Mineral Deposits and Precambrian Geology - Doctoral theses

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XRD structural assessment of peridotitic garnet with anomalous REE distribution
(2012-12-01) Ross, Kirk Campbell
This thesis explored, as its major aim, the crystallographic and compositional characteristics of a particular type of peridotitic garnet associated with kimberlite. This garnet has a highly sinusoidal rare-earth element (REE) pattern as its distinguishing feature. Before the main research question could be addressed, a technique had to be developed that enabled the rapid and straight-forward acquisition of a full profile digital X-ray diffractogram from a single sub-milligram crystal fragment. After extensive experimentation and testing, successful development and realization of a method that is capable of producing such data was achieved. The next step of this research project was to empirically investigate and assess the crystal lattice strain model (CLSM) of Blundy and Wood (1994). Careful analyses of the REE present in a suite of clinopyroxenes were compared to the crystal structure data given from the XRD technique outlined above. Clinopyroxene is particularly useful for such an assessment because the radius of the M2 site in this mineral is between the largest and smallest REE, making the distribution of REE particularly sensitive to variation of the M2 site, which in turn is a direct consequence of the overall pyroxene structure. Subsequent to illustrating that XRD data could be collected on such small material and the XRD data and structural data given from the CLSM correlate strongly, peridotitic garnets with sinusoidal REE patters were investigated. The conclusions drawn in the first two contributions – namely that it was possible to collect accurate and precise XRD data from sub-milligram specimens and that the crystal structure and REE distribution were directly related – were imperative for the deduction of conclusions in the final, major research question. The XRD analysis of many garnets with and without sinusoidal REE patterns showed the presence of a small amount of an additional phase in some of these garnets. While this phase (2 out of 3 peaks indexed as possibly orthorhombic perovskite) is not present in sufficient quantities to give rise to such a strong sinusoidal segment in the garnet REE pattern, it prompted the critical step forward in formulating a working hypothesis for the otherwise inexplicable REE patterns. Specifically, I posit that many of the sinusoidal garnets may originally have precipitated as a very high pressure phase (in the mantle transition zone or deeper) that subsequently underwent a subsolidus isochemical transformation to garnet. Possible original precursor mineralogy is a combination of two perovskites or a perovskite + iv garnet assemblage. Theoretical calculation using experimental partition coefficients demonstrated that a mixture of Ca-perovskite (CaPv) and Mg-perovskite (MgPv) REE patterns in the approximate proportions of 10% CaPv and 90% MgPv produce a REE diagram that is strikingly similar to those observed in sinusoidal single phase garnet. It has been shown experimentally that with increasing depth in the mantle, garnet plus a progressively increasing CaPv component is the stable mineral assemblage. Initial precipitation of two perovskites or CaPv + garnet as cumulates from a deep magma ocean would preserve the REE distribution of these minerals. Subsequent exhumation of such an assemblage would result in the retrogressive subsolidus phase transformation to a mineral stable at conditions of T and P of the shallow mantle environment, i.e. garnet, while retaining the REE pattern of the initial precipitate mineral assemblage. While this working hypothesis will require many more tests, its proposal may have significant implications for the mantle structure.
Regional volcanogenic massive sulphide metallogeny of the neoarchean greenstone belt assemblages on the northwest margin of the Wawa subprovince, Superior province
(Laurentian University of Sudbury, 2013-10-08) Lodge, Robert Wilfred David
The ca. 2720 Ma Vermilion, Shebandowan, Winston Lake, and Manitouwadge greenstone belts (VGB, SGB, WGB, and MGB, respectively) are located along the northern margin of the Wawa subprovince. They are interpreted to have formed in broadly similar rifted arc to back-arc environments, but their base and precious endowment and, in particular, their endowment in VMS deposits, differ markedly. These difference is metal endowment reflect differences in their metallogenic history that were examined by comparing their regional, belt-scale lithostratigraphy, chemostratigraphy, petrogenesis and tectonic history constrained by new U-Pb zircon geochronology. The MGB is the most VMS-endowed and isotopically juvenile (Pb and Nd) greenstone belt. It has a trace element chemostratigraphy that is consistent with a rifted arc to back-arc environment. The trace element chemostratigraphy of the WGB is also consistent with a rifted-arc to back arc geodynamic setting. The Winston Lake VMS deposits formed during early rifting of the arc and their timing is tightly constrained at ca. 2720 Ma by U-Pb ages of the host felsic strata and post-VMS Zenith gabbro. The Zn-dominated VMS mineralization formed from hydrothermal fluids that were <300 ° and were possibly boiling in relatively shallow water. The trace element chemostratigraphy of the VGB, SGB, and WGB indicates a plume-driven rifted arc to back-arc geodynamic settings. The composition of VMS mineralization, lithofacies, and alteration in these belts are consistent with a relatively shallower-water environment, which may have compromised VMS formation. The high-Mg andesites that are typical of, but restricted to, the SGB formed during compressional “hot” subduction, which resulted in the development of a thicker arc crust. This thicker crust may have inhibited VMS formation, but favoured the formation of magmatic sulphide and gold mineralization. New detrital and magmatic zircon U-Pb geochronology allowed comparison and correlation of lithostratigraphy and metallogeny between the greenstone belts. U-Pb ages within the VGB also defined younger, Timiskaming-type volcanic and sedimentary strata that are coeval with similar deposits in the SGB. These strata are spatially and temporally associated with gold mineralization in both belts and are coeval with similar deformation and magmatic events in the WGB and along the northern margin of the Wawa-Abitibi terrane. This indicates that the formation of Timiskaming-type pull apart basins in the northern part of the Wawa-Abitibi terrane were synchronous, and earlier than in the southern part, which is consistent with oblique convergence of the Wawa-Abitibi terrane onto the Superior Province. Detrital zircon geochronology also revealed the presence of a >2720 Ma iv zircon population within the Timiskaming-type sedimentary strata of the SGB. This is consistent with their derivation from the Wabigoon subprovince and suggests trans-terrane transport of detritus in a foreland –type basin resulting from uplift of the Wabigoon subprovince during accretion of the Wawa subprovince.
Structural controls of Ni-Cu PGE ores and mobilization of metals at the Garson Mine, Sudbury
(Laurentian University of Sudbury, 2014-01-24) Mukwakwami, Joshua
The Garson Ni-Cu-PGE deposit is located on the South Range of the 1850 Ma Sudbury structure along the contact between the Sudbury Igneous Complex (SIC) and the underlying metasedimentary and metavolcanic rocks of the Paleoproterozoic Huronian Supergroup. It comprises four ore bodies that are hosted by E-W-trending shear zones that dip steeply to the south. The shear zones formed as south-directed D1 thrusts in response to flexural-slip during regional buckling of the SIC. They imbricated the ore zones, the SIC norite, the underlying Huronian rocks and they emplaced slivers of Huronian rocks and anatectic breccia into the overlying Main Mass norite. Coexisting garnet-amphibole pairs yielded syn-D1 amphibolite facies metamorphic temperatures ranging from ~550°C to 590°C. The shear zones were coeval with the moderately southdipping South Range and Thayer Lindsley shear zones, which formed to accommodate the strain in the hinge zone as the SIC tightened with progressive D1 shortening. The SE limb of the SIC was overturned together with the D1 thrusts, which were then reactivated as steeply south-dipping reverse shear zones during syn-D2 greenschist metamorphism.Syn-D2 metamorphic titanite yield a U-Pb age of ca. 1849 ± 6 Ma, suggesting that D1 and D2 are part of a single progressive deformation event that occurred immediately after crystallization of the SIC during the Penokean Orogeny. The ore bodies plunge steeply to the south parallel to the colinear L1 and L2 stretching mineral lineations. Ore types consist mainly of pyrrhotite-pentlandite-chalcopyrite breccia ores, but also include pyrrhotite-pentlandite-chalcopyrite disseminated sulfide mineralization in norite, and syn-D2 quartz-calcite-chalcopyrite-pyrrhotite-pentlandite iv veins. In the breccia ores, matrix sulfides surround silicate rock fragments that have a strong shape-preferred orientation defining a pervasive foliation. The fragments are highly stretched parallel to the mineral lineations in wall rocks, suggesting that the ore bodies are zones of high strain. Pyrrhotite and chalcopyrite occur in piercement structures, in boudin necks between fragments, in fractures in wall rocks and in fold hinges, suggesting that the sulfides were mobilized by ductile plastic flow. Despite evidence of high strain in the ore zones, the sulfide matrix in D1 and D2 breccia ores show little evidence of strain as they consist predominantly of polygonal pyrrhotite aggregates, suggesting that they recrystallized during, or immediately after D1 and D2. However, rare elongate pyrrhotite grains aligned parallel to S2 are locally preserved only in D2 breccia ores. Exsolution of pentlandite loops along grain boundaries of elongate pyrrhotite formed S2-parallel pentlandite-rich layers in D2 breccia ores, whereas the pentlandite loops are multi-oriented in D1 contact breccia as they were exsolved along grain boundaries polygonal pyrrhotite. Because exsolution of pentlandite post-date D1 and D2, and that individual pentlandite grains neither have a shape-preferred orientation nor show evidence for cataclastic flow, the sulfides reverted to, and were mobilized as a homogeneous metamorphic monosulfide solid solution (mss) during D1 and possibly D2. This is in agreement with predictions from phase equilibria as the average Garson composition plots within the mss field in Fe-Ni-S ternary diagram at temperatures above ~400°C. Disseminated and breccia ores at Garson have similar mantle-normalized multi-element chalcophile patterns as undeformed contact-type disseminated and massive ore, v respectively, at the well known Creighton mine in the South Range. This suggests that the Garson ores are magmatic in origin and that their compositions were not significantly altered by hydrothermal fluids and deformation. The lack of variations in Ni tenors between the disseminated and breccias ores suggest that the R-factor was not the process controlling metal tenors because the disseminated sulfides do not consistently have higher metal tenors than the breccia ore. The breccia ores are enriched in Rh-Ru-Ir and are depleted in Cu-Pd-Pt-Au, in contrast to footwall-type ore at the nearby Garson Ramp mine which is enriched in the same metals. When Ni100, Rh100, Ir100, Pt100 and Pd100 are plotted against Cu100, the breccia and footwall-type ore analyses plot along model mss fractionation and sulfide melt model curves, suggesting that these two ore types are related by mss fractionation. In summary, the Garson breccia ores are mss cumulates that settled quickly at the base of the SIC via a gravity filtration process, and were mobilized as a metamorphic mss by ductile plastic flow during D1 and D2. Despite minor local hydrothermal mobilization of some metals, the study confirms findings from other studies that highly deformed Ni-Cu-PGE deposits, such as the Garson deposit, can provide important information on the genesis of the deposits.
The stratigraphy and geochemistry of the rapitan iron formation, Northwest Territories and Yukon, Canada
(Laurentian University of Sudbury, 2014-05-16) Baldwin, Geoffrey James
The Neoproterozoic was a time of major change in Earth’s surficial history, including a major rise in atmospheric oxygen, the first appearance of complex metazoan life, and a series of worldwide glaciations. A particular interesting element of these so called “snowball Earth” glacial deposits is the presence of iron formation, a distinctive Precambrian rock type that is largely absent from the post-Paleoproterozoic record. Despite being relatively poorly studied with respect to their geochemistry and sedimentology, Neoproterozoic iron formations are used to support many models for the record of oxygen concentrations of the Earth. The classical example of Neoproterozoic iron formation is the Rapitan iron formation of northwestern Canada. This hematite-jasper iron formation is associated with glaciogenic turbidites and diamictites. Despite being the archetype, the Rapitan iron formation has not been studied in the context of recent ideas about the Neoproterozoic. In this thesis, the stratigraphy, geochemistry, and basin architecture of the Rapitan iron formation are reassessed. Using the REE+Y and the redox-sensitive elements Mo and U, it is shown that the Rapitan iron formation was deposited in a partially restricted basin from biogenically reduced iron under variable redox conditions. Elemental Re and Mo isotopes further show that although oxic and ferruginous conditions predominated during deposition of the iron formation, a transition towards a sulfidic water column locally terminated deposition. Finally, regional stratigraphy and geochemistry show that the iron formation was preferentially deposited in deep, newly formed basins that were protected from significant siliciclastic sedimentation. These basins were delimited by inferred crustal-scale faults trending roughly perpendicular to the axis of the rift basin, and allowed significant changes in thickness and sedimentological character over short distances along strike. These factors help build an overall geotectonic regime under which Neoproterozoic iron formations were deposited: young, deep rift basins that had undergone marine incursion, and were intermittently sealed by an ice shelf, allowing for the generation of an anoxic, iron-rich water column. The absence of the Eu anomaly and the heavy Mo isotopic signature indicate that the open ocean was fully oxygenated at the time of Rapitan iron formation deposition, as opposed to ferruginous as previously suggested.
Regional geophysical investigation of the Sudbury structure
(Laurentian University of Sudbury, 2014-08-26) Olaniyan, Oladele Folajimi
The ca. 1850 Ma Sudbury structure (SS) lies in the Canadian Shield at the paleocontinental margin between the Archean Superior province and the Proterozoic Southern Province. It comprises the Sudbury Igneous Complex (SIC), underlying brecciated footwall rocks, and the overlying sedimentary sequence of the Whitewater Group that occupy the central depression of the SIC. Having gone through a series of syn- and post-formation orogenies, the hypothesized initial circular shape of the SIC has been deformed into an elliptical shape of about 28 km by 60 km. Contacts qualitatively interpreted from the computed directional and tilt derivatives of the magnetic field are mostly coincident with the mapped geological contacts, especially in the North Range, but show some inconsistencies in the South Range probably due to extensive alteration and deformation processes. Interpreted magnetic lineaments further highlight the continuity of some previously known faults and dykes and also revealed some new lineaments probably buried or otherwise obscured. Two-and-a-half dimensional joint magnetic and gravity forward modelling consistent with the magnetic contacts and a new interpretation of the Lithoprobe seismic section incorporated a north-verging fold into the basal portion of the SIC in the South Range. The location of the basal deformation is coincident with and could be related to a linear gravity high observed within the Sudbury Basin. Geological interpretation along six profiles was used to develop a three-dimensional geological model of the Sudbury structure and forward gravity computation suggests the existence of a dense feature, interpreted to be mafic volcanic rocks of the Elliot Lake Group, at about 5-6 km depth under the Sudbury Basin. Sulfide-rich sublayer norite could lie above this deformed zone, suggesting it to be a deep prospective zone for future investigation. Northwest-directed closure and reversal of the Huronian rift basin during the Penokean orogeny is suggested to explain the development of iv structures at the base of the SIC. This interpretation is consistent with most reflectors of the Lithoprobe seismic section.
Novel electromagnetic surveying and interpretation methods for improved near surface characterization and deep exploration
(2015-06-17) Kolaj, Michal
Inductive electromagnetic (EM) geophysical methods are often applied and their data processed for two different purposes. One is to create images of the distribution of electrical conductivity of the subsurface while the second is to derive simple robust physical estimates of the location, size and attributes of discrete conductors. In order to improve both the ability to efficiently image the subsurface and to derive simple robust physical properties of discrete conductors, several surveying methods and interpretation algorithms have been developed. To improve the lateral near-surface resolution of thin conductors, such as overburden, nickel laterites and mine waste, two interpretation methods were developed which rely on the measurement of the vertical spatial derivative of the time-varying magnetic field. The first method does not require grid or line data, but, is less accurate than the second inversion method when the spatial gradient of the resistance is strong and/or when the horizontal magnetic fields are large. When applied to data collected over an old mine tailings pond, the two methods produced similar high resolution maps of the conductance. For both shallow and deep exploration a simple and robust conductance estimation method for borehole EM data was developed. The method relies on the calculation of the vertical spatial derivative of the magnitude of the magnetic field using adjacent down-hole stations. In a field trial, a reliable conductance was calculated for two deep sulfide bodies. To improve the resolution of EM surveying, a multi-transmitter surveying processing method was developed. Individual targets are highlighted by calculating a weighted-sum of the multiple transmitter data and the method is shown to produce high signal-to-noise ratio data for high finesse surveys in complex conductor environments where many transmitter-to-target coupling angles are required and/or for deep focused exploration. A field example over an offset dyke located in the Sudbury Basin showcased the ability of the surveying and processing method to determine the location and orientation of a sulfide body. To provide geoscientists with low-cost and efficient interpretation tools, a fast approximate 3D inversion for fixed-loop surface data was developed. The method solves for the causative subsurface current system which is approximated with a 3D subsurface grid of 3D magnetic (closed loop current) or electric (current element) dipoles. Ground data from a deep massive sulfide body was inverted and the results were consistent with existing interpretation and a second example over a near-surface mine tailings pond highlighted the strength of being able to invert magnetic field data using either magnetic or electric dipoles.
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.
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.
Sedimentology, stratigraphy, geochemistry, geochemistry and diagenesis of giant lacustrine carbonate seep mounds (Ikpiarjuk Formation), Mesoproterozoic Borden Basin, Nunavut, Actic Canada
(2016-03-23) Hahn, Katherine Elizabeth
The Ikpiarjuk Formation consists of a group of very large (kms long and hundreds of metres thick), isolated, deep-water, vent-related dolostone mounds that are unlike any documented example of vent-related carbonate rocks in the geological record. The mounds lack ‘normal’ features of Proterozoic carbonates: the mounds contain thrombolites (microbially clotted carbonate), formed below the photic zone, and lack the stromatolitic fabric characteristic of most Proterozoic reef structures. The sedimentology, stratigraphy, geochemistry, and diagenetic history of the mounds were investigated through field work, standard petrographic methods, and various geochemical and other analytical techniques. Shale-normalised rare earth element patterns of various depositional components of the mounds depicts binary mixing between basin-water-derived precipitates and seep-fluid-derived carbonate. Basin fluid has characteristics that are consistent with deposition in an alkaline lake, and seep fluid features are consistent with circum-neutral groundwater that had interacted extensively with basement blocks before seeping into the lake bottom through faults. The sedimentology and mineralogy of the mounds indicates that they were formed by thrombolites that were made up of a microbiota that was not photosynthetic or aerobic. The Ikpiarjuk Formation thrombolite microbiota was probably dominated by an anaerobic chemoautotrophic or chemoheterotrophic metabolism, and records visible preservation of a community that has never before been described from the Mesoproterozoic. The diagenetic history of the Ikpiarjuk Formation is complicated. Dolomitisation occurred early during diagenesis, and subsequently, five different fluids flowed through primary pore space in the mounds and precipitated successive generations of cement. One of the cements contains sulphides inclusions and may correlate with base-metal bodies elsewhere in the basin.
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.
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
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.
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.
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.
A study of the late-stage mineralogy of agpaitic environments: borosilicates, niobate hydrates and development of laser ablation ICP-MS as a tool to analyze for high concentrations of light elements (Li, Be, and B)
(2017-10-25) Haring, Monika Magdaline-Marion
To date, over 400 mineral species have been discovered at Mont Saint-Hilaire (MSH) many of which are new to science. These new minerals tend to occur as late-stage phases with an enrichment high field strength elements (HFSE) (i.e. Nb, Ti, Zr) as well as light elements (i.e. Li, Be and B). Characterization of these minerals has been hindered by their small crystal sizes (i.e. μm scale) as well as enrichment in light elements. This thesis discusses the chracterization and overall implications of franconite group minerals as well as the new boro silicate minerals steedeite and nolzite. Finally the effectiveness of laser ablation ICP-MS as a method of accurately measuring major concentrations of light elements is discussed. Franconite group minerals (FGM) are hydrous-alkali niobates, inferred to have formed from Nbrich precursor minerals at low temperatures (~150 °C). The structures of these minerals are held together by hydrogen bonds and are highly flexible with Na, Ca, Mg and possilbly Fe2+ and K substituting for one another. Although FGM are Nb-rich, they are notably devoid of Ta despite the two elements being geochemical twins. The FGM are crystallo-chemically similar to scandia octahedral molecular sieves, a group of synthetic compounds known for their cation exchange properties. Twelve borosilicate phases, including the new minerals steedeite and nolzeite are known from MSH. These are crystallo-chemically similar, both minerals containing of single loop-branched dreier chains in their structures. Such chains occur in synthetic compounds (e.g. Li2Mg2[Si4O11]) but are not known to occur in other naturally occurring minerals. Steedeite is highly ordered with respect to Si and B whereas nolzeite contains a mixed Si/B site despite the two minerals being associated with one another. Accurate chemical analysis of late-stage minerals, rich in light elements, is hindered by the fact that such elements have low atomic masses. Laser ablation ICP-MS analysis of a variety of mienrals with major concentrations of light elements gave measured concentrations of Li, Be and B that were within ± 10% of expected ideal values for most minerals. Beam widths varying from 6 to 30 μm were employed however; they had no effect on the measured light element concentrations. Four standards including the NIST610 & 612 glasses, natural beryl, and lithium metaborate were used. Of these NIST610 is considered the most suitable as it has the smallest associated errors (1σ error: ± 0.05 to ± 1.68 ppm). Based on this LA-ICP-MS is considered to be a suitable alternative to secondary ion mass spectrometry for analysis of light elements.
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.
The volcanic and deformation history, geodynamic setting, and metallogenesis of the Chisel Sequence, Snow Lake, Manitoba, Canada
(2018-11-08) Stewart, Margaret Signe
The largest Paleoproterozoic VMS district in the world is hosted in the Flin-Flon Glennie Domain of the Trans-Hudson Orogen. Most of the VMS deposits occur within the Flin Flon and Snow Lake arc assemblages of the Flin Flon belt. The Chisel sequence of the ca. 1.89 Ga Snow Lake assemblage is host to six economic VMS deposits; however, its deformation history prior to ca. 1.86 Ga and its ore-forming volcanic environment are poorly constrained. The Photo Lake deposit occurs within the Chisel sequence and exhibits anomalous Au and Cu enrichment when compared to the other Chisel sequence deposits, but the source of this metal enrichment and its stratigraphic position are unknown. Detailed field mapping, including lithofacies and structural mapping, was completed at 1:2,000 scale, with local detailed areas at 1:100 or 1:200 scale. Core from 20 Hudson Bay Exploration and Development (HBED) drill holes were re-logged and an additional 39 drill holes were examined using whole-rock geochemistry and drill logs provided by HBED to trace the extent of units. Eighty-two outcrop samples and 65 drill core samples, including 32 sulfide samples, were analyzed for whole rock major and trace element geochemistry. Eight of the outcrop samples were also analyzed for whole rock Nd, Hf, and Pb isotopes. Scanning electron microscopy provided the semi-quantitative compositions of mineral phases in least altered and altered samples, and quantitative mineral chemical data for pyrite and pyrrhotite were acquired using an electron microprobe. Trace element maps were produced by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for individual pyrite grains in four of the sulfide samples. Detailed field mapping, re-logging of selected drill holes, and whole rock geochemistry completed in this study indicate: 1) the volcanic rocks of the Snow Lake assemblage underwent a folding and thrusting event prior to 1.86 Ga due to intraoceanic accretion or early accretion to the Amisk collage; 2) with the exception of the Threehouse unit, the entire Upper Chisel sequence represents folded and thrust-repeated Lower Chisel sequence strata; 3) the Chisel sequence VMS deposits, including the Photo Lake deposit, formed within a single time-stratigraphic ore interval during a protracted arc-rifting event; 4) Au enrichment in the Photo Lake deposits is syngenetic and can be attributed to a magmatic input; 5) the Anderson VMS deposits formed during widespread arc rifting, whereas the Chisel VMS deposits formed during localized extension above a magma chamber in the extended arc.
Structural geology, stratigraphy, and gold deposits of the New Britannia Mining District of the paleoproterozoic Snow Lake arc assemblage (Snow Lake, Manitoba, Canada)
(2019-02-21) Rubingh, Kate Elizabeth Louise
Orogenic gold deposits in the Snow Lake area in the southeastern Trans Hudson Orogen, Manitoba, include the New Britannia deposit which, with a past production of a 1.4M oz Au (43 699 kg), is the largest Proterozoic gold deposit in Manitoba and Saskatchewan. The deposits are hosted by the ca. 1.89 Ga Paleoproterozoic Snow Lake arc (SLA) assemblage of the Flin Flon Glennie Complex (FFGC). The FFGC is bound by a sedimentary basin to the north, namely the Kisseynew Domain. It represents an oceanic protocontinent within an ancestral ocean, the Manikewan Ocean, which occupied the region between three Archean cratons (Hearne, Sask, Superior). The deposits occur within a sequence of explosive, submarine, bimodal volcanic rocks emplaced during an early episode of rifting and subsidence of the SLA. Deformation of these rocks began with their imbrication along brittle thrust faults during a D1 event. It continued during a ca. 1.84- 1.82 Ga D2 event with thrusting of the Kisseynew basin and FFGC above the colliding Sask craton. The D2 event produced a penetrative regional foliation axial planar to map-scale isoclinal folds (NorAcme Anticline), sheath-like gneiss domes along the Kisseynew-FFGC boundary, and regional southwest-directed ductile thrust faults, such as the McLeod Road Thrust. Final collision of the FFGC and Sask craton with the Superior craton during a ca. 1.83-1.80 Ga D3 event reactivated the ductile thrust faults, folded the gneiss domes, and enabled the development of an orogen-parallel regional stretching lineation during lateral flow parallel to the cold Superior craton. The New Britannia deposit and nearby gold deposits were emplaced in the hinge of the Nor-Acme Anticline early during the D2 event, were folded during tightening of the fold, stretched parallel to the regional stretching lineation, and transposed along a late crosscutting shear zone, the Howe Sound fault, which formed as a transfer fault during thrusting. The deposits formed at amphibolite facies conditions during a prograde metamorphic event that culminated during the collision of the FFGC and Sask craton with the Superior craton during the D3 event. Thus, the deposits are atypical compare to most other Proterozoic and Archean orogenic gold deposits which typically form at greenschist facies conditions.
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.
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.

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