Mineral Deposits and Precambrian Geology - Doctoral theses

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Large-scale resistivity structure of the Superior Craton, Canada
(2023-12-19) Roots, Eric Alexander
The Superior Craton, located within the Canadian Precambrian Shield, hosts significant mineral wealth largely concentrated within its southeastern portion. However, little is known about a) the deep geophysical signatures, and b) the processes involved in the metal emplacement in the Archean greenstone belts of the Superior province. Furthermore, despite several decades of research involving a variety of geophysical methods, the electrical resistivity structure of the Superior Craton remains enigmatic. Via the analysis, inversion, and interpretation of new and legacy magnetotelluric data, this study aims to reveal the large-scale resistivity structure of the Superior Craton, and thus gain insights into the geodynamic processes which were active during its complex history as well as the underlying mechanisms responsible for the disparate levels of shallow metal endowment across the Superior. New 3D inverse modelling results of broadband MT stations from southeastern Superior, including the well-endowed Abitibi and Pontiac subprovinces, reveals the details of a ‘whole-of-crust’ magmatic-hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment, while upper and mid-crustal cross trends suggest mineralized fluids flowed along narrow pathways within and/or oblique to the fault planes. These resistivity features delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids which may be genetically related to a late-stage pulse of ore-bearing magmatism, possibly as a result of slab break-off or delamination. Similar lower-crustal structures are imaged within the less endowed western Superior, where inverse modelling results reveal curvilinear conductors within the lower crust trending sub-parallel to major terrane boundaries and their associated fault systems. Similar to the southeastern Superior, these conductive features are interpreted as the geo-electric signatures of syn- to post- orogenic magmatism during crustal delamination. At depths of 100–200 km, the model is dominated by north-trending bands of alternating low and high resistivity, interpreted to be artefacts of electrical anisotropy. Subsequent anisotropic modelling of the data image an electrically anisotropic layer within the lithosphere, interpreted to represent channelized metasomatism as a result of mid-Proterozoic tectono-magmatic activity.
Factors contributing to metal endowment in the western Wabigoon and southern Abitibi subprovinces: a machine learning approach to Precambrian greenstone belts
(2023-09-20) Montsion, Rebecca M
Mineral exploration workflows are including more quantitative techniques and probabilistic targeting to capture subtle or convoluted relationships to gain insight about geological processes. However, related methodological improvements often focus on efficiency and sensitivity, leaving geologically representative feature engineering underdeveloped. Improving modeling capabilities alone is insufficient, and both geological plausibility and representation of complex processes or features are critical to generate robust predictive models. Geological features involved in Magmatic Ni-Cu-PGE, Volcanogenic Massive Sulfide (VMS) Cu-Zn-Pb-Ag(-Au), and Orogenic Au mineral system prospectivity from two Archean greenstone belts from the southern Superior Province near Timmins and Dryden, Ontario are examined and compared using a variety of statistical techniques. Specifically, this PhD research explores 1) current knowledge and characteristic geological features for both greenstone belts, 2) methods to enhance geological knowledge using whole rock geochemistry, 3) methods to reduce bias and improve repeatability when mapping structural complexity, 4) how data science and geological understanding of mineral systems can be integrated for enhanced feature engineering 5) which factors control, or are most strongly associated with mineralization, and 6) why the greenstone belts near Timmins and Dryden, Ontario have contrasting orogenic Au endowment. Data-related outcomes of this research include multi-disciplinary geoscientific databases (e.g., structural, field observations, geochemistry), new bedrock geology maps for each area, and reprocessed aeromagnetic grids. Methodological outcomes of this research include new geochemical classification diagrams for ultramafic to felsic (including tonalite- trondhjemite -granodiorite and lamprophyre) Archean igneous rocks, Igneous Rock Favorability indices, automatic mapping of structural complexity from bedding measurements and aeromagnetic lineaments, mapping pre-deformation fluid path distances, mapping rheological and chemical contrast, semi-discrete interpolation of characteristic element ratios, as well as mapping mobile element gain/loss. Geological knowledge outcomes include the importance ranking of factors controlling magmatic, volcanogenic, and orogenic prospectivity from random forests as well as geological insight about contrasting orogenic Au endowment in the Timmins and Dryden areas. Overall, this research demonstrates the importance of integrative studies that leverage multi- disciplinary data, methods, and knowledge to improve existing geological understanding, maximize data utility, and generate robust exploration targets. These improvements may enhance exploration under difficult conditions, such as in data sparse environments, regions affected by clustered/partial data representation, or targets under cover.
Tectono-depositional characterization and sedimentary provenance of the Mesoproterozoic Fury and Hecla Basin, Nunavut, Canada
(2022-09-20) Patzke, Mollie Claire McKenna
Understanding the original depositional environments, architecture, and sediment sources of intracratonic basins is integral to terrane analysis in Precambrian cratons. Proterozoic intracratonic basins are widespread throughout Arctic North America, yet their inception and evolution are poorly understood. The late Mesoproterozoic amalgamation of the supercontinent Rodinia represents, specifically, an exemplary time to investigate intracratonic basin development due to many basins being formed. Four basins in eastern arctic Canada and Greenland, known as the Bylot basins, are thought to be genetically linked due to broadly similar lithologies, age, and geographic proximity. Southernmost in this basin system is the Fury and Hecla Basin, which straddles the homonymous strait and is exposed on Baffin Island and Melville Peninsula. Establishing a correlative framework between the Fury and Hecla Basin and the better characterized Borden Basin is key to a refined understanding of the broader Bylot basins. The late Mesoproterozoic strata filling the Fury and Hecla basin, cumulatively known as the Fury and Hecla Group, comprise six formations: Nyeboe, Sikosak Bay, Hansen, Agu Bay, Whyte Inlet, and Autridge; of these, three (the Nyeboe, Sikosak Bay, and Whyte Inlet formations) are sandstone-dominated. Facies analysis indicates that the lowermost Nyeboe Formation records a range of depositional environments including terrestrial, backshore eolian, marine intertidal, wave-dominated marine shelf, and marine-offshore transitional. The Sikosak Bay and Whyte Inlet formations were largely deposited in a wave-dominated marine-shelf realm and show evidence of large sandwave buildup due to prolonged longshore drift. Seven sandstone samples were analyzed for detrital zircon U-Pb geochronology from the Fury and Hecla Group, and yielded grains with ages ranging from ~3350 to ~1695 Ma. Altogether, detrital zircon grains show a bimodal age distribution with peaks at ~2.7 Ga and ~1.9 Ga. Notably, the Archean age peak is more prominent in the lower stratigraphy, whereas the Paleoproterozoic peak is more significant in the upper stratigraphy. Facies and provenance analyses suggest that the Fury and Hecla Basin opened in a half- graben setting, with its master fault located on the present-day northern side of the basin. Initially, sediment was likely derived from erosion of local basement rocks. Facies mapping suggests that the basin expanded its catchment area with time. This trend was accompanied by increasing recycling of older siliciclastic successions from nearby basins – a provenance pattern that dominated over local sediment sourcing in the upper strata. As such, despite broadly similar lithologies and age, the Fury and Hecla Basin shows different depositional environments and a unique evolution, when compared to the known data, from the other Bylot basins.
Assessing primary versus secondary features in two pegmatite swarms with implications for the nature of pegmatitic systems
(2023-03-24) Pfister, Jérémie Damien
Understanding the processes behind the formation and evolution of pegmatites, which host globally significant resources of strategic metals (e.g., Li, Ta, Nb, Sn, Cs, Rb, Y, REE) critical to the global renewable energy transition, is paramount for their exploration and exploitation. This study investigates the nature, origin and evolution of two distinct pegmatite swarms (i.e., the LCT-type Little Nahanni Pegmatite Group, NWT, Canada, and the NYF-type Highway 69 Pegmatite Group, Ontario, Canada) in order to address several unresolved aspects of pegmatite genesis, such as the origin of pegmatite-forming melts (i.e., anatexis versus fractional crystallization), nature of various pegmatitic textures (i.e., primary versus secondary), metasomatism and re-mobilization of rare-metal mineralization, and influence of the host rocks on pegmatites (i.e., open- versus closed systems). This study integrates various aspects of field observations that include detailed mapping, field and petrographic observations (transmitted light, BSE, SEM-CL), bulk whole-rock and trace-elements geochemistry (ICP-AES, ICP-MS), in situ mineral analysis (SEM-EDS), in situ trace-elements analysis (LA-ICP-MS), bulk and in situ stable isotopes (O, H) geochemistry (DI-IRMS, SIMS), and in situ U-Pb geochronology (LA-ICP-MS). The principal findings suggest that: (1) both anatexis and fractional crystallization of a parental melt are viable processes of pegmatite formation; (2) most macroscopic pegmatitic textures are primary and are generally preserved during sub-solidus evolution of pegmatites; (3) late-stage, fine-grained albitic and micaceous units associated with significant rare-metal mineralization are secondary; (4) secondary micro-features related to coupled dissolution-precipitation processes in pegmatites are profuse, despite being rarely reported, and are frequently associated with accessory rare-metal mineralization; and (5) pegmatites evolve as partially open systems during both magmatic and metasomatic stages with influence from the host rocks, and in some cases, external fluids.
Geology, mineralogy, geochemistry, and petrogenesis of Ni-Cu-(PGE) and PGE-(Cu)-(Ni) deposits in China
(2023-04-01) Lu, Yiguan
Nickel, copper, and platinum-group minerals are regarded as “Critical Minerals” that are crucial to the national economy and sustainable development. Magmatic sulfide deposits account for approximately 93% of Chinese nickel resources, 7.3% of Chinese copper resources, and more than 90% of Chinese platinum-group element resources. This study focuses on the mineralogy, geochemistry, and isotope geochemistry of magmatic sulfide deposits in China, including detailed studies of the Jinbaoshan and Bamazhai deposits in the Emeishan Large Igneous Province, leading to a metallogenic model for the spectrum of magmatic sulfide deposits in China. The 260 Ma Jinbaoshan platinum-group element (PGE) deposit in SW China is a sulfide-poor magmatic PGE deposit that experienced multiple phases of post-magmatic modification. It is characterized by: 1) high-temperature violarite-pyrite1-millerite-chalcopyrite and 2) low-temperature violarite-(polydymite)- pyrite2-chalcopyrite assemblages with more than 16 varieties of platinum-group minerals. Postmagmatic hydrothermal fluids enriched the mineralization in lead, silver, cadmium, and zinc. Cobalt and platinum were also added into violarite, and expelled palladium to the margins of high-temperature violarite and millerite, which also caused the formation of pallidum-enriched minerals. Late-temperature violarite inherited palladium, rhodium, iridium, and osmium from primary pentlandite. Overall, the atypical sulfide assemblages in Jinbaoshan deposit result from multiple overprinted post-magmatic processes, but they did not significantly change the platinum-group element contents of the mineralization, which is interpreted to have formed at high magma:sulfide ratios (500~50000) through interaction of crustal sulfide and a hybrid high-magnesium magma derived by melting of a modified region of the Earth’s mantle. The ~259 Ma Baimazhai nickel-copper-(platinum-group element) deposit is a typical magmatic sulfide deposit in Emeishan Large Igneous Province. The economic No. 3 intrusion is lenticular and concentrically-zoned from finely dispersed through “net-textured” to massive sulfides (margin to core). The sulfide assemblage comprises pyrrhotite, chalcopyrite, and pentlandite, with lesser magnetite, cobaltite, violarite, and galena. The mineralization is enriched in nickel, copper, and cobalt relative to platinum-group elements. Combined with the geochemical features of Baimzhai host rocks, the sulfides appear to have formed from a PGE-depleted magma derived from mantle source that was modified by crustal contamination and formed at moderate magma:sulfide ratios (100–1000). Post-magmatic alteration modified the primary sulfide assemblage, resulting in secondary sulfides enriched in nickelcobalt and antimony-lead-silver-gold. The tectonic and petrogenetic settings of Baimazhai and other deposits in China highlight the potential of nickel-copper deposits to occur in post-subduction settings and exploration potential remains for the Ailaoshan orogenic belt to host additional magmatic sulfide deposits. Unlike other magmatic sulfide deposits in the world, many of which are older and formed primarily in extensional settings, all known Chinese deposits are younger and many are inferred to have formed in compressional settings. Mineral chemical, whole-rock geochemical, ore geochemical, and isotopic data for 18 typical deposits have been used to aid in the assessment of their genesis and prospectively. Most deposits in mountain belts appear to have been derived from magmas formed by partial melting of a modified but originally PGE-depleted mantle source with minor crustal contamination. Most deposits in the Eemeishan Large Igneous Province appear to be hosted by rocks derived from magmas generated from originally more enriched mantle sources with variable degrees of crustal contamination. Deposits related to the breakup of Rodinia exhibit transitional geochemical characteristics. Taken together with the geochemical and isotopic evidence, it is suggested many Chinese magmatic sulfide deposits were derived by melting modified mantle, most likely produced by interaction of recycled oceanic crust with depleted asthenospheric mantle.
Multifocusing seismic imaging of complex geological structures
(2022-04-05) Jodeiri Akbari Fam, Hossein
We introduce two highly effective stacking approaches, 2.5D multifocusing (MF) and 3D generalized spherical multifocusing (GSMF) algorithms, to improve stacked seismic sections. The proposed methods can be applied to crooked-line or 3D seismic data with arbitrary recording geometry from areas with complex near-surface/subsurface to generate either a 2D crooked stacked section or a 3D stacked volume. Both methods simultaneously correct for 3D normal moveout and 3D azimuth-dependent dip moveout, and transform multi-coverage prestack seismic data into a stacked section that is equivalent to a synthesized zero-offset wavefield. In addition, the 3D GSMF approach accounts for the elevation and spatial coordinate of all source and receiver positions, so can be applied for seismic data collected in areas with irregular topography. The 2.5D MF moveout operator is accurate for quasi-hyperbolic reflections, whereas, the 3D GSMF traveltime surface is a more general higher-order moveout operator with a closed-form implicit formulation that can account for non-hyperbolic moveout. The proposed methods efficiently extract valuable 3D structural information when applied to crooked-line/3D seismic surveys. The introduced methods are data-driven algorithms and can perform automatically with a coherence-based global optimization search. We also deployed an efficient processing sequence for applying the developed methods, which mainly consists of building super common-midpoint bins, wavefield attributes analysis, multifocusing moveout correction, data enhancement/stacking, and prestack/poststack dip-independent RMS velocity analysis and migration. Our wavefield analysis uses a multidimensional differential evolution algorithm to simultaneously determine optimal wavefield parameters, which improves the efficiency and accuracy of the estimation. The performance of the proposed methods is tested using 3D synthetic data with both 3D and crooked-line surveys over various curvatures ranging from low (gently curved interface or planar layer with curvature of zero) to high (spherical reflector or point diffractor). The numerical tests demonstrate that the new approximations extract dips from seismic data accurately and are accurate for gently to highly curved interfaces beneath low (multiple homogeneous layers) to relatively high heterogeneous overburden (vertical/horizontal background velocity with a gradient of 1/s for the 2.5D MF method and vertical background velocity with a gradient of 2/s for the 3D GSMF method). Applying the 2.5D MF approach to the Larder Lake crooked-line transect with subsurface complexity, as well as a real data set collected in a thrust-belt area, focused the steeply dipping reflections more coherently and mapped new reflections that are not visible in the conventional results. Application of the 3D GSMF method on a 3D low-fold real seismic dataset, acquired over a complex thrust-belt area with rugged terrain, yielded high-resolution and an accurate stacked seismic volume with a high signal-to-noise ratio, compared to the conventional 3D stacking.
Metallogeny of the Powell Block, Rouyn-Noranda Mining District, Québec
(2023-02-08) Schofield, Marina Decker
The Rouyn-Noranda mining district of Québec is located along the southern margin of the Abitibi greenstone belt of the Archean Superior Province. It hosts 22 Cu-Zn-Au-Ag volcanogenic massive sulfide (VMS) deposits, distributed amongst four fault blocks (Hunter, Flavrian, Powell and Horne). The Powell Block hosts the Au-rich Quemont VMS deposit and separates the Horne deposit, a world class Au-rich VMS deposit, from conventional VMS deposits (≤ 1 g/t Au) north of the Beauchastel fault. The structural and stratigraphic evolution of the Powell Block is poorly constrained and as a result, the time-stratigraphic position of the Quemont deposit relative to the Horne deposit and deposits of the Flavrain Block is uncertain, as are the processes responsible for the anomalous gold content of the Horne and Quemont deposits. Gold-quartz-carbonate and quartz-sulfide (Cu-Ag-Zn) epigenetic veins are also present, but their origin and relative timing remain enigmatic. The Powell Block can be subdivided into two distinctive domains, the Brownlee and Joliet domains, representing two overlapping volcanic centres. These two domains both consist of lower Blake River Group volcanic strata that differ slightly in age and volcanic history. The Brownlee domain is characterized by mafic dominated, bimodal volcanism (ca. 2701 Ma), whereas the Joliet domain is characterized by a thick sequence of felsic coherent and volcaniclastic units (≥2702 Ma). Two base metal mineralizing hydrothermal events are recognized. An early event is related to voluminous felsic volcanism, concomitant subsidence and the formation of the Quemont and Horne deposits in the Joliet domain and Horne block, respectively. These two deposits formed within the district’s largest monogenic felsic volcanic centre in a subaqueous setting that was locally emergent. Given this setting, the Au-rich Quemont and Horne deposits may have had a more direct magmatic input resulting in higher Au grades than the Cu-Zn VMS deposits of the Flavrian Block, which formed in a mafic dominated, bimodal, effusive and perhaps, a deeper water volcanic setting. A later hydrothermal event at ca. 2699~2695 Ma is related to the formation of disseminated, vein and breccia hosted (e.g. Joliet Breccia) quartz-sulfide (Cu-Ag-Zn) mineral occurrences, with variations in deposit characteristics reflecting differences in depth of formation and proximity to a magmatic source. The lateral zoning of alteration types, compositional gains and losses, and metal tenor are similar to VMS footwall alteration zones within the district and elsewhere, and these breccias and veins are part of a broad continuum of subsurface, cross-stratal magmatic-hydrothermal mineralization to seafloor, stratiform, VMS mineralization, a spectrum akin to the porphyry-epithermal continuum of modern subaerial volcanic arcs. Approximately 30–40 m.y. later, gold-quartz-carbonate veins were emplaced during N-S shortening of the volcanic rocks. Superposition of alteration related to these younger veins perturbed and masked regional alteration patterns related to the Quemont and Horne deposits, but due to their restricted distribution and comparatively low-grade, did not substantially affect the original gold tenor of these VMS deposits.
The evaluation and application of tourmaline crystal chemistry as an indicator of porphyry-style Cu-Mo-Au mineralization
(2021-12-15) Beckett-Brown, Christopher Edward
As the exploration for porphyry Cu-Mo-Au deposits becomes increasingly challenging, there exists a need for exploration techniques that can detect buried deposits. Tourmaline supergroup minerals (TSM), a chemically and physically robust phase in the surficial environment, is investigated for its indicator mineral potential to identify porphyry Cu-Mo-Au systems. Samples of TSM from mineralized porphyry systems (Cu, ±Au, ±Mo) including Casino (Yukon Territory, Canada), Coxheath (Nova Scotia, Canada), Donoso Breccia-Los Bronces (Chile), Highland Valley Copper (British Columbia, Canada), New Afton (British Columbia, Canada), Schaft Creek (British Columbia, Canada), and Woodjam (British Columbia, Canada) were examined. Results show that TSM are generally an early hydrothermal mineral predating both mineralization and alteration assemblages. They form sub- to euhedral crystals that are exclusively black (in hand sample) and can be found throughout a porphyry system across all zones of alteration. Major-element chemistry of the TSM is unremarkable, with most having NaMg-Al (dravite) compositions (~80 % of grains) with lesser amounts of schorl (Fe). They exhibit consistent concentrations of ~2.0 apfu Mg with most plotting along the oxy-dravite–povondraite trend reflecting the dominance of Al3+ ↔ Fe3+ substitution. An investigation of TSM trace-elements (LA-ICP-MS) revealed key elements (Sr, Pb, Zn, Cu, and Ga) and select element ratios (Sr/Pb and Zn/Cu) that can be used as discriminators of its environment of formation. Utilizing an understanding of both intrinsic and extrinsic controls on TSM chemistry is an effective approach to distinguish TSM forming in porphyry bedrock samples. A decision tree was developed and applied to TSM grains recovered surrounding two known porphyry deposits including Casino-YT and Woodjam-BC to assess its effectiveness. Using both textural and chemical features of TSM from mineralized porphyry systems highlighted above, porphyry- related grains were identified down ice and downstream of both deposits respectively, highlighting the potential for this method to be used in exploration. The crystal chemistry of TSM has been demonstrated to be sensitive to its environment of formation allowing for the discrimination and identification of TSM forming in porphyries from that forming in other geologic environments and ore systems.
Understanding the South Range Sudbury impact structure: a study of shear zones, impact breccias and PGE occurrences in the Vermilion and Crean Hill mines area, Sudbury, Canada
(2022-12-15) Généreux, Carol-Anne
The Sudbury impact structure formed when a bolide collided with the southern Superior craton margin ca. 1.85 Ga ago. During the impact, the target rocks were brecciated and melted, which resulted in the formation of a thick impact melt sheet, the Sudbury Igneous Complex (SIC), underlain by brecciated basement rocks. Nickel-copper-platinum group elements (Ni-Cu-PGE) deposits formed at the base of the SIC as contact deposits, in association with offset dikes that flowed from the melt sheet into the basement rocks (offset deposits), and as sulphide veins and pods of disseminated PGE mineralization that were emplaced into the brecciated basement rocks (footwall deposits). The deposits and their host rocks were subsequently modified by orogenic events that reworked the South Range of the impact structure. The thesis discusses the formation of syn-impact breccias below the SIC-basement contact, the structural and metamorphic modification of the impact structure during subsequent orogenic events, and the effect of syn- and post-impact processes on the formation and remobilization of (Ni-)Cu-PGE mineralization at the contact-type Crean Hill deposit and offset-type Vermilion deposit. The breccias formed by in situ shock melting of the footwall rocks during the impact, and were modified by contact metamorphism during cooling of the SIC and by syn-tectonic deformation and metamorphism after the impact. The latter resulted in the formation of regional folds and south-side-up-dextral shear zones during northwest-directed shortening of the impact structure. (Ni-)Cu-PGE mineralization at the Crean Hill and Vermilion deposits was emplaced as sulphide melts immediately after the impact and was remobilized by metamorphic semi-metal melt and hydrothermal fluids into the breccias and shear zones during and after regional deformation. These results illustrate how both syn- and post-impact processes contributed to the formation and modification of Ni-Cu-PGE deposits in Sudbury.
The incorporation of geophysical, petrophysical and geological constraints in gravity modeling to resolve structures at depth.
(2022-12-21) Della Justina, Fabiano
Gravity modeling is an important tool for interpreting and understanding geological structures in the subsurface. In forward modeling or inverse modeling, the main goal is to modify a geophysical/geological model to accomplish an acceptable level of reproducibility in the observed data. However, due to the non-uniqueness of potentialfield data, more than one model might fit the observed data. In order to reduce the number of acceptable models, constraints are commonly incorporated into the model. There are countless studies available in the literature demonstrating the necessity of constraining gravity and magnetic models. However, typically they do not demonstrate the individual enhancements that come as a consequence of integrating each constraint into the geophysical model. This study demonstrates how the model, either inverse or forward, is improved as new constraints are built into the modeling workflow. The constraints include information from a density compilation, high-resolution seismic sections, geological maps as well as geological interpretations. The mapped surface geology and the density of this surface data were important to explain the gravity variations associated with faults and to estimate the dip and the error in this dip estimation. The high-resolution seismic sections were helpful to identify reflective features that were most likely lithological contacts where there could be changes in density. Incorporating some of these deeper features and the gradual changes in depth evident in the seismic data resulted in changes in the thickness of the near-surface rocks that was more consistent along strike. Information from previous studies in the area, such as geological interpretations of seismic sections, were required to ensure the geological feasibility of gravity models.
Geological, fluid-chemical and petrochronological studies of the East Kemptville Sn(-Cu-Zn-Ag-In) deposit and its Devonian host batholith (Nova Scotia, Canada).
(2021-07-21) Bickerton, Luke
Primary tin ores form in magmatic-hydrothermal systems related to highly-evolved granites enriched in lithophile elements (e.g., Li, Cs, Rb, U). Causative magmas are sourced from melting (or contamination by) enriched crust, or from melting hydrothermally-altered mantle. These granites form in large, multi-phase complexes (batholiths) and concentrate tin through fractional crystallization. The latter increases volatiles (H2O, F) in residual melts that are emplaced at shallow crustal depths, and thus the hydrothermal component to Sn-systems. Volatiles exsolve into aqueous fluids that contain soluble Sn2+. These fluids typically separate into highly-saline brines and vapours, and precipitate ore (SnO2) via oxidation of the Sn2+ . The fluids focus along fractures in the crust and their accumulation is dependent on fluid- versus lithostatic pressures. The northern Appalachian evolution included incremental emplacement of numerous batholiths and tin occurrences. The largest of these is the South Mountain Batholith (SMB) in Nova Scotia, which was emplaced deep in the crust; although is host to multiple evolved granites, the SMB contains only one significant tin deposit at East Kemptville (EK). To establish the depositional setting of EK at a greater crustal depth, this thesis analyses its geology and fluid chemistry as well as stable (O) and radiogenic (Re-Os, U-Pb, Lu-Hf) isotopes for both EK mineralization and the zircon minerals that represent the SMB. The study addresses the absence of other significant tin deposits in the SMB by evaluating the source of metal endowment. Due to its deep emplacement, the fluids at EK show no evidence of phase separation, yet abundant evidence of pressure-cycling. The latter allowed for replenishment of ore fluids during deposit formation; whereas initial tin formed via fluid-rock exchange, later ores formed from mixing with foreign fluids. This segmented hydrothermal evolution at EK is reflected by a range of mineralization ages. The zircon ages and chemistry indicate: 1) the SMB formed over 15-20 m.y. from altered mantle melts that underwent contamination by host rocks; and 2) the EK host is temporally and isotopically distinct from the SMB, and likely evolved from a lower crust-derived melt. The distinct source suggests other tin occurrences in the region share a similar origin.
Structural geology, tectonostratigraphy, and unconformity-related uranium mineralization of the Aberdeen Lake area, northeast Thelon Basin, Nunavut, Canada
(2021-06-04) Hunter, Rebecca Corrine
The Aberdeen Lake supracrustal belt is located alongside the northeast Thelon Basin in southcentral Nunavut. The belt hosts recently-discovered unconformity-related uranium occurrences, including the Tatiggaq, Qavvik and Ayra prospects. New geological mapping, structural analysis, whole rock lithogeochemistry, U-Pb zircon geochronology, and fluid inclusion analysis were conducted to interpret the stratigraphy, tectonic setting, and structural geology of the belt, and its uranium metallogeny. The revised lithostratigraphy of the Aberdeen Lake supracrustal belt consists of: 1) a ca. 2750 Ma Lower Sequence of komatiite, TTG (tonalite-trondhjemitegranodiorite) intrusions, and mafic gneiss; 2) a ca. <2687 Ma Middle Sequence of ca. psammopelitic gneiss, iron formation and a ca. 2680 Ma felsic gneiss; and 3) a ca. < 2650 Ma Upper Sequence of pelitic to psammopelitic gneiss with minor iron formation and arkosic gneiss. Transecting these Neoarchean supracrustal rocks are the ENE-trending Thelon and Judge Sissons faults, which record long-lived, dextral transcurrent movement with complex reactivation and fluid flow histories. A ca. 1830-1760 Ma initial faulting event produced damage zones consisting of multiple fracture sets, quartz veins and hydrothermal breccias, surrounding core zones of mosaic to chaotic breccias and cataclasites with dextral slip. A ca. 1760-1750 Ma epithermal faulting event comprises cross-cutting comb, crustiform-cockade, and lattice bladed quartz veins associated with Pitz Formation volcanism. Late reactivation events at ca. 1600-1300 Ma are expressed as irregular, fractures, non-cohesive crackle to mosaic breccias and gouges, which were the primary pathways for uranium-bearing hydrothermal fluids. The Tatiggaq, Qavvik and Ayra prospects are located between or along ENE- to NE-trending secondary faults associated with the Thelon and Judge Sissons fault zones. Steep-dipping fracture and non-cohesive breccia zones along these faults host most of the uranium mineralization within medial to proximal alteration halos characterized by strong clay, sooty sulphide and hematite. Fluid inclusion work suggests that a ≥300ºC, reduced, sulphur-bearing basement fluid acted as the primary reductant for uranium mineralization. This fluid was transported by fault-valve seismically-driven fluid injections into a large interconnected and reactivated fault system, which initially formed during dextral transcurrent movement along the Thelon and Judge Sissons faults.
Assessing the origin of some representative Archean-Paleoproterozoic porphyry Cu ± Au deposits
(2021-06-04) Meng, Xuyang
Most known porphyry Cu ± Au deposits are exclusively associated with oxidized and sulfurrich Phanerozoic arc-related magmas, but Precambrian analogues are rare. The genesis of some rare examples of such deposits (e.g., Tongkuangyu in Trans-North China Orogen) remains debated, and in particular, it remains unclear as to whether similar metallogenic processes that typify the Phanerozoic deposits also operated in the Precambrian. To resolve these issues, three study areas were chosen to variably characterize relevant aspects of these deposits in the context of porphyry-type metallogenies (i.e., the nature of mineralization, the redox states and volatile element abundances of the causative magmas): (1) the ~2.1 Ga Tongkuangyu deposit in Trans-North China Craton; (2) the ~1.88 Ga Haib porphyry Cu deposit, Southern Namibia; and (3) representative porphyry-type Cu ± Au deposits (Côté Gold, St-Jude, and Croxall) in the ~2.7 Ga Abitibi subprovince. At the Tongkuangyu deposit, remapping of the distribution of the host rocks, alteration, and mineralization support a porphyry-type model where ore formation occurred at ~2.1 Ga. This age is significantly later than the host granodiorite porphyry (~2.18 Ga) and schists (~2.5–2.2 Ga) and indicates that porphyry-type mineralization need not be linked directly to a causative magma exposed at surface. Instead, the latter is interpreted to be represented by subjacent porphyritic stocks and dikes. At the Haib deposit, the host calc-alkaline plutonic rocks and mineralization are dated to ca. 1886–1881 Ma. Estimations of magmatic fO2 and S, based on zircon geochemistry, apatite µXANES spectra, and apatite S analysis, demonstrate that oxidized, sulfur-rich arc magmas associated with porphyry Cu mineralization already existed in the late Paleoproterozoic. In addition, zircon Hf-O isotopes and whole-rock geochemistry support magmas of mantlederivation with minor crustal contributions that experienced amphibole ± plagioclase fractionation. These features of the Haib magmas are thus similar to those for Phanerozoic porphyry Cu deposits. For the ~2.7 Ga TTG (tonalite-trondhjemite-granodiorite-diorite)-related porphyry-type Cu ± Au deposits in the Abitibi subprovince, whole-rock geochemistry, zircon Hf-O isotopes, apatite S contents, and multiple oxybarometers suggest that variable magmatic origin and fO2 for the causative magmas. The rarity of porphyry-type Cu-Au deposits in these older settings may be attributed to either location restriction of these favorable metallogenic conditions and/or preservation, or exploration bias.
Exploration targeting for gold deposits using spatial data analytics, machine learning and deep transfer learning in the Swayze and Matheson greenstone belts, Ontario, Canada
(2021-05-19) Maepa, Mothepana Francisca
The rate of mineral deposit discovery has declined in the past decade despite increasing efforts from mining and government. The low rate of deposit discovery and the massive historical data available from brownfield exploration sites has prompted geoscientists to apply scale-integrated, empirical, and conceptual targeting approaches to exploration targeting. Applications of the mineral systems approach as a conceptual targeting method together with mineral prospectivity mapping has become the focus of predictive modelling for mineral exploration targeting. Evaluating the essential ingredients that make up a mineral system at various scales with data science machine learning tools could potentially help improve exploration discovery. This study was aimed at mineral exploration targeting gold deposits in the Abitibi greenstone belt using various spatial analysis, machine learning, and transfer learning methods. The multi-scale spatial analysis of gold prospects in the Swayze greenstone belt revealed orogenic mineral systems display fractal characteristics at regional and deposit scales, that gold prospects are clustered within 2 -4 km distances, and that clustering within camps can be attributed to the occurrences of lower-order fault densities or intrusive source rocks. Analyzing spatial correlations between prospect distributions and geological features was instrumental in identifying the physical controlling parameters at various scales, which were primarily D2 structures at regional scales and 2nd and 3rd order structures and competency contrast at prospect scales. Furthermore, the mineral prospectivity maps generated from the various machine learning methods such as support vector machines, random forest, radial basis function neural networks, and deep neural networks were not only beneficial in predicting prospective regions with > 80% accuracies but were essential for emphasizing important geoscience predictor layers that correlate well with mineral prospects. Deep transfer learning attempted for exploration targeting aimed at training a deep neural network model on the Swayze greenstone belt and using the learnt knowledge to make predictions of prospectivity on the Matheson region resulted in over 70% prediction accuracies. Deep transfer learning was valuable in showing that pre-trained models can be used to generate prospectivity predictions in relatively greenfield exploration site where the distributions of prospects are unknown. Overall, this study demonstrates that data integration and applications of data science tools is effective for exploration targeting today.
Gold metallogeny of the southern Swayze Area, Abitibi Greenstone Belt
(2021-04-29) Hastie, Evan Carman George
Understanding ore forming processes and their control on mineralization is essential for the proper classification of Archean, and also younger, Au deposits and mineral exploration success. This study evaluates Archean Au-forming processes using examples across the Superior Province (Canada), with an emphasis on the Swayze area in the Abitibi greenstone belt. It combines detailed field mapping with various discriminants (e.g., in-situ stable isotopes, whole rock and trace-element geochemistry, LA-ICP-MS elemental mapping and chemistry of native gold) to differentiate between orogenic and intrusion-related Au deposits. Based on previous work and newly generated data on 44 deposits, the dataset suggests that Ag, Te and Hg are the only metals consistently associated with Au across different deposit types. Although these metals represent universal elemental vectors for Au exploration, other criteria such as rock and mineral textures, timing of Au mineralization, stable isotope data, and bulk geochemistry must be used to differentiate between Au-deposit types. An important finding is the recognition of a distinct subset of intrusion-related Archean deposits related to sanukitoid-type magmatism. These deposits, generally younger than 2690 Ma, predate shearing and are characterized by Te and Hg elemental associations, relatively high fO2, and δ34S ≤ -5‰. They differ from other intrusion related deposits in terms of their timing and geochemistry and, thus, require a different exploration strategy. Of importance to both academia and industry is the origin of late-stage high-grade Au zones in many deposits. This study suggests such zones result from coupled dissolution-reprecipitation of Au-bearing sulfides and generation of polymetallic Au melts and/or Au nanoparticles, which aggregate as coarse native gold. Both of the processes can elegantly explain ultra high-grade gold zones and are widely applicable to many Au deposits formed throughout Earth’s history.
Metallogeny and characterization of late cretaceous superimposed porphyry Cu-Au-Mo and epithermal Au-Ag systems in the Dawson Range, Yukon, Canada: case study on the Klaza deposit
(2021-03-23) Lee, Well-Shen
The Dawson Range Gold Belt (DRGB; Yukon’s richest mineral district by resource) lies in the Yukon segment of the North American Cordillera and is dominated by Late Cretaceous (77–74 Ma) porphyry-epithermal systems. Mineral occurrences in the DRGB have seen limited exploration due to: (1) poor surface exposure; (2) incoherent classification of intrusive rocks; and (3) outdated exploration models. A multidisciplinary study utilizing: (1) field observations (drill core logging and mapping); (2) geochronology (U-Pb in zircon by LA-ICP-MS and CA-TIMS; Ar-Ar in muscovite; Re-Os in molybdenite); (3) whole-rock geochemistry; (4) zircon trace element geochemistry; (5) petrography (SEM-EDS, optical microscopy); and sulfide mineral geochemistry (LA-ICP-MS element maps) is designed to address the above challenges through a detailed investigation on the well-preserved Klaza deposit using 2011-2020 drilling data. Results suggest the presence of six intrusive phases of mafic to intermediate compositions. Intrusive activity occurs in four pulses spanning the Late Triassic to the Late Cretaceous. The Late Cretaceous magmatic pulse is protracted (80–65 Ma) and displays timedependant compositional changes. The youngest plutonic suites: (1) display enrichments in LREEs relative to the older suites; (2) are related to garnet-bearing sources (depleted HREEs, high La/Yb); (3) are hydrous (presence of hornblende-biotite); and (4) reflect a dynamic magma chamber (magma mixing textures). Two clusters of hydrothermal ages are constrained (ca. 77 Ma and ca. 71 Ma), correlating with Casino suite and Prospector Mt. suite magmatism, respectively. Higher temperature A-, B-and EDM-like veins are related to the 77 Ma event and cut by fault-veins related to the 71 Ma event. The fault-veins consist of four stages. Gold is hosted in early (Stage 2a, 2b) arsenopyrite and pyrite lattices and later liberated through late (Stage 2d) Copper-bearing fluids, whereas silver occurs later (Stage 2c) as native silver and sulfosalts. The Klaza system is best described as a porphyry-related intermediate sulfidation epithermal deposit superimposed on an older, unrelated porphyry system. Similar observations of spatial-temporal overprinting are documented throughout the DRGB (e.g., Casino deposit and Freegold Mt. district), suggesting these Late Cretaceous porphyry systems are linked to a fertile metallogenic event spanning 15 million years. This study is the first detailed characterization of Late Cretaceous porphyry systems in the DRGB and presents the first use of machine learning assisted paragenetic study of sulfide minerals for exploration and improving ore body knowledge.
Metallogeny of the Cornwallis Zn-Pb district, Arctic Canada
(2020-10-22) Mathieu, Jordan
The Cornwallis Zn-Pb district in the Canadian Arctic archipelago contains numerous carbonatehosted mineralised showings, including the past-producing Polaris Zn-Pb mine on Little Cornwallis Island. The showings’ mineralogy is primarily Zn-Pb, with anomalous Cu showings. The volume and geographic/stratigraphic positions of mineralisation are variable, with the majority being hosted by Middle Ordovician Thumb Mountain Formation limestone/dolostone adjacent to extensional faults. Multiple in situ micro-analytical techniques were used on samples from throughout the district to determine the origin, character, flow path, and age of the mineralizing fluid(s) responsible for the showings in the district, and to re-evaluate models of the Polaris deposit. Geochemical (fluid inclusion microthermometry, trace and rare earth elements, oxygen isotopes) results indicate that a single, regional fluid was involved in mineralisation at all of the showings. Sulphur isotopes indicate that pre-mineralisation sulphur accumulations at individual showings affected early mineralisation δ34S values until thermochemical sulphate reduction (TSR) of the regional fluid became the dominant sulphur supply, and that varying degrees of TSR completion resulted in showing-specific δ34S values, rather than a uniform regional value. The main control on the volume of mineralisation was the amount of fluid flux that could be accommodated by the showing, with Polaris being much larger than the rest because of its tectonic location, which allowed for copious amounts of mineralising fluid to interact with host rocks. The Storm copper showing on Somerset Island indicates that Cu showings in the Zn-Pb district are related to the regional fluids of the Zn-Pb showings, but local strata affected the mineralogy. The geochemical signatures of the paragenetic stages indicate that pre-ore fluids were related to the regional Zn-Pb fluid, whereas the main-ore fluid is related to a fluid that displaced the regional Zn-Pb-mineralising fluid during ongoing orogenesis. An underlying Proterozoic redbed contains a record of diagenetic sequences and fluids that is similar to those responsible for the Storm copper showing, indicating that the geographically limited redbed was probably responsible for the anomalous Cu-rich showing in the district.
New methods for the interpolation and interpretation of lineaments in aeromagnetic data
(2020-06-16) Naprstek, Tomas
Aeromagnetic data is one of the most widely collected types of geophysical data. In mineral exploration it can assist in mapping geological features, as well as indicate potential locations of economic interest. Due to the method in which aeromagnetic surveys are flown, an interpolation process must be completed before any map-based interpretation can be accomplished. One artifact common to many interpolation methods is that of “beading”, which is a discontinuous sequence of circular magnetic features that are at acute angles to the traverses, often caused by thin, linear geologic features such as dykes. Developing an interpolation method that “trends” or images these beads as continuous features on magnetic images would allow automatic and reliable quantitative methods to be used for interpretation by geologists and geophysicists. First, a new interpolation method was developed for aeromagnetic data. Utilizing a Taylor derivative expansion and structure tensors, it iteratively enhances trends evident across flight lines to manifest as linear features on the interpolated grid. When applied to both synthetic data and field data, the new method showed improvement over standard bidirectional gridding, minimum curvature, and kriging methods for interpolating thin, linear features at acute angles to the flight lines .Following this, a machine-learning interpolation approach was developed for aeromagnetic data using support vector machines and random forests. By using multiple standard interpolation methods as input to the machine-learning models, a filter-like approach was developed. These models could produce aeromagnetic maps that were overall more accurate than any single interpolation method, but not as effective as the Taylor derivative expansion method on lineament features. Finally, convolution neural networks were applied to estimate the source parameters characterizing lineament anomalies. A synthetic aeromagnetic data modeler was used to vary relevant physical parameters, and a representative dataset of approximately 1.4 million images was developed. These were then used for training convolution neural networks to estimate the strike and depth of sources. Applying the trained networks to a real-world dataset that was interpolated by the Taylor derivative expansion method, they located a dyke and estimated a depth consistent with a previous borehole investigation.
Fluid-chemical studies of gold systems: case studies of an Archean greenstone belt, Nunavut, and a Paleozoic slate belt, Meguma terrane, Nova Scotia.
(2019-12-16) Kerr, Mitchell James
Despite several decades of study, the source(s) of gold and mineralizing fluids, and the mechanisms responsible for gold precipitation remain controversial. In particular, there is a need to improve: (i) the identification of secondary gold upgrading processes that lead to very highgrade gold zones; (ii) the understanding of precipitation mechanisms in carbonaceous material (CM)-bearing metasedimentary systems (i.e., slate belts); and (iii) the exploration criteria for the cryptic, finely disseminated gold mineralization in CM-bearing metamudstone lithologies. The combined results of an extensive in situ microanalytical protocol (SEM, confocal Raman microspectroscopy, microthermometry, decrepitate mound analysis, LA-ICP-MS, cathodoluminescence, SIMS) has idenitified evidence of secondary gold enrichment at the Madrid deposit, Hope Bay Greenstone Belt, NU, Canada, resulting from the addition of new gold onto an earlier orogenic quartz-carbonate vein deposit by magmatic-hydrothermal fluids. This conclusion is based on robust textural, mineralogical, and fluid inclusion evidence. The geological setting and mineral-chemical features suggest an intrusion-related (i.e., porphyry), or intermediate-sulfidation epithermal mineralization style for the later event. This work provides another example of the importance of compositionally distinct cumulative hydrothermal events in the development of high-grade gold deposits in orogenic settings. The Dufferin deposit, NS, Canada, is a vein-type system hosted in CM-bearing metasedimentary rocks. A detailed fluid inclusion study strongly suggests that Au mineralization occurred through coupled fluid fO2 reduction (via interaction with CM) and pH increase, leading to efficient Au precipitation from an Au-undersaturated aqueous-carbonic fluid. This study presents one of the few examples of measured Au concentrations in ore fluids in an economic orogenic Au deposit and reports direct evidence for the genetic relationship between Au and CM in metasedimentary lithologies. The results of this study show that, despite mineralizing fluids being Auundersaturated, they may still produce economic deposits through highly efficient precipitation processes. The Moose Rive anticline, NS, Canada, hosts a number of, commonly inconspicuous, metasediment-hosted disseminated Au deposits. An initial study using an on-line, rock-crushing gas chromatographic technique identified bulk fluid differences (CO2 and hydrocarbon abundances) between gold-bearing and gold-barren parcels of metasedimentary rock. This work suggests that bulk fluid characteristics may be a viable exploration vector for disseminated gold systems.
Genesis of mafic-ultramafic inclusions in sublayer and inclusion quartz diorite and implications for the formation of associated NICU-PGE mineralization in the Sudbury igneous complex
(2019-10-30) Wang, Yujian
The lowermost, discontinuous units of the impact-generated Sudbury Igneous Complex (SIC), Sublayer, Footwall Breccia (FWBX), and Inclusion Quartz Diorite (IQD), are distinguished from overlying Main Mass norite rocks by the presence of abundant inclusions and Ni-Cu-PGE (PGE – platinum group element) mineralization. The majority of the felsic-mafic inclusions appear to be derived from exposed country rocks, but volumetrically important mafic-ultramafic inclusions have only rare equivalents in the surrounding country rocks and appear to be preferentially associated with sulfide mineralization. Establishing the petrogenesis of the mafic-ultramafic inclusions and the nature of their association with the Ni-Cu-PGE mineralization are therefore critical to understand the evolution of the impact melt, genesis of Sublayer, FWBX, and IQD, and the formation of one of the world’s largest accumulations of Ni-Cu-PGE mineralization. Petrographic, mineralogical, geochemical, and Sm-Nd and Re-Os isotopic data indicate three origins for the olivine-bearing mafic-ultramafic inclusions: (1) Anteliths, comprising olivine melanorites and olivine melagabbronorites in the Whistle and Levack embayments on the North Range, which are characterized by igneous textures, Zr/Y, Zr/Nb, Nb/U, and Zr/Hf ratios similar to igneous-textured Sublayer matrix (ITSM), unradiogenic εNd1850 Ma values (-8 to -5), and slightly unradiogenic to radiogenic γOs1850 Ma values (-8 to +94). They likely crystallized from a local mixture of SIC impact melt and a more mafic melt derived by melting of the widespread Huronian volcanic and subvolcanic units in the region. (2) Local xenoliths, comprising wehrlites and olivine clinopyroxenites in the Levack embayment and olivine melanorites in the Foy Offset on the North Range, which are characterized by shock mosaic and recrystallized textures, and trace element patterns (e.g., negative Th-U, Nb-Ta-(Ti), Sr, and Zr-Hf anomalies) similar to and Nb/U ratios overlapping with a layered mafic-ultramafic intrusion in the footwall of the Levack and Fraser deposits. They were likely derived from local mafic-ultramafic protoliths that were petrogenetically-related to the layered mafic-ultramafic intrusion in the footwall of the Levack and Fraser deposits. (3) Exotic xenoliths, comprising phlogopite/feldspar lherzolites in the Trill, Levack, and Bowell embayments and the Foy Offset dike on the North Range, which are characterized by variably igneous, tectonic-metamorphic, and shock-metamorphic textures, and orthopyroxene reaction rims against igneous-textured Sublayer matrix (ITSM), indicating disequilibrium with the impact melt. One composite inclusion exhibits igneous layering of feldspar lherzolite and olivine gabbro, suggesting derivation from an unexposed older layered mafic-ultramafic intrusion. The calculated parental magma for one particularly well-preserved feldspar lherzolite inclusion is similar to continental arc basalt formed by up to 5% partial melting of garnet peridotite. Ol-Cpx-Pl thermobarometry of several exotic inclusions indicate equilibration at 900o C – 1120o C and 210 – 300 MPa, suggesting emplacement into upper-middle crust (7.7 – 10.9 km), prior to being incorporated into the lower parts of the proto-SIC during impact excavation and/or thermomechanical erosion of target rocks. Most analyzed inclusions, ITSM, and Main Mass lithologies are enriched in highly incompatible elements with negative Nb-Ta-Ti anomalies, unradiogenic Nd, and radiogenic Os isotopic signatures. These features suggest that the impact sampled rocks that were derived from subduction-metasomatized mantle, including the widespread Huronian volcanic and intrusive rocks adjacent to the SIC. Melting of these volcanic and intrusive rocks and the underlying Neoarchean Superior Province upper-middle crustal rocks would produce the observed geochemical characteristics of the SIC lithologies and inclusions. The Main Mass has a very homogeneous Hf isotopic composition, indicating that the impact melt sheet was well mixed. However, Sublayer, IQD, and overlying basal Main Mass norites vary widely in terms of Pb-S-(Os) isotopic compositions. Most mafic-ultramafic inclusions, except for anteliths, contain no sulfides and exhibit no signatures of Ni-Cu-PGE depletion caused by prior sulfide saturation, which suggest that the association between mafic-ultramafic inclusions and NiCu-PGE sulfide mineralization is attributable to the hydrodynamic equivalence of less dense but larger silicate inclusions and denser but smaller sulfide melt droplets during transport and/or settling. Anteliths, locally-derived inclusions, and local variations in Pb-S-(Os) isotopes must have been generated in situ, requiring significant degrees of assimilation of footwall rocks via thermomechanical erosion, whereas most exotic inclusions other than shocked feldspar lherzolite were derived from deeper mafic-ultramafic protoliths, generated during impact excavation and/or thermomechanical erosion, and physically transported into their current locations. Thus, thermomechanical erosion played an important role in the generation of embayments, incorporation of xenoliths and sulfide xenomelts from the mineralized country rocks (e.g., EBLINipissing-Huronian), and formation of isotopic heterogeneity in the basal parts of the SIC. Convective- and gravity-driven mass flow contributed to the horizontal transportation of inclusions and sulfide xenomelts into the embayment when the impact melt contained <45% inclusions, but became less significant as proto-Sublayer incorporated more inclusions.

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