T6 Mineralogical Society of America at 100: The Many Faces of Tourmaline—From Crystallographic Complexity to Recorder of Crustal Evolution

Sunday, 22 September 2019
8:00 AM - 12:00 PM
Room 131ABC, North Building

TOURMALINE-MUSCOVITE PAIRS AS RECORDERS OF ORE-FORMING PROCESSES AT THE PANASQUEIRA W-SN-CU DEPOSIT (PORTUGAL)

Paper number: T6-6
Program: Technical Sessions
Time: 9:20 AM – 9:40 AM

Authors

Abstract

Tourmaline and muscovite are common gangue minerals in magmatic-hydrothermal ore deposits. Tourmaline chemical and B-isotope compositions are widely used to constrain ore-forming conditions, sources and processes. Muscovite occurs in the same environments as tourmaline and is a significant boron host, but its geochemical information has not been exploited to the same extent. We report on a study of B-isotope (SIMS) and chemical (microprobe, LA-ICP-MS) variations in coexisting tourmaline and muscovite from the Panasqueira W-Sn-Cu deposit in Portugal. This is the largest W-deposit in Europe and it consists of sub-horizontal, meter-thick W-Sn-Cu-bearing quartz veins hosted by clastic metasediments above the greisen cupola of a late-Variscan peraluminous granite.

Tourmaline is the main hydrothermal mineral formed during alteration of the metasediments but it is absent from the greisen. The tourmaline has intermediate schorl-dravite compositions and is chemically zoned, with increases in Fe/Mg and F from core to rim, while Ca and Al contents decrease. The B-isotope composition (δ11B) of tourmaline ranges from -7 and -11‰ in 90% of the grains, consistent with a granitic source. There is no isotopic zoning but a slight variation with distance from the ore veins. Muscovite is abundant in the greisen and it forms cm-thick vein selvages, where it is locally intergrown with tourmaline. The median δ11B values of muscovite from the greisen and selvages are the same (-17 to -18‰), whereas late muscovite has lower values (to -23‰). The B-isotopic compositions of tourmaline-mica pairs are consistent with isotopic equilibrium as the system cooled from about 500° to 300°C. Muscovite has relatively high concentrations of lithophile elements (Li, Rb, Cs, Tl, Nb, W, Sn, and Ta) compared with tourmaline, which is richer in V, Sr, Ti, and Zn. This contrast relates partly to crystal-chemical differences but also to the fact that tourmaline formed mainly by mineral replacement in the host rocks whereas muscovite formed directly from the hydrothermal fluids and thus better reflects their composition.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-334169

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