Origin of massive calcite veins in the golden cross low-sulfidation, epithermal Au-Ag deposit, New Zealand

At Golden Cross, andesitic lavas and volcaniclastic rocks host epithermal v eins that formed in the shallow part (<500 m depth) of a hydrothermal syste m. Calcite is a trace mineral in precious metal-bearing quartz Veins and a common replacement mineral in the surrounding intensely altered host rocks. Late barren calcite veins crosscut the precious metal-bearing quartz-sulfi de veins and were a significant source of dilution in the underground worki ngs of the mine; where large, they also posed significant problems for grou nd control. These veins range up to 10 m in width and contain more than 99 percent calcite, predominantly as massive coarse crystals, with only trace amounts of quartz, pyrite, and clays. Fluid inclusion data indicate that much of the late barren calcite formed b etween 160 degrees and 220 degrees C, overlapping the temperature range of fluid inclusions from the precious metal-bearing quartz-sulfide veins. Ice melting temperatures range from 0.0 degrees to -1.1 degrees C. Slight vapor bubble expansion during crushing of a few calcite-hosted fluid inclusions indicates the presence of dissolved carbon dioxide. These results indicate that the hydrothermal solutions responsible for late calcite deposition wer e very dilute (<2 NaCl wt percent equiv) and contained up to approximately 2.5 wt percent dissolved carbon dioxide. The best interpretation of the ste ep Th vs. T-m cooling trend is carbon dioxide gas loss through phase separa tion combined with variable amounts of mixing. The delta(18)O composition of calcite from the altered country rock and lat e veins ranges from 3.4 to 15.4 per mil, with the bulk of the data correspo nding to equilibrium delta(18)O water compositions of -2 to -6 per mil; thi s range of compositions is 0 to greater than or equal to 2 per mil lower th an the delta(18)O compositions for the waters in equilibrium with quartz fr om precious metal bearing quartz-sulfide veins. The delta(13)C composition of calcite ranges from -3.1 to -9.0 per mil. The equilibrium delta(13)C com positions of carbon dioxide for most of these data fall between -7 and -9 p er mil. Electron microprobe analyses indicate that calcite contains less than 10 mo le percent combined Mn, Mg, and Fe. Replacement calcite and veinlet calcite show greater substitution by these elements compared to calcite in massive veins, which is nearly pure. The minor element compositions of calcite app ear to be primarily controlled by solution composition, and these constitue nts may be locally derived from the country rock. Using the knowledge from active geothermal systems of the Taupo Volcanic Zo ne as a framework for interpretation, we propose that the late massive calc ite veins were deposited from downward-moving, CO2-rich, steam-heated water . This water was heated and locally reached vapor saturation as it descende d into the former upflow zone of the hydrothermal system during waning acti vity. The reverse solubility of calcite accounts for the selective depositi on of calcite over all other common hydrothermal phases, and condensation o f steam into local ground water accounts for the slightly lower delta(18)O water values. From this we suggest that, for some low-sulfidation epitherma l prospects, the occurrence of barren calcite veins may be indicative of CO 2-rich, steam-heated waters that formed as a result of boiling.