Microchemical signatures of gold in copper-gold porphyries, BC

Project code: 2014-007

Project type: Minerals and Mining

Proponent: Robert J. Chapman and Thomas J. Mileham

Project location: BC copper mines: Mount Milligan, Mount Polley, Ajax and Copper Mountain


Summary
Tiny gold grains smaller than the head of a pin were collected from four copper-gold mines across central British Columbia and found to have unique chemical characteristics. This is valuable information for explorers searching for new copper deposits.

The Need
Demand for copper is expected to soar as much as 43% by 2035 as the transition to a 'green' economy gathers pace (link to ref). Finding new copper deposits and mining them responsibly provides essential materials for this transition. Mineral explorers need reliable earth science information and knowledge to quickly and efficiently find these complex deposits that are often created in multiple phases over millions of years.

Project Goals
The project was designed to:

  • Develop innovative field techniques for sample collection, adding a new tool to the explorer's toolbox for companies looking for giant copper deposits in BC and around the world
  • Analyze the chemical composition on detrital/placer gold grains and their potential as 'pathfinders' for alkalic porphyry copper deposits
  • Match the chemical characteristics of the grains to well-known, economically-significant alkalic copper-gold porphyries in BC; Mt Milligan, Mt Polley, Ajax, and Copper Mountain
  • Explore how the presence of palladium and mercury in gold grains can discriminate between different deposit types
  • Stimulate on-the-ground exploration activities that could lead to new mineral discoveries.

Project Benefits
In British Columbia, numerous copper-gold mineral deposit types co-exist along the 'spine' of the province, known as the northern Cordillera, stretching from BC, through Yukon and into Alaska. When gold grains form in these different deposits, they typically include trace amounts of other metals such as silver, copper, mercury and palladium. Matching the 'geochemical DNA' contained in the tiny gold grains to the type of deposit they came from creates a valuable tool that helps mineral explorers to find the source of the gold and, potentially, new mineral deposits. The search for new copper-gold deposits brings mineral exploration investment to the province, with the potential for new jobs and mining revenue. Understanding the characteristics of these resources also helps communities with economic development and land planning.

Survey Area
Gold grains were collected from four mine sites, each around 200 million years old, alkalic copper-gold porphyry deposit within the Quesnel arc terrane in BC:

  • The Mt Milligan open pit copper-gold mine is located approximately 155 kilometres northwest of Prince George between the forestry--based communities of Mackenzie (population approximately 5,200) and Fort St. James (population approximately 1,900) in central BC. The Mount Milligan deposit is a tabular, near-surface, alkalic copper-gold porphyry and the mine has a 22-year mine life. Operated by: Centerra Gold.
  • The Mt Polley open pit copper-gold mine is located in south-central BC, 27 kilometres southwest of Likely and 56 kilometres northeast of Williams Lake. Mine employees live in the nearby communities of 150 Mile House, Beaver Valley, Big Lake, Morehead Lake, Hydraulic, Little Lake and Likely. The Mount Polley copper-gold porphyry deposit has a nine-year mine life. Operated by: Mount Polley Mining Corporation (MPMC), a subsidiary of Imperial Metals.
  • The Ajax copper-gold mine is under development close to Kamloops in southern BC. It has an estimated 23-year mine life and is operated by KGHM.
  • The Copper Mountain open pit copper-gold mine is located about 20 kilometres south of Princeton in southern BC. It is a alkalic porphyry with an expected mine life of 17 years.

How was the data collected?
Natural gold grains exhibit several features which may be used in their classification and usually comprise a metal alloy (mix of metals) which can be observed and used as an indicator, or pathfinder, of a particular mineralizing event. These porphyry copper indicator minerals (or PIMS) collected during routine stream sediment samples can be used as a predictive exploratory tool.

In 2014, Dr Rob Chapman and masters candidate Thomas Mileham from the University of Leeds (UoL) visited the mine sites in BC to collect drill core and rock samples, and panned samples from nearby streams. The gold grains from the samples were isolated and analysed at UoL.

In the laboratory, the grains were mounted in resin and polished to a specified thickness. Under a Scanning Electron Microscope (SEM), the polished grains were scanned with a focused bean of electrons to identify the characteristics and composition of each grain. Tiny bubbles of fluid trapped in the gold grains -- fluid inclusions -- were also probed with a microbeam of electrons to determine the chemical composition.

What was found?
Although the use of detrital gold as an exploration tool has been examined before, this study was the first semi-quantitative attempt to characterize the mineralogical signatures of detrital gold derived from different source styles based on a large sample set. The study generated a template to distinguish gold derived from alkalic porphyries from other deposit types.

Final Deliverables

  • Geoscience BC Report 2016-06: Detrital gold Grains as Indicators of Alkalic Copper-Gold Porphyry Mineralization - Released November 2, 2017 (PDF, 3MB)

The Detail

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