Abstract

In order to analyze the concentrations of Ag, As, Au, Cd, Cu, Hg, Mn, Mo, Ni, Pb, Se, Sb, Te, Tl, W, and Zn in deep hydrothermal chloride waters at 195° to 320°C, and in shallow boiled chloride waters at 160° to 230°C, we sampled wells drilled to 3-km depth in geothermal systems of the North Island, New Zealand. Six of the systems are located in a segment of volcanic arc in the central Taupo Volcanic Zone, and the seventh system is associated with an intraplate mafic (felsic) volcanic center.

The concentrations of metals range widely from 0.1 to >1,000 μg/kg with a large degree of intersystem variability. Some of the largest contrasts in Au, Ag, Pb, and Te concentrations are observed in the two nearest systems, Rotokawa and Wairakei, which are only 10 km apart. The correlations between metals are poor, except for Ag-Au-Pb-Te, and As-Sb. The correlations between metals, Cl, and H2S are also poor, with the exception of Rotokawa where the highest concentrations of Ag, Au, Cu, and Te correlate with the highest concentration of aqueous H2S.

Speciation calculations indicate that the dominant aqueous species of Ag, Au, Cu, Pb, and Zn involve HS complexes. The calculations also show that the states of saturation range from undersaturated conditions for acanthite, arsenopyrite, and gold to oversaturated conditions for chalcopyrite, sphalerite, and tellurides. Notably, the Au- and Ag-transporting capacities of the deep chloride waters are much larger than the measured aqueous concentrations. These results suggest that fluid-mineral equilibria and the concentrations of ligands exert weak influence on metal concentrations at the temperatures and depths of sampling.

The complex trends in hydrothermal metal concentrations strongly suggest that the deep-seated sources of metals, comprising magmatic intrusions, deep country rock, and their related fluids, limit the hydrothermal supplies of metals. Between the geothermal systems, hydrothermal fluxes of Ag (6–8,000 kg/y), Au (0.9–66 kg/y), Cu (30–23,500 kg/y), and Te (2–10,400 kg/y) are variable. The highest concentrations and fluxes of Ag, Au, and Te in Rotokawa and Mokai are attributed to direct fluid inputs from intrusions of andesitic and basaltic magmas, respectively.

Compared to their deep counterparts, boiled chloride waters are strongly depleted in Ag, Au, Cd, Cu, Pb, and Te, because these metals deposit in sharp response to gas loss and cooling in the well. By comparison, the As, Mn, Mo, Ni, Sb, Tl, and Zn concentrations are measurably less depleted in boiled waters, making them available to form metal anomalies at shallow depths and in the peripheral parts of the epithermal environment.

Periods of strong metal flux through the geothermal system combined with deep boiling favor epithermal ore formation and the development of large precious metal deposits. However, even moderate metal fluxes can produce Au and Ag mineralization as long as the duration of focused fluid flow and boiling can be sustained. There is no evidence that the compositions of deep magmatic intrusions, mafic or intermediate, limit the ore-forming potential of a geothermal system.

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