The Kerr-Addison gold mine (Ontario, Canada) sits in the structural footwall of the crustal-scale Larder Lake – Cadillac deformation zone (LLCDZ) in the southern Abitibi greenstone belt and ranks as one of the largest past-producing gold deposits in Canada with over 11 Moz produced gold. Despite its size and significance, however, the number of geological studies available on the deposit is limited and date back to the early 1990’s. With recently renewed exploration, the volume of new geological and geochemical data offered the possibility for a much needed updated model for the deposit. Ultramafic and mafic volcanic units in the area underwent multi-stage alteration in the presence of a NaCl-KCl-CO2 hydrothermal fluid that also carried H2S, As, Sb and Au. The mineral paragenesis shows spatial and temporal progression during the chemical evolution of the system, primarily driven by pH and relative element concentration changes due to fluid-rock interaction. The diversity of alteration assemblages are explained by differences in host rock compositions interacting with the same fluid. The initial hydrothermal stage is characterized by pervasive carbonate ± sodic alteration traceable for over 700 m lateral distance in the LLCDZ footwall. In areas distant to mineralization, the advanced hydrothermal stage is present as an overprinting, pervasive to fabric-controlled chlorite alteration defining the footprint chlorite-carbonate facies. As a contrast, the initial carbonatization is followed by sulfidation of the host rock in the alteration envelope associated with mineralization, producing contrasting alteration and mineralization types as a function of Fe2+ availability. At this stage, pyrite-arsenopyrite mineralization associated with advanced albite alteration and economic gold grades (historically termed “flow ore”) occurred in the mafic volcanic units, while disseminated pyrite in the komatiite is generally sparse, and elevated gold grades, if present, are associated with As- and Sb-bearing Ni-sulfide species (gersdorffite, skinnerite). Ultramafic flow type mineralization occurs in locally derived, graphitic interflow sediments, where sulfidation of the relatively Fe-poor host is suggested to have occurred through redox reactions caused by graphite. Subsequent potassic alteration is uniformly defined by K-mica (sericite or fuchsite) in both rock types as the system became fluid buffered. Several generations of gold-hosting quartz-carbonate±albite veins developed in the potassic altered komatiite, representing the primary mineralization style in the ultramafic host (“green carbonate ore”). Although visible gold is present in the veins, it is frequently found in subgrain zones of the deformed vein material intergrown with calcite, post-dating vein formation. Excess Ca is a product of a late fluid event that can be traced along major faults in the deposit area, and may be responsible for the partial remobilization of gold. The hydrothermal system is broadly coeval with regional D2 deformation with development of S2 fabric following the onset of the sulfidation and advanced sodic alteration stage. Potassic alteration is primarily controlled by S2 foliation and gold-bearing veins are progressively deformed during the same deformation event. A magmatic source for the mineralizing fluid, as concluded by former deposit models based on the presence of albitized (“albitite”) dikes, is not evidenced in the current study.