Abstract

The Rotokawa geothermal system is less than 20,000 yrs old and is situated in a sequence of Pleistocene and Recent volcanic rocks and underlying Mesozoic graywacke basement in the Taupo volcanic zone, a region dominated by extensional tectonics. At lease 12 different hydrothermal explosition breccias are preserved in surface outcrops, the largest eruptive site being a crater of 1.5 km in diameter which formed about 6,060 years ago and is presently occupied by an acid lake (Lake Rotokawa) with a pH approximately 2. Significant bedded sulfur deposits which formed in an earlier crater lake exist beneath the present lake. The deep fluids are dilute chloride (0.022 mole/kg) waters having higher reduced sulfur concentrations (H 2 S approximately 7X10 (super -3) mole/kg) than any previously studied active geothermal system. A maximum temperature of 320 degrees C has been measured in the system. Water-rock interaction has given rise to a zone of propylitic alteration at depth which is overlain by a region of advanced and intermediate argillic alteration. Extensive boiling occurs at depth an sulfide minerals such as sphalerite, galena, chalcopyrite, and argentite occur sporadically with ubiquitous pyrite. Argentiferous alunite occurs in the advanced argillic alteration zone. clasts of silicified lake beds in the hydrothermal explosion breccias commonly contain pyrite and carry subeconomic gold values of up to 0.1 g/metric ton. Thermal waters discharging on the northeast shore of the lake are depositing gold ore grade arsenic and antimony sulfide-rich muds which also contain high concentrations of tungsten, thallium, mercury, silver, and gallium. This surface deposit is estimated to contain about 250 kg (8,000 oz) of gold and has been deposited during the past 1,800 yrs. Since the formation 6,060 yrs ago of the large hydrothermal explosion crater in which Lake Rotokawa is now situated, about 12.0 million oz. (370 metric tons) of gold may have been transported into the environment beneath the crater. It is estimated that 1.7 to 3.3 million oz. of gold may have been deposited in the upper 300 to 400 m underlying Lake Rotokawa, from hydrothermal fluids initially saturated with gold.

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