long, 1,025 ft wide, ranges in thickness from 1–15 ft, and has been drill tested to about -322 ft depth. It consists of numerous stacked quartz veins but only Vein 2 and Vein 4 are currently considered economic.
The deposit consists of stacked, discrete, quartz-veins hosted in the Jualin Diorite.
The Jualin deposit consists of north-trending, steeply east-dipping shear veins hosted within a sub-parallel, tightly foliated, shear zone. The vein thickness and orientation are controlled by the prevalent C–S fabric of the shear with blow outs occurring along jogs in the system. Occasional west-dipping, gouge-filled faults offset the vein as much as 40 ft in a reverse left-lateral sense similar to the overall structure fabric of the region.
The large veins (2 and 4) within the system tend to be high grade. The veins dominantly contain pyrite, chalcopyrite, galena, sphalerite, and native gold. Mineralization occurs as seams, blebs, and disseminated grains throughout the veins that rarely continues into the host diorite, and associated extensional veins are only weakly mineralized.
Elmira Deposit
The Elmira deposit is located about 2,500 ft east of the main Kensington deposit and consists of one main quartz vein with associated splays.
The Elmira deposit is about 2,200 ft long, 1,400 ft wide, ranges in thickness from 1–20 ft, and is drill tested to about -50 ft elevation.
The deposit consists of stacked, discrete, quartz veins hosted in the Jualin Diorite.
Mineralization within the veins ranges from widely-disseminated pyrite to stringers, blebs, and pyrite breccias. The mineralization does not seem to favor either the footwall or hanging wall, though it tends to be more abundant near margins or smaller internal structures within the veins.