Assessment of pyrite and arsenopyrite compositions, in situ S isotopes, and bulk Pb isotopes from the Cape Spencer gold deposit, New Brunswick, Canada

• LA-ICP-MS data indicated that at Cape Spencer gold occurs as solid solution within the crystal lattice and as nano- and micro-inclusions in pyrite. • A sequential shift towards more δ 34 S negative values may be induced by wall-rock sulphidation or phase separation. • Both the in situ S and bulk Pb isotope data indicate that ore-forming fluids could have been derived from a mixed source. • In situ δ 34 S isotope analysis can be rapidly performed using LA-ICP-QQQ-MS measurements on thin sections. The Cape Spencer gold deposit is situated proximal to the boundary between two major lithotectonic zones of the Canadian Appalachians, the Caledonia and Meguma terranes, within a fold-thrust belt in southern New Brunswick along the Minas Fault Zone. Gold mineralization occurs in quartz-dominat veins with 2–5 % sulphides hosted by the highly deformed and sheared rocks of the Millican Lake Granite and the Cape Spencer Formation. In this research, in situ S isotope and trace-element concentrations of pyrite in combination with bulk pyrite Pb isotope analyses are presented to better understand the metal sources, fluid channels, and mechanisms of ore deposition. Based on textural characteristics and trace element concentrations, five generations of pyrite from the hydrothermal stage were identified in the ore bodies: Py1a, Py1b, Py2a, Py2b, and Py3. The first pyrite generations, Py1a and Py1b (substage I), contain higher concentrations of Co and Ni than pyrite from substages II and III and have positive δ 34 S values ranging from +6.8 to +18.7 ‰. Economic concentrations of gold are associated with Py2a and Py2b (substage II), which occur as spongy and porous pyrite; Py2a pyrites contain visible gold along fractures and margins. Py1a to Py2b display an Au-Ag-Pb-Bi-Te association, also supported by the presence of hessite, petzite, sylvanite, and native bismuth in the gold ores. Pyrites from substage II display slightly less positive values of δ 34 S of +1.0 to +12.3 ‰. The last identified pyrite generation, Py3 (substage III), contains no gold and is characterized by high concentrations of As and Cu compared with pyrites from the previous generations and negative δ 34 S values of −9.8 to −3.8 ‰. This sequential shift towards more negative values from Py1 to Py2 may be induced by phase separation and wall-rock sulphidation processes, in addition to interaction with rocks of the Lancaster Formation for Py3. The generally low gold contents (<0.3 ppm) in pyrite and visible gold within fractures and along pyrite grain margins suggest post-depositional dissolution of pyrite. The influx of ore fluids, consistent with cyclic decompression, induced the removal of iron from Fe-bearing minerals, mainly specular hematite that led to an increase of the Fe in the fluid, destabilizing the A u ( H S ) 2 - complexes and promoted the depletion of S and subsequent gold precipitation. The ore-forming fluids were transported along major structures and derived from a mixed source that includes intrusive and metasedimentary rocks from the Avalonia and Meguma terranes, sharing similar characteristics to those values from the Meguma gold deposits, as indicated by both the S and Pb isotope signatures