Philipp Weis

Geologic controls on supercritical geothermal resources above magmatic intrusions

A new and economically attractive type of geothermal resource was recently discovered in the Krafla volcanic system, Iceland, consisting of supercritical water at 450 °C immediately above a 2-km deep magma body. Although utilizing such supercritical resources could multiply power production from geothermal wells, the abundance, location and size of similar resources are undefined. Here we present the first numerical simulations of supercritical geothermal resource formation, showing that they are an integral part of magma-driven geothermal systems. Potentially exploitable resources form in rocks with a brittle–ductile transition temperature higher than 450 °C, such as basalt. Water temperatures and enthalpies can exceed 400 °C and 3 MJ kg−1, depending on host rock permeability. Conventional high-enthalpy resources result from mixing of ascending supercritical and cooler surrounding water. Our models reproduce the measured thermal conditions of the resource discovered at Krafla. Similar resources may be widespread below conventional high-enthalpy geothermal systems.

Salt precipitation in magmatic-hydrothermal systems associated with upper crustal plutons

Magmatic-hydrothermal systems associated with upper crustal plutons strongly influence volcanic and geothermal processes and form important mineral deposits. Fluids released from plutons are commonly saline and undergo phase separation into high-salinity brines and low-salinity vapors upon ascent. While brine-vapor immiscibility has been extensively studied, precipitation of solid salt during phase separation in magmatic-hydrothermal systems has generally been considered a rare phenomenon. Here we show that most porphyry deposits exhibit fluid inclusion evidence best interpreted by solid salt precipitation from ore-forming solutions. This interpretation naturally links thermodynamics, numerical simulations, and independent estimates of porphyry ore formation depths. Salt precipitation imposes major changes on the permeability of the system. Moreover, salt precipitation has implications for ore formation along the liquid-vapor-halite curve. The recognition of salt-saturated systems is challenging, but very relevant for understanding the evolution of magmatic-hydrothermal systems.

Trace elements in fluid inclusions of sediment-hosted gold deposits indicate a magmatic-hydrothermal origin of the Carlin ore trend

The Carlin-type deposits in Nevada (western USA) constitute the world9s second-largest gold ore province. These structurally and stratigraphically controlled, sediment-hosted ore bodies are characterized by carbonate dissolution attending hydrothermal precipitation of gold-rich arsenian pyrite. The origin of the mineralizing fluids and the source of the gold remain debated. Conceptual models, favoring either sedimentary, metamorphic, or magmatic fluid sources, are based on isotopic tracers, giving ambiguous results. Here we use the trace element compositions of fluid inclusions to separate geochemical signals of the large-scale fluid source from effects of deposit-scale fluid interaction with the sedimentary host rocks. Specifically, we compare the ratios of Rb, K, B, As, Sr, and Ba between clearly magmatic-hydrothermal Cu-Au ores at Copper Canyon in the Battle Mountain–Eureka trend with the Gold Quarry and Chukar Footwall deposits on the Carlin trend that contain high-grade gold in similar sedimentary host rocks. Results indicate that both ore districts can be related to upper crustal hydrous magmatic intrusions, but are now exposed at different levels of erosion and formed at different distances from their magmatic fluid source. Fluid compositions are best explained by separation of a deep magmatic fluid into Rb-K–enriched brine and B-As-Au–enriched vapor, followed by cooling and contraction of the magmatic vapor phase to an epithermal liquid, which reacted with Sr-Ba–bearing sedimentary rocks during ascent and eventual precipitation of Au-rich arsenian pyrite.

Constraining timescales of ore-formation by numerical simulations of magmatic-hydrothermal systems

This journal supplement has title: Awards Ceremony Speeches and Abstracts of the 19th Annual V.M. Goldschmidt ConferenceThis journal supplement has title: Awards Ceremony Speeches and Abstracts of the 19th Annual V.M. Goldschmidt Conference