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[Display omitted] • Mineral prospectivty mapping for Au. • Random forest algorithm for integrating geoscience data. • Knowledge-based algorithms. • Superior tectonic province. This paper uses the random forest (RF) algorithm to produce two maps of orogenic gold prospectivity (MPM) across the entire Superior geologic province of Ontario and Quebec, Canada. One MPM of this entire study area is based on a compilation of mapped faults while the other is based on a manual interpretation of lineaments from airborne magnetic data. In addition, we have created four MPMs over the Abitibi region as the MPM map of the entire Superior is characterized by a mapping bias due to more intense mapping of faults over the Abitibi portion of the study area with respect to the rest of the Superior province. There are three RF maps generated over the Abitibi region based on faults, fault density and fault intersection density, faults + magnetic data and faults + gravity + magnetic data. To reduce the effect of the fault mapping bias, we developed a fourth map based on a knowledge driven (weighted sum technique). Statistically the best MPM is based on the faults, gravity and magnetic data. The major predictors of gold are NW-SE, NE-SW, EW trending faults, fault intersection density between EW and NW trending faults and to a lesser extent fault density and a magnetic vertical gradient image. In addition, we compare two greenstone belts in three dimensions with respect to their potential for gold exploration. We conclude that the Larder Lake greenstone belt is more fertile with respect to gold mineralization than the Swayze greenstone belt due to deeper penetrating faults which have greater potential for tapping gold mineralized fluid from the mantle or lower crust. Our final MPM map has identified prospective areas that contain known gold mines as well as areas that do not contain any known gold mines. These prospective areas may be prime for orogenic gold exploration. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Esker intrusive complex and associated chromitite deposits are modeled in 3D. • Shear zone refinement suggests late dextral apparent movement with ∼ 1.5 km offset. • A pre-shear reconstruction reveals multiple intrusive centers fed by faults. • The intrusive centers inflated and merged into a single complex with time. • 3D ore models suggest mechanical sorting contributed to chromitite mineralization. A 3D geological model of the Archean Esker ultramafic–mafic intrusive complex of the Ring of Fire intrusive suite of northwestern Ontario (Canada) is presented, providing insight into the nature of its magmatic plumbing system and allowing for speculation on the formation of its valuable Ni-Cu-(PGE) and chromitite deposits. The 3D model was constrained by exploration drillholes and high-resolution aeromagnetic data, which allowed the interpretation of the broad internal structure of the intrusive complex and two shear zones, which define for a major part of their strike extent the contacts between ultramafic and overlying mafic intrusive rocks. Restoring the post-emplacement component of strike-slip displacement and subsequently rotating the intrusive complex back from its tilted to sub-horizontal orientation of emplacement, provides a three-dimensional rendition of a 14 km-long dominantly tabular-shaped intrusive complex. The complex is defined by intact basal intrusive contacts with localized keel-shaped ultramafic promontories parallel to a nearby blade-shaped magmatic conduit. The keels extend in a subparallel orientation to depths of up to 1 km into the underlying tonalitic rocks and coincide with significant offsets of the basal intrusive contact, leading to speculation that their intrusion may have been guided by pre-existing normal faults. Two of these magmatic conduits host Ni-Cu-(PGE) deposits/prospects that formed from gravitational segregation of sulfides. The chromitite deposits, hosted in ultramafic rocks at higher stratigraphic levels, were modeled based on drill hole assay data. Zones of dominantly massive to semi-massive chromitite were defined by the ≥ 35% Cr 2 O 3 threshold, while intercalated sequences of chromitite in dunite/peridotite were defined by the ≥ 15% to ≤ 35% Cr 2 O 3 percentage range. The 3D modeled surfaces fitted to these constraints exhibit ore zones with lateral extents from several hundreds of meters for massive bodies up to a maximum of 2 km for intercalated chromitite ore that are dominantly conformable to the layering of the ultramafic host rocks. The lack of lateral continuity of chromitite ore zones beyond this scale, suggests that the chromitite layers are not continuous along the entire strike extent of the Esker intrusive complex, which is consistent with bifurcations of the 3D modeled ore shells signifying pinch-outs of both the massive chromitite layers and the dunitic interlayers. This discontinuous distribution of chromitite ore resembles that in the stratiform deposits of the Stillwater Complex and may support recent models of mechanical segregation of chromitite from cotectic chromite-olivine slurries. Based on our 3D model, we propose that the Esker intrusive complex evolved as a series of individual intrusions that were emplaced during multiple magmatic pulses that eventually coalesced to form a composite ultramafic–mafic complex prior to being dissected and partly dismembered by post-ore shear zone deformation. [ABSTRACT FROM AUTHOR]

In Archean greenstone belts, magmatism is dominated by intrusive and volcanic rocks with tholeiitic affinities, as well as tonalite- and granodiorite-dominated large-volume batholiths, i.e., tonalite–trondhjemite–granodiorite (TTG) suites. These intrusions are associated with poorly documented mineralization (Cu-Au porphyries) that, in the Neoarchean Abitibi Subprovince (>2.79 to ~2.65 Ga), Superior Province, Canada, are associated with diorite bearing plutons, i.e., tonalite–trondhjemite–diorite (TTD) suites. The importance of TTG versus TTD suites in the evolution of greenstone belts and of their magmatic-hydrothermal systems and related mineralization is unconstrained. The aim of this study was to portray the chemistry and distribution of these suites in the Abitibi Subprovince. The study used data compiled by the geological surveys of Québec and Ontario to evaluate the chemistry of TTG and TTD suites and uncovered two coeval magmas that significantly differentiated (fractional crystallization mostly): 1) a heavy rare earth elements (HREE)-depleted tonalitic magma from high pressure melting of an hydrated basalt source; and 2) a hybrid HREE-undepleted magma that may be a mixture of mantle-derived (tholeiite) and tonalitic melts. The HREE-depleted rocks (mostly tonalite and granodiorite) display chemical characteristics of TTG suites (HREE, Ti, Nb, Ta, Y, and Sr depletion, lack of mafic unit, Na-rich), while the other rocks (tonalite and diorite) formed TTD suites. Tonalite-dominated magmatism, in the Abitibi Subprovince, comprises crustal melts as well as a significant proportion of mantle-derived magmas and this may be essential for Cu-Au magmatic-hydrothermal mineralizing systems. [ABSTRACT FROM AUTHOR]

One of the major consequences of photosynthesis is locally increased carbonate alkalinity, which stimulates the precipitation of carbonate minerals in aqueous environments. On the modern Earth, reef–type ecosystems occupy the most productive end of the oxygen– and carbonate–producing spectrum. However, prior to about 700 million years ago, photosynthetic bacteria fulfilled this role, building reef–like microbialite (e.g., stromatolite) structures, and eventually whole carbonate platforms, out of the carbonate minerals they help precipitate as a consequence of their alkalinizing effect. The most ancient examples of these systems, dating back to ca. 3.0 Ga, remain little explored. The EARTHBLOOM project examines in detail the stratigraphy, major element and trace element geochemistry of the 2,940 +/- 2 Ma to 2,925+/- 3 Ma Ball assemblage, Red Lake Greenstone belt, N. Ontario, Canada (dates from Corfu and Wallace, 1986, Can. J. Earth. Sci. 23, 27–42). This deposit contains over 400m of metasedimentary rocks that include over 200m of stromatolitic calcite and dolomite, making it the earliest known significant accumulation of carbonate on the planet. Field studies and examination of multiple industry drill cores reveal that in the east, deep-water facies (BIF, chert) are overlain by carbonates with pervasive microbial fabrics, while in the west, carbonates showing few microbial textures were deposited on and interbedded with abundant siliciclastics. Trace element data indicate general anoxia during precipitation of carbonate lithofacies. Relatively abundant pyrite and potential pseudomorphs after gypsum suggest evaporative concentration of sulfate, and sulfur isotope studies are currently underway. Carbonate C and O isotope compositions are consistent with precipitation from Archean seawater. Carbonate C isotopes, however, do show some variation between facies, with lighter values in deeper water facies. Organic carbon isotopes show a significant range, with values spanning from -30 to as heavy as -10 permil. The statistical distribution of organic carbon isotope data appears to suggest different organic carbon sources in near-shore carbonate-dominated environments versus off-shore cherty or shaley environments. A diagenetic origin of this signal appears unlikely based on C- and O- isotope systematics. The ensemble of data available to date indicates that the Red Lake carbonate platform represents an evaporitic setting, perhaps with locally concentrated sulfate, and remarkably, with little evidence for the presence of cyanobacteria during carbonate precipitation. [ABSTRACT FROM AUTHOR]