Hydrothermal alteration and hydrologic evolution of the Golden Cross epithermal Au-Ag deposit, New Zealand

Golden Cross, located in the Coromandel peninsula, New Zealand, is a classi c example of a volcanic rock-hosted, low-sulfidation epithermal gold-silver deposit. Gold and silver ore is confined to the Empire vein system and sha llow-level stockwork. The veins are hosted by Miocene to early Pliocene and esites and dacites of the Waipupu Formation and the Waiharakeke Dacite that are unconformably overlain by the postore Whakamoehau Andesite. Hydrothermal alteration minerals display distinct spatial and temporal zona tion around veins, as defined along three cross sections (1,000 m long x 45 0 m deep), located 200 m apart, which transect the Empire vein system. Alon g these sections replacement quartz, chlorite, and pyrite are ubiquitous wi th the abundance of quartz veinlets increasing toward major veins. Replacem ent adularia envelops the Empire vein system and shallow stockwork in each section. It is coextensive with, and is variably replaced by, illite that p rogressively grades upward and laterally into a zone of interstratified ill ite-smectite that mantles the deposit. Replacement calcite and minor sideri te formed contemporaneously with, and also overprint, the above minerals, w hereas late barren calcite veins crosscut mineralized quartz veins. Kaolini te +/- pyrite veinlets, together with rare, very local alunite, formed duri ng late-stage hydrothermal activity. Clay mineral zonation is well developed. Illite occurs at depth and close t o the veins, grading outward and upward into illite-smectite, with minor sm ectite occurring similar to 600 m east of the Empire vein system. This over all zonation reflects paleothermal gradients of similar to 150 degreesC on the periphery to > 220 degreesC near the veins, consistent with the observe d TI, range of 150 degrees to 240 degreesC for fluid inclusions in quartz, platy calcite, and late barren calcite veins. Final ice-melting temperature s for inclusions mostly range from 0.0 degrees to -1.4 degreesC, correspond ing to apparent salinities of less than 2.4 wt percent NaCl equiv. Ice-melt ing temperatures combined with vapor bubble expansion on crushing indicate the presence of dissolved CO, in some platy calcite and late-stage barren c alcite. The CO? content is estimated to range from 0.35 to 3.5 wt percent, with the lower limit set by fluid inclusion vapor expansion during crushing and the upper limit by the absence of any observable clathrates. Depth est imates based on inclusions in platy calcite suggest that the shallow-level stockwork zone formed about 100 m below the paleowater table under hydrosta tic conditions. Veins and alteration minerals at Golden Cross formed in the shallow part (< 400 m) of a hydrothermal system analogous to geothermal systems in the nea rby Taupo Volcanic Zone. The assemblage of quartz, adularia, chlorite, illi te, calcite, and pyrite reflects the upflow of near-neutral pH to weakly al kaline chloride waters. Con temporaneously, steam-heated CO2-rich waters fo rmed on the margins and reacted with the wall rock to form the assemblage o f calcite, siderite, smectite, illite-smectite, and kaolinite. Eventual the rmal collapse and invasion of these peripheral CO2-rich waters into the for mer upflow zone produced late-stage barren calcite veins and an overprint o f clay-carbonate alteration minerals. At the same time, localized steam-hea ted acid-sulfate waters, generated above the water table, descended to form the late overprint of kaolinite, pyrite, and alunite.