Geology

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A fluid inclusion study of the Little Nahanni LCT-Type Pegmatite Group, NWT Canada: implications for the nature and origin of fluids in LCT-Type Pegmatites and Pegmatite evolution
(2016-12-19) Burns, Michael George Garrett
The Cretaceous (82 Ma) Little Nahanni Pegmatite Group , Canada, is a 15 km long swarm of LCT-type pegmatite dikes that intrude Precambrian to Lower Cambrian siliciclastic metapelites and carbonate rocks. The dykes are dominated by a primary assemblage of quartz, spodumene, K-feldspar ± muscovite lacking both graphic textures and core zones, but characterized by combtextured spodumene and K-feldspar intergrowths. Varying degrees of albitization and lesser phyllic alteration accompany ore-grade oxide enrichment (Ta-Nb and Sn, respectively).A comprehensive fluid inclusion study using a wide variety of methods (petrography , CL, microthermometry, SEM/EDS evaporate mound analysis, LA ICP-MS, laser Raman spectroscopy, δ13C) on minerals (quartz, K-feldspar, albite, spodumene, muscovite, garnet, and tourmaline) from barren and mineralized samples indicate the fluid inclusions are secondary and appear to post-date pegmatite crystallization. The presence of secondary fluid inclusions may instead represent entrapment during the late-stage (sub-solidus to post crystallization) metasomatic evolution stage of the pegmatites. Three fluid types occur, and listed in paragenetic order are: (A) an aqueous-carbonic fluid (≤1 wt. % eq. NaCl) with varying XCO2 (0.1 to 0.78), purity (TmCO2= -57o to -61oC), and density; (B) a low-salinity (≤2 wt. % eq. NaCl) aqueous fluid; and (C) an aqueous-methane-nitrogen fluid. SEM-EDS analysis indicates that the fluids are Na (-K) - dominant, but with significant S and enrichment in transition and chalcophile elements. Fluid inclusion extracts have δ13C values (-9.7 to -27.7‰; n=7) which indicates exchange of the fluids with the immediate, carbon-bearing wall rocks occurred. Collectively the data indicate: (1) fluids were trapped during the later, metasomatic stage of pegmatite evolution; (2) fluids were trapped at <2.5 kbars and <350°C with pressure fluctuating between lithostatic and hydrostatic; (3) variable degrees of fluid:rock interaction occurred with the exocontact zone, resulting in varying XCO2and CO2:CH4 values; and (4) metasomatism and rare metal mineralization occurred in the presence of meteoric water. These findings indicate that the LNPG pegmatites are not a closed system, some of the carbonic component of H2O-CO2 fluids is exotic, and that low-salinity fluids of meteoric origin infiltrated during the subsolidus stage of pegmatite evolution.
Sedimentology, stratigraphy, and U-Pb detrital zircon geochronology of the Mesoproterozoic Husky Creek Formation, Coppermine River Group, Nunavut, Canada
(2019-01-08) Meek, Robert David
The Husky Creek Formation (part of the Coppermine River Group, western Nunavut, Canada) is a <1.23-billion-year-old siliciclastic sedimentary unit that represents the uppermost portion of the Sequence A stratigraphic subdivision – a package of Proterozoic volcanic and sedimentary rocks correlative across the Canadian Arctic. Sedimentologic indicators suggest a continental, temperate to arid depositional environment for the Husky Creek Formation, which included fluvial channel belts and floodplains subject to local aeolian winnowing. U-Pb detritalzircon geochronology resolved derivation from rocks related to the Mackenzie Igneous Event, or recycling from older sediments such as the Hornby Bay Group. The Husky Creek Formation was deposited in the waning stages of the Mackenzie Igneous Event, in a restricted basin that was carved atop an extensive mafic volcanic plateau. A dominantly west-northwestward palaeoflow suggests that sediment transport from the core of the Laurentian craton was not significantly disrupted by crustal doming related to large-igneous-province emplacement.
Volcanic reconstruction of the paleoproterozoic powderhouse formation, Snow Lake, Manitoba, Canada: implications for controls on volcanogenic massive sulfide formation
(2019-02-08) Friesen, Vanessa C.
The Powderhouse formation of the Paleoproterozoic Snow Lake arc assemblage comprises the stratigraphic footwall to six volcanogenic massive sulfide deposits (VMS) at Snow Lake, Manitoba, Canada. It is interpreted to be a product of voluminous pyroclastic eruptions and concomitant subsidence followed by a period of relative volcanic quiescence that was dominated by suspension sedimentation, the reworking of these previously deposited pyroclastic units by debris flows and bottom currents, and localized emplacement of rhyolite domes. The rhyolite domes are spatially associated with the Chisel, Chisel North, Lost, Ghost, Photo and Lalor deposits. The Chisel, Lalor and Lost members comprise the Powderhouse formation and are subdivided into thirteen lithologically and chemically distinct lithofacies and allows, for the first time, correlation between the South Chisel basin and Lalor area. The Chisel and Lalor members contain lithofacies and bedforms that are characteristic of emplacement by subaqueous pyroclastic mass flows and concomitant subsidence. The Chisel member also contains coarse volcaniclastic breccias emplaced by mass debris flows derived from movement along fault scarps after early pyroclastic eruptions, and during continued subsidence. The Lost member consists of lithofacies deposited by mass flows generated from faults scraps during continued subsidence, but also contains lithofacies reworked by bottom currents and those deposited by suspension sedimentation, and locally coherent rhyolite. The Lost member represents a time stratigraphic interval, the “Ore Interval”, that marks contemporaneous rhyolite dome eruption, VMS formation, and a hiatus in explosive volcanism.
Gravity data acquisition and potential-field data modelling along Metal Earth's Chibougamau transect using geophysical and geological constraints
(2019-07-26) Ghahfarokhi, Amir Maleki
The Metal Earth (ME) project aims to understand the underlying geological mechanisms that differentiate mineral endowments in Precambrian greenstone belts of the Canadian Shield. The ME project acquires and collates various geological and geophysical data along 13 transects to create valid models of subsurface features in order to identify components that contribute to the mineralization processes that result in mineral endowment. In this thesis, gravity observation along ~128 line kilometers in the Chibougamau transect is considered. The acquired data were checked for quality, processed to calculate the complete Bouguer anomaly and combined with existing gravity data provided by the Geological Survey of Canada. Gravity and compiled magnetic data were forward modelled along four sections and constrained by surficial geological observations, seismic sections, and petrophysical properties to estimate and improve the geometry and depth of plutonic bodies, and identifying the subsurface features such as dykes and faults. These improvements will help others to identify components that contribute to mineralising processes.
Deformation history and gold mineralization of the Cadillac Group north of the Larder Lake-Cadillac deformation zone, southern Abitibi greenstone belt, Quebec
(2019-08-16) Samson, Brendon
The Cadillac Group is located immediately north of the Larder Lake–Cadillac deformation zone (LLCDZ) along the southern margin of the Abitibi greenstone belt in the Archean Superior Province, Quebec. The LLCDZ is a major 250 km long structure known for hosting several gold deposits. It is generally east-trending over a distance of 170 km, and then it swings to the southeast over a distance of 35 km along the so-called Malartic segment, before resuming its easterly trend for another 45 km. With its thick sequence of <2687 Ma turbiditic sandstone interlayered with mudstone and polymictic conglomerate, the Cadillac Group provides favourable rocks for studying the structural history of the Malartic segment which, due to its unique trend, is important for understanding the overall tectonic evolution of the LLCDZ. Deformation of the Cadillac Group began with large-scale isoclinal folding and the formation of a regional cleavage parallel to the trend of the Malartic segment (D1 event). These structures overprint a 2675.5 ± 2 Ma quartzfeldspar-phyric (QFP) intrusion providing a maximum age for the D1 event and a minimum age for the deposition of the Cadillac sequence. The deformation of the Cadillac Group resumed with bedding-parallel sinistral shearing and the emplacement of gold-bearing extensional quartz veins (D2 event), and continued with bedding-parallel dextral shearing, Z-shaped folding, the formation of a cleavage oriented oblique (anticlockwise) to the trend of the Malartic segment, and by the rotation and segmentation of the gold-bearing veins (D3 event). Contrary to previous interpretations of the Malartic segment as an early pre-D1 accretionary structure, it is interpreted as the short limb of a broad Z-shaped flexure that formed during D3 folding of the LLCDZ and that postdates the emplacement of gold-bearing veins in the Cadillac Group. Mineralized veins at the nearby worldclass Canadian Malartic deposit were emplaced during the same D2 event but they differ in alteration mineralogy and geochemistry to the Cadillac veins suggesting different sources for the mineralizing fluids. Mineralization at the Lapa deposit is similar in sulphide mineralogy and metal association to the Cadillac veins but was emplaced earlier during the deformation history of the LLCDZ.
Emplacement of sharp-walled sulphide veins during reactivation of impact-related structures at the Broken Hammer Mine, Sudbury, Ontario
(2019-08-26) Hall, Marshall Francis
Broken Hammer is a hybrid, Cu-Ni-Platinum Group Element (PGE) footwall deposit in the North Range of the ca. 1.85 Ga Sudbury impact structure. The sulphide vein system and associated low sulphide PGE mineralization were mined as an open pit operation over a 15-month period, providing a unique opportunity to study a complete 90-meter vertical section across a footwall deposit. The deposit is hosted within Archean basement rocks and impact-induced Sudbury breccia, 1.5 km north of the Sudbury Igneous Complex (SIC) – basement contact. The low sulphide mineralization consists of disseminated to blebby chalcopyrite (<5%), minor pyrite, chalcocite, galena, sphalerite and platinum group minerals, associated with Ni-bearing chlorite overprinting alteration patches of epidote, actinolite and quartz. The veins comprise massive chalcopyrite and minor magnetite, chalcocite, millerite, and rare sperrylite, surrounded by thin epidote, actinolite and quartz selvedges. They are grouped into five, steeply-dipping, NE-, SW-, SE-, S- and EWstriking sets, which intersect in a common line controlling the plunge (60°) and trend (220°) of ore shoots. The veins were emplaced along syn-impact fractures that were reactivated multiple times during stabilization of the impact crater floor. Early reactivation of the fractures created pathways for the migration of hydrothermal fluids from which quartz and chlorite precipitated sealing the fractures. Renewed slip and reactivation shattered the quartz-chlorite veins into fragments that were incorporated in massive sulphide veins that crystallized from strongly fractionated sulphide melts or high temperature (400°C-500°C) hydrothermal fluids which migrated outward into the basement rocks from a cooling and crystallizing impact melt sheet represented by the SIC. Hydrothermal fluids syn-genetic with the epidote-actinolite-quartz alteration distributed the PGE into the footwall rocks, or late hydrothermal fluids associated with the Ni-bearing chlorite leached Ni and PGM’s from the sulphide veins and re-distributed them within the footwall rocks, forming the low-sulphide high-PGE mineralization. During post-impact tectonic events, reactivation and slip at temperatures below the brittle-ductile transition for chalcopyrite (<200°C-250°C) produced striations along the vein margins. The Broken Hammer deposit exemplifies how Cu-Ni-PGE footwall deposits formed by the reactivation of syn-impact fractures that provided conduits for the migration of melts and hydrothermal fluids.
Can magnetic susceptibilities measured on outcrops be used for modelling (and constraining inversions of) aeromagnetic data?
(2019-09-07) McNeice, William John
Magnetic susceptibilities measured on outcrop and drill-core samples using hand-held instruments have been shown in the literature to be useful for identifying mineralogical changes. It is not yet clear how useful these measurements are for constraining magnetic modelling and inversion. We have generated estimates of the apparent magnetic susceptibility of the ground by mathematical transformation of an aeromagnetic data and assumed that these values can be used to model the magnetic data. In the same area we have a large number of measurements on outcrop and have compared these two independent estimates. When the measured values are below 1x10-3 S.I., there is a no correlation between the measured and apparent values, interpreted to be likely due to the influence or interference from nearby or underlying magnetic sources. Hence, in this case the measured values cannot be used to constrain modelling and inversion. When the measurements are above this value there is a limited correlation, with values only agreeing to within a factor of about 10, so these values can be used as very rough constraints. The poor correlation is interpreted as due to the presence of remanent magnetization or heterogeneity of the magnetic susceptibility within the rock. A large database of outcrop measurements gives an indication of the range of the variation in magnetic susceptibility values that could be used in modelling.
Estimating overburden thickness in resistive areas from on-time airborne em data
(2019-09-27) Bagley, Thomas
We propose a method to invert two-component time-domain EM data to a thick-sheet over half-space model, as a solution representing an overburden on top of bedrock. We first estimated the conductivity of the lower half-space using a combination of inversion for half-space (if appropriate) or a thin sheet over half-space (also when appropriate). This yielded a number of estimates, which could be combined to give a reasonable estimate of the lower half-space conductivity for the survey area. With this estimation an equation solver was used to solve the thick-sheet over half-space model for sheet thickness and sheet conductivity. The output of the algorithm was generally stable when applied to GEOTEM data in an area of moderately resistive overburden over a generally more resistive half-space. Although it did not reliably reproduce the overburden thicknesses as measured in the reference drill holes, it did give an estimate that was reasonable in the conductive areas.
Compositional and textural analysis of host-rock diamictite matrix at the Kakula copper deposit, Democratic Republic of Congo
(2019-10-11) Dabros, Quinn
The Kakula deposit is a high-grade sedimentary-rock-hosted Cu deposit (628 Mt, 2.72% Cu indicated resource, 1% cut-off) ~10 km south of the Kamoa deposit (759 Mt, 2.57% Cu indicated resource, 1% cut-off) in the central African copperbelt, Democratic Republic of Congo. Copper-sulphide ore (chalcocite, bornite, chalcopyrite) at Kakula is predominantly disseminated in the fine-grained matrix of clast-poor (≤20% clasts ≥2 mm) subaqueous debrite (diamictite), at the base of the nearly flat-lying midNeoproterozoic “grand conglomérat” (Mwale formation). Scanning electron microscopy was used to document matrix texture and composition to develop a matrix paragenesis, recorded in five phases of the matrix evolution: sedimentation, pre-ore diagenesis, mainore mineralisation, post-ore alteration, and weak tectonism. The ore-zone matrix is porous, up to 12.5%, and consists of clay- to silt-sized muscovite, quartz, chlorite, Kfeldspar, dolomite, and biotite, whereas least-altered matrix, several hundred metres above copper-sulphide mineralisation, consists of clay- to silt-sized quartz, albite, chlorite, K-feldspar, calcite, and dolomite. Copper-sulphide precipitation is contemporaneous with chlorite and biotite (+/- hematite, quartz, and K-feldspar) and fit in a paragenetic sequence between diagenetic pyrite (± Fe-dolomite) and later muscovite. Hematite is ore-stage and most abundant in areas containing chalcocite. Areas of mm- to cm-scale “aligned” matrix (nearly vertical microfabric of aligned, elongated grains) consists of a higher abundance of muscovite, locally elongated copper-sulphides, and a lower concentration of copper than non-aligned matrix, suggesting that copper-sulphide development pre-dated fabric development and that copper-sulphide grains were later dissolved and possibly remobilised. Although determining the original mineralogy and texture of the diamictite matrix is challenging, the depositional matrix characteristics (clast-rich versus clast-poor diamictite) and the availability of reactive agents (e.g., diagenetic pyrite) may have been important controls on copper grade and distribution. More work is required to constrain the absolute timing of mineralisation, which is a major debate at Kakula and Kamoa.
Provenance and depositional age of the Cryogenian ‘grand conglomérat’, host of the Kamoa Cu-deposit, Democratic Republic of Congo
(2019-10-11) Trudel, Philippe Gerald Eric
The Kamoa copper deposit (759 Mt, 2.67% Cu at 1% cut-off) is one of the largest copper deposits in the central African copperbelt. Mineralisation at Kamoa is hosted by the Cryogenian ‘grand conglomérat’, which was deposited in an intracratonic rift and its sub- basins associated with the break-up of Rodinia, and is equated to the ‘Sturtian’ glaciation, the oldest of the Neoproterozoic glacial events. Using detrital zircon geochronology, the provenance of synglacial and underlying pre-glacial units is compared, to evaluate the temporal dynamics of the sediment supply at Kamoa. The detrital zircon age distributions suggest that the ‘grand conglomérat’ was derived from locally exposed basement of the Mesoproterozoic Kibaran Supergroup and from significant recycling of underlying pre-glacial units, with minor contributions from distal, primary source areas. The stratigraphic differences in the distribution of zircon age populations of the strata at Kamoa may reflect changes in glacial dynamics or of rifting during deposition of the ‘grand conglomérat’. A Neoproterozoic zircon age population ~700 Ma is probably derived from rift-related volcanic rocks and is significant because it repositions the timing of mid-Neoproterozoic rifting in the Katangan basin and the maximum depositional age of the ‘grand conglomérat’. The new age constraints suggest that the midNeoproterozoic ‘Sturtian’ glaciation was globally diachronous.
Litho-and chemostratigraphy of the lower to middle cambrian Mount Clark, Mount Cap and Saline river formations in Hornaday River Canyon, Carcajou Range, and Norman Range, Northwest Territories
(2019-12-06) Bouchard, Mélanie Lynn
The Lower to Middle Cambrian Mount Clark (sandstone-dominated), Mount Cap (sandstone and dolostone), and Saline River (dolostone, gypsite, and siltstone) formations were deposited in an epicratonic basin that spans northwestern mainland Canada from the eastern Mackenzie Mountains to the Arctic coast (>500 km distance). A cross-section through the three Cambrian formations was developed based on stratigraphic sections documented at surface exposures on the northeastern margin of the basin (Hornaday River canyon), in the west-central part of the basin (Norman Range), and in two southwestern locations near the ancient Mackenzie arch (Carcajou Range). Carbon isotopic (δ13Cvpdb) data for the Mount Cap and Saline River formations show correlatable regional trends that are consistent with part of the Cambrian global composite carbon isotope curve (including the global ROECE and DICE negative carbon isotope excursions), but also highlight local excursions that record phenomena characteristic of specific, localised depositional paleoenvironments near the Mackenzie arch. Carbon isotope chemostratigraphy is here demonstrated to be a robust tool that can augment biostratigraphy in future studies that require correlation of lower Paleozoic strata in northwestern Canada.
The use of sulfide mineral chemistry to understand PGE mineralization processes in the Northern Limb of the Bushveld Igneous Complex
(2020-01-10) Mkhonto, Sharlotte C.
The Platreef is a complex lithological unit defining the base of the Northern Limb of the Bushveld Igneous Complex (BIC) and typically characterized by complex magma-footwall interaction. It is of interest primarily because it contains significant PGE-Ni-Cu mineralization and there is no consensus regarding the mineralization processes, particularly on the role of footwall assimilation. One puzzling aspect is the apparent lack of correlation between sulfide abundance and PGE-Ni-Cu contents. Another aspect is the possible relationship between PGE-Ni-Cu mineralization in the Northern Limb and PGE-Ni-Cu mineralization elsewhere in the BIC (e.g. the Merensky Reef). This study focuses on the mineral chemistry (major and trace elements) of major sulfides: pyrrhotite (Po), pentlandite (Pn), and chalcopyrite (Ccp) in Platreef samples intersected by Ivanplats during deep drilling in the Turfspruit area. A drill core (UMT094), for which Sr and S isotope stratigraphy was collected previously, was used as reference because the isotopic data helped define the degree of footwall assimilation. Whole rock geochemistry (Cu, Ni, S, PGE) showed the existence of at least two distinct sample populations in UMT094: one defined by relatively low S/Ni (with no evidence of footwall assimilation and stratigraphically associated with PGE mineralization), the other defined by relatively high S/Ni (and stratigraphically associated with footwall assimilation and with low PGE grades). The project had three objectives. The primary goal was to assess whether the differences identified in whole rock geochemistry were also expressed in the composition of major sulfides (pyrrhotite, pentlandite, chalcopyrite) in UMT094. Having established those parameters, the second goal was to use the same approach (and assess the degree of footwall assimilation) on samples (from drill cores UMT231 and UMT233) considered representative of the main PGE-Ni-Cu mineralization (i.e. the Flatreef) in the Turfspruit area. Finally, the data obtained in this study were compared to similar data from the Merensky Reef (from published peer-reviewed literature) to evaluate possible matches between sulfide chemistry in assemblages associated with PGE mineralization in the Northern Limb and the Western Limb of the Bushveld Igneous Complex (to assess a possible common origin). After careful petrographic observations and documentation, major and trace element contents in sulfides were acquired using electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The results showed that there are no significant differences in major element chemistry (e.g. Fe/Ni in pentlandite, Fe/S in pyrrhotite) between the two populations identified. However, the trace element data validate the hypothesis tested and show that in UMT094: (a) Selenium contents in all sulfides associated with PGE-Ni-Cu mineralization are higher than in sulfides associated with footwall assimilation; and (b) Other trace elements (such as Co and Ru in pyrrhotite; Co, Pd and Ru in pentlandite; Cd and Sn in chalcopyrite) are also useful in distinguishing between sulfides in intervals with no evidence of footwall assimilation and sulfides (lower in the stratigraphy) associated with footwall assimilation. When applied to the Flatreef samples (UMT231 and UMT233) the results show that, in general, trace element content in sulfides from the Flatreef also varies with stratigraphy: sulfides from Flatreef intervals with high PGE grades (and intervals above) have trace element contents that are closer match to sulfides from the unaltered (magmatic) intervals in UMT094. Finally, when data from this study is compared to similar data from the Merensky Reef the results show significant overlap. This is interpreted as compelling evidence to conclude that the mechanisms that operated in the formation of PGE-Ni-Cu mineralization in the Flatreef were similar to those involved in the formation of the Merensky Reef (for which local footwall assimilation is not considered a significant factor). Thus, footwall assimilation was not the primary process in PGE-NiCu mineralization. These results show that trace element data in major sulfides can be an additional and useful tool to distinguish among different sulfide mineralization events
Mobilization of komatiite-associated Ni-Cu-(PGE) mineralization in the Southern Manneville fault zone, Southern Abitibi subprovince, Quebec
(2020-03-25) Shirriff, Danielle; Shirriff, Danielle
The Cubric showing and Marbridge deposit occur in the 2714 ± 2 Ma La Motte-Vassan Formation, 20-25 km north of Malartic, Québec. Mineralization at the Cubric showing is hosted by a silicate-magnetite facies iron formation as well as gabbro, but the high Ni/Cu and Ir/Pd ratios of the mineralization suggest that it formed from a komatiitic magma, similar to the mineralization at the historic Marbridge deposit. Normal abundances of most metals in the mineralization in the gabbro suggest that it was mechanically mobilized into the gabbro as a high-temperature monosulfide solid solution, whereas low abundances of Cr and Ir-Ru-Rh in the mineralization in the iron formation suggest that Ni-Cu-Pt-Pd-Au and other metals were mobilized into the iron formation by metamorphic-hydrothermal fluids. The host rocks and country rocks have experienced two phases of deformation during regional metamorphism, and it is likely that the mineralization was mobilized from unexposed/eroded mineralized cumulate komatiites during this deformation and metamorphism, which occurred after emplacement of the younger 2680 ± 1.5 Ma gabbro.
Emplacement history of mafic-ultramafic volcanic rocks in the northern Pontiac subprovince, Quebec, Canada
(2020-05-15) Rehm, Adrian Gerhard Nutting
Detailed geological mapping of volcanic, sedimentary and intrusive rocks in the northern Pontiac subprovince of the Superior Province has identified multiple volcanic-sedimentary contacts at the margins of a 30 km long volcanic package of high-Fe tholeiites and Munro- to Barbertontype komatiitic flows and sills. Breccias displaying blocky, globular and tendril-like juvenile clasts in a massive sedimentary matrix were observed along the contacts of mafic-ultramafic intrusions with their host sedimentary rocks. These textures are consistent with peperite, a rock formed by the mixing and mingling of magma with wet, unconsolidated sediments. Geochemical data for one of the peperites indicates that the igneous component has a similar trace element composition to spinifex-bearing komatiite flows and the sedimentary component has a similar composition to the Pontiac Group wackes. Additionally, conformable unstrained contacts were observed between effusive volcanic rocks and turbiditic wackes. These relationships indicate a previously unrecognized ca. 2682 Ma mafic and ultramafic volcanic event synchronous with the deposition of the Pontiac subprovince sedimentary rocks. This suggests that the mafic-ultramafic volcanic package is synsedimentary and autochthonous, which is not consistent with previous interpretations of these rocks as structural thrust panels. We propose that the mafic-ultramafic volcanic package was emplaced during a previously unrecognized plume-related extensional event that facilitated the upward flow of mantle-derived magmas along major, mantle-penetrating structures during Pontiac sedimentation. Shortly after, tectonic compression folded and imbricated the Pontiac rocks and accreted them to the southern margin of the Abitibi subprovince.
Modelling the airborne electromagnetic response of a sphere underlying a uniform conductive overburden
(2020-05-15) Zamperoni, Anthony
Electromagnetic geophysical methods are used in mineral exploration to detect conductors at depth. In igneous and metamorphic settings, the background half-space is often largely resistive. In such cases, it is important to consider the interaction between the target conductor and any thin, conductive overburden that might exist above the half-space. The overburden is often comprised of glacial tills and clays or the weathering of basement rocks to more conductive material. This situation can be approximated using a discrete conductor model consisting of a “dipping sphere” in a resistive half-space underlying a uniform conductive overburden. A semianalytical solution that considers the first-order interaction of the sphere and overburden has been derived to calculate the electromagnetic response. The simplicity and efficiency of this solution makes it well suited to be implemented when computation time and immediacy of results are desirable. To this end, we have developed a graphical user interface (GUI) based program to model the electromagnetic response of this model. The program allows users to change the parameters of the survey and target body and quickly view the resulting changes in the shape and decay of the electromagnetic response. The program was tested on airborne electromagnetic data from the Forrestania test range in western Australia. The sphereoverburden model as implemented in the program was able to fit the anomalous data with a spherical body buried 156 m deep and having a dip of 35 degrees to the north.
A study of millerite from Cu-Ni-PGE footwall veins, Sudbury, ON: crystal-chemistry, morphology, & geological implications
(2020-05-20) Gore, Thomas Edward
Millerite (-NiS) is a common accessory mineral in magmatic Cu-Ni-PGE deposits, such as those occurring in Sudbury, Ontario, Canada. In such localities, millerite develops as platy crystals, often forming large, cleavable masses. This is in stark contrast to the more widespread occurrence of millerite as a hydrothermal precipitate within vugs in carbonate basins, where it develops an acicular morphology, with crystals often exhibiting exaggerated aspect ratios (l:w > 20). The reason for these contrasting morphologies is attributed directly to the genesis of magmatic millerite that initially crystallizes as the high-temperature mineral crowningshieldite (-NiS), later inverting to millerite upon further cooling. The morphology exhibited by platy millerite is thus representative of crowningshieldite via a paramorphic relationship. Millerite of magmatic origin also frequently exhibits secondary twinning on {011̅2}, resulting from a postcrystallization increase in pressure. Development of these twins produces a variable response under BSE microscopy that is attributed to orientation contrast.
Structural evolution and orogenic gold metallogeny of the western Wabigoon subprovince, Canada
(2020-09-15) Zammit, Kendra
Some of the world’s largest orogenic gold deposits are hosted within granite-greenstone subprovinces of the Superior Province of Canada. The apparent gold endowment of the western Wabigoon subprovince is much less than that of the Abitibi subprovince, despite both Neoarchean subprovinces sharing similar structural and metallogenic histories. Further, the relationship between regional deformation and orogenic gold mineralizing events in the western Wabigoon subprovince is poorly constrained in comparison to other greenstone belts within the Superior Province. New structural mapping of the Dryden area of the western Wabigoon subprovince indicates that it experienced a N-S shortening event (D1), 2705-2695 Ma, which reactivated, or initiated the formation of, the E-W-trending Wabigoon and Mosher Bay-Washeibemaga deformation zones (Wdz and MBWdz, respectively). Following the D1 event, after 2695 Ma, a protracted period of NNW-SSE shortening (D2) localized transpression along the Manitou Dinorwic deformation zone (MDdz), which is spatially associated with orogenic gold systems, including the Goldlund deposit (~1.7 Moz Au) and Kenwest prospect (~0.3 Moz Au). During D2, the MDdz experienced sinistral transpression, while the Wdz and MDdz experienced dextral-sense movement. U-Pb geochronology of vein-hosted xenotime from the Goldlund deposit and Kenwest prospect indicate that hydrothermal events, and likely gold mineralization, occurred syn- to late- D2 deformation at 2664 ± 8.3 Ma and 2580 ± 12 Ma, respectively. The structural evolution and orogenic metallogeny of the western Wabigoon subprovince is similar to that of anomalously gold- rich subprovinces of the Superior Province. Therefore, it is proposed that the structural evolution of individual greenstone belts is not the primary controlling factor on their bulk gold endowment. Nevertheless, orogenic gold deposits are spatially associated with major deformation zones and linked to widespread hydrothermal event(s), spanning 2680-2580 Ma across the southern Superior Province.
Sulfide textures, geochemistry, and genesis of the Komatiite-Associated Eagle’s Nest Ni-Cu- (PGE) Deposit, McFaulds Lake Greenstone Belt, Superior Province, Ontario, Canada
(2020-09-28) Zuccarelli, Natascia
The Eagle’s Nest nickel-copper-(platinum-group element) deposit occurs within the 2.73 billion- year-old Double Eagle intrusive complex of the Ring of Fire intrusive suite, in the McFaulds Lake greenstone belt in northern Ontario. It contains 11 million tonnes of proven and probable mineralization grading 1.68 wt.% nickel, 0.87 wt.% copper, 0.89 g/t Pt, 3.1 g/t Pd, and 0.18 g/t Au, and 9 million tonnes of inferred mineralization grading 1.0 wt.% nickel, 1.14 wt.% copper, 1.16 g/t Pt, and 3.49 g/t Pd. Mineralization occurs along the northern margin of a subvertical blade- shaped dike~500 m wide (N-S) x ~85 m thick (E-W) x >1500 m deep composed of harzburgite, lherzolite, and wehrlite. It is interpreted to have been emplaced as a subhorizontal blade-shaped intrusive body into tonalitic rocks below the Double Eagle intrusion and rotated into its present subvertical orientation. Four sulfide textural facies have been defined spatially and geochemically through detailed core logging, petrography, and whole-rock and mineral geochemical analyses: 1) disseminated (fine-grained olivine and pyroxene with <15 wt % uniformly distributed interstitial sulfide), 2) net (fine-grained olivine and minor pyroxene with 15-35 wt% semi-continuously to continuously distributed interstitial sulfide), 3) semi-massive (50-80 wt% sulfide with 20-40% anteliths and gabbroic xenoliths), and 4) massive (>80 wt% sulfide with <20% xenoliths). There are relatively few areas containing 35-50 wt% sulfide mineralization. Five subfacies of net texture have been identified: 1) bimodal olivine bearing leopard-net texture (most common), 2) fine- grained patchy-net texture with irregular patches of sulfide (uncommon), 3) orthopyroxene- bearing pinto-net texture (coarse altered orthopyroxene similar to white spots on pinto-coloured horse), 4) inclusion-net texture, each characterized by different peridotite inclusion populations, and 5) localized zones of ‘disrupted-net texture” containing 3-5 cm thick zones of amoeboid cross- cutting barren pyroxenite. All textural facies are characterized by a typical magmatic sulfide assemblage of pyrrhotite – pentlandite – chalcopyrite with minor platinum-group minerals. Massive sulfides are localized in two embayments along the basal contact that are separated by a topographical high, grading upwards to rare semi-massive, laterally more continuous net texture, and disseminated sulfides, with gradational contacts between textures except with massive sulfide. The similar mean ore tenors of the different sulfide textural facies, Ni100 ~7.5 and Cu100 ~4.8, suggest that the majority of the mineralization formed from similar magma compositions at similar magma:sulfide ratios, but the presence of different inclusion populations (peridotite, gabbro, chromitite) and the presence of disrupted-net texture indicates that the olivine, inclusions, and sulfide melts accumulated from multiple pulses in a dynamic system. The smaller, blade-shaped, sulfide-rich, chromite-poor Eagle’s Nest body does not appear to be the feeder to the overlying larger, oblate, sulfide-poor, chromite-rich Double Eagle body. This highlights the need to understand the fluid dynamics of entire plumbing systems when exploring for these deposit types and the significance of smaller, more dynamic magmatic conduits as environments favourable for nickel-copper-(platinum-group element) mineralization and of larger, less dynamic chonoliths/sills, as environments favourable to chromite mineralization.
Integrated seismic interpretation of the Larder Lake area, Southern Abitibi greenstone belt, Ontario, Canada
(2020-10-21) Mpongo, Elton
The southern Abitibi greenstone belt is characterized by a series of complex metavolcanic and metasedimentary rocks intruded by granitic plutons and batholiths, which makes the area a hardrock environment. In the Larder Lake area, these stratigraphic units are truncated by two major breaks, the Lincoln Nipissing shear zone (LNSZ) and Cadillac-Larder Lake deformation zone (CLLDZ) which trend NE-SW and E-W, respectively. This thesis is focused on the quantitative interpretation of a ∼44 km long seismic transect acquired as part of the Metal Earth project in the Larder Lake area. Seismic imaging and interpretation in a hardrock environment are challenging due to the lack of continuity of reflections and smaller acoustic impedance contrasts between different stratigraphic units. Hence, structural interpretation of the seismic data is favoured rather than stratigraphic interpretation. The application of curvelet transforms and seismic attribute analysis significantly increased the signal to noise ratio (SNR) of the seismic data. Seismic data have pitfalls in imaging the subsurface geology in a hardrock environment due to a strong degree of structural heterogeneity and complex geometry of targets. These pitfalls are overcome by the integration of seismic data with complementary geophysical methods. This study aims to determine the structural architecture of the area using seismic data and other depth resolving geophysical methods using an integrated modelling approach. The integrated modelling is achieved by extracting anomalies from the geophysical inversion models. The study uses the empirical relationship between physical properties derived from all the datasets and integrates these in a spatial domain. Physical properties from the individual models are differentiated by applying unsupervised learning algorithms for characterization. The recovered physical properties show correlations with the seismic data along deformation zones.
Integrated interpretation of the Blake River Group using geophysical data from metal earth's Rouyn-Noranda transect, Quebec
(2020-11-09) Rapolai, Thobejane Robert
The study of crustal structure in Precambrian greenstone belts is crucial for understanding of mineral deposits and their genesis. This thesis takes on an integrated interpretation of geophysical data for Metal Earth’s Rouyn-Noranda transect as part of a large collaborative research. Regional scale seismic reflection, magnetotelluric and gravity data as well as magnetic and surface geology maps are used to conduct the study. The seismic reflection data was enhanced by the application of the curvelet enhancement method and seismic attributes for to improve interpretation ability. Two sets of density models were used, namely, constrained and unconstrained models. A 2-D conjugate gradient inversion of Magnetotelluric data was done after the phase tensor and skew angle were analyzed to determine dimensionality and directionality. The data was interpreted together in the form of integrated interpretation in order to understand the structure and architecture of the crust along the transect. The seismic data is characterized by poorly reflective upper crust, strongly reflective mid crust, exhibiting complex reflection structure sub-horizontal reflections truncated by gently dipping reflections. The lower crust is characterized by sub-parallel reflections with weak continuity. The magnetotelluric model is characterized by a resistive upper crust with conductive pathways penetrating through the resistive bodies; and a conductive mid-to-lower crust. Integration of the data reveals a good correlation between the data sets and the upper crustal conductive conduits correlate directly with surface geology map structures a well as lineations from the magnetic map. The highly reflective mid crust correlates with the conductive feature in the mid crust in which the correlation is attributed to partial melts in the mid-crust. The density models also show a good correlation with the seismic data and were used to constrain the depths and dimensions of plutonic rocks along the transect.

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