Petrogeochemical assessment of rock units and identification of alteration/mineralization indicators using portable X-ray fluorescence measurements: Applications to the Fire Tower Zone (W-Mo-Bi) and the North Zone (Sn-Zn-In), Mount Pleasant deposit, New Brunswick, Canada

Abstract

The Mount Pleasant polymetallic deposits are located along the southwestern margin of the Late Devonian Mount Pleasant Caldera Complex in southwestern New Brunswick, Canada. The Fire Tower Zone (W-Mo-Bi) and the North Zone (Sn-Zn-In) comprise the main mineralized zones within the Mount Pleasant deposit. The rock units examined at surface in both zones are highly altered and are surface weathered to some extent. In order to classify these rock units with confidence, the Olympus X-5000 portable X-ray Fluorescence (pXRF) spectrometer was used in this study to rapidly obtain the geochemical composition of these rocks, identify lithologic discriminants, and find alteration/mineralization indicators in these areas. The immobile elements Ti, Zr, Nb, Y, and Th were selected to discriminate among various rock types. The Little Mount Pleasant Formation has the highest Ti and Zr and the Mount Pleasant Granitic Suite (MPGS, GI and GII) contains the highest Y, Nb, and Th. The McDougall Brook Granitic Suite (MBGS) has an intermediate composition between them. The element ratios of Zr/Ti, Nb/Ti, Y/Ti, and Th/Ti effectively discriminate these rock units and increase from the Little Mount Pleasant Formation, to the MBGS, and then to the MPGS.Greisen and propylitic alteration are reflected by strong depletion of K and Rb in some samples. The Fe and Mn typically are leached out of the mineralization system during the greisen alteration. Principal component analysis (PCA) was performed on the pXRF datasets. The first principal component (PC1) of the pXRF data from both the Fire Tower Zone and the North Zone mainly extracts information on Ti, Zr, Ce, Cr, V, Nb, Y, U, and Th, and thus represents different rock units. The PC2 is characterized by high-positive loadings of K, Rb, Fe, and Mn and negative loading of As and Mo. It is interpreted as the W-Mo mineralization indicator showing by the enrichment in Mo and depletion of K, Rb, Fe, and Mn associated with quartz+topaz+sericite+fluorite alteration. The PC3 consists of positive loadings of Sn, Zn, Cu, and S and slightly negative loadings of Sr and Ba, and thus represents the Sn-Zn mineralization in this area.