Spectral characterization of the Umbuzeiro Doce skarn target (Brazil): insights for the exploration of a W-Mo-gemological vesuvianite system

Abstract

The Umbuzeiro Doce skarn target in the Seridó Belt (NE Brazil) was studied using reflectance spectroscopy and petrography. Interpretation of the 61 reflectance spectra obtained in the study established the following zones: marble zone, with diagnostic absorption features at 2,340 and 2,475 nm (C-O); tremolite marble zone, with main absorption at 1,393 (OH⁻) and 2,313 nm (Mg-OH); garnet-vesuvianite zone, with the main absorption at 2,215 nm (OH⁻); diopside-hornblende zone, marked by a broad absorption at 1,153 nm (Fe²⁺); late wollastonite zone, which lacks a diagnostic spectral signature; and a local violet vesuvianite-bearing marble, characterized by potential gemological vesuvianite marked by Cr³⁺ absorption features (~548 and 680 nm). Centimeter-sized molybdenite aggregates occur at the interface of the marble and calc-silicate alteration zones. Spectral indices were produced to automate identification of potential zones to explore Mo, W, and gemological vesuvianite. Indices for the zones of marble, tremolite marble, garnet-vesuvianite, diopside-hornblende and violet vesuvianite were produced and tested. This methodology appears as a powerful exploratory guide for Mo, W, and violet vesuvianite occurrences or deposits that show a mineral zoning like the Umbuzeiro Doce target.

Keywords:
reflectance spectroscopy; spectral indices; hydrothermal alteration; violet vesuvianite; metal exploration