Digging Deep: In Pursuit of Porphyries

By: Christa Pellett, Vice President, Minerals and Brady Clift, Manager, Minerals

Geoscience BC has funded a series of MDRU-Mineral Deposit Research Unit minerals research projects that identify minerals associated with porphyry deposits in till or bedrock samples at the surface. This research has created new tools for mineral exploration in British Columbia.

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Geoscience BC and MDRU and co-hosted a webinar about these projects, focusing on the latest research results, on October 21, 2020. Click below to watch the video on YouTube.

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The Challenge
More than 80% of British Columbia’s copper and gold endowment is hosted in porphyry copper systems. Mineral exploration in BC’s central interior, which hosts a number of BC’s copper-gold deposits, is hampered by extensive veneers of till and related glacial sediments. The creation and application of exploration techniques that can help to identify fertile pluton targets, or better to detect evidence of alteration and mineralization at depth to essentially see through the glacial cover, would be an enormous benefit for mineral exploration. While early Geoscience BC projects such as QUEST and QUEST-West applied geophysics and geochemistry to help tackle the issue from a regional perspective, the University of British Columbia’s MDRU-Mineral Deposit Research Unit recognized that the development of new exploration techniques could target porphyry deposits cheaply and effectively by identifying those minerals that are associated with porphyry deposits in till or bedrock samples collected at the surface.

Kimberlite Indicator Minerals (KIMS) have long been used in the diamond exploration industry to help narrow in on the location of kimberlite host rocks. These specific minerals, which are resistant to weathering, originate from rocks hosting the deposit, but survive erosion and glacial transport, and when coupled with an understanding of the glacial history of a region, can help point explorers “up-ice” towards a prospective deposit. Back in 2009, MDRU proposed a proof-of-concept study to Geoscience BC to examine the use of weathering-resistant minerals such as apatite, rutile, titanite and titanomagnetite to help pinpoint porphyry deposits. These were termed PIMS, or Porphyry Indicator Minerals, as they are hydrothermal or alteration-modified minerals which are associated with the formation of copper bearing porphyry deposits in BC.

Research first focused on alteration textural and chemical characteristics of PIMS and demonstrated that there is a correlation between apatite luminescence and magnetite replacement textures with the degree and intensity of porphyry alteration, and that porphyry-altered apatite and magnetite can be recognized both visually and geochemically. Given that success, follow-up work aimed specifically at BC’s alkalic type porphyry deposits.

Figure 1. Characteristics of apatite in fresh, k-silicate altered, and muscovite altered host-rock. Figure from Bouzari, F., Hart, C.J.R., Barker, S. and Bissig, T. (2012): Porphyry Indicator Minerals (PIMS): A New Exploration Tool for Concealed Deposits in south-central British Columbia; Mineral Exploration Roundup 2012 poster [September 2020].

Porphyry Fertility Indicators
Following on the success of the PIMS alteration work, Geoscience BC supported a new MDRU study examining how certain minerals show magmatic processes that were necessary to make porphyry mineralization in fertile plutons of BC. Minerals such as apatite and zircon had previously been observed to have certain characteristics where associated with mineralization, hence they could potentially be used as indicators of metal “fertility” in a given pluton. Being able to identify and distinguish porphyry-fertile from barren plutons quickly and cheaply in the earliest stages of mineral exploration would add a valuable tool to the mineral exploration toolbox in BC.

Early work focused on using titanite and apatite, where key mineralogical and geochemical characteristics were identified in BC’s Quesnel terrane. For example, it was shown that apatite mineral grains associated with fertile plutons become progressively depleted in chlorine and sulphur during crystallization of fertile plutons. This chemical change is reflected optically as well: apatite luminescence changes from brown to green as they become more depleted in chlorine and sulphur. The project resulted in the development of a porphyry fertility toolkit: a list that can be used to assess various characteristics of porphyry fertility in a given pluton.

Figure 2. Correlation of apatite texture with composition: (left) zoned apatite from the Woodjam Creek unit of the Takomkane batholith, showing a core with brown luminescence and a rim with green luminescence, numbers represent location of spots analyzed by EPMA; (right) binary diagram showing correlation of apatite luminescence in grain shown on left. Figure from: Bouzari, F., Hart, C.J.R., Bissig, T. and Lesage , G. (2018): Mineralogical and geochemical characteristics of porphyry-fertile plutons: Guichon Creek, Takomkane and Granite Mountain batholiths, south-central British Columbia (NTS 092I, P; 093A, B); Geoscience BC Report 2018-17, MDRU Publication 412, 36 p [September 2020]

Most recently, MDRU researchers have expanded these studies to focus on how the mineral zircon can be used to evaluate the fertility of a given pluton. The most recent report was released in September 2020, and demonstrates not only that zircon mineral grains’ geochemical composition can be used to help mineral explorers identify rocks that potentially host copper deposits, but that its texture and zoning can provide a fast and cost-effective method to evaluate and rank BC’s plutons.

Figure 3. Textural characteristics of zircon showing that fertile plutons have zircons with oscillatory zoning, particularly those with regular zoning patterns. Figure from Bouzari, F., Hart, C.J.R. and Bissig, T. (2020). Assessing British Columbia Porphyry Fertility in British Columbia Batholiths using Zircons. Geoscience BC Report 2020-08, MDRU Publication 450, 24p. [September 2020].