Tourmaline breccia pipes of the Giant Copper porphyry system: Extending the Cascadia porphyry district into southern British Columbia, Canada

ABSTRACT

The A. M. breccia is part of the Giant Copper property of British Columbia, Canada. It is the only well-defined tourmaline breccia pipe (TBP) in the Canadian Cordillera. The A. M. breccia shares similarities with other TBPs, most notably those of South America. The A. M. breccia demonstrates concentric zonation with regard to breccia texture, consisting of an outer rim of shingle breccia surrounding a fragmental breccia core. Copper grades correlate to breccia zonation, with higher Cu grades within shingle breccias relative to the fragmental core. Hypersaline fluid inclusions were identified in quartz cement within the A. M. breccia, which reflects conditions expected in porphyry copper deposits. Breccia textures, mineralization, and fluid inclusions at the A. M. breccia are nearly identical to other well-studied porphyry-related TBPs. The strong similarities suggest the A. M. breccia is a porphyry-related TBP. This interpretation encourages deep drilling to identify further mineralization within the A. M. breccia pipe as well as discovery for conventional porphyry mineralization at depth. Similarities can also be drawn to Ancestral Cascadia arc porphyry copper deposits and associated TBPs. The proximity of Giant Copper to the Ancestral Cascadia porphyry district infers a genetic relationship and the interpretation that the Giant Copper porphyry system developed from magmatism related to the Ancestral Cascadia subduction. Classifying Giant Copper as an Ancestral Cascadia porphyry system extends the district into southern British Columbia, where Giant Copper marks its northernmost extent.

RÉSUMÉ

La brèche A. M. fait partie de la propriété The Giant Copper en Colombie-Britannique, au Canada. Il s’agit de la seule cheminée de brèche à tourmaline (TBP, de l’anglais tourmaline breccia pipe) bien définie dans la Cordillère canadienne. La brèche A. M. présente des similitudes avec d’autres TBP, notamment celles d’Amérique du Sud. La brèche A. M. présente une zonation concentrique en ce qui concerne la texture de la brèche, consistant en une bordure extérieure de brèche à galets entourant un noyau de brèche fragmentaire. Les teneurs en cuivre sont en corrélation avec la zonation des brèches, les teneurs en Cu sont plus élevées dans les brèches à galets que dans le noyau fragmentaire. Des inclusions de fluides hypersalins ont été identifiées dans le ciment de quartz de la brèche A. M., ce qui correspond aux conditions attendues dans les gisements de cuivre porphyrique. Les textures de la brèche, la minéralisation et les inclusions fluides de la brèche A. M. sont presque identiques à d’autres brèches porphyriques bien étudiées liés à des porphyres. Les fortes similitudes suggèrent que la brèche A. M. est un TBP lié au porphyre. Cette interprétation encourage les forages profonds pour identifier d’autres minéralisations dans la brèche A. M. ainsi que la découverte de minéralisations porphyriques conventionnelles en profondeur. Des similitudes peuvent également être établies avec les gisements de cuivre porphyrique de l’arc Ancestral Cascadia et les TBP associés. La proximité de Giant Copper avec le district porphyrique de l’Ancestral Cascadia implique une relation génétique et l’interprétation que le système porphyrique de Giant Copper s’est développé à partir d’un magmatisme lié à la subduction de l’Ancestral Cascadia. La classification de Giant Copper en tant que système porphyrique d’Ancestral Cascadia étend le district au sud de la Colombie-Britannique, où Giant Copper marque son extension la plus septentrionale.

KEYWORDS:

• Canada
• Cascades
• Copper porphyry deposits
• Exploration vectoring
• Tourmaline breccia pipes (TBPs)

MOTS-CLÉS:

• Canada
• Cascades
• cheminées de brèche à tourmaline
• gisements porphyriques de cuivre
• vecteur d’exploration

ACKNOWLEDGMENTS

We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for grants to DDM, and Imperial Metals Corporation for funding and supporting this doctoral research. Carol-Anne Nicol is thanked for help with the fluid inclusion measurements. Lastly, Peter Baldazzi and Derek Saxton of Imperial Metals Corporation are thanked for assisting with geologic mapping and fieldwork.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors.

REVIEW STATEMENT

This article was reviewed and approved for publication by the Geological Society of the Canadian Institute of Mining, Metallurgy and Petroleum.

ETHICS APPROVAL AND CONSENT TO PARTICIPATE

There are no ethical issues associated with this manuscript.

Additional information

Funding

We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for grants to DDM, and Imperial Metals Corporation for funding and supporting this doctoral research.

Notes on contributors

W. T. Fischer

W. T. Fischer is the Principal Greenfields Geologist of North America for South32. In 2022, he earned his doctorate at Simon Fraser University in British Columbia, where his research focused on using tourmaline as a vectoring tool in mineral exploration. Specifically, Dr. Fischer’s research emphasizes identifying spectral, chemical, and isotopic variations of tourmaline in hydrothermal systems and the physical description and formation of ore-bearing breccias. He received a MSc degree in 2014 from the University of Nevada, Las Vegas, and a BSc degree in 2012 from St Norbert College, Wisconsin.

D. D. Marshall

D. D. Marshall is professor emeritus at Simon Fraser University. He received his doctorate at Universite de Lausanne in Switzerland and his MSc and BSc from Carleton University in Ottawa. His research interests involve the application of a variety of methods of pressure, temperature, and age determination to studies in ore deposits, metamorphic petrology, aqueous geochemistry, and tectonics. His research focuses upon applying various analytical techniques, specifically fluid inclusions, to ore deposits, alteration, and metamorphic rocks in order to establish the tectonic histories and refine the genetic models for ore systems.

J. Miller-Tait

J. Miller-Tait has worked in exploration, development, and mining of various deposits within North America for over 30 years. He joined Imperial Metals in 2009 as Exploration Manager and was appointed Vice President Exploration in 2018. Prior to this, Jim was Vice President Exploration with Selkirk Metals and Cross Lake Minerals.