LA-ICP-MS analysis of volatile trace elements in massive sulphides and host rocks of selected VMS deposits of the Bathurst Mining Camp, New Brunswick: methodology and application to exploration

Abstract

The application of volatile trace element contents of pyrite, chlorite, and white mica in selected polymetallic Zn-Pb-Cu-Ag volcanogenic massive sulphide (VMS) deposits of the Bathurst Mining Camp, northern New Brunswick (Armstrong A, Brunswick No. 12, Canoe Landing Lake, Halfmile Lake Deep Zone, Key Anacon, Louvicourt, and Restigouche) as a vectoring tool in VMS exploration has been investigated. In situ volatile trace element contents (As, Cd, Hg, In, Sb, Tl, etc.) were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Pyrite is a ubiquitous sulphide in altered host rocks to VMS mineralization and can accommodate a wide range of volatile trace elements. Most analyzed pyrite grains are arsenian with As contents up to 7.3 wt%, Sb (up to 2910 ppm), Tl (up to 4110 ppm), Au (up to 73.2 ppm), Hg (up to 220 ppm), In (up to 49 ppm), and Cd (up to 270 ppm). Variations in the spot chemical compositions of pyrite in stratigraphic profiles of the examined deposits reveal distinct volatile trace element features. Pyrite in the footwall alteration zones typically displays systematically increasing volatile trace element contents with decreasing distance stratigraphically upward to the ore horizon. However, in the hanging-wall alteration zones, there are no consistent pyrite chemical compositional trends. Pyrite in the hanging wall of some deposits, such as Brunswick No. 12, Canoe Landing Lake, Restigouche, and Key Anacon, show high As in the upper portions and, to a lesser extent, higher abundances of other volatile trace elements. Volatile element contents of chlorite and white mica (muscovitic to phengitic in composition) indicate that these elements are crystal lattice-bound. White mica typically contains As (up to 1.01 wt%), Sb (up to 4750 ppm), Tl (up to 698 ppm), In (up to 563 ppm), Hg (up to 67 ppm), Cd (up to 83 ppm), and Bi (up to 185 ppm). In comparison to white mica, chlorite is preferentially enriched in Cd (0.07-420 ppm) and Bi (0.02-185 ppm). The spot LA-ICP-MS analyses of chlorite and white mica demonstrate that volatile trace element contents of hanging-wall- and footwall-altered rocks increase with decreasing distance toward mineralization. Our data indicate that pyrite, chlorite, and white mica are the residence sites of significant abundances of volatile trace elements, in particular As, Sb, Tl, Bi, and to lesser extent Cd, In, and Hg. Therefore, these minerals can serve as the primary widespread dispersal proxies for volatile elements in the studied drillholes. Systematic trends in volatile trace element distribution patterns in pyrite, chlorite, and white mica can be used to vector toward VMS mineralization in the Bathurst Mining Camp. This methodology may be applicable in other areas where polymetallic deposits occur, and may complement other geochemical and geophysical exploration methods.