Update: Brucejack Gold Project, BC, Canada

Overview

The Brucejack Project is a gold-silver deposit in northern British Columbia, Canada. The Brucejack project is 100% owned by Pretium Resources (TSX:PVG). This project is in the development phase of it’s life cycle, with published reserves and resources along with ongoing exploration and mine construction. The latest reserves update from December 2016 shows 8.7 million ounces of gold Proven and Probable and 31.9 million ounces of silver Proven and Probable. Commissioning of an underground mine at Brucejack is scheduled for 2017 and construction is underway.

Regional Geology

The Brucejack project is located in the “Stikinia” terrane, an ancient volcanic arc dating to early Mesozoic times and later accreted to older basement rock in the region. The entire area was subjected to the folding and thrusting of the late Cretaceous Laramide Orogeny, a period of mountain building that resulted in the Rocky Mountains of North America. A large number of similar project are located in the same geologic terrane including Seabridge’s KSM, Barrik Gold’s Eskay Creek, Novagold’s Galore Creek and Dolly Varden’s Torbrit Mine.  The area is known as BC’s “Golden Triangle”. 

Deposit Geology

The deposit occurs in highly altered volcano- sedimentary package. The package was deposited in an actively growing basin hence it is cut by numerous growth faults, which later served as conduits for mineralization. The high grade quartz-carbonate veins are found in a large, much lower grade alteration halo, of quartz-sericite-pyrite which roughly follows the contact between a conglomerate and an andesitic fragmental volcanic. In places intense silica alteration is found. It is thought that this silica fractured repeatedly leading to boiling and the deposition of extremely high grades of gold (41.1 (41,100 grams) kg per tonne)

Gold (and silver) at the Brucejack is found in structurally controlled vein systems from millimetres to tens of centimetres in size. It has been has been interpreted as a structurally deformed meso to epithermal style deposit. The extremely high grade from recent drilling the “Valley of the Kings” zone of the project would seem to support an epithermal model as these types of deposits host very high grade, lower tonnage vein systems.

Mineralization occurs as native gold and silver, electrum (a gold silver alloy) and in sulfide minerals. Much like the nearby KSM project, the mineralization appears to have occurred over several generations of hydrothermal activity. The age of mineralization ranges from 200-180 Ma (million years ago). This is because the heat from the high level porphyry deposit drove the hydrothermal system at Brucejack and when the porphyry cooled, Brucejack cooled as well.

Pretium’s nearby Snowfield Project is a lower grade gold deposit similar in style to a copper porphyry system and immediately neighbouring the KSM. The diagram to the right shows Pretium’s model for the relationship between the two deposits: Snowfield a porphyry-style system and the Brucejack a epithermal-style gold system consisting of high grade veins at shallower depths.

The whole area was deformed and metamorphosed during the Skeena Orogeny, and later brought to the surface within the last few million years as the Coast Range has risen up.

Discussion

Despite early challenges in defining and measuring the deposit, the company should be commended for bringing the Brucejack Project into the construction and development stage during what has been a difficult number of years for the mining industry. This probably would not have been possible if not for the very high grades afforded by an epithermal gold system, and a smart and thorough exploration program undertaken by company geologists. As one of the premier gold discoveries of the last 20 years Brucejack showcases the potential and promise of BC’s golden triangle. 

Further Reading

Our Previous Coverage of Pretium

Pretivm Resources (Company Website)

Feasibility Study and Technical Report NI 43-101 June 2013 (PDF)

Stikine Terrane Cu, Cu-Au & Mo Porphyry Deposits  – BC Geological Survey (PDF)