Pine Point is a major carbonate-hosted Pb-Zn deposit, but key questions remain regarding its formation. The Pb-Zn mineralization is commonly found in spatial association with marcasite, but the genetic relationship that gives rise to this association is unclear. Here, we provide new insights into the geochemical processes behind the observed paragenetic associations. We present petrographic, geochemical and fluid-inclusion data that indicate early marcasite deposition, followed by a series of hydrothermal replacement reactions. Collectively, these replacement reactions suggest that an early iron-rich stage of mineralization gives way to an iron-poor stage later in the paragenesis. These results furthermore suggest that in at least some cases, early marcasite is partially consumed during the later iron-poor stage, providing a convenient, in-situ source of reduced sulfur to feed the later mineralization.