Kensington Group - Kensington, Raven, Jualin

Alaska, USA

Main commodities: Au
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The Kensington gold deposits, Kensington, Raven and Jualin are located ~80 km northwest of Juneau, Alaska, within the Berners Bay mining district, at the northernmost edge of the Juneau gold belt.

The Juneau gold belt is a 200 km long, 15 km wide structural zone that hosts the A-J, Treadwell, Kensington and Jualin deposits and many smaller gold occurrences. This belt has a historic gold production estimated at >200 tonnes of gold, ~2 t of which were derived from the coming from the Berners Bay district. Between 2012, when modern production commenced, and 2016, 14 t of gold were produced from the Kensington deposits (Deal et al., 2016).

The Kensington deposits lie to the west of the Coastal megalinament, located between the Wrangellia Terrane and the Treadwell Formation of the Gravina Belt (Deal et al., 2016).

The veins of the deposit complex are low-sulphide, mesothermal, gold-quartz veins with strong structural controls. The bulk of the economic deposits are restricted to the Cretaceous Jualin diorite intrusion, which intrudes the Triassic basalts of the Wrangellia Terrane and lies unconformably over the Gravina Belt, although mapping indicates other host lithologies (Deal et al., 2016).

The vein systems are generally composed of shear-hosted veins or vein packages, occurring as extensional vein arrays, sheeted extensional veins and stacked en echelon shear veins, as follows:
Kensington vein system, which is made up of NNW striking shear veins which dip east, with extensional vein arrays occurring between vertically stacked shear veins;
Raven vein system, comprising two cross-cutting massive shear veins that strike WNW and north respectively and dip to the east;
Jualin vein system, that includes at least five stacked, en echelon shear veins striking NW and dipping to the east (Deal et al., 2016).

The mineralogy of the veins varies from deposit to deposit, although the district is characterised by the presence of a high tellurium-gold ratio, with much of the ore occurring as gold tellurides, the most common of which are calaverite (AuTe2) and petzite (Ag3AuTe2). Gold mineralisation in the Kensington vein system is generally associated with pyritic zones, whilst ore in the Jualin vein system is generally high fineness free gold (<100 µm in size) and is most commonly associated with galena, tennantite and, to a lesser extent, pyrite. The Raven vein system has characteristics that are a combination of both the Jualin and the Kensington systems hosting both free gold with a weak chalcopyrite association and gold tellurides associated with pyritic zones (Deal et al., 2016).

Chemical analyses of ore-bearing veins suggest a decreasing temperature of the mineralizing fluid from north to south across the vein complex (Deal et al., 2016).

Published reserve and resource estimates at the end of 2015 (Coeur Mining Inc, 2016) were:
    Proved + probable reserves - 2.563 Mt @ 6.8 g/t Au;
    Measured + indicated resources - 1.662 Mt @ 9.7 g/t Au;
    Inferred resources - 1.868 Mt @ 11.5 g/t Au.
  Resources are exclusive of reserves.

The most recent source geological information used to prepare this summary was dated: 2016.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Deal, M., Robinson, W., Beasley, L., Nerup, M. and Oduro, I.,  2016 - Kensington Property: Mesothermal Vein System with Increasing Potential: in   Roundup 2016: Association of Mineral Exploration British Columbia,   Abstracts, Technical Sessions: Wednesday, pp. 8-9.

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