Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (11) Citing Articles via Google Scholar Google Scholar Articles by Simmons, S. F. Articles by Mauk, J. L. Search for Related Content GeoRef GeoRef Citation

Origin of Massive Calcite Veins in the Golden Cross Low-Sulfidation, Epithermal Au-Ag Deposit, New Zealand

Stuart F. Simmons

Geothermal Institute and Geology Department, University of Auckland, Private Bag, 92019, Auckland, New Zealand

Greg Arehart^*

Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand

Mark P. Simpson

Geology Department, University of Auckland, Private Bag, 92019, Auckland, New Zealand

Jeffrey L. Mauk

Geology Department, University of Auckland, Private Bag, 92019, Auckland, New Zealand

Corresponding author: e-mail, sf.simmons{at}auckland.ac.nz

At Golden Cross, andesitic lavas and volcaniclastic rocks host epithermal veins that formed in the shallow part (<500 m depth) of a hydrothermal system. 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-sulfide veins and were a significant source of dilution in the underground workings of the mine; where large, they also posed significant problems for ground 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 between 160° and 220°C, overlapping the temperature range of fluid inclusions from the precious metal-bearing quartz-sulfide veins. Ice melting temperatures range from 0.0° to –1.1° 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 were very dilute (<2 NaCl wt percent equiv) and contained up to approximately 2.5 wt percent dissolved carbon dioxide. The best interpretation of the steep T_h vs. T_m cooling trend is carbon dioxide gas loss through phase separation combined with variable amounts of mixing.

The ¹⁸O composition of calcite from the altered country rock and late veins ranges from 3.4 to 15.4 per mil, with the bulk of the data corresponding to equilibrium ¹⁸O water compositions of –2 to –6 per mil; this range of compositions is 0 to > 2 per mil lower than the ¹⁸O compositions for the waters in equilibrium with quartz from precious metal bearing quartz-sulfide veins. The ¹³C composition of calcite ranges from –3.1 to –9.0 per mil. The equilibrium ¹³C compositions of carbon dioxide for most of these data fall between –7 and –9 per mil.

Electron microprobe analyses indicate that calcite contains less than 10 mole 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 appear to be primarily controlled by solution composition, and these constituents may be locally derived from the country rock.

Using the knowledge from active geothermal systems of the Taupo Volcanic Zone as a framework for interpretation, we propose that the late massive calcite veins were deposited from downward-moving, CO₂-rich, steam-heated water. This water was heated and locally reached vapor saturation as it descended into the former upflow zone of the hydrothermal system during waning activity. The reverse solubility of calcite accounts for the selective deposition of calcite over all other common hydrothermal phases, and condensation of steam into local ground water accounts for the slightly lower ¹⁸O water values. From this we suggest that, for some low-sulfidation epithermal prospects, the occurrence of barren calcite veins may be indicative of CO₂-rich, steam-heated waters that formed as a result of boiling.

This article has been cited by other articles:

				E. M. Bongiolo, P. Patrier-Mas, A. S. Mexias, D. Beaufort, and M. L. L. Formoso SPATIAL AND TEMPORAL EVOLUTION OF HYDROTHERMAL ALTERATION AT LAVRAS DO SUL, BRAZIL: EVIDENCE FROM DIOCTAHEDRAL CLAY MINERALS Clays and Clay Minerals, April 1, 2008; 56(2): 222 - 243. [Abstract] [Full Text] [PDF]

				J. N. Moore, R. G. Allis, M. Nemcok, T. S. Powell, C. J. Bruton, P. E. Wannamaker, I. B. Raharjo, and D. I. Norman The evolution of volcano-hosted geothermal systems based on deep wells from Karaha-Telaga Bodas, Indonesia Am J Sci, January 1, 2008; 308(1): 1 - 48. [Abstract] [Full Text] [PDF]

				A. B. Christie, M. P. Simpson, R. L. Brathwaite, J. L. Mauk, and S. F. Simmons Epithermal Au-Ag and Related Deposits of the Hauraki Goldfield, Coromandel Volcanic Zone, New Zealand Economic Geology, August 1, 2007; 102(5): 785 - 816. [Abstract] [Full Text] [PDF]

				M. P. Simpson and J. L. Mauk The Favona Epithermal Gold-Silver Deposit, Waihi, New Zealand Economic Geology, August 1, 2007; 102(5): 817 - 839. [Abstract] [Full Text] [PDF]

				J. L. Mauk and M. P. Simpson Geochemistry and Stable Isotope Composition of Altered Rocks at the Golden Cross Epithermal Au-Ag Deposit, New Zealand Economic Geology, August 1, 2007; 102(5): 841 - 871. [Abstract] [Full Text] [PDF]

				M. J. Begbie, K. B. Sporli, and J. L. Mauk Structural Evolution of the Golden Cross Epithermal Au-Ag Deposit, New Zealand Economic Geology, August 1, 2007; 102(5): 873 - 892. [Abstract] [Full Text] [PDF]

				J. Naden, S. P. Kilias, and D.P. F. Darbyshire Active geothermal systems with entrained seawater as modern analogs for transitional volcanic-hosted massive sulfide and continental magmato-hydrothermal mineralization: The example of Milos Island, Greece Geology, July 1, 2005; 33(7): 541 - 544. [Abstract] [Full Text] [PDF]

				D. M. Hudson Epithermal Alteration and Mineralization in the Comstock District, Nevada Economic Geology, April 1, 2003; 98(2): 367 - 385. [Abstract] [Full Text] [PDF]

				K. Faure, K. Faure, Y. Matsuhisa, H. Metsugi, C. Mizota, and S. Hayashi The Hishikari Au-Ag Epithermal Deposit, Japan: Oxygen and Hydrogen Isotope Evidence in Determining the Source of Paleohydrothermal Fluids Economic Geology, May 1, 2002; 97(3): 481 - 498. [Abstract] [Full Text] [PDF]

				M. P. Simpson, M. P. Simpson, J. L. Mauk, and S. F. Simmons Hydrothermal Alteration and Hydrologic Evolution of the Golden Cross Epithermal Au-Ag Deposit, New Zealand Economic Geology, July 1, 2001; 96(4): 773 - 796. [Abstract] [Full Text] [PDF]

				S. F. Simmons, S. F. Simmons, and P. R. L. Browne Hydrothermal Minerals and Precious Metals in the Broadlands-Ohaaki Geothermal System: Implications for Understanding Low-Sulfidation Epithermal Environments Economic Geology, August 1, 2000; 95(5): 971 - 999. [Abstract] [Full Text] [PDF]