Differentiating tourmaline species via chemistry and reflectance spectroscopy at the Giant copper porphyry deposit and associated tourmaline breccia pipes; testing tourmaline as a mineral vector

• Published in: Economic geology and the bulletin of the Society of Economic Geologists Vol. 118; no. 4; pp. 883 - 902
• Main Authors: Fischer, Bill T, Marshall, Daniel D, Hanchar, John M, Riley, Dean, Hiebert, Scott
• Format: Journal Article
• Language: English
• Published: Economic Geology Publishing Company 01-06-2023
• Subjects: airborne methods; breccia pipes; British Columbia; Canada; Canadian Cordillera; copper ores; Economic geology; electron probe data; hyperspectral analysis; ICP mass spectra; intrusions; mass spectra; metal ores; mineral composition; mineral deposits, genesis; mineral exploration; mineralization; North America; North American Cordillera; pipes; porphyry copper; reflectance; remote sensing; ring silicates; satellite methods; silicates; spectra; tourmaline group; Western Canada
• ISSN: 0361-0128; 1554-0774
• DOI: 10.5382/econgeo.4987

The A.M. breccia is part of the Giant Copper porphyry deposit in southern British Columbia. It is the only well-defined zoned tourmaline breccia pipe in the Canadian Cordillera. Tourmaline is a common alteration mineral within the A.M. breccia and is spatially associated with Cu mineralization. Observed changes in tourmaline chemistry range from alkali (schorlitic-dravitic) to calcic (feruvitic-uvitic). Tourmaline subspecies vary based on their spatial location within the A.M. breccia. Tourmaline outside of the pipe contains higher concentrations of Mg, whereas tourmaline preferentially incorporates Fe within the pipe. These chemical variations are indistinguishable in hand specimens. Spectral reflectance data were collected from 587 tourmaline grains to determine if discerning chemical changes in tourmaline can be made field-based and thus more cost-effective. Spectral reflectance differentiates tourmaline associated with mineralization and breccia textures from tourmaline occurring distal to the pipe contact or within barren tourmaline breccia pipes. Fe-rich tourmaline within the A.M. breccia shows spectral characteristics of end-member schorl (Fe-rich) spectra. Tourmaline distal to the A.M. breccia and within barren pipes demonstrates spectra of end-member dravite (Mg-rich). This grouping suggests that tourmaline subspecies can be inferred by spectral reflectance, enhancing the efficiency of tourmaline as a mineral vector. Tourmaline was also identified via airborne spectral surveys. However, the airborne spectral survey did not identify the end-member spectral properties identified by in situ analysis. Airborne spectral surveys can rapidly identify tourmaline breccia pipe exposures and expedite early stages of exploration in ore districts where tourmaline is a known gangue mineral.