The Birthday Reef was the most productive gold producer during historic mining of the Reefton goldfield on the west coast of the South Island of New Zealand. Deep exploration drill holes (up to 1.6 km long) intersected the mineralised quartz vein zone and adjacent hydrothermal alteration halo beneath the historic mine workings. The Paleozoic metasedimentary host rocks contain between 0.5 and 8 ppb Au and between 4 and 30 ppm As, and metamorphic pyrite typically contains 0.1 to 1 ppm Au in solid solution. The Au and As halo, above these background values, extends < 20 m from the Birthday Reef, and other petrographic indicators of alteration are also confined to this narrow envelope. Porphyroblasts of pyrite and arsenopyrite, with minor Au enrichment, grew across the metamorphic cleavage in the alteration halo before emplacement of the Birthday Reef and associated shearing, and reflect an earlier, late metamorphic precursor to the main gold mineralisation phase.