The Coromandel Volcanic Zone [CVZ] was an active continental volcanic arc between 18 – 1.9 Ma that was responsible for bimodal andesitic–dacitic–rhyolitic volcanism. Following the emplacement of these volcanic formations, some became locally hydrothermally altered to generate ≈ 50 low sulphidation Au–Ag deposits within the CVZ (Hauraki Goldfield). The Martha Hill deposit is a rift–low sulphidation adularia–sericite gold deposit situated in the township of Waihi, New Zealand. Notably, the origin of a black breccia appearing in core logs at the Martha Hill deposit was previously unknown. The focus of this study was to identify the origin of the black breccia through analyses of three separate drill cores (pertaining to the black breccia), and to discover its significance in the paleo–geothermal field at Martha. Analytical techniques included core logging, microscopy, x–ray diffraction [XRD], x–ray flourescence for geochemical majors [XRF] and scanning electron microscopy [SEM]. Initial core logging and analyses recognised 10 new facies relating to the black breccia. These recognised facies fell under three separate categories; two facies for the weakly altered, five for the highly altered and three for the breccia facies. One of the weakly altered facies best reflects the andesite country rock of the Waipupu Formation. The other weakly altered facies is a foreign clast inclusion of a breccia deposited during the original emplacement of the andesite lavas. The five highly altered facies are related to the varying degrees and overprinting events of the paleo–hydrothermal fluid. One of the breccia facies related to the original emplacement of andesite lava as an autobreccia, whilst the other breccia facies was the product of hydraulic fracturing in the sub–surface. Based on core logging, petrographical and geochemical analyses, the black breccia was discovered to be related to diatreme formation. Diatreme formation can vary among deposits due to the post–eruptive hydrothermal environment (high versus low sulphidation) and the nature of the mechanism utilised to generate the diatreme (phreatomagmatism or hydrothermal eruption). However, the poorly sorted, matrix supported nature of the black breccia that was observed during core logging is widely associated with volume expansion (a hydrothermal brecciation mechanism responsible for diatreme formation). Diatreme formation is the result of an increase in temperature or depressurisation. Facies that may suggest the presence of a feeder dike responsible for generating volume expansion and phreatomagmatism was not observed during core logging, but may be present at greater depths or in other drill core. A more likely scenario for the formation of black breccia was due to depressurisation and volume expansion in the near surface due to hydrothermal eruptions. Indeed, initial steam bursts may have ejected cover materials at the surface resulting in depressurisation and boiling in the near–surface. Brecciation and fragmentation would have likely continued and propagated further downward until the steam supply rate was insufficient to further brecciate and uplift rock fragments. Carbonate veins (calcite) that were present during core logging indicates that the paleo–geothermal field reset after the formation of the black breccia and associated diatreme. The presence of adularia indicated by XRD analyses within the black breccia matrix, signifies that the paleo–hydrothermal fluid temperatures after the emplacement of black breccia was ≈ 180 – 350°C.