The study of deposits is carried out at several scales, from the mineral to the mining camp. Areas surveyed vary in size from a few micrometres to several kilometres and can cross different geological units. Exploration guides resulting from these studies often rely on the identification of specific features that, observed elsewhere, can lead to the definition of prospective zones for mineralization. However, criteria based on spatial and temporal relationships between deposits of the same nature, based on regional geodynamic characteristics, are sometimes not very noticeable in these guides. Can recognized elements within a deposit be found elsewhere in a geological province? In this questioning, the geodynamic context in which the mineralization genesis and evolution of a region takes place is a valuable tool that should guide any exploration process. Using various examples, this conference session aims to demonstrate the critical importance of designing and choosing a geodynamic model for discovering new deposits.
November 19, 2019
Structural Geology: Tectonic Models and Their Role in Prospecting
Tectonic-mineralization relationships in hot lithospheres: Applications for Precambrian cratons
Denis Gapais, keynote speaker
Université de Rennes 1
Many Archaean and Paleoproterozoic cratons subjected to compressive tectonics show deformation domains that differ from those of modern orogenic zones. Deformation is marked by distributed crustal thickening with subhorizontal shortening and penetrative subvertical stretching, which combines with horizontal creep in ductile crust undergoing localized partial melting. Similarly, regional metamorphic indicators generally indicate high-temperature mid-crustal conditions, attesting to hot continental crusts.
A synthesis of data from several field examples and the results of analog modelling reveal that vertical tectonics characterized by the burial of upper-crust ore deposits in the hot and weak underlying crust (pop-down tectonics) is the best explanation for the structures. Buried portions of the upper crust are restricted to subvertical deformation zones where they can achieve partial melting conditions.
These zones of intense subvertical deformation are geometrically correlated with the areas of concentrated mineralization of considerable magnitude that characterize ancient cratons. The relationship between structures and mineralization in several Archean and Paleoproteroic belts shows that the pop-down tectonics model provides a promising structural framework for mineral exploration in ancient cratons.
Ophiolites as witnesses to the geodynamic evolution of the oceanic lithosphere: Tectonic and metallogenic implications
Ophiolites are fragments of oceanic lithosphere carried onto a continental margin during an orogenic collision, usually marking the trace of subduction sutures. They are found in the geological records of the Paleoproterozoic and are extensively documented in those of the Phanerozoic, but are rather rare, if not absent, in the Archean. Ophiolites are evidence of oceans that have since disappeared and can be used to conduct a “dry land” study of the secular changes in primary structures and the nature of the deep oceanic crust and upper mantle. Recent studies suggest that the composition and structure of modern oceanic lithosphere are primarily controlled by ridge spreading rates that range from a low of 2-3 cm/year to a high of 15-20 cm/year. These ranges can be extrapolated to ophiolites around the world to distinguish slow- versus fast-spreading sequences.
This presentation focuses on a lithological and structural comparison of two slow-spreading Phanerozoic ophiolitic sequences: the Jurassic Mirdita ophiolites (165 Ma) in the Dinarides of Albania and the Ordovician ophiolites (500 to 480 Ma) in the Appalachians of southern Québec. The Mirdita ophiolites form a 4,000 km2 allochthonous nappe that has experienced very little post-obduction deformation and which contains an “ocean core complex” (OCC), an early structure of exhumed deep crust and mantle that facilitated hydrothermal fluid circulation. Pre-obduction structures, possibly associated with the OCC, are also recognized in the Québec ophiolites, even though they have undergone two phases of post-obduction deformation. The U-Pb and 40Ar/39Ar ages of the Thetford Mines ophiolite indicate that obduction (s.s.) was spread over 15 Ma. Despite an age gap of 300 Ma, the Dinaric and Appalachian ophiolites share several lithological and structural similarities, including (i) a discordant syn-obduction piggyback basin sedimentary cover, and (ii) early extension structures that drained magmatic intrusions and/or hydrothermal fluids at different crustal levels. Both sequences carry Cr, PGE, Ni and Cyprus-type synvolcanic sulphide mineralization, the latter being the source of ore in the Munella mine in Albania.
The existence of Archean ophiolites is one of the key elements in the debate on the nature of Archean tectonics. The absence of mantle rocks or evidence supporting major displacements of Archean oceanic sequences are arguments against the principle of uniformitarianism for the Archean Earth.
Orogenic gold: What to expect in high intensity metamorphic terrains
Gold from “orogenic” deposits accounts for ~75% of world production. These deposits are distributed in Archean to Phanerozoic volcano-sedimentary belts metamorphosed mainly to greenschist facies (chlorite assemblage). Mineralization is composed of quartz-carbonate veins with low sulphide content (pyrite±arsenopyrite) in shear zones characterized by an alignment of phyllosilicates (schistosity) and alteration marked by sericite, chlorite and iron carbonates. The abundance of quartz veins is the result of brittle behaviour in the seismogenic zone, induced by high fluid pressure. Seismic movements create openings where fluid depressurization causes quartz to precipitate (vein formation). The dip of these mineralized shear zones is vertical to moderate (>45°), and the depth extension of mineralization is kilometric (>2 km).
In greenschist terrains, where biotite and actinolite occur along schistosity planes in deformation zones, the ore contains significantly fewer quartz veins. Moreover, the veins are smaller, boudinaged and dismembered. Carbonates are commonly absent. Pyrite coexists with pyrrhotite, and both sulphides have the same trace metal signatures, demonstrating that they are cogenetic. These characteristics can be explained by a higher temperature of metamorphism. Behaviour is more ductile than brittle within the shear zones, and consequently fewer openings form, limiting quartz vein formation and the precipitation of carbonates. Sericite (K) alteration and chlorite (Fe) alteration are replaced by a single mineral, biotite (K-Fe), which is stable at higher temperatures. The geometry of the mineralized shears varies from vertical to subhorizontal. The depth extension of mineralization is dependent on the proximity of amphibolite facies.
At amphibolite facies, in which hornblende is present by definition, deformation is ductile and manifests as an alignment of silicate minerals (foliation). The capacity to concentrate hydrothermal fluids within these seismic deformation zones is minimal. Therefore, the deposits correspond either to metamorphized and remobilized ancient mineralization or to retrograde events with hydrothermal alteration expressed by phyllosilicates.
Metamorphic facies, defined by temperature, controls the mineralogy and geometry of gold mineralization and the potential for depth extensions. We will use several deposits as case examples: Lapa (Neoarchean, Canada), Boungou (Paleoproterozoic, Burkina Faso), WG-03 (Neoproterozoic, Sudan) and Macreas (Phanerozoic, New Zealand).
Application of an unstable stagnant lid model to explain the geology and metallogeny of the Archean
A critical review shows that the volcanic, plutonic, metamorphic and sedimentary facies of Archean terranes do not resemble the typical rocks of modern arcs and ridges. This refutes the presumption that plate tectonics was ubiquitous in the Archean. We assess a model involving the periodic destabilization of a stagnant lid (as on Venus). The stable phases correspond to periods of stratified convection in the mantle. The upper mantle would be geochemically depleted, having generated basaltic magmas during previous overturn events. The thermal gradient with the surface would produce convection currents, which would gradually cool and densify the upper mantle. The undepleted lower mantle, on the other hand, would gradually become less dense over time because it would heat up, having kept its radioactive elements (U-Th-K). Eventually, after about 100 to 300 Ma, there would be a density inversion, producing huge upwellings of the lower mantle for periods of 50 to 150 Ma. The decompression of hot fertile mantle would produce the effusions of tholeiitic basalt that dominate greenstone belts. In addition to the typical ensialic effusions, these basalts would also form a thick mafic oceanic crust rarely preserved. Archean basalts show little or no change in composition over time and appear to have derived from a uniform but relatively undepleted mantle reservoir that is very different from the reservoir generating modern ridge basalts.
The long duration of the upwellings and their periodic recurrence would gradually remelt and cannibalize pre-existing crust to produce TTG and calc-alkaline suites, and, eventually, continental crust. This crustal recycling and distillation would take place above the mantle upwellings with each mantle overturn. The radial flow of mantle associated with these upwellings could have disaggregated pre-existing continental blocks and aggregates, producing rift basins favourable for komatiite Ni-Cr ore deposits. More complete disaggregation would create transient oceanic basins (Abitibi?). The lateral currents induced by these mantle overturns would also set in motion distal continents by pushing on the deep lithospheric keels below the cratons, thus creating hot accretionary orogens such as the eastern and western portions of the Superior Province. In these orogens, gold deposits would develop along faults channelling metamorphic fluids. A comparison with Ishtar Terra on Venus shows that such accretionary orogens can form in the absence of ridges and arcs.
The enigmatic hidden structures of the deep crust and upper mantle associated with Au and Ni-Cr-Cu-PGE mineralization in the Superior Province: New data, tectonic interpretations and exploration targets
Long-lived deep crustal and upper mantle (SCLM) structures exert primary controls on cratonic architecture and the location of mineral deposits in diverse tectonic settings. Structures in metasomatized SCLM and deep crust may focus the flow of volatile-rich mantle-sourced fluids and magmas related to a range of mineral deposit types; their reactivation creates host structures for mineralization in the mid- to upper-crust.
Aeromagnetic data for the Superior Province, enhanced to highlight deep crustal +/- SCLM features, portray: (i) regional-scale, rectilinear faults and margins to discrete, competent mafic and felsic granulite blocks at high angles to both regional mapped structures and sub-province boundaries, commonly with little to no direct, surface expression and, especially in the western Superior, (ii) concentric elliptical structures up to ca.185 km in diameter that are spatially associated with orogenic Au, porphyry, and magmatic Ni–Cu–PGE–Cr mineralization. Magnetotelluric data for the W Superior (Ontario) indicate that deep ca. N-S crustal faults and elliptical structures also have a SCLM expression. Elliptical structures resemble coronae on Venus which radar and gravity interpretations suggest formed above mantle plumes. Emplacement of mafic-ultramafic bodies hosting Ni-Cr-PGE mineralization within these concentric structures at their intersection with ca. N-S faults (phi structures), along with their location along the S margin to the N Superior Craton, are consistent with mantle upwellings portrayed in numerical models of a mantle plume beneath a craton with a deep lithospheric keel. Early N-S faults preserved in the lower crust beneath broad E-W Neoarchaean dextral ductile transcurrent to transpressional shear zones and younger discrete faults acted as conduits for magma transport into the overlying crust and focussed hydrothermal fluid flow.
Observations imply decoupling in the mid-crust, viz. a ‘millefeuille’ lithospheric strength profile with competent SCLM and deep as well as upper crustal layers; geophysical interpretations cannot be explained by conventional arc accretion models but they support parautochthonous, plume-related rift models for the formation of Archaean greenstone terrains. Geophysical enhancements, newly identified deep structures related to Au and Ni–Cu–PGE–Cr mineralization, and the alternative model to plate tectonics confirmed in this study open the way for the generation of new exploration targets, including for areas with little to no outcrop.
Research was funded by NRCan’s TGI Program. N. Cleven and C. Guilmette contributed to James Bay interpretations. Geophysical processing used Geosoft Oasis Montaj®.