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Abstract

The Southern Alps mountain range, New Zealand, is a collisional orogenic belt in which rapid uplift (near 10 mm/yr) has caused high near-surface geothermal gradients which commonly exceed 50°C/km. Thermal modeling suggests that conductive heat flow perturbations reach a steady state after about 2.5 Ma, with temperatures as high as 300°C within 5 km of the surface. The high heat flow results in enhanced fluid flow in the upper 6 km of the crust. Uplifting greenschist facies rocks release gold-bearing metamorphic water produced during metamor-phism by dehydration of chlorite. Gold precipitation occurs due to cooling and/or dilution where this>300°C fluid with salinities up to 4 equiv wt percent NaCl mixes with convecting lower salinity <250°C meteoric water. Uplifting amphibolite facies rocks release relatively small quantities of an aqueous fluid which is CO2-bearing but contains little or no gold. Influx of this fluid results in alteration of biotite to chlorite, causing progressive decrease in XH2 0 to values as low as 0.6. The resulting CO2-rich fluid mixes with meteoric water while migrating to the surface, where CO2-bearing hot springs emanate. Minor base metal mineralization occurs throughout the zone penetrated by meteoric water. The mineralization model can be extended to the Archean, when higher geothermal gradients prevailed. For an initial geothermal gradient of 40°C/km, uplift rates as low as 1 to 2 mm/yr extended over about 5 Ma could have produced near-surface thermal anomalies similar to those observed in the Southern Alps.

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