THE O-P TREND IN TOURMALINE: INDICATOR OF HIGH SALINITY AND SELECT METAL DEPOSITS

The occurrence of a compositional trend between the oxy-tourmaline species oxy-dravite and povondraite (the so-called O-P trend) appears to reflect mineral formation in a high-salinity environment and may serve as a guide to some types of metal deposits. Tourmalines developed in low-temperature evaporite deposits typically have compositions that are relatively Na-, Fe³⁺- and W-site O-rich reflecting their chemical environment of formation – and typically follow the O-P trend. Two localities that illustrate chemical characteristics of tourmaline associated with evaporite deposits are (1) the cap rock of a salt dome at the Challenger Knoll, Gulf of Mexico and (2) the brecciated, weakly metamorphosed caprock of a salt dome in the central Andes, Bolivia. This tourmaline trend can be used as a guide to the type of chemical signals tourmaline might develop within low-temperature hypersaline environments and appears to be diagnostic of an oxidizing environment with high Na and high salinity in the fluid with the resulting reduction of H₂O activity. The O-P trend may also be preserved in evaporite deposits that have been subjected to medium-to-high grade metamorphism such as the amphibolite facies Neoproterozoic meta-evaporites from Namibia and the upper amphibolite facies Paleoproterozoic metaborate deposit from Liaoning Province, China. This O-P trend, that appears to be characteristic of tourmalines developed in hypersaline evaporitic environments, has also been found in other settings. These include some types of economically important metal deposits such as the Cu-Mo-Au deposits of the Proterozoic Coxheath porphyry granite system of Nova Scotia, Canada; the Cu-Au deposits in the Ghagri area, Rajasthan, India; and the porphyry-style Darasun Au deposits in Transbaikalia, Russia. The O-P trend in tourmaline is a potential prospecting guide for several types of Cu, Mo, Sn and Au deposits as well as an indicator of hypersaline environments.