Fluids and Drains: Metal Transfer in Archean and Modern Crust – Implications for Exploration in Québec

Mining exploration generally involves several geoscientifc disciplines, such as structural geology, geochemistry, geophysics, petrography, hydrogeology, spatial analysis, geostatistic tools, etc. Although mineralizing processes imply fluids and their circulation modes in permeable zones of the crust, numerical modelling of fluid dynamics is rarely presented as an exploration tool. Realistic fluid properties (temperature in relation to fluid viscosity, temperature and pressure in relation to fluid density) can be used to identify the conditions that support thermal convection. In the field of deep geothermics, this numerical tool demonstrated its predictive potential (Guillou-Frottier et al., 2013; https://doi.org/10.1016/j.jvolgeores.2013.02.008). In the field of economic geology, one can assume, at first approximation, that mineralized bodies correspond to areas where the cooling rate is maximal. This cooling rate – as defined by the Rock Alteration Index (O.M. Phillips, 1991) – depends on the fluid velocity and the local temperature gradient, two variables that can be computed by coupling heat equation, Darcy’s law and mass conservation. This approach was applied to orogenic gold deposits (Harcouët-Menou et al., 2009; https://doi.org/10.1111/j.1468-8123.2009.00247.x) and to perigranitic gold deposits (Eldursi et al., 2018; https://doi.org/10.1016/j.jafrearsci.2018.01.011). The reproduction of the location of mineralized bodies demonstrated the predictivity potential of the numerical modelling approach. In the case of magmatic fluids, the particular case of LCT pegmatites can be investigated with the same tool, as soon as physical properties, such as the very low viscosity of pegmatitic magmas, are well adapted. New numerical experiments on fluid circulation in faulted zones show that the location of mineralized areas depends on fault zone permeability and dip angle. For subvertical fault zones, mineralized bodies would locate along the footwall continuously, but when permeability is high, they would locate along the hanging wall. Conversely, fault zones with a low dip angle would favour the appearance of multiple disconnected mineralized zones.

The continental crust concentrates rare metals (e.g., Li, Sn, Nb, Ta, W) from the silicate Earth, and rare metal deposits have formed in orogens in relation with crustal magmatism since the late Archean. The Superior Province, and more specifically the Eeyou Istchee James-Bay territory, records a strong interest in LCT (Lithium, Cesium et Tantalum) pegmatite exploration, especially for spodumene pegmatite. For this type of deposit, a detailed and well-supported 3D litho-structural model is key for a mineral resource estimation model.

This presentation will show examples of LCT pegmatite occurrences in order to discuss their distribution and the litho-structural controls on their emplacement. This talk will also provide an overview of the state of the art on the formation of these magmatic-hydrothermal deposits related to the superposition of a series of processes including partial melting of aluminous crustal rocks, magmatic differentiation and the nature of protolith.

Magmatic-hydrothermal deposits such as porphyry Cu-Mo-Au and related epithermal Au-Ag systems are a major source of global copper and gold production and supply significant amounts of molybdenum and silver, among other critical elements including rhenium. How the metals are loaded into the hydrothermal fluids and from where they are trapped over long periods of economic geology. With this in mind, here we consider the key drivers of analogous porphyry-epithermal systems found in subduction zones, where porphyry-epithermal systems are formed by the efficient transport and accumulation of ore metals that precipitate predominantly as sulphides from hydrothermal fluids released from evolving arc magmas. To this end, this paper presents a decadal review of our research team in the Southern Volcanic Zone of the Andes (SVZ) (Chile).

The SVZ volcanic arc segment represents a unique natural laboratory for studying the interaction between tectonic structures and volcanic processes, for which the proposed methodology is to trace metal origins, mainly highly detectable/soluble metals such as copper, by using isotope concentrations and noble gas ratios in volcanic rocks and associated geothermal gases. According to previously published material on active magmatic systems using volcanic rocks, on the one hand, evidence is found in mafic mineral fluid/as melt inclusions from most primitive MORB values of atmospheric corrected 3He/4He (Rc/Ra) ratios that copper is present in early magmatic vapor as fluid phases in most paroxysmal eruptions. On the other hand, review of now published fumarole datasets in the SVZ confirms the diverse field of Cu fertility of volcano-tectonic context transtensional and transpressional developed at the convergent margin of the SVZ related to the Liquiñe-Ofqui Fault System (LOFS) and the Andean Transverse Faults (ATF).

According to Rc/Ra and copper as sulphur/CO₂ ratio tracers, we distinguish two tendencies; (1) active fault systems misoriented for reactivation are favorable for ore formation (fluids with higher Cu/SL and lower Rc/Ra), (2) the presence of fault systems optimally oriented for reactivation is unfavorable for the genesis of coeval magmatic-hydrothermal ore deposits (fluids with lower Cu/SL and higher Rc/Ra). This internal Chilean analogue system provides key understanding mechanisms of tectonic environments controlling the formation of IOA and IOCG deposits, e.g. those associated with the sinistral strike-slip Atacama fault system during the Early Cretaceous, where Fe-(±Ti-V)-rich and Cu-S-poor IOA deposits occur mostly along the NNE main trace of the fault, while Cu-S-rich IOCG systems often occur along NW structures, indicating structurally driven Fe vs. Cu partitioning.

Noble gases are rare, inert elements and are used in a wide range of Earth science applications, particularly to determine the sources of crustal fluids, including the origin of hydrothermal fluids, which often carry metals in the Earth’s crust. The isotopic signature of these gases dissolved in the fluids reflects that of the fluids’ sources, as chemical reactions do not modify them. Helium is particularly useful, as it has two isotopes, mass 3 (3He) and mass 4 (4He), with different origins. The first is a primordial isotope enriched in the earth’s mantle, and the other is produced in the crust by decay of U and Th. The ratio, therefore, makes it possible to determine whether magmatic sources or water-rock interaction processes are associated with the enrichment of certain metals. This presentation will show examples of how these isotopes measured in sulfide-associated fluid inclusions and hydrothermal springs in Mexico and Chile can be used to determine which fluids interact in the formation of gold deposits and copper porphyries, as well as in geothermal fluids, analogous to those leading to the formation of epithermal deposits in the past.