Boiling and condensation of saline geothermal fluids above magmatic intrusions

Abstract

Numerical simulation of subaerial, magma‐driven, saline hydrothermal systems reveals that fluid phase separation near the intrusion is a first‐order control on the dynamics and efficiency of heat and mass transfer. Above shallow intrusions emplaced at <2.5 km depth, phase separation through boiling of saline liquid leads to accumulation of low‐mobility hypersaline brines and halite precipitation, thereby reducing the efficiency of heat and mass transfer. Above deeper intrusions (>4 km), where fluid pressure is >30 MPa, phase separation occurs by condensation of hypersaline brine from a saline intermediate‐density fluid. The fraction of brine remains small, and advective, vapor‐dominated mass and heat fluxes are maximized. We thus hypothesize that, in contrast to pure water systems, for which shallow intrusions make better targets for supercritical resource exploitation, the optimal targets in saline systems are located above deeper intrusions.

Permeability Changes Resulting from Quartz Precipitation and Dissolution around Upper Crustal Intrusions

“…While the solubility of quartz is largely pHindependent in fluids with a pH below~9 (e.g., [37]), a value above that generally measured in primary geothermal fluids [58], quartz solubility depends strongly on salinity [18,39,43] and the concentrations of volatile gases such as CO 2 [42,43]. Phase separation of NaCl-rich water at temperatures above the critical temperature of pure water may lead to brine formation and halite precipitation [59], which may significantly reduce porosity and permeability near the intrusion [60]. Thus, these models are mainly appropriate for geothermal systems that are recharged by meteoric fluids and therefore contain dilute groundwater, rather than seawater.…”

“…The basal parts of sheeted dyke complexes within the Troodos ophiolite contain elongated (up to 1 km wide), pod-shaped, quartz-rich epidosites [67], similar to the quartz-rich, low permeability zones that form in our simulations with initially high permeability host rocks (see Figure 9(c)). However, the dynamics of quartz precipitation in subseafloor hydrothermal systems containing seawater as the pore fluid will differ from systems containing pure water because of phase separation at temperatures above the critical point of pure water, brine formation, and halite precipitation [18,59,60].…”

Utilizing supercritical geothermal systems: a review of past ventures and ongoing research activities

“…Scott et al (2016) modeled the temporal evolution of high enthalpy geothermal systems associated with shallow intrusions, and observed that host rock permeability and composition, intrusion depth, intrusion geometry, and strain rate all play important roles in the thermal structure. Scott et al (2017) noted that, for saline hydrothermal systems, the depth of the magmatic intrusions powering geothermal systems impacts the efficiency of heat transfer. In their models, for intrusions >4 km deep, heat transfer is maximized by phase separation occurring via condensation, whereas less efficient heat transfer occurs via boiling in shallower (<2.5 km) systems.…”

The Jbel Saghro Au(–Ag, Cu) and Ag–Hg Metallogenetic Province: Product of a Long-Lived Ediacaran Tectono-Magmatic Evolution in the Moroccan Anti-Atlas

“…In large Mexican epithermal silver deposits of intermediate sulfidation state, Wilkinson et al [26] suggested that the ore forming fluids, were injected into a geothermal circulation system in response to the ascent of a magmatic intrusion. Such hydrothermal diapirs would be sourced from a stratified hyper-saline brine reservoir, formed in response to incremental exsolution of magmatic fluid, and intense brine condensation at depth, with halite precipitation, being stored above the source magma reservoirs [26,192,193]. By contrast, Essarraj et al [40,41,49] suggested that the ore-forming fluids were related to deep-basinal sedimentary brines and that metals had no genetic relationship with Neoproterozoic magmatism, on the basis of numerous deposits they investigated in the Eastern and Central Anti-Atlas.…”

“…We here suggest that all ore deposits described above are related to the Ediacaran magmatic-hydrothermal complex emplaced from 575 to 530-520 Ma. The concept of stored hypersaline brines is emphasised following Scott et al [193]. Nevertheless, while a magmatic origin of such stored brines is obvious, the origin for Ca-rich brines has yet to be defined.…”