Abstract Teruggite is the dominant phase in a soft, off-white, poorly-layered and weakly-cemented surface crust, 10–15 mm thick, occurring in the high-temperature El Tatio geothermal field of Chile. Other minerals present include halite, which is present throughout but also forms a thin (<0.5 mm), brittle, cratered surface to the deposit, nobleite, ulexite and opal-A, with possible traces of illite-smectite and at least one unidentified phase. With the exception of ulexite, none of the minerals associated with teruggite at El Tatio has been reported from other occurrences of this mineral, nor do they occur with nobleite in its sole other known occurrence in Death Valley. EDS and XPS analyses of the main mass of the deposit show the presence of Ca, As, B, Na, and Cl, consistent with the identified mineral assemblage, but with elevated concentrations in Ca and Cl that are presumably associated with a further phase. Little Mg is present and the El Tatio teruggite appears deficient in this element, with Ca presumably replacing Mg in the structure. Unlike earlier documented occurrences of teruggite, that at El Tatio is evaporitic, modern and surficial. It is located some 50 m from the nearest hot (~50°C) pool and there is no evidence of association with fluid discharge. As such, the deposit has presumably derived from a fluid moving in the uppermost levels of the El Tatio field; perhaps a heavily modified version of the brines found in the deep wells.
Abstract To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic. The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.
Geologic controls on development of high-flux hydrothermal conduits that promote epithermal ore formation are evaluated at large and small scales for geothermal systems of the Taupo Volcanic Zone, New Zealand. Most geothermal systems occur within a rifted volcanic arc (~150 km long) dominated by silicic volcanism, and they occur in association with major faults near caldera structures or within accommodation zones that transfer ex tension between rift segments . The geothermal systems are hosted in a thick sequence (1:>3 km) of young vol canic deposits that rest unconformably on weakly metamorphosed Mesozoic argillite and graywacke. Flow regimes and permeability controls in one extinct (Ohakuri) and six active (Broadlands-Ohaaki, Waiotapu, Roto kawa, Waimangu, Te Kopia, and Orakeikorako) geothermal systems show that in general, hydrothermal fluid flow is controlled by (1) heat from magmatic intrusions which drives convective circulation; (2) intergranular host-rock porosity and permeability; (3) fault-fracture network permeability produced by tectonism, volcanism, and/or diking; (4) pipelike vertical conduits produced by volcanic and hydrothermal eruptions; and (5) hydro thermal alteration and mineral deposition that may cause heterogeneity in the porosity and permeability of a fluid reservoir. Such controls influence fluid flow within three distinctive depth zones: (1) a feed zone (>2,000 m depth), (2) an epithermal mineralization zone ( 100 km 3 that encloses geothermal reservoirs and high-flux fluid conduits. Fracture-dominated flow becomes important with decreasing porosity induced by hydrothermal alteration. In the discharge zone, the re duction in confining pressure, combined with mineral deposition and alteration, hydrothermal eruptions, and interplay of hot and cold waters create complex, but strongly localized flow paths that feed hot springs. The permeability structure conducive to epithermal vein formation is analogous to a geothermal well: short in horizontal dimension (10s:100s m) but long in vertical dimension (>1,500 m) and possibly pipelike in shape. Episodic high-flux occurs over time scales of tens to thousands of years to accumulate sufficient amounts of gold and silver to form orebodies. During these episodes when faults and fractures are dilated, development of an upward-expanding column of boiling fluid promotes rapid ascent and high mass flow but also promotes silica and calcite precipitation, which can quickly reduce hydrothermal flow. Seismic activity and/or dike intru sion create and reactivate these high-flux pathways through extension and extensional shearing, caused by low differential stresses. The Taupo Volcanic Zone is highly prospective for epithermal-style mineralization, but the predominance of weak porous host rocks at shallow depths is prone to disseminated-style mineralization (e.g., Ohakuri). Structurally controlled mineralization forms in volcanic rocks where they have been embrittled by silicification through seismicity and fault displacement, caldera-forming eruptions, and dike intrusion.
Abstract Better tools are needed to map the thermal structure of ore deposits. Here, carbonate clumped isotope thermometry is applied for the first time in epithermal, skarn, and carbonate-hosted deposits to identify the conditions involved in metal transport and deposition. Clumped isotope temperature calibrations were tested by measurement of carbonates from three geothermal fields in the Taupo volcanic zone, New Zealand, that record growth temperatures between 130° and 310°C. Results for modern Taupo volcanic zone calcites were paired with known fluid δ18O values and these indicate precipitation in equilibrium with produced geothermal waters. Measurements carried out at the Waihi low sulfidation deposit in New Zealand, the Antamina polymetallic skarn in Peru, and the Mount Isa sediment hosted Pb-Zn and Cu deposit in Queensland, Australia, demonstrate that clumped isotope values are sensitive to temperature gradients defined using other methods. At Waihi, an andesite-hosted deposit, temperature controls the majority of variation in carbonate mineral δ18O. At Mount Isa, ~300° to 400°C temperatures were recorded in a 1.5 Ga orebody, which are consistent with fluid inclusion values, highlighting the longevity of clumped isotope archives in dolomite minerals. Collectively, these results demonstrate the potential for clumped isotopes to delineate the heat footprint around deposits that contain carbonates, and to more effectively disentangle magmatic and meteoric fluid δ18O signals.
The formation of hydrothermal calcite relates to the movement of carbon dioxide in a geothermal system as governed by boiling, dilution, and condensation. In this paper we show how these processes control the occurrence, distribution, and stable isotope composition of calcite based on a study at Broadlands-Ohaaki. The two principal calcite occurrences in the Broadlands-Ohaaki geothermal system are: (1) as replacement of rock forming minerals and volcanic glass; and (2) as platy crystals infilling voids. Both are stable over a broad temperature range from 160 degrees C to 300 degrees C. Replacement calcite is widespread and forms through hydrolysis reactions involving calcium alumino-silicates and sub-boiling liquids that contain 0.3 to 0.75 m CO ~2~ . Platy calcite, in contrast, forms over a restricted vertical interval of a few hundred meters within the upflow zone. It precipitates from boiling fluids through exsolution of carbon dioxide as indicated by coeval liquid-rich and vapor-rich fluid inclusions and its formation in the two-phase zone. Fluid inclusion data help to define the boiling paths of fluids from which platy calcite formed. Homogenization temperatures range from 160 degrees C to 310 degrees C and are consistent within the present geothermal regime. Ice melting temperatures range from 0.0 to -1.0 degrees C and indicate the presence of up to 0.5 m dissolved carbon dioxide. Model boiling curves calculated to match these data show how the concentration of dissolved carbon dioxide in the preboiled fluid dictates the depth of first boiling. Most fluid inclusion data lie along a model boiling path characteristic of the centre of the upflow zone, in which the rising fluid (initially containing 0.75 m CO ~2~ ) begins to boil at approximately 320 degrees C and approximately 2000 m depth; data from well Br-18 instead matches a curve in which the rising fluid (initially containing 0.53 m CO ~2~ ) begins boiling at approximately 245 degrees C and approximately 900 m depth. The shallowing of the depth of first boiling likely results from dilution of dissolved carbon dioxide in the parent chloride water, as it rises and mixes with marginal waters. Calcite precipitates from both shallow formed steam-headed groundwater and deeply derived chloride water, and these waters are isotopically distinct. At Broadlands-Ohaaki, the delta ^18^ 0 values of calcite at 200 degrees C range from 0.5 to 7.5 per mil, whereas delta ^18^ 0 values of calcite at 200 degrees C range from 4 to 10 per mil. Taking appropriate temperature dependent fractionation factors into account, these data indicate equilibration with chloride water (delta ^18^ 0 ~H2O~ = -4.5 per mil) and steam-heated groundwater (delta ^18^ O ~H2O~ = -7.0 per mil), respectively. Oxygen isotopes of hydrothermal calcites in the nearby Wairakei and Waiotapu geothermal systems show similar patterns, consistent with the occurrence of both chloride and steam-heated waters there. Calcite formation is explained by a model that describes the distribution of two-phase conditions and aqueous carbon dioxide concentrations in a column of hydrothermal fluid rising through a rock matrix of isotropic permeability. In this ideal situation, platy calcite forms along the inner margin of the two-phase zone, having the shape of an inverted cone, whereas replacement calcite mostly forms in the surrounding one-phase liquid-only zone. The sparse occurrence of calcite at less than or equal to 800 m depth in the central upflow of the Ohaaki sector at Broadlands-Ohaaki is compatible with this model and appears related to the exsolution of dissolved carbon dioxide through boiling deeper in the system.
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