Mineral Deposits and Precambrian Geology

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Application of physical properties measurements to lithological prediction and constrained inversion of potential field data, Victoria Property, Sudbury, Canada.
(2015-11-30) Mahmoodi, Omid
In recent years the number of near-surface deposits has decreased significantly; consequently, exploration companies are transitioning from surface-based exploration to subsurface exploration. Geophysical methods are an important tool to explore below the surface. The physical property data are numerical data derived from geophysical measurements that can be analyzed to extract patterns to illustrate how these measurements vary in different geological units. Having knowledge of links between physical properties and geology is potentially useful to obtain more precise understanding of subsurface geology. Firstly, down-hole density, gamma radioactivity, and magnetic susceptibility measurements in five drillholes at the Victoria property, Sudbury, Ontario were analyzed to identify a meaningful pattern of variations in physical property measurements. The measurements grouped into distinct clusters identified by the fuzzy k-means algorithm, which are termed ‘physical log units’. There was a meaningful spatial and statistical correlation between these physical log units and lithological units (or groups of lithological units), as classified by the geologist. The existence of these relationships suggests that it might be possible to train a classifier to produce an inferred function quantifying this link, which can be used to predict lithological units and physical units based on physical property data. A neural network was trained from the lithological information from one hole, and was applied on a new hole with 64% of the rock types being correctly classified when compared with those logged by geologists. This misclassification can occur as a result of overlap between physical properties of rock types. However, the predictive accuracy in the training process rose to 95% when the network was trained to classify the physical log units (which group together the units with overlapping properties). Secondly, lithological prediction based on down-hole physical property measurements was extended from the borehole to three-dimensional space at the Victoria property. Density and magnetic susceptibility models were produced by geologically constrained inversion of gravity and magnetic field data, and a neural network was trained to predict lithological units from the two physical properties measured in seven holes. Then, the trained network was applied on the 3D distribution of the two physical properties derived from the inversion models to produce a 3D litho-prediction model. The lithologies used were simplified to remove potential ambiguities due to overlap of physical properties. The 3D model obtained was consistent with the geophysical data and resulted in a more holistic understanding of the subsurface lithology. Finally, to extract more information from geophysical logs, the density and gamma-ray response logs were analyzed to detect boundaries between lithological units. A derivative method was successfully applied on the down-hole logs, and picked the boundaries between rock types identified by geologists as well as additional information describing variation of physical properties within and between layers not identified by the geologist.
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.
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.
Characterization of banded iron formations associated with gold mineralization: primary geochemical signatures and exploration implications
(2016-04-15) Gourcerol, Blandine
Algoma-type banded iron formations (BIFs), which represent chemical sedimentary rocks characterized by alternating layers of iron-rich minerals and chert intercalated with Eoarchean to late Paleoproterozoic volcano-sedimentary sequences, act as a favorable host rock for orogenic gold mineralization within several Archean cratons (i.e., Pilbara, Kaapvaal, Superior, Slave and Churchill). Besides this economic aspect, these Fe-rich sequences have long been appreciated as an important contributor to furthering our understanding of the geochemical evolution of the Earth. However, these deposits are in general tectonically deformed, metamorphosed and dismembered, thus making reconstruction of their depositional setting and overall geologic setting difficult. Based on four Canadian BIF-hosted gold deposits (the Meadowbank, Meliadine, Musselwhite and Beardmore-Geraldton deposits), this thesis aims to establish the depositional setting of the Algoma-type BIF using the abundance of REE+Y of chert material used as proxy of the primary signature, as well as assess gold enrichment processes based on textures and trace element zoning of variable sulfides (i.e., pyrite, arsenopyrite and pyrrhotite) and finally study if there is a particular geochemical type of Algoma-type BIF associated with gold mineralization. Laser ablation-inductively coupled plasma-mass spectrometric (LA-ICP-MS) analyses performed on chert material suggest that BIFs from the four deposits show common depositional settings illustrated by deposition in semi-closed to closed basin under variable influence of high-temperature (>250ºC) hydrothermal fluids input and detrital contamination. Moreover, evidence of late diagenetic processes involving O isotopic exchange between chert precursor (i.e., opaline material) and seawater origined fluid have been documented. According to their primary v signature, it appears that barren versus mineralized BIFs do not show any geochemical divergence suggesting that the depositional setting may influence the epigenetic gold mineralization. Based on quantitative element distribution maps combined with line traverse and spot analyses by LA-ICP-MS on sulfides, a common gold mineralizing event characterized by intense stratabound sulfide-replacement of Fe-rich material was reported in three studied deposits exhibiting a Au-As-Se-Te-Ag element association. This result suggests that metamorphic/hydrothermal orogenic processes driven by devolatilization of a common weakly to unmetamorphosed source rock have ledto generation of gold-bearing fluid which channelled into Algoma-type BIF via major crustal faults and/or shear zones within low tensile strength rocks
Characterization of barren and mineralized hydrothermal systems in an impact structure: the granophyre unit and footwall of the Sudbury Igneous Complex, Ontario, Canada
(2017-04-03) Stewart, Robert Craig
The Sudbury Igneous Complex (SIC) is the product of a differentiated impact melt sheet and one of the best preserved impact sites globally. The top 1.5 km of the 2.5 km-thick crystallized melt sheet, referred to as the granophyre unit (GR), exhibits strong textural, mineralogical, geochemical, fluid inclusion, and oxygen isotopic evidence for a long and complex hydrothermal history. This hydrothermal system has not previously been documented, and is characterized by an early magmatic-hydrothermal stage that has been overprinted by a later hydrothermal stage. The magmatic-hydrothermal stage was dominated by fluids exsolved from the GR and resulted in pervasive saussuritization of plagioclase, alteration of alkali feldspar and plagioclase to endmember compositions, and precipitation of epidote and muscovite in pits and void spaces. The overprinting hydrothermal stage was dominated by modified seawater sourced from the hydrothermal system in the overlying Onaping Formation and produced a pervasive alteration assemblage characterized by epidote, ferro-actinolite, stilpnomelane, biotite, and titanite with lesser muscovite, chlorite, and ferrotschermakite. The results from this study record a previously unidentified complex and protracted post-crystallization history of fluid-rock interaction for the GR that is related to the ore-forming hydrothermal system in the overlying Onaping Formation. The relationship between the hydrothermal system in the GR and the mineralized systems in the footwall to the Sudbury Structure (SS) has not previously been established. The Nickel Rim South (NRS) Cu-Ni-PGE deposit, located in the east range of the Sudbury Structure, Ontario, records textural and geochemical evidence of a complex, multi-stage hydrothermal history and provides an excellent comparison to the GR. Results from fluid inclusion petrography and microthermometry are used in conjunction in order to characterize the hydrothermal system at NRS. The combined results of fluid inclusion petrography and microthermometry, evaporate mound analysis, oxygen isotopic values of quartz, and trace element chemistry of alteration minerals reveal a complex hydrothermal history at NRS related to the cooling SIC, which contributed to hydrothermal remobilization of metals into the footwall. The hydrothermal fluids are notably different from fluids documented in other mineralized footwall systems that are represented by high temperature, multi-solid fluid inclusions inferred to be critical for ore remobilization. Instead, the hydrothermal system at NRS shares more similarities with the GR hydrothermal system, bringing into question the role of fluids represented by high temperature, multi-solid inclusions in ore remobilization.
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.
Evolution of the Sudbury igneous complex southern metamorphic aureole and controls on anatexis
(2017-03-28) Jørgensen, Taus Richardt Chræmmer
The South Range metamorphic aureole around the 1.85 Ga Sudbury Igneous Complex (SIC) is spatiotemporally connected to the world-class ore deposits of the Sudbury mining camp. Defining the physiochemical expression and understanding the evolution of the metamorphic aureole are therefore of economic interest to mineral exploration efforts. The importance of studying the SIC metamorphic aureole is highlighted by new insights into low-P/high-T (LP-HT) metamorphism of basalts including, LP-HT mineral assemblages, partial melting, melt mobilization, phase equilibria modelling of metabasalts at relatively LP-HT conditions, element mobility during metamorphic processes, and retrograde zircon formation with concurrent mobilization and fractionation of Zr-Hf. The South Range metamorphic aureole is best preserved in Paleoproterozoic Elsie Mountain Formation (EMF) metabasalts that form a large proportion of the immediate footwall to the SIC along its southern margin, which also includes the Murray and Creighton granites. Mapping of the metamorphic aureole in the EMF metabasalts defines 3 metamorphic zones: 1) an up to ca. 500 m wide pyroxene-hornfels zone (PHZ) extending from the SIC contact and characterized by a peak metamorphic mineral assemblage of plagioclase-clinopyroxene-orthopyroxene-magnetite-ilmenite estimated to reflect peak temperatures of ≥925 °C; 2) a pyroxene-granofels zone (PGZ) extending from the PHZ and up to 750 m from the SIC contact characterized by a similar twopyroxene assemblage, but typically with abundant retrograde high-Ti hornblende; 3) a hornblende-hornfels zone (HHZ) extending from the PGZ and to at least 1000 m from the SIC contact characterized by a hornblende-plagioclase-quartz-ilmenite ± biotite ± magnetite assemblage indicating temperatures of up to 680 °C. Field evidence for partial melting and melt mobilization in the EMF metabasalts consist of mainly macroscopic leucocratic patches that locally coalesce. Microtextural evidence for partial melting includes optical continuous quartz domains containing plagioclase and pyroxenes locally with euhedral crystal faces, and relatively low-Ca plagioclase and quartz frameworks around mainly relatively high-Ca plagioclase representing nucleation from a melt onto existing crystals. Phase equilibria modelling using bulk rock compositions indicate that partial melting resulted in 10-20% melt generation in the PHZ, and probably even higher degrees of melting is recorded locally. Compared to the granites where partial melts have been traced as dikes for hundreds of meters injecting back into the SIC, no backinjections were documented to emanate from the EMF metabasalts. This indicates that while a high-T metamorphic aureole developed in the metabasalts, the granites were continuously experiencing high degrees of partial melting preventing the development of a metamorphic aureole even some time after solidification of the SIC. Thus, the width of the high-T contact aureole is wider in the EMF metabasalts than in the granites. This is also true in a comparison to the contact aureole documented in the North Range Archean gneisses. Furthermore, the estimated peak contact metamorphic temperatures in the EMF metabasalts are in better agreement with previous thermal models that required substantial thermomechanical erosion (800 m) of North Range footwall rocks to match the width of the observed contact aureole. Thus, the process of thermomechanical erosion might have been less significant in the EMF basalts and perhaps other mafic lithologies. Trace element geochemistry of the EMF metabasalts successfully permits a subdivision of the PHZ into Hornfels A and B zones. The Hornfels A zone defines the metabasalts are in better agreement with previous thermal models that required substantial thermomechanical erosion (800 m) of North Range footwall rocks to match the width of the observed contact aureole. Thus, the process of thermomechanical erosion might have been less significant in the EMF basalts and perhaps other mafic lithologies. Trace element geochemistry of the EMF metabasalts successfully permits a subdivision of the PHZ into Hornfels A and B zones. The Hornfels A zone defines the inner most ca. 250 m, and is characterized by metabasalts that generally show relative depletion in LILE, REE and HFSE. Thus, trace element systematics in the EMF metabasalts of the Hornfels A zone accentuates the potential for metamorphic processes including devolatilization reactions and partial melting to severely mobilizing not only relatively easy mobilized elements, e.g., LILE, but also the relatively immobile HFSE. Thus, the trace element systematics has the potential to identify high-T parts of the metamorphic aureole where the micro- and macroscopic petrographic evidence has subsequently experienced obliteration by tectonometamorphic events. The defining geochemical characteristic of Hornfels A samples is a pronounced negative Zr-Hf anomaly (Zr/Zr* < 0.67) that is associated with sub-chondritic Zr/Hf values. Furthermore, zircon with uncharacteristic textures forming poikilitic, branching, and interstitial networks are observed exclusively in Hornfels A samples, and yield an U-Pb age of 1850 ± 24 Ma. The zircon textures, age, relation to high-T mineral assemblage, and chemistry suggests crystallization from trapped melt films during retrograde cooling. In combination with the whole rock trace element geochemistry these observations provides strong circumstantial evidence that Zr-Hf was mobilized in silicate melts, and that a 250 m zone from the SIC contact experienced melt segregation. Important to mineral exploration efforts is the observation that the width of the contact aureole in the EMF basalts appear to correlate with the thickness of the SIC that is thought to have a primary control on the location of contact deposits. Also, the width of the high-T contact aureole might provide a limiting factor for the extent to which low-S Cu-PGE rich mineralization can penetrate into the footwall.
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.
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.
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.
Forward modelling and imaging method studies for cross-hole radio imaging (RIM) data.
(2017-06-06) Li, Yongxing
The radio imaging method (RIM) is a cross-hole electromagnetic (EM) method which employs radio frequency EM waves to estimate the electric properties between boreholes. RIM is applied in hard rock mining to find and delineate sulfide mineral deposits. A basic and relatively simple method for imaging and interpreting RIM data is the straight-ray method. However, the strengths and weaknesses of the straight-ray method and other more sophisticated methods have not been studied thoroughly. In the first part this research, I modelled RIM data using a finite element package, Comsol Multiphysics. To validate the Comsol approach, I compared the Comsol model data with the analytical solution of an electric dipole in a homogeneous whole-space model, and some published analytical solutions and numerical solutions of models with conductive objects. The Comsol generated data are consistent with the analytical and the published results. Secondly, I used Comsol synthetic data to assess the effectiveness of the straight-ray method for interpreting RIM data and to study the characteristics of the radio-frequency EM fields. I studied four sets of models with conductive objects embedded in resistive environments, which resemble ore deposits in mining settings. The experiments show that the characteristics of the EM fields mainly depend on the wavelength. Longer wavelengths are associated with lower frequencies. In this condition, EM induction is strong. Shorter wavelengths are associated with higher frequencies. In this condition, the scattering effects of EM waves dominate. In the radio-frequency range, I concluded that the straight-ray method cannot always provide high quality imaging results for RIM data. To account for the scattering effects, I adopted the contrast source inversion (CSI) method, which was originally developed for microwave tomography in medical imaging, to invert the RIM data. The CSI method was tested with Comsol synthetic data and field data. The synthetic studies show that the CSI method provides images with more accurate locations and shapes of the conductive objects when compared with the straight-ray method. The case studies show that CSI imaging results are more consistent for data collected at different frequencies and are easier to interpret geologically.
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.
The geochemical behavior of metalloids and their effect on the highly siderophile elements during the crystallization of a magmatic sulfide liquid in relation to the formation of Ni-Cu-PGE magmatic sulfide deposits
(Laurentian University of Sudbury, 2015-07-08) Cafagna, Fabio
The highly siderophile elements (HSE), which include Re, Au and the platinum-group elements (PGE: Ru, Rh, Pd, Os, Ir, Pt) are of scientific and economic interest because they are tracers of geological processes (e.g., planetary differentiation) or have industrial or investment value. In most ore deposits, HSE are usually mined as by-products of Ni-Cu mineralization, such as those of the Sudbury district (Canada). Although at high temperature HSE are expected to be in lattice of sulfides they are usually found as discrete phases or platinum-group minerals (PGM). Many of PGM have metalloids (As, Se, Sb, Te and Bi) as essential constituents but, despite this relationship, the role of metalloids on HSE mineralization is still unclear. This dissertation documents the results of three experimental studies undertaken to investigate the effect of metalloids on the geochemistry of HSE. The first study examined whether the metalloids can mobilize HSE in an anhydrous environment. The results show that metalloids do not transport the HSE but also that Au, Pd and Pt can be efficiently fractionated from the other HSE and transported away from a sulfide assemblage through a relatively low-porosity, low-permeability medium in absence of hydrothermal fluids. The second study investigated the solubility of metalloids in monosulfide solid solution (mss) and intermediate solid solution (iss), to asses the conditions required for the formation of immiscible metalloid liquids and their effect on HSE partitioning. The results show that the metalloids form two separate phases, one that is As-rich and strongly sequestered Pt from the sulfides and another that is Bi-Te rich and affected mostly Pd and Au. The results also show that the other HSE (Ru, Rh, Re, Os, Ir) are largely unaffected by the presence of metalloids. In addition the results show that mss can dissolve significant amounts of metalloids (hundreds of ppm) before metalloid V saturation can occur. The third study constrained the geochemical behavior of metalloids and HSE in a pyrite-bearing sulfide system. The results show that Co-Ni-rich and HSE bearing pyrite can form during cooling of a sulfide melt and also that the HSE partition roughly equally between mss and pyrite, whereas Pd that partitions preferentially into mss and Au partitions preferentially into iss. Moreover, pyrite showed compositional zonation of many elements similar to that documented in natural pyrite samples that are also Co-rich and HSE-bearing. Thus, identification of such pyrite in natural assemblages is consistent with a magmatic origin.
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.
Geology of the Archean Côté Gold Au(-Cu) intrusion-related deposit, Swayze Greenstone Belt, Ontario
(2016-04-05) Katz, Laura Rachel
The Côté Gold Au(-Cu) deposit is a low-grade, large-tonnage type deposit with an indicated resource of 269 Mt averaging 0.88 g/t Au (7.61 M oz) and an inferred resource of 44 Mt averaging 0.74 g/t Au (1.04 M oz) at a cut-off grade of 0.3 g/t Au. The deposit is located in the Swayze greenstone belt (SGB), part of the larger and gold-rich Abitibi Subprovince and is the first large gold deposit to be discovered in the SGB. The deposit is hosted by multi-phase tonalite, diorite and quartz diorite intrusions of the Chester intrusive complex (CIC), a multi-phased, laccolith-shaped synvolcanic intrusion. Crosscutting relationships suggest coeval emplacement of the host rocks and high-precision UPb ID-TIMS zircon geochronology results constrain the CIC to ca. 2741 to 2739 Ma. Although a close temporal relationship exists between the tonalite and dioritic phases, they appear to be petrogenetically unrelated. The CIC is demonstrated to be petrologically, texturally and geochemically similar to low-Al tonalite-trondhjemite-diorite composite intrusions known to underlie VMS-type mineralization. The CIC was emplaced into tholeiitic mafic metavolcanic rocks of the Arbutus Formation that are interpreted to have formed in a back-arc environment. The CIC is coeval and cogenetic with the felsic to intermediate rocks of the Yeo Formation. Emplacement of the CIC into a shallow crustal level is inferred based on the incorporation of screens and inclusions of the Yeo Formation, consistent with the presence of textures, such as granophyres, miarolitic cavities and pegmatites, and Al-in hornblende geobarometry results of ≤1.3 ± 0.6 kbars. The intrusive complex is intruded by several younger and petrogenetically unrelated dike phases, as well as overprinted by regional deformation events. iv The deposit has a large ore envelope (1200 m by 300 m) within which several styles of mineralization (breccias, disseminations and veins) that are co-spatial and cogenetic to hydrothermal alteration are recognized. The hydrothermal stages consist of: (1) a rare series of gold-bearing amphibole-rich veins and breccias spatially restricted to the south and central parts of the deposit; (2) an extensive, main-stage biotite alteration that is centred on Au- and Cubearing hydrothermal biotite breccia bodies. Outwards from the breccia are veins (stockworks and sheeted arrays) and disseminations; (3) an extensive, Au ± Cu-bearing fracture-controlled and replacement-style muscovite alteration in the centre of the deposit; (4) an unmineralized fracture-controlled and disseminated epidote alteration that is localized to the north of the deposit; (5) a texturally destructive albite alteration occurs in the central part of the deposit; and (6) chlorite alteration that overprints biotite alteration. Drill core observations suggest that all the hydrothermal events, except chloritization, broadly overlap with magmatism and are crudely zoned on the deposit scale; chloritization is considered to be metamorphic in origin. To constrain the timing of hydrothermal events U-Pb ID-TIMS, SHRIMP and LA ICPMS dating of four hydrothermal titanites from amphibole and albite alteration assemblages and Re-Os dating of two molybdenite samples were analyzed. The hydrothermal titanite ages are centred on a ca. 2740 Ma and the molybdenite ages are 2736.1 ± 11.4 Ma and 2746.8 ± 11.4 Ma. These ages overlap with magmatism and strongly support a syn-intrusion timing for alteration and gold mineralization. Importantly, this age represents a new metallogenic gold event in the Abitibi Subprovince. In addition, two rutile ages at ca. 2667 Ma were obtained and indicate the deposit was affected by subsequent thermal events, such as regional metamorphism. Several of the alteration types (i.e., amphibole, biotite, muscovite and epidote) are considered to be genetically related to the dioritic phases of the CIC based on relative timing, v distribution, mineralogy and mineral chemistry and whole-rock geochemistry. The progressive crystallization of the dioritic magma chamber is considered to have resulted in the generation of mineralized magmatic-hydrothermal fluids that are thought to be responsible for generating the Côté Gold deposit. The sequence of alteration types, their zonation in the deposit and their chemistry (i.e., gains and losses, REE patterns, metal associations and isotopes) is consistent with the early hydrothermal stages (i.e., amphibole and biotite) having formed from high temperature magmatic-derived dioritic fluids and the later alteration types (i.e., muscovite and epidote) having formed from cooling magmatic fluids mixed with seawater. The development of albite alteration is thought to be the result of volatile exsolution of tonalite based on its syn-intrusion timing and the Na-rich nature of the fluids. Although albite alteration may be gold-bearing, the erratic distribution of the mineralization and the leaching capacity of fluids suggest earlier gold was remobilized during this event. Considering the characteristics of the host rocks, the demonstrated overlap of magmatic and hydrothermal events at ca. 2740 Ma, the nature and distribution of alteration, the style of the associated mineralization and the geochemical characteristics of the hydrothermal fluids the Côté Gold deposit is considered to be intrusion-related in origin and formed in a manner similar to Phanerozoic porphyry-type deposits.
The geology of the Beardmore-Geraldton belt, Ontario, Canada: geochronology, tectonic evolution and gold mineralization
(2018-11-07) Tóth, Zsuzsanna
The Beardmore-Geraldton belt (BGB) is a greenstone belt straddling the boundary between the eastern Wabigoon subprovince and the Quetico metasedimentary subprovince. It consists of shearbounded panels of metavolcanic and metasedimentary rocks which were imbricated during an early thrusting event and regionally folded and sheared during subsequent deformation events. It hosts several historic gold mines with a combined past production exceeding 4 Moz gold. New gold resources were delineated within the last ca. 10 years at the Hardrock deposit in the eastern part of the belt which highlighted the need for a better understanding of the controls on gold mineralization in the belt. The BGB comprises three panels of metasedimentary rocks, representing a southward transition from fluvial to deltaic to deep oceanic basin plain environments, overlying three panels of older, ca. 2725 Ma, metavolcanic rocks, representing back arc, island arc, and oceanic crust. Detrital zircon geochronology of the BGB and adjacent northern Quetico metasedimentary rocks suggests that these rocks formed from sediments derived by the erosion of ca. 2700 Ma to 2900 Ma source rocks and older >3200 Ma Mesoarchean craton of the eastern Wabigoon subprovince. The emplacement of crosscutting 2694±1 Ma feldspar-quartz porphyry (FQP) dikes, marks the end of sedimentation in the belt. The panels were subsequently imbricated during an early thrusting event (D1) which ended with the emplacement of the stitching 2690±1 Ma Croll Lake stock. The composition of the FQP dikes and Croll Lake stock (i.e. SiO2, ferromagnesian oxide and transition element contents, [La/Yb]N ratio) indicate a shift from tonalite-trondhjemite-granodiorite (TTG) suite to sanukitoid suite magmatism over a 4 Ma period. The formation of the sanukitoid melts, which involves the addition of a mantle melt component, is consistent with their generation during delamination or slab break-off as the BGB metavolcanic and metasedimentary panels were thrustimbricated and accreted to the Wabigoon subprovince during closure of the Quetico basin. D1 structures are expressed by early, isoclinal, F1 folds with an axial planar S1 cleavage. They are folded by regional F2 folds with an axial planar, regional, S2 cleavage, which becomes more pronounced along belt-parallel, sinistral, high strain zones characterized by asymmetrical, Sshaped F2 folds and NE-striking, en echelon, quartz-carbonate veins. The association of regional folds and cleavage with sinistral high strain zones suggest that these structures formed during regional sinistral transpression (D2) across the belt. During subsequent dextral D3 transpression, the regional F2 folds and S2 cleavage were overprinted by Z-shaped F3 folds and by a second regional cleavage (S3), and the D2 high-strain zones were reactivated as dextral shear zones. Within the reactivated high strain zones, F1, F2 and F3 fold axes are parallel to a strong, shallowlyplunging, stretching lineation that formed during D2 and was accentuated and rotated towards the orientation of the fabric attractor during D3. The fabric attractor is oblique to the intersection lineation between the reactivated S2 cleavage and dextral shear bands, suggesting that the deformation during D3 had triclinic symmetry. The Hardrock gold deposit is one of several deposits hosted by the Bankfield-Tombill deformation zone near Geraldton in the eastern part of the BGB. Gold was emplaced during early D1 thrusting and D2 sinistral transpression in association with strongly folded, early D1, quartz-carbonate veins (V1) and E- to NE-striking, syn-D2, tourmaline-rich (V2) and quartz-carbonate veins (V3) veins. Gold is present in association with inclusion-rich pyrite with elevated Au, Ag, As, Bi, Co, Ni, Pb, Sb, Te concentrations and along fractures and as inclusions in the veins and sulfides. The deposit is surrounded by a broad (≤250 m) alteration envelope of carbonate and sericite with elevated pathfinder S, Te, As, W, and Bi concentrations. Early introduction of gold in the belt during thrusting and sinistral transpression differs from previous studies which favored late tectonic emplacement of gold in D3 shear zones and fold hinges.
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.
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.
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.
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.

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