Digging Deep: In Pursuit of Porphyries
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].