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.摘要：针对巴西东北部塞里多造山带（Seridó Belt）的Umbuzeiro Doce夕卡岩（skarn）靶区，本研究采用反射光谱学（reflectance spectroscopy）与岩石学方法开展研究。本次研究获取的61条反射光谱数据经解译后，划分出以下矿物分带：大理岩带，其特征吸收谱带位于2340 nm与2475 nm处（对应碳氧键C-O）；透闪石大理岩带，主要吸收峰位于1393 nm（羟基OH−）与2313 nm（镁羟基Mg-OH）处；石榴子石-符山石（garnet-vesuvianite）带，主要吸收峰位于2215 nm处（羟基OH−）；透辉石-角闪石带，以1153 nm处的宽吸收谱带为特征（对应二价铁Fe2+）；晚阶段硅灰石带，无特征光谱标识；局部含紫符山石大理岩，以具宝石学价值的三价铬（Cr3+）吸收特征（约548 nm与680 nm）的符山石为典型标志。在大理岩带与钙硅酸盐蚀变带的界面处，可见厘米级规模的辉钼矿集合体。本研究构建光谱指数以实现潜在找矿带的自动化识别，用于勘探钼（Mo）、钨（W）及宝石级符山石。针对大理岩带、透闪石大理岩带、石榴子石-符山石带、透辉石-角闪石带及紫符山石带，分别构建并测试了对应的光谱指数。该方法可作为具备类似Umbuzeiro Doce靶区矿物分带特征的钼、钨及紫符山石矿点或矿床的高效找矿指导工具。