Recognizing Tourmaline in Mineralized Porphyry Cu Systems: Textures and Major-Element Chemistry

Abstract

Samples of tourmaline supergroup minerals from seven mineralized porphyry systems (Cu, ±Au, ±Mo), including Casino (Yukon Territory, Canada), Coxheath (Nova Scotia, Canada), Donoso breccia-Los Bronces (Chile), Highland Valley Copper (British Columbia, Canada), New Afton (British Columbia, Canada), Schaft Creek (British Columbia, Canada), and Woodjam (British Columbia, Canada), were examined at a variety of scales to evaluate their relationships with mineralization. Data from paragenetic observations show that tourmaline supergroup minerals are generally early hydrothermal minerals that predate both mineralization and alteration (e.g., overgrown and crosscut by). In general, tourmaline supergroup minerals occur as sub- to euhedral crystals that are black in hand sample and can be found in a variety of mineralized settings (including breccias, veins, and disseminations) and alteration assemblages (including potassic, sodic-calcic, phyllic, propylitic, and argillic). As tourmaline supergroup minerals are physically and chemically resilient and occur throughout a given porphyry system, they are comprehensive recorders of the type and extent of various geochemical processes that exist during the complex genesis of these systems. Data from BSE imaging shows two primary zonation types: concentric and sector. These are interpreted to reflect conditions of rapid crystallization and disequilibrium. Results from SEM-EDS analyses show that most tourmaline supergroup minerals are dravite (∼80% of grains), with the remainder being primarily classified as schorl. Porphyry tourmaline supergroup minerals exhibit remarkably consistent ∼2.0 apfu Mg values (range: 0.69–2.89), with the majority of tourmaline supergroup minerals plotting along the oxy-dravite–povondraite trend, reflecting the predominance of the Al3+ ↔ Fe3+ substitution at constant Mg values. This pattern starts from the povondraite side (reflecting the oxidizing nature of early porphyry mineralizing fluids) and trends toward oxy-dravite as a porphyry system evolves, a feature that can, in turn, be interpreted to reflect relative emplacement depths. In mineralized porphyry systems, tourmaline supergroup minerals exhibit remarkably similar physical and chemical characteristics among the systems examined, suggesting that the source and geological processes must be extraordinarily similar. Unfortunately, these characteristics are not unique to porphyry systems and such observations should be integrated with additional data, such as trace element mineral chemistry, to effectively discriminate tourmaline supergroup minerals that have formed in porphyry systems.