Round hill shear zone-hosted gold deposit, Macraes Flat, Otago, New Zealand: Evidence of a magmatic ore fluid

The 0.8-Moz Round Hill shear zone-hosted gold deposit is one of several loc ated on, or near, the Hyde-Macraes shear zone, Macraes Flat, New Zealand. F ield relationships show stockwork veins are the oldest part of the hydrothe rmal system, followed by flat and ramp veins. Hanging-wall shear veins form ed early but have been modified by late deformation. Fluid inclusion microthermometric results for dominant type I inclusions sh ow that flat and ramp veins have the same average wt percent NaCl equiv val ues (2.2 +/- 0.6%) and were precipitated from the same fluid. Salinity data for these veins overlap with those from hanging-wall veins (average 1.3 +/ - 0.3%) and stockwork veins (1.6 +/- 0.6% for vein margins; 0.9 +/- 0.3% fo r vein centers), although both of the latter trend toward lower over all wt percent NaCl equiv values, suggesting mixing between relatively dilute and more saline end members. All of the above veins have similar temperatures of homogenization (T-h), ranging between 110 degrees and 188 degrees C. Flu id inclusion trapping temperatures are considered to be similar to 300 degr ees C, equating to a similar to 10-km depth of fort-nation. Carl,on dioxide dissolved in the hydrothermal fluid is responsible for up to one third of fluid inclusion apparent salinities. Stockwork and hanging-wall veins thus have true salinities of <1 wt percent NaCl equiv (<0.17 molal) and flat and ramp veins have true salinities of <2 wt percent (<0.34 molal). Fluid inclusion volatiles from Round Hill and other Macraes deposits are do minated by H2O (99.02-99.69 mol %), with lesser CO2 (0.14-0.76 mol %), N-2 (0.03-0.32 mol %), and CH4 (0.06-0.17 mol %). Noticeable concentrations of hydrocarbons occur in many of the samples. Fluid inclusion cation and anion data show few components of relatively law concentration are dissolved in the hydrothermal fluid, consistent with CO2- corrected microthermometric me asurements. A range in fluid inclusion delta D-H2O values obtained for the Round Hill d eposit is coincident with the different vein types and also indicates mixin g between two fluids. One fluid is considered to be O isotope shifted meteo ric water as trapped by the early stockwork veins, and has the lowest delta D-H2O values of -80 per mil. The other fluid is represented by flat and/or ramp and hanging-wall veins that have delta D-H2O values up to -40 per mil . The Macraes delta D-H2O data show a trend toward the magmatic water end m ember for geothermal systems associated with convergent plate boundaries an d are consistent with a 25 to 50 percent magmatic component in the Macraes hydrothermal system.