Interpretation of Magnetic Anomalies over 2D Fault and Sheet-Type Mineralized Structures Using Very Fast Simulated Annealing Global Optimization: An Understanding of Uncertainty and Geological Implications

Identification of intraterrane dislocation zones and associated mineralized bodies is of immense importance in exploration geophysics. Understanding such structures from geophysical anomalies is challenging and cumbersome. In the present study, we present a fast and competent algorithm for interpreting magnetic anomalies from such dislocation and mineralized zones. Such dislocation and mineralized zones are well explained from 2D fault and sheet-type structures. The different parameters from 2D fault and sheet-type structures such as the intensity of magnetization (k), depth to the top (z1), depth to the bottom (z2), origin location (x0), and dip angle (θ) of the fault and sheet from magnetic anomalies are interpreted. The interpretation suggests that there is uncertainty in defining the model parameters z1 and z2 for the 2D inclined fault; k, z1, and z2 for the 2D vertical fault and finite sheet-type structure; and k and z for the infinite sheet-type structure. Here, it shows a wide range of solutions depicting an equivalent model with smaller misfits. However, the final interpreted mean model is close to the actual model with the least uncertainty. Histograms and crossplots for 2D fault and sheet-type structures also reveal the same. The present algorithm is demonstrated with four theoretical models, including the effect of noises. Furthermore, the investigation of magnetic data was also applied from three field examples from intraterrane dislocation zones (Australia), deep-seated dislocation zones (India) as a 2D fault plane, and mineralized zones (Canada) as sheet-type structures. The final estimated model parameters are in good agreement with the earlier methods applied for these field examples with a priori information wherever available in the literature. However, the present method can simultaneously interpret all model parameters without a priori information.

Geophysical Studies in the Southwest Pacific : Primarily Studies of Crustal Structure between New Zealand and Antarctica

<p>Geophysical data - primarily magnetic field measurements, bathymetry, and seismicity data - are presented for the area between New Zealand and Antarctica from approximately 145[degrees]W to 155[degrees]E. The data are used to determine the structure of the Pacific-Antarctic boundary, the oceanic part of the Pacific plate and the area of intersection of the Indian, Pacific and Antarctic plates. In the southwest Pacific basin the magnetic anomalies are very clear and an extensive pattern of anomaly lineations with some offsets is mapped. The magnetic anomalies show that the uniform Pacific basin area formed between about 83 and 63 mybp. The Pacific-Antarctic boundary is shown to differ either side of about 175[degrees]W. To the east it is a relatively uniform aseismic spreading ridge, offset by some transform faults. West of 175[degrees]W, to 161[degrees]E, the boundary consists of a seismically active zone of disturbed bathymetry and magnetic anomalies striking about N.70[degrees]W. The zone, the Pacific-Antarctic fracture zone, probably consists of several fractures striking about N45[degrees]W. The area between the Pacific-Antarctic boundary and the southwest Pacific basin represents the interval 10 to -55 mybp, and only in the east are anomaly lineations clear. The Indian-Antarctic Pacific triple junction is near 61.5[degrees]S, 161[degrees]E and is a stable ridge-fault-fault junction; the Indian-Antarctic boundary being the ridge. Plate tectonics is applied to the area and the structure is shown to fit, and be explained by a different rotation pole for each of the major intervals indicated by the structure, i.e. 0-10 mybp, 10-63 mybp and 63-80 mybp. The poles, with rotation rates deduced from the magnetic anomalies, are used to reconstruct the position of New Zealand relative to Antarctica at 80 mybp. The two continents probably started to separate at close to 83 mybp. The times of the major changes of structure and plate movement in the area are shown to coincide with major plate movement changes in the southwest Pacific area and in the rest of the world. A new method for determining poles of rotation, based only on epicentre locations is presented, The method is applied to independently determine the Indian-Pacific, Pacific-Antarctic and Indian-Antarctic poles. The poles should form a consistent. set and they do. The method yields effectively instantaneous poles, is quantitative, and is applicable to most plate boundaries. Earthquake magnitude-frequency relationship b values for the plate boundaries in the area are determined. Comparisons with results from elsewhere indicate an association of high b with high temperature and conversely. Several factors which have previously been suggested as determining b value are shown to not be determinants. A revised and extended magnetic reversal time scale based on model studies of the southwest Pacific basin anomalies is presented. Other model studies indicate that a magnetized layer thickness of at least 2 km is probable. Variations of anomaly amplitudes are studied. A detailed study of the application of numerical correlation techniques to magnetic anomalies is presented. It is concluded that horizontal scale variations and discontinuities in profiles can be critical. Methods for over-coming some of the problems, and for determining quantitative error estimates, are. given. The methods, and conclusions, are applicable to any correlation problem.</p>

Geophysical Studies in the Southwest Pacific : Primarily Studies of Crustal Structure between New Zealand and Antarctica

<p>Geophysical data - primarily magnetic field measurements, bathymetry, and seismicity data - are presented for the area between New Zealand and Antarctica from approximately 145[degrees]W to 155[degrees]E. The data are used to determine the structure of the Pacific-Antarctic boundary, the oceanic part of the Pacific plate and the area of intersection of the Indian, Pacific and Antarctic plates. In the southwest Pacific basin the magnetic anomalies are very clear and an extensive pattern of anomaly lineations with some offsets is mapped. The magnetic anomalies show that the uniform Pacific basin area formed between about 83 and 63 mybp. The Pacific-Antarctic boundary is shown to differ either side of about 175[degrees]W. To the east it is a relatively uniform aseismic spreading ridge, offset by some transform faults. West of 175[degrees]W, to 161[degrees]E, the boundary consists of a seismically active zone of disturbed bathymetry and magnetic anomalies striking about N.70[degrees]W. The zone, the Pacific-Antarctic fracture zone, probably consists of several fractures striking about N45[degrees]W. The area between the Pacific-Antarctic boundary and the southwest Pacific basin represents the interval 10 to -55 mybp, and only in the east are anomaly lineations clear. The Indian-Antarctic Pacific triple junction is near 61.5[degrees]S, 161[degrees]E and is a stable ridge-fault-fault junction; the Indian-Antarctic boundary being the ridge. Plate tectonics is applied to the area and the structure is shown to fit, and be explained by a different rotation pole for each of the major intervals indicated by the structure, i.e. 0-10 mybp, 10-63 mybp and 63-80 mybp. The poles, with rotation rates deduced from the magnetic anomalies, are used to reconstruct the position of New Zealand relative to Antarctica at 80 mybp. The two continents probably started to separate at close to 83 mybp. The times of the major changes of structure and plate movement in the area are shown to coincide with major plate movement changes in the southwest Pacific area and in the rest of the world. A new method for determining poles of rotation, based only on epicentre locations is presented, The method is applied to independently determine the Indian-Pacific, Pacific-Antarctic and Indian-Antarctic poles. The poles should form a consistent. set and they do. The method yields effectively instantaneous poles, is quantitative, and is applicable to most plate boundaries. Earthquake magnitude-frequency relationship b values for the plate boundaries in the area are determined. Comparisons with results from elsewhere indicate an association of high b with high temperature and conversely. Several factors which have previously been suggested as determining b value are shown to not be determinants. A revised and extended magnetic reversal time scale based on model studies of the southwest Pacific basin anomalies is presented. Other model studies indicate that a magnetized layer thickness of at least 2 km is probable. Variations of anomaly amplitudes are studied. A detailed study of the application of numerical correlation techniques to magnetic anomalies is presented. It is concluded that horizontal scale variations and discontinuities in profiles can be critical. Methods for over-coming some of the problems, and for determining quantitative error estimates, are. given. The methods, and conclusions, are applicable to any correlation problem.</p>

Drone-based magnetic and multispectral surveys to develop a 3D model for mineral exploration at Qullissat, Disko Island, Greenland

Mineral exploration in the West Greenland flood basalt province is attractive because of its resemblance to the magmatic sulphide-rich deposit in the Russian Norilsk region, but it is challenged by rugged topography and partly poor exposure for relevant geologic formations. On northern Disko Island, previous exploration efforts have identified rare native iron occurrences and a high potential for Ni-Cu-Co-PGE-Au mineralization. However, Quaternary landslide activity has obliterated rock exposure at many places at lower elevations. To augment prospecting field work under these challenging conditions, we acquire high-resolution magnetic and optical remote sensing data using drones in the Qullissat area. From the data, we generate a detailed 3D model of a mineralized basalt unit, belonging to the Asuk Member (Mb) of the Palaeocene Vaigat formation. A wide range of legacy data and newly acquired geo- and petrophysical, as well as geochemical-mineralogical measurements form the basis of an integrated geological interpretation of the unoccupied aerial system (UAS) surveys. In this context, magnetic data aims to define the location and the shape of the buried magmatic body, and to estimate if its magnetic properties are indicative for mineralization. High-resolution UAS-based multispectral orthomosaics are used to identify surficial iron staining, which serve as a proxy for outcropping sulphide mineralization. In addition, high-resolution UAS-based digital surface models are created for geomorphological characterisation of the landscape to accurately reveal landslide features. UAS-based magnetic data suggests that the targeted magmatic unit is characterized by a pattern of distinct positive and negative magnetic anomalies. We apply a 3D magnetization vector inversion model (MVI) on the UAS-based magnetic data to estimate the magnetic properties and shape of the magmatic body. By means of using constraints in the inversion, (1) optical UAS-based data and legacy drill cores are used to assign significant magnetic properties to areas that are associated with the mineralized Asuk Mb, and (2) the Earth’s magnetic and the paleomagnetic field directions are used to evaluate the general magnetization direction in the magmatic units. Our results indicate that the geometry of the mineralized target can be estimated as a horizontal sheet of constant thickness, and that the magnetization of the unit has a strong remanent component formed during a period of Earth’s magnetic field reversal. The magnetization values obtained in the MVI are in a similar range as the measured ones from a drillcore intersecting the targeted unit. Both the magnetics and topography confirm that parts of the target unit were displaced by landslides. We identified several fully detached and presumably rotated blocks in the obtained model. The model highlights magnetic anomalies that correspond to zones of mineralization and is used to identify outcrops for sampling. Our study demonstrates the potential and efficiency of using multi-sensor high-resolution UAS data to constrain the geometry of partially exposed geological units and assist exploration targeting in difficult, poorly exposed terrain.

Crustal Density and Susceptibility Structure beneath Achankovil Shear Zone, India

The Southern Granulite Terrain (SGT) is a large tract of exposed Archean continental crust, divided into the Madurai Block (MB), Trivandrum Block (TB), and Nagercoil Block (NB). These crustal domains are linked with the NW-SE trending Achankovil Shear Zone (AKSZ). We combine gravity and magnetic data with previously published ground observations and geochronological data to re-evaluate the crustal architecture, evolution of the AKSZ, and possible extension of AKSZ into Madagascar. Analyses indicate that the long wavelength trends of the magnetic anomalies originate at ~20 km depth of different SGT blocks. These observations are corroborated with the gravity as well as computed gravity gradient anomalies. The presence of khondalite outcrops in Trivandrum Block implies that high magnetization crust is the main source of positive magnetic anomalies. Such magnetic anomalies advocate that SGT preserves the remanent of Archean crustal blocks in South India, a part due to variation in thermal and geochemical processes. The AKSZ, TB, and MB exhibit contrasting magnetic crustal signatures. The joint modeling results reveal a three-layered crustal configuration with varying Moho ranging from 41 to 34 km in NE to SW, respectively. It is also noted that AKSZ is a narrow and deep structure near to the Western Ghats Escarpment while it is wide and shallow in the far-east, which implies that the evolution of the Western Ghats is a late geological event.