Earth Sciences / Sciences de la Terre

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Application of Occam’s inversion to airborne time-domain electromagnetics
(2009-03-01) Vallée, Marc A.; Smith, Richard S.
Airborne time-domain electromagnetics (ATDEM) methods are regularly used for mining, hydrocarbon, and groundwater exploration. A large quantity of data is collected along survey lines from an aircraft, and there is an incentive to interpret these data in a systematic way. When the geology is appropriate, the use of 1D inversion methods is justified. Among these methods are: conductivity-depth transform (CDT) (Wolfgram and Karlik, 1995), layered-earth inversion (Sattel, 1998), Zohdy's method (Sattel, 2005), and Occam's inversion (Constable et al., 1987; Sattel, 2005). These methods either require considerable tuning to get realistic results, are limited to step response data, or require considerable experimentation with the initial guess to ensure a reasonable result. The advantage of the Occam's algorithm is that it can be easily adapted to different ATDEM methods and is not strongly dependent on the initial guess. Furthermore, there are not a lot of parameters to tune in order to get a reasonable result. The weakness of the Occam's inversion is that for ATDEM data, the process requires a great deal of computer time. In this paper, we review details of the application of Occam's method to ATDEM data and we present the results of some of our experiments.
Airborne electromagnetic methods: applications to minerals, water and hydrocarbon exploration
(2010-03) Smith, Richard S.
Case history of combined airborne time-domain electromagnetics and power-line field survey in Chibougamau, Canada
(2010-03-01) Vallée, Marc A.; Smith, Richard S.; Keating, Pierre
Exploration for volcanogenic massive sulfides requires good geologic understanding. Geologic knowledge often is limited by a lack of outcrops. This is especially true in Canada under residual glacial covers. Geologic information must therefore be complemented by information obtained using means such as geophysical and geochemical observations. Electromagnetic (EM) methods extend lithological understanding to depths beyond the overburden. Massive sulfides are highly conductive and, depending on their depth and volume, may be detected easily by airborne EM surveys. They are more often equant than graphitic sediments, which typically have longer strike length. Current EMtechniques that identify massive sulfides operate in the frequency or time domain, the latter being more common. Additional information can be provided by using power-line fields as a source of EM signals when the powerlines are appropriately located in the area of interest. We have worked in an active exploration area near Chibougamau, Canada, known for a large occurrence of massive sulfide deposits. The geology is a sequence of volcanic formations with felsic and mafic intrusions. Our magnetic technique responded well to mafic rocks. An airborne time-domain EM survey mapped localized and intrasedimentary conductors in that area. We learned in our study that power-line EM fields can be used to map large-extent conductive formations and narrow geologic faults.
Metalliferous mining geophysics — State of the art after a decade in the new millennium
(2011-06-03) Vallée, Marc A.; Smith, Richard S.; Keating, Pierre
Mining exploration was very active during the first decade of the twenty-first century because there were numerous advances in the science and technology that geophysicists were using for mineral exploration. Development came from different sources: instrumentation improvements, new numerical algorithms, and cross-fertilization with the seismic industry. In gravity, gradiometry kept its promise and is on the cusp of becoming a key technology for mining exploration. In potential-field methods in general, numerous techniques have been developed for automatic interpretation, and 3D inversion schemes came into frequent use. These inversions will have even greater use when geologic constraints can be applied easily. In airborne electromagnetic (EM) methods, the development of time-domain helicopter EM systems changed the industry. In parallel, improvements in EM modeling and interpretation occurred; in particular, the strengths and weaknesses of the various algorithms became better understood. Simpler imaging schemes came into standard use, whereas layered inversion seldom is used in the mining industry today. Improvements in ground EM methods were associated with the development of SQUID technology and distributed-acquisition systems; the latter also impacted ground induced-polarization (IP) methods. Developments in borehole geophysics for mining and exploration were numerous. Borehole logging to measure physical properties received significant interest. Perhaps one reason for that interest was the desire to develop links between geophysical and geologic results, which also is a topic of great importance to mining geologists and geophysicists.
A grid implementation of the SLUTH algorithm for visualizing the depth and structural index of magnetic sources
(2012-01-01) Smith, Richard S.; Thurston, Jeffrey B.; Salem, Ahmed; Reid, Alan B.
The SLUTH method requires first-order derivatives at two or more different heights above the ground and can estimate the location and depth of source bodies from magnetic data. Results of this method are independent of a specific model type and can be used to estimate the most appropriate model (structural index). This paper presents a grid implementation of the SLUTH method to visualize both depth and structural index from magnetic anomaly data. The implementation uses the Geosoft GX programming language. The method has been tested using theoretical magnetic gridded data and of two methods have been used for estimating depth; the estimate from the width of the imaged feature gives an underestimate and the estimate from the rate of fall off of the field with height gives an overestimate. The practical utility of the algorithm is demonstrated using field data from the Saskatoon area of Canada.
How to make better use of physical properties in mineral exploration: The exploration site measurement
(2012-03-01) Smith, Richard S.; Shore, Mark; Rainsford, Desmond
In recent years, there has been a growing awareness that a better understanding of physical property information is required in mineral exploration. As a consequence, there has been a strong push to collect more data and to use these data more intelligently. There are a multiplicity of reasons behind this impetus: geophysicists want more information about physical property data to enable better surveys to be planned and better interpretations to come from the data acquired and geologists want physical properties to provide addition information about the geology that might allow them to see variations in rocks that are not easy to see using traditional or more expensive methods (hand specimen examination, thin sections, lithogeochemistry, assays, etc.). If a hole is drilled on a geophysical target, then a physical property measurement of the core or the rocks surrounding the core can confirm if the target was intercepted and provides data that can be used to model the target response.
Using combinations of spatial gradients to improve the detectability of buried conductors below or within conductive material
(2012-12-12) Smith, Richard S.
The detection of conductive bodies is an important capability when exploring for massive sulfide deposits or looking for unexploded ordnance. When these bodies are buried below conductive overburden or embedded in conductive material, the use of an electromagnetic system to identify the bodies becomes problematic because the response of the overlying conductive material can be much greater that the response of the buried conductor. I calculated the response of five models representing different conductivity distributions (a buried conductor, a uniform overburden with changes in the system altitude, a paleochannel, a thicker overburden, and a thinner overburden). The subtle response of the buried conductor was difficult to identify because it looked very similar to the responses of other structures that are not necessarily of interest. The spatial gradients for the same five models showed that the greatest improvement in the relative size of the anomalous gradient response compared with the background gradient came for the cases in which the material closest to the surface changes, in particular the paleochannel and thickening overburden models. However, identification of the deeper buried conductor was still problematic because of the large background gradients. In theory, the cylindrical symmetry of a dipole transmitter over a layered earth ensured that there were exact relations between the spatial derivatives. Hence it was possible to define two specific combinations that should be zero over a layered earth. Calculating these combinations for the five models showed that the anomalous zones stood out with significantly greater anomaly-to-background ratios. The measurement of the gradients and the calculation of these combinations therefore provided a means of identifying anomalous zones in and below a conductive earth. Different relative sizes and shapes of the two combinations for different models provided a way of discriminating between the vertical conductor model and the four other models.
Qualitative geophysical interpretation of the Sudbury Structure
(2013-07-26) Smith, Richard S.; Olaniyan, Oladele; Morris, Bill
The Sudbury Structure is one of the most studied geologic structures in the world due to its enigmatic nature and mineral wealth. The available geologic work from the literature and mining industry operations accumulated for more than a century was recently assessed and compiled into a bedrock geologic map. Most regional geophysical investigations of the Sudbury Structure have been quantitative — modeling and depth estimation without a clear definition of surface control. Airborne total magnetic intensity data over the Sudbury Structure were compiled, processed, and interpreted, to define magnetic stratigraphy boundaries and near-surface lineaments. Traditional directional and normalized derivatives were computed to enhance the high-frequency information in the magnetic field. Available airborne frequency-domain electromagnetic (EM) data were also interactively interpreted along profiles and in a gridded format to isolate conductive structures. On-screen geographic information system-based information extraction from multiple derivatives was used to interpret the magnetic contacts, dykes, and lineaments. The magnetic interpretation was compared with published bedrock maps of the Sudbury Structure. Magnetic contacts based on the qualitative classification of the magnetic texture did not always correspond to the geologic boundaries on the existing maps. Some magnetic lineaments corresponded with well-defined geologic structures, some were further extensions of partially mapped structures, and others are newly identified linear structures. Conductive locations identified from the EM profiles were probably due to responses from conductive ore bodies, faults, dykes, lithological contacts, and cultural objects.
Processes at the mineral-water interface in the acid soils of the Sudbury area
(Laurentian University of Sudbury, 2013-10-16) Lanteigne, Sonia
Over a century of mining activities and smelting in the area of Sudbury, Ontario, Canada have resulted in the contamination of the local soils with metal(loid) bearing particulates. Minor and trace elements associated with these phases are released during their weathering. This release is therefore strongly dependent on the mineralogical and chemical character of the metal(loid) bearing phases. The metal(loid)s are then subject to transport before being attenuated through their incorporation into secondary phases. Elevated concentrations of metal(loid)s in silica rich alteration layers has recently been described for altered surfaces at the solid-water and solid-atmospheric interfaces in tailings, and in the vicinity of smelters, respectively. To determine if similar coatings occur in soils, samples were taken from areas around three major smelting centers in the area. Coated grains were extracted from these samples and individually mounted to be analysed. Particulate matter (representing primary metal(loid)-bearing phases) and coatings (secondary metal(loid)-bearing phases) were analysed using scanning electron microscopy, Raman spectroscopy, Laser-Ablation Inductively-coupled plasma mass spectroscopy, Micro-X-ray fluorescence, and X-ray photoelectron spectroscopy. The particulates were divided into three main groups: smelter-derived particles, sulfides, and nickel-oxides. Smelter derived particles contained the most elevated concentrations of metal(loid)s in their sulfide inclusions and metal(loid)-rich rims. The mobility of metal(loid)s in the identified mineral phases found within particulates mirrored the transport observed in the soil column; Zn>Cu>Ni>Pb. Once mobilized, these elements are subject to transport before being attenuated by secondary phases. Micro-coatings were found to be composed of hematite, schwertmannite, ferrihydrite, silica, and jarosite group minerals. Coatings are distinguished on the basis of their atomic Si:Fe ratios: FeOx coatings have Si:Fe <1, Si–FeOx coatings have Si:Fe between 1-10, and SiOx coatings iv have Si:Fe>10. Iron-rich coatings (FeOx) and silica-rich coatings (SiOx) have lower trace-metal concentrations than Fe-SiOx coatings. Micrometer-thick coatings are predominantly composed of hematite, schwertmannite, ferrihydrite and (amorphous) silica and contain elevated metal(loid) concentrations in the form of metal(loid)-rich phosphate minerals (mainly minerals of the jarosite group). A general model is developed that describes the formation of mineral coatings in acid soils and their important role in the uptake and retention of metal(loids). Here, micrometer-thick Fe-silica coatings form through adsorption, co-precipitation and dehydration processes involving amorphous silica and iron hydroxides. Metal(loid)-bearing phases nucleate within a gel-type matrix and are subsequently preserved during dehydration and solidification. Aluminum-rich surfaces form on mineral grains once the pH has been raised sufficiently high (pH~5-6) so as to lead to the complete removal of sulfate-bearing phases. The implications of this model are widespread in terms of the attenuation of metal(loid)s in acid soils and their retention or subsequent remobilization in recovered soils with near neutral pH.
Application of the cross-hole radio imaging method in detecting geological anomalies, MacLennan township, Sudbury Ontario
(Laurentian University of Sudbury, 2013-10-30) Sharif, Ladan Karimi
The occurrence of conductive sulphide in an otherwise highly-resistive host rock is the ideal situation for exploring using high-frequency electromagnetic methods. The FARA radio imaging (RIM) system was deployed to explore the rock properties between two boreholes MAC104 and MAC100G, which are about 182 m apart, on the Nickel Rim South property (MacLennan Township) 22 km northeast of Sudbury. Tomographic data were collected and processed at 625 kHz and 1250 kHz. One data set has the transmitter in MAC100G and the receiver in MAC104; the other “reciprocal” data set has the transmitter in MAC104 and the receiver in MAC100G. The amplitude data were reduced, edited, and processed to generate tomograms employing the SIRT algorithm. Separate tomograms were created for the reciprocal data sets in the ImageWin software. A sensitivity analysis was conducted to assess the influence that perturbations in the ImageWin processing parameters have on the resulting tomograms. The sensitivity study of the tomograms along with the information obtained from the value of fit analysis can be used to select appropriate processing parameters. Finally, the two reciprocal sets of conductivity values were averaged and imported into Geosoft to create a final tomogram for the panel. The resistivity values of the studied zone obtained from the FARA modeling package agree fairly well with the conductivity data set generated by the ImageWin modeling package when compared using the Geosoft and GOCAD visualization software. Differences between the two tomograms are attributed to the different solver methods employed by FARA and ImageWin and the statistical analysis used for averaging the attenuation value over ray paths. Furthermore, it is shown that the tomographic results are consistent with the location of conductive zones that were identified using down-hole geophysical logging. The main focus of the project is to understand how the radio imaging (RIM) data is processed with the ImageWin software to construct an attenuation tomogram. This research showed that both tomograms created by ImageWin and FARA illustrate the same pattern with two conductive zones at the same depth; however, the values of conductivity are slightly different. The FARA resistivity values obtained for the upper zone is a factor of two lower than the resistivity calculated by ImageWin. The resistivity values obtained for the lower zone using the FARA processing is a factor of eight lower than the resistivity calculated by ImageWin. Also, there is a slight discrepancy in the orientation of the upper and lower zones on the two tomographic images generated using the two processing packages. In the tomograms generated by FARA software both upper and lower zones are continuous linear zones from one hole to the other with dips from MAC104 towards MAC100G, whereas in the tomograms created by ImageWin the upper and lower zones are less linear and do not have obvious dips.
Mapping lateral changes in conductance of a thin sheet using time-domain inductive electromagnetic data
(Society of Exploration Geophysicists, 2013-11-05) Kolaj, M.; Smith, Richard S.
With the inductive electromagnetic geophysical method, the laterally varying conductance of thin sheet models can be estimated either through a direct transform of the measured data or through inversion. The direct transform (called the simplified solution) does not require grid or line data and is simple enough to be performed in the field because the conductance at a location is calculated directly from the ratio of two measured magnetic fields (the vertical spatial and temporal derivative of the vertical magnetic field) at that location. However, the simplified solution assumes that the secondary horizontal magnetic fields are zero and/or that the sheet has a uniform conductance. Our nonapproximate solution (called the full inversion) does not make these assumptions, but requires gridded data, measurements of the secondary horizontal magnetic fields, and more complicated inversion algorithms. Through forward modeling, we found that the full inversion provides better results than the simplified solution when the spatial gradient of the resistance is strong and/or when the horizontal magnetic fields are large. Because the simplified solution may be preferable due to its simplicity, we introduce two unreliability parameters, which assess the unreliability of the conductance calculated using the simplified solution. A comparison of the simplified solution and full inversion in a fixed in-loop survey collected overtop a dry tailings pond in Sudbury, Ontario, Canada, revealed that there were small differences around large conductance contrasts, which coincided with elevated unreliability parameters. The simplified solution is recommended if fast in-field interpretations are required, or additionally, as a first-pass survey that can be performed with sparse station spacing to identify areas of interest. Denser grid data can then be collected, for the more reliable full inversion, over areas of interest and/or zones where the simplified solution is expected to be unreliable as predicted by the unreliability parameters.
Characteristics of diagenetic fluids affecting two major carbonate units on Victoria Island, Northwest Territories
(Laurentian University of Sudbury, 2014-03-17) Mathieu, Jordan-Paul
Diagenetic histories of Proterozoic and Paleozoic carbonate strata on Victoria Island, in the Canadian arctic, are poorly understood, and their potential to be associated with base metals or petroleum is unknown. Using fluid inclusion and geochemical techniques, it was determined that the diagenetic fluid compositions of two major carbonate units, the Wynniatt Formation and the “Victoria Island formation”, were largely controlled by fluid-rock reactions in reservoirs and by mixing of multiple fluids. Diagenesis of the Wynniatt Formation resulted from the progression from a shale-dominant fluid mixture to a meteoric-dominant mixture. Fluid composition of “Victoria Island formation” was a shale-dominant mixture. A change in fluid:rock from low to high was recorded during diagenesis of both units. Metals and hydrocarbons transported to the study sites were ac-quired by the fluids during interaction with the respective source reservoirs. Mixing of diagenetic fluids follows the established ‘mixing model’ used to explain many other min-eralised locations. The diagenetic fluids that affected the strata in this study were compa-rable to those that produced the Polaris Zn-Pb deposit. This similarity suggests that there is potential for mineralisation on Victoria Island.
Robust conductance estimates from spatial and temporal derivatives of borehole electromagnetic data
(Society of Exploration Geophysicists, 2014-05-01) Kolaj, M.; Smith, Richard S.
The conductance of an infinite uniformly conductive thin sheet can be calculated using the ratio of the temporal gradient and the spatial gradient in the normal direction of any component (or combination of components) of the secondary magnetic field. With standard borehole electromagnetic (BHEM) systems, the temporal gradient can either be measured or readily calculated from transient-magnetic-field data, and the spatial gradient in the normal direction can be estimated using adjacent stations. Synthetic modeling demonstrates that, for a finite thin sheet, the magnitude of the field provides a robust and reliable apparent conductance in typical three-component BHEM survey configurations. The accuracy in which the apparent conductance can be calculated is hindered by low spatial gradient signal values and can only be reliably estimated where the fields are large (i.e., in close proximity to the target). In a field example of BHEM data collected over a massive sulfide deposit in Sudbury, Ontario, Canada, the spatial gradient could be calculated over a roughly 100-m-wide zone, and a consistent apparent conductance could be calculated at each delay time using the magnitude of the field. Increases in the apparent conductance with increasing delay time are likely due to currents migrating into more conductive parts of the body. The apparent conductance values were also consistent with Maxwell models and time constant derived conductance estimates. This simple and robust apparent conductance is ideal as a first-pass estimate for target discrimination, grade estimation, and starting values for forward and/or inversion modeling.
High tenor NI-PGE sulfide mineralization of the south manasan ultramafic intrusion, Thompson Nickel Belt, Manitoba
(Laurentian University of Sudbury, 2014-05-16) Franchuk, Anatoliy
The South Manasan ultramafic intrusion (ca. 1880 Ma) located in the Early Proterozoic Thompson Nickel Belt (TNB) contains Ni and platinum group element (PGE) mineralization hosted by disseminated sulfide. Whole-rock Ni values range from 0.3 to 1.7 wt. % and total precious metals (TPMs) range from 0 to 1.3 ppm Pt + Pd + Au and equate to tenor values (i.e., metal in 100% sulfide) of 11-39 wt. % Ni and 8-27 ppm TPMs. The South Manasan intrusion is a steeply dipping sill-like body with a boudinaged outline having a strike length of approximately 1200 m, average width of 125 m and a minimum depth extent of 1000 m. The intrusion is composed of approximately 25% fresh dunite, 50% serpentine altered dunite and 25% tectonized and carbonate altered dunite. The most intense alteration is found near the intrusion’s margin where it is in contact with metasedimentary rocks of the Pipe Formation, part of the surrounding Ospwagan Group. In fresh dunite the sulfide assemblage characterized by an intercumulate texture is dominated by pentlandite with accessory pyrite; the latter having a symplectic-like texture. The pentlandite-pyrite assemblage in the serpentinized dunite, although still characterized overall by an intercumlate-texture, has well developed platy intergrowths with chlorite and serpentine. In the most intensely modified unit (the carbonate altered dunite) the sulfide assemblage consists primarily of pyrrhotite and pentlandite. Whole-rock geochemical data (n=360), modal mineralogy and mineral chemistry obtained on representative drill core throughout the South Manasan intrusion have been used to establish a type section in order to evaluate the relative roles of primary magmatic versus secondary (i.e., serpentinization, carbonate alteration and deformation) processes. These data indicate that the primary silicate-sulfide assemblage was systematically modified during : serpentinization, carbonate alteration and deformation of the South Manasan intrusion such that a sequence of primary versus secondary events can be established. Intrusion of the original komatiitic magma and formation of the South Manasan intrusion took place at a shallow level into consolidated Ospwagan Group sediments with subsequent contamination of this melt with crustal S. This triggered sulfide saturation and generation of an immiscible sulfide melt. Calculated Ni and TPM tenor values constrain the R factor to between 500 and 2500. The early crystallization of olivine inhibited the sulfide melt from settling to the bottom of the magma column and as a consequence, the sulfides now have a primary interstitial magmatic texture. The current sulfide association dominated by pentlandite>>pyrite>chalcopyrite has a mineral paragenesis that is consistent with subsolidus re-equilibration of a primary pentlanditepyrrhotite- chalcopyrite assemblage. The subsequent processes of serpentinization, deformation and carbonate alteration resulted in modifying the primary sulfide assemblages and their textures (i.e., to platy habits), but did not greatly alter the bulk composition, in particular metal contents, except for addition of volatiles (H2O, CO2). It is concluded therefore that the enrichment of the sulfide assemblage at South Manasan in Ni and PGEs is a consequence of a primary magmatic process involving high R factor and that the effects of later overprinting processes (alteration, deformation) are not responsible for the presently observed high-tenor sulfide association.
Genesis of Cu-PGE-rich footwall-type mineralization in the Morrison deposit, Sudbury
(Laurentian University of Sudbury, 2014-05-21) Nelles, Edward William
The Morrison deposit, located at the Levack mine in the City of Greater Sudbury, is a footwall-type Cu-Ni-platinum-group-element (PGE) deposit hosted within a zone of Sudbury Breccia in the Archean Levack Gneiss Complex beneath the North Range of the Sudbury Igneous Complex. It consists of sharp-walled, sulfide-rich veins that are enriched in Cu-Pt-Pd-Au relative to contact-type mineralization and can be subdivided based on vein geochemistry, mineralogy, texture, and morphology into a pyrrhotite-rich upper domain, a chalcopyrite-rich lower domain, and a pyrrhotite equal to chalcopyrite middle domain. All domains contain steeply to vertically dipping first-order sulfide veins, irregular and discontinuous second-order sulfide veins, and disseminated sulfides in country rocks. First- and second-order veins can be further subdivided into inclusion-free veins typically within Sudbury breccia matrix or along clast-matrix boundaries, and very irregular and inclusion-rich veins associated with leucosomes in mafic gneiss clasts and granophyric-textured dikes. First-order veins consist of pyrrhotite > chalcopyrite = pentlandite > magnetite in the upper domain, pyrrhotite = chalcopyrite > pentlandite > cubanite > magnetite in the middle domain, and chalcopyrite >> pentlandite > pyrrhotite = cubanite > magnetite in the lower domain. Second-order veins consist of pyrrhotite = chalcopyrite > pentlandite > magnetite and chalcopyrite = millerite = pentlandite in the middle domain, and chalcopyrite >> millerite, millerite > chalcopyrite, bornite >> chalcopyrite, and millerite > bornite > chalcopyrite in the lower domain. Second order veins are adjacent to and in contact with epidote, amphibole, chlorite, carbonate, quartz, and magnetite alteration minerals. Sulfide mineralization in the Morrison deposit is similar to other footwall mineralization associated with the SIC. The veins appear to have been emplaced preferentially into zones of Sudbury Breccia that were within ~400m of the basal contact of the SIC, because that lithology is more permeable and because those zones are within the thermal aureole of the cooling SIC permitting penetration of sulfide melts. The mineralogical, textural, and geochemical zoning in the chalcopyrite-pentlandite-pyrrhotite-rich parts of the Morrison deposit are best explained by partial fractional and/or equilibrium crystallization of MSS and ISS. Bornite ± millerite-rich mineralization are interpreted to have formed by reaction of residual sulfide melts with wall rocks, consuming Fe and S to form actinolitemagnetite- epidote-chlorite-sulfide reaction zones and driving the sulfide melt across the thermal divide in that part of the Fe-Cu-Ni-S system to crystallize borniteSS ± milleriteSS. Gold-Pt-Pd appear to have been more mobile than other metals, forming localized zones of enrichment, although it is not clear yet whether they were mobile as Au-Pt-Pd-Bi-Te-Sb-rich melts or aqueous fluids.
Structural and stratigraphic setting of the Rey de Plata volcanogenic massive sulfide deposit in the Guerrero Composite Terrane, Mexico
(Laurentian University of Sudbury, 2014-07-24) Monter, Ahiram Ramirez
The Rey de Plata volcanogenic massive sulfide (VMS) deposit is located in southern Mexico and within the Teloloapan subterrane of the Guerrero Composite Terrane. It is hosted by the Middle to Late Cretaceous Villa Ayala Formation, which is itself subdivided into three informal members. The lower Ahuehuetla member consists of basaltic flows, sills, and volcaniclastic rocks. The middle Rey de Plata member, which hosts the Rey de Plata deposit, consists of rhyolitic to rhyodacitic flows, sills, and volcaniclastic rocks. The timing of felsic volcanism and the formation of the Rey de Plata deposit is constrained between 139.1 ± 0.4 Ma to 129.4 ± 0.7 Ma by U/Pb dating of zircons from an aphyric flow. The upper Villa Ayala member consists of basaltic amygdaloidal sills and volcaniclastic rocks. The slightly LREE enrichment, negative Nb and positive Zr anomalies of the mafic rocks suggest that they formed either in a primitive to slightly evolved arc to backarc setting with minor contamination by older arc (or continental) crust. The slight LREE enrichment, flat HREE pattern, and pronounced negative Nb anomaly of the felsic volcanic rocks are consistent with FII rhyolites, which indicate their formation and emplacement during rifting within an arc or nascent back-arc environment; an interpretation consistent with the geochemical signature of the associated basaltic rocks. The Rey de Plata and Villa Ayala members are unconformably overlain by argillite of the Acapetlahuaya and turbiditic sandstone of the Miahuatepec Formations, which are capped by Eocene conglomerates of the Balsas Group. Detrital zircons from the Acapetlahuaya Formation have U/Pb ages ranging from 115 ± 4 Ma to 152 ± 4 Ma, similar to the age of volcanic rocks of the Villa Ayala Formation. This suggests that the Acapetlahuaya sedimentary rocks were deposited in an intra-arc basin or in a back-arc basin, which separated the Villa Ayala arc from continental Mexico. This basin was closed during deposition of the younger Miahuatepec Formation sedimentary rocks as these strata yielded older, 800 Ma to 1200 Ma, zircon populations derived from the erosion of older terranes previously accreted to continental Mexico. D1 at Rey de Plata corresponds to the Upper Cretaceous to Lower Tertiary Siever- Laramide Orogeny. A shallowly SW-dipping S1 cleavage, L1 stretching lineation, and NE-verging F1 folds formed in altered volcanic rocks, argillite and turbiditic sandstone during NE-directed D1 thrusting. D2 extensional collapse of the orogen occurred either during or shortly after D1. It produced asymmetrical W-verging folds folding S1, and an S2 shear band cleavage (C’), indicating normal, west-directed slip down the dip of the S1 foliation. During D2, gold- and silver-rich epithermal veins were emplaced. These veins may have contributed to the high grade gold and silver zones of Rey de Plata deposit. Deformation of the Teloloapan subterrane resumed in the Eocene with the formation of D3 normal brittle faults ii linked by strike-slip brittle faults. W-verging F3 drag folds are associated with the normal faults. Extensional basins formed during D3 and were filled by fluvial conglomerate and ignimbrites of the Balsas Group. Late, NW-SE-directed, D4 compression produced upright to inclined, NE- and SW-verging F4 folds that overprint S1 and S2. Rey de Plata is a bimodal-felsic type VMS deposit. It formed during rifting of an arc or backarc and consists of stratigraphically stacked Zn-Cu-Pb massive sulfide ore lenses with high Ag-Au grades. The lenses range in thickness from 3m to 60m, in length from 300m to 1500m, and in width from 100m to 500m. They consist of massive, semi-massive, and disseminated pyrite ± sphalerite ± galena ± chalcopyrite ± silver sulphosalts ± gold, together with gangue minerals of quartz ± barite. Extensive quartz-sericite-pyrite alteration surrounds the ore lenses. Graphitic argillite units up to 40m in thickness typically occur at the top and bottom contacts of the ore lenses. The ore lenses underwent the same deformation history as their host rocks. During D1, they developed a strong S1 foliation, they were stretched parallel to L1, and their base metal zonation reflects clockwise rotation of the lenses (looking NW). Thereafter, during D2 and D3, the ore lenses were displaced southwestward by 200 meters along normal faults and shears and then openly folded by F4 during D4.
Early neoproterozoic marine redox conditions recorded in black shale from the little Dal Group, Northwest Territories, Canada
(Laurentian University of Sudbury, 2014-08-11) O’Hare, Sean Patrick
Black shale in the Little Dal Group (ca. <817 Ma), Mackenzie Mountains Supergroup (<1005 Ma; >779 Ma), was deposited during the early Neoproterozoic, and is one of the few known black shale deposits from this crucial time in Earth’s evolutionary history. Relative iron enrichment (FeT/Al) and conventional iron speciation (DOP), along with enrichment in molybdenum, total sulphur, and total organic carbon, were studied. Iron systematics (FeT/Al >0.5 and DOP <0.80) indicate ferruginous, anoxic, and possibly oxic bottom-water conditions over the time of deposition of the entire black shale unit. The enrichment factors of several of the authigenic redox-sensitive trace elements (U, Mo, V) are strongly correlated, and appear to be related to both the FeT and the organic carbon content of the black shale. Molybdenum enrichment (<10 ppm) is limited, which is in very good agreement with data from Mesoproterozoic black shales, but is much lower than Mo enrichments in Paleozoic black shales (typically >100 ppm). Several black muddy siltstones yielded similar results, but authigenic iron was greatly overwhelmed by siliciclastic sedimentation. These new data support the theory that ocean bottom-waters returned from sulphidic to ferruginous prior to development of oxygenated conditions in the Ediacaran open ocean. This study documents a predominantly open-marine basin that was characterised by ferruginous conditions, similar to Archean and early Paleoproterozoic conditions, with brief intervals when oxic conditions developed.
A procedure for collecting electromagnetic data using multiple transmitters and receivers capable of deep and focussed exploration
(Society of Exploration Geophysicists, 2014-11-26) Lymburner, J.; Smith, Richard S.
Many ground controlled-source electromagnetic (EM) systems have been deployed, and under ideal conditions these systems are capable of detecting large conductors to depths of approximately 800 m; however, more common detection limits are less than 400 m. Although these systems have been used with great success, they may experience two weaknesses when exploring for deeper conductors: poor coupling with the target and small signal-to-noise ratios (S/Ns), both of which decrease the quality and interpretability of the data. We evaluated a novel time-domain EM procedure that addresses these weaknesses. The coupling weakness was addressed through multiple transmitter locations and multiple receiver locations, and the S/N was increased by spatial stacking of measurements (from the various transmitter-receiver combinations). A field test of this procedure was undertaken. Reciprocity data indicated that the noise levels of the vertical component data we acquired were about −0.004μV/Am2. Spatial stacking of the data can reduce the noise levels by a factor of seven. This means that a small conductor previously only visible to 150 m could be seen to 275 m and a conductor visible to 300 m could be seen to 575 m. One challenge of the new procedure was the time required to collect all the transmitter-receiver combinations — this time can be reduced using the principle of reciprocity and not repeating approximately reciprocal measurements. Another challenge was to visualize and interpret the large volumes of data collected using the procedure — this has been partially addressed by creating equivalent-dipole depth sections. Synthetic and real equivalent-dipole depth sections appeared very similar and illustrated that these images of the subsurface could be interpreted. However, the features appeared too deep on the sections, so better visualization techniques could be developed.
Modelling radio imaging method data using electric dipoles in a homogenous whole space
(Laurentian University of Sudbury, 2014-12-17) Naprstek, Tomas
Information as to how the signal of the Radio Imaging Method (RIM) changes when the system parameters change or the rock properties change is not well documented. Having a better understanding of the impact of these changes would assist in the design of surveys and the interpretations of RIM data. To quantify the impacts, a modelling program was created by representing the transmitter as an electric dipole. It outputs the amplitude and phase of the electric field in a homogeneous whole space. The system parameters were varied to investigate their effects on the measured signal. It was found that increasing the conductivity or the magnetic permeability resulted in amplitude attenuation and sharper anomalies, while increasing the dielectric permittivity resulted in increased amplitude and broader anomalies. A case study was performed using data from Drury Township, near Sudbury, Ontario, Canada. The mostly homogeneous section of field data was fit with synthetic data whose conductivity values ranged in the 10-3 S/m magnitude. A better fit was found using a conductivity of 3*10-4 S/m, by increasing the relative dielectric permittivity from 1 to between 18 and 20. It was concluded that the program was effective at fitting homogeneous sections of field data, and was developed into RIM forward model software for easy use.
A comparative study of hand-held magnetic susceptibility instruments.
(Laurentian University of Sudbury, 2015-01-27) Deng, Deng Ngang
A study to compare six magnetic susceptibility (MS) instruments (the KT-10 supplied by Terraplus Inc., RT-1 produced by Fugro, SM30 produced by ZH Instruments, MS2K & MS2C produced by Bartington Instruments and MPP-EMS2+ probe produced by GDD Instruments (denoted as GDD)) was conducted to characterize the equipment on the basis of their accuracies, resolution, reproducibility, ease of use and response to drift. MS data were collected on BQ core from 3 holes, NQ core samples, 2 rock samples and 2 calibration samples at the Vale office in Thompson, Manitoba. The results show that the GDD and MS2K are most affected by temporal drift whereas the KT-10 and MS2C gave more repeatable results. The MS2C, MS2K and GDD generally gave higher susceptibility readings than the rest of the meters. It was also noted that measurements on the flat face of half-core samples were always higher compared to measurements on their respective whole core samples. There is a correlation between instruments, frequencies and sensitivities, but no relationship between frequencies of operation and temporal drift. Keywords

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