Tourmaline at diagenetic to low-grade metamorphic conditions: Its petrologic applicability

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Tourmaline developed under low-temperature conditions (<~300. °C) typically has distinctive morphological and chemical characteristics that reveal significant information about the environments in which they crystallize. Under diagenetic conditions, tourmaline can develop as overgrowths on detrital tourmaline cores or as new, authigenic crystals. Tourmaline, as monopolar crystal overgrowths, is found in sandstones, shales, conglomerates, dolostones and limestones nucleating exclusively at the positive end of the c axis (. +c pole) on the detrital grain, a feature related to the mineral's polar nature. These overgrowths tend to have compositions dominated by X-site vacant tourmaline species (e.g. foitite) and can be linked to growth in aqueous environments low in Na, with other chemical characteristics related to the host rocks. As temperature increases, tourmaline overgrowths develop at both the +c pole and -. c pole, but continue to exhibit a significant degree of morphological and chemical asymmetries. These differences at either pole diminish as a function of increasing temperature such that it can be used as an intramineral geothermometer.Authigenic tourmalines without detrital tourmaline nuclei are found in many sedimentary or weakly metamorphosed metasedimentary rocks as well as fluid-dominated geothermal systems. Here, tourmalines are commonly foititic, but have Mg-Fe contents influenced by the host rock composition. Low-temperature tourmalines developed in hypersaline environments (e.g. salt domes) have distinct chemical characteristics - high Na, Fe 3+ and WO, and follow a trend between "oxy-dravite" and povondraite (O-P trend). This compositional trend can be used as a guide to indicate tourmaline development within low-temperature hypersaline environments and appears to be diagnostic of an oxidizing environment with high Na in the fluid and high salinity with the resulting reduction of the activity of H 2O. The O-P trend is retained in some meta-evaporite deposits that are relatively high grade and is found in tourmaline from some types of economically important metal deposits. Although tourmalines in low temperature environments are largely overlooked, they have potential to be a useful indicator of the chemical and thermal environments in a regime largely absent of such indicators. © 2012 Elsevier B.V.

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