Correlation of gold in siliceous sinters with 3He 4He in hot spring waters of Yellowstone National Park

Abstract

Opaline sinter samples collected at Yellowstone National Park (YNP) were analyzed for gold by neutron activation and for other trace elements by the inductively coupled plasma optical emission spectroscopy (ICP-OES) method. No correlation was found between Au and As, Sb, or total Fe in the sinters, although the sample containing the highest Au also contains the highest Sb. There also was no correlation of Au in the sinter with the H 2 S concentration in the discharged hot spring water or with the estimated temperature of last equilibration of the water with the surrounding rock. The Au in rhyolitic tuffs and lavas at YNP found within the Yellowstone caldera show the same range in Au as do those outside the caldera, while thermal waters from within this caldera all have been found to contain relatively low dissolved Au and to deposit sinters that contain relatively little Au. Therefore, it is not likely that variations in Au concentrations among these sinters simply reflect differences in leachable Au in the rocks through which the hydrothermal fluids have passed. Rather, variations in [H 2 S], the concentration of total dissolved sulfide, that result from different physical and chemical processes that occur in different parts of the hydrothermal system appear to exert the main control on the abundance of Au in these sinters. Hydrothermal fluids at YNP convect upward through a series of successively shallower and cooler reservoirs where water-rock chemical and isotopic reactions occur in response to changing temperature and pressure. In some parts of the system the fluids undergo decompressional boiling, and in other parts they cool conductively without boiling. Mixing of ascending water from deep in the system with shallow groundwaters is common. All three processes generally result in a decrease in [H 2 S] and destabilize dissolved gold bisulfide complexes in reservoir waters in the YNP system. Thus, different reservoirs in rocks of similar composition and at similar temperatures may contain waters with different [H 2 S] and [Au]. The [H 2 S] in a subsurface reservoir water is difficult to assess on the basis of analyses of hot spring waters because of uncertainties about steam loss during fluid ascent. However, the same processes that result in low [H 2 S] in reservoir waters also tend to result in decreases in the ratio of 3 He 4 He(R) "> 3He4He(R) dissolved in that water. Values of R relative to this ratio in air ( Ra ) attain values > 15 in YNP thermal waters. To date, all of the thermal waters at YNP that have R Ra "> RRa values <9 have been found to deposit sinters with relatively low gold concentrations. These include all of the thermal waters that discharge from 180–215°C reservoirs at Upper, Midway, and Lower Geyser Basins within the western part of the Yellowstone caldera, and thermal waters at Norris Geyser Basin, outside the Yellowstone caldera, where some of the waters flow directly to the surface from a reservoir where the temperature is about 300°C. A high 3 He 4 He "> 3He4He ratio in thermal water discharged at the surface does not guarantee high gold concentrations in the sinter deposited by this water. Boiling with loss of steam (the gas phase takes a separate route to the surface) during rapid upflow from the shallowest reservoir to the surface decreases the [H 2 S] and total He dissolved in the residual liquid without appreciably changing the 3 He 4 He "> 3He4He ratio. This is because the isotopic composition of the He of the initial bulk fluid is unchanged and there is too little time for much radiogenic 4 He to build back into the liquid during this rapid ascent from the near-surface reservoir. However, if boiling with phase separation and loss of steam occurs deep in the system, the 3 He 4 He "> 3He4He ratio in the residual liquid, now depleted in H 2 S and total He, will be susceptible to dilution with radiogenic 4 He that is acquired during the longer residence time underground. Some or all of the Au that comes out of solution when an initial gold bisulfide complex breaks down as a result of loss of H 2 S may be swept up to the surface as solid (probably colloidal) particles, depending on the rate of flow of the mixture of water and steam, and the geometry of the channel. Where colloidal silica also forms as a result of this boiling, free Au apparently becomes attached to the colloidal silica and deposits where the silica deposits.