Biogenicity of gold- and silver-bearing siliceous sinters forming in hot (75deg C) anaerobic spring-waters of champagne pool, Waiotapu, North Island, New Zealand

Journal of the Geological Society, Nov 2001 by Jones, Brian, Renaut, Robin W, Rosen, Michael R

Abstract: Champagne Pool, a large hot spring at Waiotapu in North Island, New Zealand, is rimmed by a subaerial sinter dam and a shallow subaqueous shelf that is composed of orange sinter rich in metallic sulphides. Orange siliceous flocs, also rich in sulphides, are in constant circulation in the spring pool and form loose sediment on the shelf. The orange sinters and floes are rich in As, Sb, TI, and Hg, and have high concentrations of Au (>100 ppm) and Ag (>330 ppm). Most metallic sulphides are amorphous and disseminated throughout the sinter, instead of forming distinct mineral phases.

The shelf sinters are domal and resemble stromatolites. The neutral chloride waters (pH 5.5; temperature 75 deg C), however, are virtually anaerobic. Examination of the sinters by scanning electron microscopy confirms that they are laminated and contain an abundant, low-diversity assemblage of filamentous, bacilliform, and coccoid microbes. The flocs are similarly composed of very small, silicified filaments. Silicification involved replacement of the cell walls and extensive encrustation by opal-A. Based on size and morphology, these microbes are probably anaerobic bacteria or archaea. By providing substrates for nucleation of the silica, the microbes are indirectly contributing to the formation of the gold-bearing sinters.

Keywords: New Zealand, sinter, stromatolites, gold, epithermal deposits.

Precious metals are commonly precipitated with the siliceous sinters that form around hot springs and geysers (Henley & Ellis 1983). Some of the highest natural gold (>80 ppm) and silver (>175 ppm) concentrations reported from recent deposits are found in the sinters around Champagne Pool (Figs I & 2a), a large hot spring with neutral chloride waters located in the Waiotapu geothermal area of the North Island of New Zealand (Weissberg 1969; Weissberg et aL 1979; Hedenquist 1983, 1986b; Hedenquist & Henley 1985). These sinters, together with ore-grade precipitates in the fractured feeder conduits that are inferred to underlie the modern springs, have commonly been used in the development of models for low-sulphidation epithermal gold mineralization (e.g. Weissberg 1969; Goldie 1985; Hedenquist & Henley 1985; Hirner et al. 1990; White & Hedenquist 1990; Karpov 1991). It has been assumed that the formation of these sinters and their associated metalliferous sulphides is due to abiotic processes (Weissberg 1969; Hedenquist & Henley 1985).

Most of the hard orange sinter substrate on the shallow subaqueous shelf around the margins of Champagne Pool (Fig. 2b) displays domal and hemispheroidal structures that resemble stromatolites (Fig. 2c-g). The spring water, which is virtually anaerobic, has a nearly uniform temperature of 75 deg C, which exceeds the upper temperature tolerance of cyanobacteria (Brock 1978). Samples of these metalliferous sinters, together with loose orange sediments on the shelf and orange siliceous flocs suspended in the spring water, were collected to examine their fabrics and composition, and to determine if microbes contribute to their formation. In this paper, we show that the opaline silica and amorphous sulphides are precipitating mainly on filamentous microbes and biofilms, which form templates for both the metal-rich sinter and the suspended flocs. We then consider the role that the thermophilic microbes play in formation of the metalliferous sinters. The sinters of Champagne Pool provide a rare modern example of primary silicification of prokaryotic microbes under essentially anaerobic conditions.

Methods

Small (

Large (7 x 5 cm) thin sections were used to document the main textural features of the sinters. Small fractured samples of the sinter, particles from the orange sediment, and the orange flocs were examined on a JOEL 6301F Field Emission Scanning Electron Microscope (FE-SEM) at an accelerating voltages of 0.5-5.0 kV. Energy dispersive X-ray (EDX) analysis was done at an accelerating voltage of 15 kV. Low porosity samples were coated with chromium or examined uncoated, whereas those with high porosity received a thin gold coating to reduce charging. The latter method, however, produced surface artifacts when taking photomicrographs at very high magnification. Polished thin sections of some sinter samples were carbon coated and analysed both qualitatively and quantitatively using a JEOL JXA 8600 electron microbe.

Water temperature and pH were measured in the field. Duplicate water samples were collected in the field, one of which was acidified on site. Water samples were analysed, within one month of collection, by atomic absorption spectrometry (Na, K, Ca, Mg, Si), ion chromatography (SO^sub 4^), ion specific electrodes (Cl, F, Li), and titration (HCO^sub 3^, CO^sub 3^). Trace elements were determined by ICP-MS using a Perkin Elmer ELAN 5000. Au and Ag were analysed separately after pre-treatment with aqua regia. Aqueous As was determined by ICP-AES. Analysis of the sinter, sediment, and floes was done by ICP-MS except As, which was determined by XRF. Analysis of the dissolved gases in the water was determined from a sample collected from the upper 3 cm of the pool using a Giggenbach bottle (Giggenbach 1975).

Environmental setting

Champagne Pool lies centrally in the Waiotapu geothermal area, about 25 km SSW of Rotorua (Fig. lb). This area, which includes many hot springs and fumaroles that eject both neutral chloride and acid sulphate waters, is located in a region of ash flows and volcaniclastic and lacustrine sediments that have been deposited over the last 300 000 years (Hedenquist & Henley 1985; Wood 1994). Champagne Pool (Figs 1c & 2a) is a large subcircular hot spring that is c. 65 m in diameter, 150 m deep, and has a surface area of c. 3000 m^sup 2^ (Lloyd 1959; Hedenquist & Henley 1985). The spring fills a hydrothermal explosion crater that formed about 900 years ago.