ALBERT

Description: Ore precipitation in porphyry copper systems is generally characterized by metal zoning (Cu–Mo to Zn–Pb–Ag), which is suggested to be variably related to solubility decreases during fluid cooling, fluid-rock interactions, partitioning during fluid phase separation and mixing with external fluids. Here, we present new advances of a numerical process model by considering published constraints on the temperature- and salinity-dependent solubility of Cu, Pb and Zn in the ore fluid. We quantitatively investigate the roles of vapor-brine separation, halite saturation, initial metal contents, fluid mixing and remobilization as first-order controls of the physical hydrology on ore formation. The results show that the magmatic vapor and brine phases ascend with different residence times but as miscible fluid mixtures, with salinity increases generating metal-undersaturated bulk fluids. The release rates of magmatic fluids affect the location of the thermohaline fronts, leading to contrasting mechanisms for ore precipitation: higher rates result in halite saturation without significant metal zoning, lower rates produce zoned ore shells due to mixing with meteoric water. Varying metal contents can affect the order of the final metal precipitation sequence. Redissolution of precipitated metals results in zoned ore shell patterns in more peripheral locations and also decouples halite saturation from ore precipitation.

Description: Porphyry copper deposits provide most of the world’s, half its molybdenum reserves and are resources for Zn, Pb, Au, and Ag. The porphyry mineralization is inferred to form on time scales between 50 and 100kyrs whereby the mineralization forming magma chamber is generally built up by multiple intrusive events. The overall source magmatic system can be active for several millions of years. We used the Complex System Modeling Platform (CSMP++) to simultaneously model sill injection, heat transfer, the release of metal-bearing magmatic fluids, the multi-phase flow of saline hydrothermal fluids, and dynamic permeability variations with a continuum porous medium approach. Our modeling studies the volumetric injection rate and its impact on the growth of the magma chamber and the Cu-ore shell but also investigates the influence of hydrothermal convection and fluid release. The setup of each modeling run is changed slightly, either by changing the influx rate, changing the geometry of the magma chamber, or changing the location of fluid release. CSMP was modified to produce vtk and vtu files every 100 years which were read into the Paraview 4.3.1 software to perform the post-processing (including the calculation of the copper enrichment factor and the pore fluid factor). Paraview was then used to produce the displayed videos.

Description: Rock at temperature 〉400 °C can be reached at 〉10-20 km depth globally. Although natural rock permeability at such depths is prohibitively low for energy extraction, laboratory studies have shown that high-pressure fluid injection into superhot rock under high confining pressures (〉500 bar) can generate fine-scale fracture networks permeable to fluid flow. In this study, we focus on modeling the thermal and hydraulic behavior of an enhanced geothermal system at such depths to evaluate its potential for thermal energy extraction. Our models show that such systems can achieve high power output with a low spatial footprint if bulk permeability in the stimulated volume can be maintained near 10-15 – 10-14 m2. While this permeability range is several orders of magnitude higher than the natural maximum permeability of ductile crust at this depth during natural fluid-driven processes (e.g. fault zone metamorphism), much remains to be understood about the response of nominally ductile rock to fluid injection and thermal stress cracking. As additional rock mechanical data describing the permeability response of nominally ductile rock undergoing pressurization and thermal shocking become available, the experimental constraints can be incorporated into the model. Although our existing model points at an exciting potential, the development of such a model would enable more accurate predictions of the long-term sustainability and commercial viability of deep geothermal systems. The integration of interdisciplinary research and collaboration between academia, industry, and government agencies will be critical to the success of future deep EGS projects.

Description: Abstract. Numerical models can provide unique insights into the temporal and spatial relationships of ore-forming processes. We use a model for magma reservoir growth to investigate the impact of sill injection rates on the hydrothermal system. The simulations with more episodic, low injection rates (〈1.3 x10-3 km³/y) result in a highly variable fluid plume which allows almost pure magmatic fluids to migrate to shallower and cooler regions where they can phase separate and potentially form epithermal ore deposits. The modelling results point towards a relatively short time span of potential ore formation of a few thousands of years until the magmatic fluid plume retreats. Long-lived magma reservoirs which are forming at higher injection rates hamper the formation of highgrade epithermal deposits, but are more favourable for high-grade porphyry Cu deposits

Description: The epithermal Pirquitas SnAg -Pb-Zn mine in NW Argentina is hosted in a domain of metamorphosed sediments without geological evidence for volcanic activity within a distance of about 10 km from the deposit. However, recent geochemical studies of ore-stage fluid inclusions indicate a significant contribution of magmatic volatiles. We tested different formation models by applying an existing numerical process model for porphyry-epithermal systems with a magmatic intrusion located either at a distance of about 10 km underneath the nearest active volcano or hidden underneath the deposit. The results show that the migration of the ore fluid over a 10-km distance results in metal precipitation by cooling before the deposit site is reached. In contrast, simulations with a hidden magmatic intrusion beneath the Pirquitas deposit are in line with field observations, which include mineralized hydrothermal breccias in the deposit area.

Description: Continental crust at temperatures 〉 400 °C and depths 〉 10–20 km normally deforms in a ductile manner, but can become brittle and permeable in response to changes in temperature or stress state induced by fluid injection. In this study, we quantify the theoretical power generation potential of an enhanced geothermal system (EGS) at 15–17 km depth using a numerical model considering the dynamic response of the rock to injection-induced pressurization and cooling. Our simulations suggest that an EGS circulating 80 kg s−1 of water through initially 425 ℃ hot rock can produce thermal energy at a rate of ~ 120 MWth (~ 20 MWe) for up to two decades. As the fluid temperature decreases (less than 400 ℃), the corresponding thermal energy output decreases to around 40 MWth after a century of fluid circulation. However, exploiting these resources requires that temporal embrittlement of nominally ductile rock achieves bulk permeability values of ~ 10–15–10–14 m2 in a volume of rock with dimensions ~ 0.1 km3, as lower permeabilities result in unreasonably high injection pressures and higher permeabilities accelerate thermal drawdown. After cooling of the reservoir, the model assumes that the rock behaves in a brittle manner, which may lead to decreased fluid pressures due to a lowering of thresholds for failure in a critically stressed crust. However, such an evolution may also increase the risk for short-circuiting of fluid pathways, as in regular EGS systems. Although our theoretical investigation sheds light on the roles of geologic and operational parameters, realizing the potential of the ductile crust as an energy source requires cost-effective deep drilling technology as well as further research describing rock behavior at elevated temperatures and pressures.