Earth Sciences / Sciences de la Terre

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Airborne electromagnetic methods: applications to minerals, water and hydrocarbon exploration
(2010-03) Smith, Richard S.
An airborne electromagnetic system with a three-component transmitter and three-component receiver capable of detecting extremely conductive bodies
(Society of Exploration Geophysicists, 2018-08-28) Smith, Richard S.
Extremely conductive bodies, such as those containing valuable nickel sulfides, have a secondary response that is dominated by an in-phase component, so this secondary response is very difficult to distinguish from the primary field emanating from the transmitter (because by definition they are identical in temporal shape and phase). Hence, an airborne electromagnetic (AEM) system able to identify the response from the extremely conductive bodies in the ground must be able to predict the primary field to identify and measure the secondary response of the extremely conductive body. This is normally done by having a rigid system and bucking out the predicted primary (which will not change significantly due to the rigidity). Unfortunately, these rigid systems must be small and are not capable of detecting extremely conductive bodies buried deeper than approximately 100 m. Another approach is to measure the transmitter current and geometry and subtract the primary mathematically, but these measurements must be extremely accurate and this is difficult or expensive, so it has not been done successfully for an AEM system. I exploit the geometric relationship of the primary fields from a three-component (3C) dipole transmitter. If the transmitter is mathematically rotated so that one axis points to the receiver, then linear combinations of the fields measured by a 3C receiver can be combined in such a way that the primary fields from the transmitter sum to zero and cancel. Alternatively, the measured transmitter current and response could be used to estimate the transmitter-receiver geometry and then to predict and remove the primary field. Any residual must be the secondary coming from a conductive body in the ground. Hence, extremely conductive bodies containing valuable minerals can be found. An AEM system with a 3C transmitter and a 3C receiver should not be too difficult to build.
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.
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.
Approximate semianalytical solutions for the electromagnetic response of a dipping-sphere interacting with conductive overburden
(Society of Exploration Geophysicists, 2016-06-06) Desmarais, J.K.; Smith, Richard S.
Electromagnetic exploration methods have important applications for geologic mapping and mineral exploration in igneous and metamorphic terranes. In such cases, the earth is often largely resistive and the most important interaction is between a conductor of interest and a shallow, thin, horizontal sheet representing glacial tills and clays or the conductive weathering products of the basement rocks (both of which are here termed the “conductive overburden”). To this end, we have developed a theory from which the step and impulse responses of a sphere interacting with conductive overburden can be quickly and efficiently approximated. The sphere model can also be extended to restrict the currents to flow in a specific orientation (termed the dipping-sphere model). The resulting expressions are called semianalytical because all relevant relations are developed analytically, with the exception of the time-convolution integrals. The overburden is assumed to not be touching the sphere, so there is no galvanic interactions between the bodies. We make use of the dipole sphere in a uniform field and thin sheet approximations; however, expressions could be obtained for a sphere in a dipolar (or nondipolar) field using a similar methodology. We have found that there is no term related to the first zero of the relevant Bessel function in the response of the sphere alone. However, there are terms for all other zeros. A test on a synthetic model shows that the combined sphere-overburden response can be reasonably approximated using the first-order perturbation of the overburden field. Minor discrepancies between the approximate and more elaborate numerical responses are believed to be the result of numerical errors. This means that in practice, the proposed approach consists of evaluating one convolution integral over a sum of exponentials multiplied by a polynomial function. This results in an extremely simple algorithmic implementation that is simple to program and easy to run. The proposed approach also provides a simple method that can be used to validate more complex algorithms. A test on field data obtained at the Reid Mahaffy site in Northern Ontario shows that our approximate method is useful for interpreting electromagnetic data even when the background is thick. We use our approach to obtain a better estimate of the geometry and physical properties of the conductor and evaluate the conductance of the overburden.
Assessing the extent of local assimilation within the Platreef, Northern Limb of the Bushveld Igneous Complex, using sulfur isotopes and trace element geochemistry
(2019-04-15) Keir-Sage, Evan
The proximity to metasedimentary footwall units in the Northern Limb of the Bushveld Igneous Complex (BIC) has resulted in a complex local contamination of in this maficultramafic intrusive body, including the units containing platinum group element (PGE) mineralization. To assess the extent of incorporation of non-magmatic material and its effects on PGE mineralization, geochemical and isotopic data were collected from drill core UMT094 on the Turfspruit project, where core logging has shown a clear macroscopic division between mineralization and local footwall contamination. The S isotopic data combined with whole rock geochemistry data (including CaO/Al2O3, V/Ti, Ni/Cr, S/Se, LOI) present substantial evidence to assess the range of incorporation of local footwall material. A δ34SVCDT profile shows a steady decrease from the footwall assimilation zone (δ34S = +8 ‰) to near constant δ34S values (δ34S < +4 ‰) below mineralization. Through PGE mineralization, the δ34S data converge to the range that has been documented for the Merensky Reef in the Eastern and Western Limbs of the BIC (δ34S: ~ 0 to +3 ‰). Other geochemical parameters through mineralization, such as S/Se and CaO/Al2O3, also match the ranges documented for the Merensky Reef. In addition, parameters such as whole rock V/Ti are shown to be useful indicators of the type of contaminant (e.g. V/Ti > 2 for intervals assimilating shales and V/Ti < 1 for intervals assimilating carbonates; 1 < V/Ti < 2 for uncontaminated magmatic units). The results indicate that there is negligible local contamination through mineralization and that the primary mechanism of PGE mineralization in the Platreef was no different than the mechanism that generated the Merensky Reef in the Eastern and Western limbs of the BIC.
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.
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.
Characterization of metamorphic assemblages and assessment of Cu-Pb-Ag-Au-Zn mobility at the Lalor deposit, Snow Lake, Manitoba
(2018-12-21) Lam, Judy
The Lalor VMS deposit is located within the Snow Lake arc assemblage at the easternmost end of the Paleoproterozoic Flin Flon belt. A regional metamorphic event at 1.81 Ga, up to middle amphibolite facies (550˚C, 5kbar), recrystallized the mineral assemblages in the massive sulfide and hydrothermally altered rocks that are associated with the formation of VMS deposits in the area. The Lalor deposit differs from most VMS deposits in the area in that it contains low-sulfide Au rich zones that are proximal to but separate from the massive sulfide lenses. Gold mineralization primarily occurs in three rock types at the Lalor deposit: massive sulfides, and calc-silicates to carbonate silicates, and Fe-Mg altered rocks. Electrum is the dominant form of gold, but gold also occurs in sulfosalt phases such as aurostibite (AuSb2) and in tellurides such as petzite (Ag3AuTe2). It is often associated with hessite (Ag2Te), altaite (PbTe), chalcopyrite (CuFeS2), and galena (PbS). Gold mineralization occurs along fractures, grain boundaries, cleavage planes, and as discrete inclusions in metamorphic minerals. There are two dominant metal associations with gold: Cu-Au and Pb-Au. The Cu-Au association is more common and occurs in a variety of rock types (including massive sulfides and Fe-Mg altered rocks), whereas the Pb-Au association is restricted to calc-silicate to carbonate silicate altered rocks. In the massive sulfides, gold content and distribution are a function of primary VMS zone refining processes; however, Au has been locally remobilized during metamorphism and deformation. In the Fe-Mg altered rocks proximal to massive sulfide, the distribution and tenor of Au reflects primary zone refining and local remobilization due to metamorphism and deformation, whereas in Fe-Mg altered rocks distal to massive sulfide, gold distribution is largely a product of pre-peak to peak metamorphic remobilization of primary VMS gold via fluid-dominated transportation. In the calc-silicate to carbonate silicate rocks, which contained or contain carbonate, Au distribution reflects metamorphic remobilization. Metamorphic devolatilization of primary carbonate bearing rocks is responsible for adding components such as H2O, CO2 and S2 into a fluid phase that remobilized Au and some metals (e.g. Cu, Pb) in the deposit, and in altered rocks located proximal and distal to the massive sulfide lenses. Gold was mobilized and transported as a sulfur complex, with CO2 acting as a buffer for a low salinity fluid phase such that it could maintain an elevated gold content for transport and deposition. The result of this mobilization is responsible for the Pb-Au in calc-silicate to carbonate silicate altered rocks
Combining spatial components and Hilbert transforms to interpret ground-time-domain-electromagnetic data
(Society of Exploration Geophysicists, 2015-06-19) Desmarais, J.K.; Smith, Richard S.
We have developed a method for displaying or imaging data from a ground-time-domain electromagnetic system and for extracting the geometric parameters of a small conductor. The parameters are determined directly from the data using combinations of the spatial components of the secondary fields and their Hilbert transforms. The position of the target coincides with the peaks of the energy envelope (EE) or the 𝑇-component of the response. Here, the EE is the square root of the sum of the squares of the three spatial components and their Hilbert transforms, whereas the 𝑇-component response is an analogous quantity that excludes the Hilbert transform terms. Studies on synthetic models indicate that the 𝑇-component response is sharper than the EE in most possible target orientations. Once the position of a body has been determined using the peak of the 𝑇-component response, the dip of the target can be quantified using the ratio of the full-width at half-magnitude (FWHM) of the 𝑇-component response and the 𝑇-component Hilbert transform response, which is analogous to the EE but excludes the untransformed quantities. Finally, once all other geometric parameters have been determined, the depth of the target can be evaluated using the FWHM of the 𝑇-component response. The proposed modeling method was tested over an anomaly acquired at the Coulon field site during an InfiniTEM survey in the Abitibi greenstone belt of Quebec. The extracted geometric parameters were consistent with the available geologic information.
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
A comparison of magnetic susceptibility meters using samples from the Thompson Nickel Belt, Canada
(2016-10) Deng, Deng N.; Smith, Richard S.
Constraining alteration in the footwall of the Sudbury igneous complex: a case study of the alteration footprint to the Podolsky, Cu(-Ni)-PGE deposit, Sudbury
(2018-11-19) MacInnis, Linette
The footwall environment of the Sudbury Igneous Complex (SIC) host high-grade Cu-(Ni)-PGE deposits that have become a prime exploration target due to increases in metal prices and depletion of traditionally mined contact-style deposits that are high-grade Ni-Cu rich deposits. Understanding and constraining footwall deposits has been underway for some time, still, several aspects of these deposits that need to be constrained. With respect to alteration the nature, origin, relevance, and significance of these hydrothermal minerals have not yet been constrained. A detailed study of the homogeneous grey gabbro unit that is hosted in the Podolsky 2000 deposit host’s sharp-walled sulfide veins, rich in copper and PGEs was an ideal study area to look at alteration leading up to sharp-walled sulfide veins and its association to mineralization as the vein is approached. The initial step is to define and understand what the composition of the least altered grey gabbro with respect to its petrogenetic, spatial, and chronological relationship. Once the least altered grey gabbro is characterized alteration associated to these veins can be explored in detail. Alteration assemblages associated to these sharp-walled veins could potentially be implemented into an exploration tool for mining companies local to the area in search of such footwall deposits. The least altered grey gabbro unit was not as homogenous as previously thought, after careful review of petrography, SEM-DES, geochemical bulk-rock data, isotope work, and U-Pb age dating; a suitable petrographic summary of the grey gabbro was achieved. Once a least altered summary was achieved, comparing this data to transects leading up to these sharp-walled sulfide veins and various samples of most altered grey gabbro were petrographically, geochemically, isotopically, compared to the least altered samples available. Generally, understanding footwall deposits and unravelling the relevance and significance of these hydrothermal minerals was summarized in this thesis, unfortunately, these alteration trends are rather small and can only be observed <30 cm away from the vein, and are most obvious against the vein. This indicated that alteration signatures are not very strong outside of these sharp-walled sulfide veins, despite these veins ~1 m thick.
Convolutional neural networks applied to the interpretation of lineaments in aeromagnetic data
(2022-01-01) Naprstek, Tomas; Smith, Richard S.
Parameter estimation in aeromagnetics is an important tool for geologic interpretation. Due to aeromagnetic data being highly prevalent around the world, it can often be used to assist in understanding the geology of an area as a whole or for locating potential areas of further investigation for mineral exploration. Methods that automatically provide information such as the location and depth to the source of anomalies are useful to the interpretation, particularly in areas where a large number of anomalies exist. Unfortunately, many current methods rely on high-order derivatives and are therefore susceptible to noise in the data. Convolutional neural networks (CNNs) are a subset of machine-learning methods that are well-suited to image processing tasks, and they have been shown to be effective at interpreting other geophysical data, such as seismic sections. Following several similar successful approaches, we have developed a CNN methodology for estimating the location and depth of lineament-type anomalies in aeromagnetic maps. To train the CNN model, we used a synthetic aeromagnetic data modeler to vary the relevant physical parameters, and we developed a representative data set of approximately 1.4 million images. These were then used for training classification CNNs, with each class representing a small range of depth values. We first applied the model to a series of difficult synthetic data sets with varying amounts of noise, comparing the results against the tilt-depth method. We then applied the CNN model to a data set from northeastern Ontario, Canada, that contained a dike with known depth that was correctly estimated. This method is shown to be robust to noise, and it can easily be applied to new data sets using the trained model, which has been made publicly available.
The cortaderas zone, Pirquitas Mine, NW Argentina: an example of miocene epithermal Ag-Zn-Pb-Sn mineralization in the Andean Tin Belt
(2016-09-07) Slater, Evan Thomas
The Pirquitas mine, located in the highly elevated Puna plateau region of NW Argentina hosts Ag-rich polymetallic mineralization that defines the southern limit of the prolific Andean Tin Belt. Approximately 500 m north of its currently active open pit is the Cortaderas Zone which hosts the large Ag-Zn-rich Cortaderas Breccia whose nature and origin were previously unknown. This thesis serves as the first academic study of the Cortaderas Zone that incorporates data pertaining to its geological setting, metal distributions, breccia bodies, mineralogy, ore textures, alteration and fluid inclusions to interpret the origin of its mineralization. The results of this study suggest that the Cortaderas Zone represents the high-level and distal expression of the richly endowed hydrothermal system once present at the Pirquitas mine. Its mineralization formed in a dynamic intermediate-sulfidation epithermal system where ore formation was facilitated by transient fluctuations in confining pressure that were caused by cyclical opening and closing of the system. These insights into the formation of the Cortaderas Zone have implications for understanding the formation and subsequent modification of ores in epithermal settings globally.
Determining the feasibility of a real-time geophysical magnetic and electric measurement system for monitoring strain underground
(Laurentian University of Sudbury, 2015-02-24) Schaub, Christoph
In deep mining environments, rockbursts can occur in areas of high stress. The unpredictable nature, and sometimes fatal consequences of the rockbursts, makes identifying increases in stress that occur prior to failure events, important for mine safety. The piezoelectric, electrokinetic, and seismo-electric/magnetic effects result in rocks that are stressed emitting electric and magnetic fields that can potentially be measured in (or close to) a rock mass. Electric, magnetic and seismic data were collected during a four-day period at Coleman Mine, Sudbury, Ontario, and later analysed. No signals due to strain were evident in the magnetic or electric data, so a more careful three-part experiment was proposed to see if strain related signals could be identified in magnetic data. In the first part of the experiment thirty-two rocks were stressed until failure. Measurable magnetic signals associated with audible cracking sounds were found for most rock samples, as well as consistent signals across all samples prior to failure. The implementation of real-time monitoring of these signals has the potential to significantly improve deep mine safety by mapping the evolution of strain underground and potentially indicating areas susceptible to failure.
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.
The effect of dielectric permittivity on the fields radiated from a radio-frequency electric dipole in a homogeneous whole space
(Society of Exploration Geophysicists, 2016-02-18) Naprstek, T.; Smith, Richard S.
The radio imaging method (RIM) is an electromagnetic cross-borehole method with applications in mineral exploration, as well as in the coal industry, where it is used across mine drives. Attenuation of the signal from conductive regions may indicate areas of mineralization, and these conductive effects in general dominate the response. In an effort to better understand the effect of a material’s dielectric permittivity on the response of the RIM, we have developed a simple program to model an electric dipole in a homogeneous whole space. When increasing the dielectric permittivity, the amplitude peak broadened and increased, whereas the phase peak sharpened and shifted negatively. To showcase the effect of dielectric permittivity on RIM data, data recorded from two transmitter positions in a moderately homogeneous zone in the Sudbury Basin were curve fit, and we concluded that despite the stronger effect that conductivity has on the signal, RIM is still sensitive to dielectric permittivity, and appropriate values must be used when developing conductivity tomograms. In addition, we found that for the given situation and frequencies used, an increase in either the conductivity or dielectric permittivity could be accounted for by a decrease of approximately the same factor in the other variable. However, the low-conductivity, high-permittivity case seemed to fit the shape of the amplitude and phase curves better. For the sulfide impregnated crystalline rocks at our field site, relative dielectric constants of 26.4 and 31 at 1250 and 625 kHz, respectively, were inferred.
Elevated magmatic sulfur and chlorine contents in ore-forming magmas at the Red Chris porphyry Cu-Au deposit, Northern British Columbia, Canada
(2018-11-01) Zhu, Jing-Jing; Richards, Jeremy Peter; Rees, Chris; Creaser, Robert; DuFrane, Andrew; Locock, Andrew; Petrus, Joseph; Lang, Jürgen
The Red Chris porphyry Cu-Au deposit is located in the Stikinia island-arc terrane in northwest British Columbia. It is hosted by the Red Stock, which has four phases of porphyry intrusions: P1, P2E, P2L, and P3. New U-Pb dating of zircon shows that these intrusions were emplaced at 211.6 ± 1.3 Ma (MSWD = 0.85), 206.0 ± 1.2 Ma (MSWD = 1.5), 203.6 ± 1.8 Ma (MSWD = 1.5), and 201.7 ± 1.2 Ma (MSWD = 1.05), respectively. The ore-forming event at Red Chris was a short-lived event at 206.1 ± 0.5 Ma (MSWD = 0.96; weighted average age of three Re-Os analyses), implying a duration of <1 m.y., as defined by the uncertainty range. This mineralization age coincides with the emplacement age of the P2E porphyry, and is consistent with crosscutting relationships that suggest P2E was the main syn-mineralization intrusion. Zircons from P1 to P3 porphyry rocks have consistently high EuN/EuN* ratios (mostly > 0.4), indicating that their associated magmas were moderately oxidized. The magmatic water contents estimated from plagioclase and amphibole compositions suggest H2O contents of ~5 wt. %. Taken together, the P1 to P3 porphyries are interpreted to be moderately oxidized and hydrous. The four phases of porphyries are differentiated by sulfur and chlorine contents. The SO3 contents of igneous apatite microphenocrysts from the mineralization-related P2 porphyries are higher (P2E: 0.30 ± 0.13 wt. %, n = 34; P2L: 0.29 ± 0.18 wt. %, n = 100) than those from the pre-mineralization P1 (0.11 ± 0.03 wt. %, n = 34) and postmineralization P3 porphyries (0.03 ± 0.01 wt. %, n = 13). The chlorine contents in apatite grains from the P2E and P2L porphyries are 1.18 ± 0.37 (n = 34) and 1.47 ± 0.28 wt. % (n = 100), also higher than those from P1 (0.51 ± 0.3 wt. % Cl, n = 34) and P3 (0.02 ± 0.02 wt. % Cl, n = 17). These results imply that the sulfur and chlorine contents of the P2E and P2L magmas were higher than in the P1 and P3 magmas, suggesting that elevated magmatic S-Cl contents in the P2 porphyries may have been important for ore-formation. Although the process that caused the increase in sulfur and chlorine is not clear, reverse zoning seen in plagioclase phenocrysts from the P2 porphyry, and the occurrence of more mafic compositions in P2L suggest that recharge of the deeper magma chamber by a relatively S-Cl-rich mafic magma may have triggered the ore-forming hydrothermal event.
Estimating the parameters of simple models from two-component on-time airborne electromagnetic data
(2022-01-01) Bagley, Thomas; Smith, Richard S.
The horizontal and vertical components of an on-time electromagnetic (EM) response can be used to estimate the parameters of simple models such as thin sheets, half-spaces, thin sheets over a lower half-space, and a two-layer model. The formulas used in these methods are valid in areas where the on-time response is essentially proportional to the conductivity or conductance, the so-called “resistive limit.”The half-space and thin sheet over lower half-space models can be combined to give an estimate of the conductivity for a lower half-space below a thick sheet that might be reasonable for the entire survey area. With this estimation, an equation solver can be used to estimate the thickness and conductivity of the overlying thick sheet over the whole sur- vey area. This latter approach seemed most appropriate for the Russell South area in the Athabasca Basin, Canada, where GEOTEM data have been collected. The output of the algorithm was generally stable. Although it did not always reliably reproduce the overburden thicknesses as measured in a set of reference drillholes, it did give an estimate that was reasonable in the relatively conductive areas.

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