Zones of advanced argillic-alteration occur in the upper parts of porphyry systems, creating a blanket of clay minerals surrounding a potentially economic copper-gold deposit at the porphyry core. These clay minerals form the largest near-surface footprint of the whole porphyry system. These zones may guide mineral explorers to the concealed porphyry copper mineralization at depth. Mineral exploration within these advanced argillic zones is traditionally difficult because of the large size of the altered areas, which can exceed 10 square kilometres.
In this study, the alteration zones in three British Columbia mineral properties that host known copper-gold porphyry deposits were characterized. This enabled a toolset to be developed that can help rapid and cost-effective exploration targeting for copper-gold porphyry related epithermal minerals in British Columbia and elsewhere.
Mineral exploration within advanced argillic zones is traditionally difficult because of the large size of the altered areas (>10 km2), the intense nature of the alteration, and the subtle mineralogical changes that can be difficult to identify. More importantly, shallow parts of advanced argillic alteration are commonly offset from their causative intrusions, which typically host porphyry copper deposits.
Considering the textural, mineralogical and geochemical trends highlighted by this helps mineral explorers to identify the presence of high-sulphidation epithermal gold and potential underlying porphyry deposits and to provide tools that point toward associated mineralization.
This project supported Geoscience BC’s Strategic Objective of ‘Identifying New Natural Resource Opportunities’ and our goal to:
- Continue regional-scale surveys that deliver large data sets in support of identifying prospective targets and increasing discovery rates of deposits.
Specifically, this project:
- Examined alteration-mineral assemblages and compositions across advanced argillic-alteration zones at three BC mineral properties (Tanzilla, Alunite Ridge and Kemess North), through a range of field and analytical techniques,
- Established and further developed pioneering techniques to map mineralization footprints both vertically and laterally, and
- Identified rock physical properties that correlate with the type and intensity of alteration.
By generating a clearer understanding of the minerals that occur above and around known copper-gold porphyry deposits in BC, this study provides data and tools to mineral explorers to find new deposits more efficiently. This will help to encourage exploration and to attract investment in BC.
The research was conducted at sites in the Northwest and North Central Regions of BC, though the findings are applicable beyond BC. Three porphyry copper-gold deposits were included in this project:
- Kemess North and Alunite Ridge in the Toodoggone area of the North Central Region; and
- Tanzilla deposit, near Dease Lake in the Northwest Region.
Geoscience BC encourages anyone planning mineral exploration work to first contact Indigenous groups in the area. The Province of British Columbia’s Consultative Areas Database can help with this (https://maps.gov.bc.ca/ess/hm/cadb/). The Association for Mineral Exploration (AME) also produces an Indigenous Engagement Guidebook.
What Was Found?
A total of 230 rock samples were collected from Kemess North, Alunite Ridge and Tanzilla copper-gold porphyry deposits. The rocks represented various alteration mineral assemblages from the zone of advanced argillic-alteration, and up to six km outside of the zone of intense alteration that surrounded each porphyry deposit. These samples were analyzed by a range of methods including:
- field observations,
- hyperspectral SWIR (short-wavelength infrared),
- scanning-electron microscope (SEM),
- magnetic susceptibility,
- porosity, and
- whole-rock major and trace element analyses.
It was found that aluminum-rich minerals (e.g. andalusite, corundum, diaspore, and topaz) occur in the zones of intense alteration and the silicification of rock occurs with pyrophyllite and/or white muscovite-clay assemblages. A new potassium-magnesium-alumina (K-Mg-Alumina) diagram is introduced in the report to map white muscovite-clay-(aluminum-rich minerals), pale-green muscovite, green muscovite-chlorite, and chlorite-dominated alteration assemblages. Another diagram (LOI-Al) is introduced that identifies the aluminum-rich mineral phases from those of the muscovite (illite)-clay assemblages. Cathodoluminescence research indicated that quartz occurring within more central parts of the advanced argillic alteration displays red luminescence and typically occurs with blue luminescent pyrophyllite and clays.
Trace element indicators are also used in the report to map the mineralization footprints vertically and laterally. These indices provide tools to compare prospects in a district, identify size and level of exposure, and direct drilling at depth. A sodium-calcium (Na-Ca) depletion index is shown that maps the intensity of two different types of alterations (phyllic and advanced argillic), providing additional tools. Measured rock physical properties are shown to correlate with the type and intensity of alteration.
Together, the observations described above provide a toolset that can help rapid and cost-effective mineral exploration of porphyry copper and related epithermal mineralization within and around advanced argillic altered rocks in BC and elsewhere.