Open AccessInvestigation of tourmaline characteristics in bedrock and surficial sediment samples from two Canadian porphyry copper systems
Author(s) -
C E Beckett-Brown,
Andy McDonald,
M B McClenaghan,
A Plouffe,
T Ferbey
Publication year - 2021
Language(s) - English
Resource type - Reports
DOI - 10.4095/327989
Subject(s) - tourmaline , geology , geochemistry , trace element , bedrock , mineralogy , geomorphology
Colour, texture, and chemistry were used to determine the source of tourmaline grains in local surficial (detrital) sediments from two Canadian porphyry systems (Woodjam and Casino). Tourmaline from porphyry bedrock is generally dark brown to black. Crystalsfrequently exhibit oscillatory and sector zoning, with overgrowth, irregular, and patchy patterns. Major-element analyses show ranges from schorl (Fe2+-rich; 0.04-3.07 apfu, av. 0.76) to dravite (Mg-rich, av. 2.00 apfu), with a minor povondraite (Fe3+) component. The alkali- and alkaline-earthelement concentrations are relatively constant, with Na>Ca>_ and OH->O2- dominant. Tourmaline in porphyry systems is characterized by high Cu (tens of parts per million) and Sr (hundreds of parts per million), and low Pb (<10 parts per million) and Zn(tens of parts per million). Detrital tourmaline occurs as euhedral grains and contains similar Fe and Mg concentrations and textural zonation patterns to porphyry tourmaline. Its trace-element content shows similarities but with larger ranges of values, likely related to the presence ofnon-porphyry detrital tourmaline. Overall, a combination of physical and chemical characteristics (oxy-dravite-povondraite trend, high Sr, low Zn and Pb) of tourmaline grains are useful in discriminating between porphyry- versus non-porphyry-derived (or related) tourmaline in both bedrock andsurficial sediments.