GROWTH TEXTURES AND CHEMICAL ZONATION IN TOURMALINE FROM CU-MO-AU PORPHYRY SYSTEMS: INSIGHTS INTO FORMATION, EVOLUTION AND GEOLOGICAL IMPLICATIONS OF A UBIQUITOUS BOROSILICATE MINERAL

Tourmaline is a common accessory mineral in mineralized porphyry systems that shows promise as an indicator mineral for porphyry (Cu-Mo-Au) mineralization. It can accommodate a wide range of elements, many having partition coefficients close to unity, meaning that tourmaline as an early hydrothermal phase is capable of recording complex physiochemical changes in fluid composition throughout its crystallization history that are reflected by growth zones. As part of a broad study, tourmaline from Canadian porphyry systems (Highland Valley Copper-BC, Schaft Creek-BC, Woodjam-BC, and Casino-YT) have been examined to investigate relationships among mineralization, trace-element and textural signatures. Four types of compositional growth textures are observed which can be further subdivided into primary and secondary features. Primary growth textures include: 1) oscillatory/concentric growth zoning influenced by changes in local fluid composition, and 2) sector/polar zoning that reflect selective partitioning of elements. Secondary growth textures include: 3) patchy/irregular growth features commonly observed in recrystallized tourmaline, and 4) overgrowth or replacement features. Dissolution re-precipitation textures are present which involve the transformation of earlier-formed, Fe-rich compositions to Mg-rich ones. Further, diffusion-like textures are also occasionally observed, this being an interesting feature in that tourmaline is historically thought to exhibit little to no diffusion. Although major-element variations define the internal growth features, trace-elements also follow these same features which will help unravel formational processes. While all these textures can be observed in tourmaline from Cu-Mo-Au porphyry systems, oscillatory zonation is the most dominant, sector being very sparse or difficult to identify, minor overgrowth/multiple generations of growth, but most prominent and intriguing is the patchy/irregular growth features. This patchy growth texture is observed in other mineralized hydrothermal systems and is potentially unique making the identification and understanding of processes related to its development critical.