Geological, fluid-chemical and petrochronological studies of the East Kemptville Sn(-Cu-Zn-Ag-In) deposit and its Devonian host batholith (Nova Scotia, Canada).

Abstract

Primary tin ores form in magmatic-hydrothermal systems related to highly-evolved granites enriched in lithophile elements (e.g., Li, Cs, Rb, U). Causative magmas are sourced from melting (or contamination by) enriched crust, or from melting hydrothermally-altered mantle. These granites form in large, multi-phase complexes (batholiths) and concentrate tin through fractional crystallization. The latter increases volatiles (H2O, F) in residual melts that are emplaced at shallow crustal depths, and thus the hydrothermal component to Sn-systems. Volatiles exsolve into aqueous fluids that contain soluble Sn2+. These fluids typically separate into highly-saline brines and vapours, and precipitate ore (SnO2) via oxidation of the Sn2+ . The fluids focus along fractures in the crust and their accumulation is dependent on fluid- versus lithostatic pressures. The northern Appalachian evolution included incremental emplacement of numerous batholiths and tin occurrences. The largest of these is the South Mountain Batholith (SMB) in Nova Scotia, which was emplaced deep in the crust; although is host to multiple evolved granites, the SMB contains only one significant tin deposit at East Kemptville (EK). To establish the depositional setting of EK at a greater crustal depth, this thesis analyses its geology and fluid chemistry as well as stable (O) and radiogenic (Re-Os, U-Pb, Lu-Hf) isotopes for both EK mineralization and the zircon minerals that represent the SMB. The study addresses the absence of other significant tin deposits in the SMB by evaluating the source of metal endowment. Due to its deep emplacement, the fluids at EK show no evidence of phase separation, yet abundant evidence of pressure-cycling. The latter allowed for replenishment of ore fluids during deposit formation; whereas initial tin formed via fluid-rock exchange, later ores formed from mixing with foreign fluids. This segmented hydrothermal evolution at EK is reflected by a range of mineralization ages. The zircon ages and chemistry indicate: 1) the SMB formed over 15-20 m.y. from altered mantle melts that underwent contamination by host rocks; and 2) the EK host is temporally and isotopically distinct from the SMB, and likely evolved from a lower crust-derived melt. The distinct source suggests other tin occurrences in the region share a similar origin.