Origin of Tin Mineralization in the Sullivan Pb-Zn-Ag Deposit, British Columbia: Constraints from Textures, Geochemistry, and LA-ICP-MS U-Pb Geochronology of Cassiterite

Abstract

Abstract Textural, geochronological, and geochemical data are presented here for cassiterite from the giant (149.7 million tonnes [Mt]) Mesoproterozoic Sullivan Pb-Zn-Ag deposit, which has been subjected to several tectonothermal events. These data provide constraints on the age and origin of the tin concentrations and new insights into related base metal mineralization. Sullivan is rare among sediment-hosted, stratiform Pb-Zn-Ag deposits in having high tin contents in ore (up to 2.5 wt %; avg 310 ppm Sn). Cassiterite occurs in all facies of this deformed and metamorphosed deposit, including (1) high-grade veins with arsenopyrite and pyrrhotite, (2) bedded Pb-Zn-Ag ores, (3) massive pyrrhotite, (4) footwall and hanging-wall tourmalinites, and (5) other altered wall rocks. New in situ U-Pb dates for Sullivan cassiterite obtained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are modeled by a multicomponent-based algorithm that yields three age peaks: 1475 ± 4 Ma (51% of the data), 1366 ± 10 Ma (25%), and 1074 ± 7 Ma (24%). These dates are attributed, respectively, to primary tin mineralization at ca. 1475 Ma, the East Kootenay orogeny at ca. 1370 to 1300 Ma, and the Grenvillian orogeny at ca. 1100 to 980 Ma. Based on the presence and local abundance of cassiterite in all ore and ore-related rocks at Sullivan, the U-Pb date of 1475 ± 4 Ma reported here represents the first direct age for ore mineralization in the deposit. Occurrence of texturally discordant rims on Sullivan cassiterite grains having U-Pb dates coeval with the East Kootenay and Grenvillian orogenies suggests that these young dates reflect dissolution-reprecipitation processes associated with channelized metamorphic fluid flow. LA-ICP-MS U-Pb dates obtained on low-U (<10 ppm) cassiterite also indicate that U-Pb dates for cassiterite from other metamorphosed deposits should be viewed with caution and not assumed to record an age of primary tin mineralization. Aqueous transport conditions for tin are evaluated to gain insights into the cassiterite mineralization at Sullivan. Based on fO2-pH topology of aqueous tin species at 250°C, tin transport was dominated by an SnCl3− complex at fO2 of about –40 and pH of <4.0, conditions that were constrained, respectively, by widespread occurrence of pyrrhotite in deep footwall siliciclastic metasedimentary rocks of the host Aldridge Formation and by release of CO2 from shallow mafic sills and resulting formation of carbonic acid in condensed brine. The low fO2 value also reflects inferred production of CH4 from heating of organic matter in the sediments during emplacement of these sills. Based on a fluid pH restriction of <4.0 and a requirement for sparse or no K-feldspar in the source, the tin likely derives from previously altered Lower Aldridge strata. This model relies on the early diagenetic dissolution of K-feldspar from these sediments by basinal brines, followed by interaction with a later, more acidic hydrothermal fluid generated during the emplacement of large mafic sills in the shallow subsurface that leached tin from accessory minerals such as titanite in siliciclastic sediments of the Lower Aldridge Formation. Mass balance calculations suggest that derivation of the tin from this sedimentary source (avg 2.0 ppm Sn) required ~40 km3 and a cylinder diameter of 3.2 km (height 5.0 km) in order to supply the 0.1 Mt of tin contained in the deposit. The presence of mafic sills in the footwall of several other tin-bearing, sediment-hosted, stratiform Pb-Zn-Ag deposits and in modern, tin-rich, sediment-hosted sulfide deposits in the northeast Pacific Ocean suggests that siliciclastic marine basins that contain mafic sills—with or without stratiform sulfide deposits—should be evaluated for possible tin mineralization.