Trace Element Characteristics of Tourmaline in Porphyry Cu Systems: Development and Application To Discrimination

Abstract

AbstractSamples of tourmaline supergroup minerals from seven mineralized porphyry systems (Cu ±Au, ±Mo) that include Casino (Yukon, Canada), Coxheath (Nova Scotia, Canada), Donoso breccia (Chile), Highland Valley Copper (British Columbia, Canada), New Afton (British Columbia, Canada), Schaft Creek (British Columbia, Canada), and Woodjam (British Columbia, Canada) were examined to characterize their trace element signatures and to evaluate discriminating variables that distinguish them from tourmaline supergroup minerals forming in other environments. Trace element analyses, conducted via LA-ICP-MS, reveal striking similarities in trace element character among the mineralized environments examined in contrast to those considered to be barren. Key concentrations of elements (Sr, Pb, Zn, Cu, and Ga) and select element ratios (Sr/Pb and Zn/Cu) are useful discriminators. These discriminators are presented and compared to that of tourmaline supergroup minerals forming in other geologic environments. The tourmaline supergroup minerals from porphyry systems are characterized by high Sr/Pb values (avg: 297) and relatively low Zn/Cu (avg: 4.8) values. A ternary plot of Sr/Pb–Zn/Cu–Ga is effective in distinguishing porphyry-related tourmaline supergroup minerals. Compared to porphyry grains, tourmaline supergroup minerals derived from felsic pegmatites have lower Sr/Pb values (avg: 2.91) and higher Zn/Cu values (avg: 472); those from orogenic Au deposits have high Sr/Pb (avg: 193) similar to porphyries but intermediate Zn/Cu (avg: 223) and lower Ga (avg: 43 ppm); and those considered metamorphic (greenschist to amphibolite) form in the center of the ternary. An effective approach for discrimination was developed utilizing an understanding of both intrinsic and extrinsic controls. More broadly, this work has implications for using tourmaline supergroup minerals recovered from surficial sediments to identify prospective areas for further porphyry exploration.