Telescoped porphyry‐style and epithermal veins and alteration at the central Maratoto valley prospect, Hauraki Goldfield, New Zealand

Abstract

At the central Maratoto valley prospect, southern Coromandel Peninsula, New Zealand, andesite flows and dacite breccias host rare porphyry‐style quartz veins that are telescoped by widespread epithermal veins and alteration. Early porphyry‐style quartz veins, which lack selvages of porphyry‐style alteration, host hypersaline fluid inclusions that contain several translucent daughter crystals, including halite and sylvite. Overprinting epithermal veins and alteration are divided into two stages. Main‐stage epithermal alteration and veins are characterised by the successive deposition of pyrite, quartz, and ankerite‐dolomite veinlets coupled with intense alteration of the wall rock to quartz, illite, interlayer illite‐smectite (≤10% smectite), chlorite, pyrite, ankerite, and dolomite. Late‐stage epithermal veins and alteration are characterised by the formation of calcite and siderite veinlets, coupled with overprinting of the wall rocks by both these minerals. Multiphase fluid inclusions in a porphyry‐style quartz vein formed at temperatures >400°C and trapped hypersaline magmatic fluid. Lower temperature secondary liquid‐rich inclusions in the porphyry‐style quartz vein homogenise between 283 and 329°C and trapped a dilute fluid with <1.8wt% NaCl equivalent. Inclusions in later epithermal quartz and calcite veins homogenise between 240 and 280°C (av. 260°C) and trapped a dilute fluid with apparent salinities of <2.9 wt% NaCl equivalent. Based on homogenisation and salinity data, secondary inclusions in porphyry‐style quartz veins may have formed 700–950 m deeper than telescoping epithermal veins. Main‐stage epithermal ankerite and dolomite have δ18O(VSMOW) values of 13.5–18.1‰, whereas late‐stage epithermal calcite has δ18O(VSMOW) values of 3.1–5.1‰. Calculated isotopic compositions for the fluid in equilibrium with ankerite‐dolomite and calcite at 260°C, averages 6 and ‐3‰, respectively. The enriched value for main‐stage ankerite‐dolomite suggests formation from waters that underwent significant water‐rock exchange, whereas isotopically lighter water that formed late‐stage calcite underwent little water‐rock interaction. We propose a three‐stage model to explain telescoped veins and alteration styles at the central Maratoto valley prospect area. Porphyry‐style quartz veins were the first to form from hot hypersaline multi‐cation magmatic fluids. These are telescoped by later widespread epithermal veins and alteration following descent of the paleowater table possibly due to rapid erosion or sector collapse of a volcanic edifice. Main‐stage epithermal alteration and deposition produced quartz, chlorite, illite, interlayered illite‐smectite, pyrite, and isotopically heavy ankerite‐dolomite from deeply circulating upwelling alkali chloride waters. Late‐stage collapse of the hydrothermal system resulted in the formation of overprinting calcite and siderite from isotopically lighter descending marginal steam‐heated CO2‐rich waters.