Predictive mapping of prospectivity for orogenic gold in Uganda

Integration of enhanced regional geo-datasets has facilitated new geological interpretation and modelling of prospectivity for orogenic gold in southwestern Uganda. The geo-datasets include historical geological maps, geological field data, digital terrain models, Landsat TM data and airborne geophysical data. The study area, bordered by the western branch of the East African Rift, covers a range of different aged terranes including the Archaean basement gneisses, Palaeoproterozoic volcano-sedimentary Buganda Toro Belt, Mesoproterozoic clastic sedimentary Karagwe Ankolean Belt and several outliers of undeformed Neoproterozoic sediments.\ud \ud The mineral systems approach to practical exploration targeting requires a framework to link conceptual models of mineralisation with available data. A conceptual model requires good understanding of key processes and their timing within the geodynamic history of an area. The challenge is that processes cannot be mapped, only their results or effects. In this study, a district-scale (1:100,000) investigation is considered appropriate given the scarcity of geological information and the absence of world-renowned gold deposits in southwestern Uganda. At this scale of orogenic gold mineral systems understanding, evidence for the source of gold, active pathways and the physical traps are considered critical. Following the mineral system approach, these processes critical to orogenic gold systems are translated into district-scale mappable proxies using available regional-scale datasets. Tectono-stratigraphic domains, mantle indicators and gold occurrences represent the "source of gold" as a critical process. Zones of hydrothermal alteration were extracted from radiometric data, structures involved in the orogenies and terrane contacts were extracted to represent the active pathway as a critical process and finally the physical throttle is represented by rheological contrasts and geological complexity. Then, the knowledge-driven multi-class index overlay method was used to integrate predictor layers representing those processes in order to model orogenic gold prospectivity into a single map. Weighting of the predictor layers, prior to integration, occurs at the level of the critical process and takes into account the relative importance of the critical process mineralisation, the representativeness of a proxy and the accuracy of the proxy. The resultant prospectivity model shows that 83% of all gold occurrences are delineated within predicted prospective areas covering 30% of the study area. Eight sub-areas, covering 2500 km2, have been recommended for follow-up exploration

Prospectivity for epithermal gold–silver deposits in the Deseado Massif, Argentina

Previous prospectivity modelling for epithermal Au–Ag deposits in the Deseado Massif, southern Argentina, provided regional-scale prospectivity maps that were of limited help in guiding exploration activities within districts or smaller areas, because of their low level of detail. Because several districts in the Deseado Massif still need to be explored, prospectivity maps produced with higher detail would be more helpful for exploration in this region.\ud \ud We mapped prospectivity for low- and intermediate-sulfidation epithermal deposits (LISEDs) in the Deseado Massif at both regional and district scales, producing two different prospectivity models, one at regional scale and the other at district-scale. The models were obtained from two datasets of geological evidence layers by the weights-of-evidence (WofE) method. We used more deposits than in previous studies, and we applied the leave-one-out cross validation (LOOCV) method, which allowed using all deposits for training and validating the models. To ensure statistical robustness, the regional and district-scale models were selected amongst six combinations of geological evidence layers based on results from conditional independence tests.\ud \ud The regional-scale model (1000 m spatial resolution), was generated with readily available data, including a lithological layer with limited detail and accuracy, a clay alteration layer derived from a Landsat 5/7 band ratio, and a map of proximity to regional-scale structures. The district-scale model (100 m spatial resolution) was generated from evidence layers that were more detailed, accurate and diverse than the regional-scale layers. They were also more cumbersome to process and combine to cover large areas. The evidence layers included clay alteration and silica abundance derived from ASTER data, and a map of lineament densities. The use of these evidence layers was restricted to areas of favourable lithologies, which were derived from a geological map of higher detail and accuracy than the one used for the regional-scale prospectivity mapping.\ud \ud The two prospectivity models were compared and their suitability for prediction of the prospectivity in the district-scale area was determined. During the modelling process, the spatial association of the different types of evidence and the mineral deposits were calculated. Based on these results the relative importance of the different evidence layers could be determined. It could be inferred which type of geological evidence could potentially improve the modelling results by additional investigation and better representation.\ud \ud We conclude that prospectivity mapping for LISEDs at regional and district-scales were successfully carried out by using WofE and LOOCV methods. Our regional-scale prospectivity model was better than previous prospectivity models of the Deseado Massif. Our district-scale prospectivity model showed to be more effective, reliable and useful than the regional-scale model for mapping at district level. This resulted from the use of higher resolution evidential layers, higher detail and accuracy of the geological maps, and the application of ASTER data instead of Landsat ETM + data. District-scale prospectivity mapping could be further improved by: a) a more accurate determination of the age of mineralization relative to that of lithological units in the districts; b) more accurate and detailed mapping of the favourable units than what is currently available; c) a better understanding of the relationships between LISEDs and the geological evidence used in this research, in particular the relationship with hydrothermal clay alteration, and the method of detection of the clay minerals; and d) inclusion of other data layers, such as geochemistry and geophysics, that have not been used in this study

Near-infrared spectroscopy of hydrothermal versus low-grade metamorphic chlorites

Although the composition of chlorite groupminerals represents a known proxy for conditions in various geological environments, few comparative studies of chlorites from different geological environments have been carried out. In this study, we compare chlorites from a hydrothermal system with those formed under low-grade metamorphic conditions. Both sets of samples were collected from the Pilbara Craton,Western Australia. Near-infrared (NIR) spectroscopy was used to determine if spectral differences record compositional differences between chlorites from the two geological environments. The spectra showed a significant difference in the Mg-OH absorption wavelength, near 2350 nm, with the hydrothermal group showing longer absorption wavelengths than the metamorphic one. A comparison of the spectral data with geochemical analyses showed a relation between the absorption wavelength and the magnesium-to-iron ratio (magnesium number) of chlorite, as well as the bulk rock composition. Metamorphic rocks have a higher magnesium-to-iron ratio than the hydrothermal ones, predominantly explained by differences in the degree of metasomatism. In the hydrothermal system,mass transfer changes the bulk rock compositionwhile for themetamorphic samples the original bulk chemistry determines the current composition of the rock

Near-infrared spectroscopy of hydrothermal versus low-grade metamorphic chlorites

Although the composition of chlorite groupminerals represents a known proxy for conditions in various geological environments, few comparative studies of chlorites from different geological environments have been carried out. In this study, we compare chlorites from a hydrothermal system with those formed under low-grade metamorphic conditions. Both sets of samples were collected from the Pilbara Craton,Western Australia. Near-infrared (NIR) spectroscopy was used to determine if spectral differences record compositional differences between chlorites from the two geological environments. The spectra showed a significant difference in the Mg-OH absorption wavelength, near 2350 nm, with the hydrothermal group showing longer absorption wavelengths than the metamorphic one. A comparison of the spectral data with geochemical analyses showed a relation between the absorption wavelength and the magnesium-to-iron ratio (magnesium number) of chlorite, as well as the bulk rock composition. Metamorphic rocks have a higher magnesium-to-iron ratio than the hydrothermal ones, predominantly explained by differences in the degree of metasomatism. In the hydrothermal system,mass transfer changes the bulk rock compositionwhile for themetamorphic samples the original bulk chemistry determines the current composition of the rock

Conceptual modeling of onshore hydrocarbon seep occurrence in the Dezful Embayment, SW Iran

Petroleum and gas seeps on the ground surface are direct indicators of accumulations of hydrocarbons in the subsurface and could reflect the migration of hydrocarbons in a sedimentary basin. Quantitative analyses of the spatial pattern of hydrocarbon seeps and their spatial associations with geological features could aid in deducing geological controls on their occurrence. In this study the Fry analysis was applied to study the spatial pattern of mapped hydrocarbon seeps, whereas spatial association analyses were implemented to quantify the spatial association of mapped seeps and their alteration products with geological features. The spatial pattern analysis of hydrocarbon seeps showed that oil seeps followed prominent NW-SE and NE-SW trends while gas seeps followed NW-SE and N-S trends suggesting that NNE-SSW and NW-SE fractures are possible migration pathways for hydrocarbons to reach the surface. The results of the spatial association analysis illustrated strong positive spatial associations of oil and gas seeps with the Gachsaran and the Mishan formations, implying upward migration of hydrocarbons through permeable micro-fractures and micro-pores in their strata. A conceptual model has proposed for the occurrence of onshore hydrocarbon seeps in the Dezful Embayment

Mapping the wavelength position of deepest absorption features to explore mineral diversity in hyperspectral images

A new method is presented for the exploratory analysis of hyperspectral OMEGA imagery of Mars. It involves mapping the wavelength position and depth of the deepest absorption feature in the range between 2.1 and 2.4 µm, where reflectance spectra of minerals such as phyllosilicates, carbonates and sulphates contain diagnostic absorption features. For each pixel of the image, the wavelength position maps display the wavelength position of the deepest absorption feature in color and its depth in intensity. This can be correlated with (groups of) minerals and their occurrences. To test the validity of the method, comparisons were made between wavelength position maps calculated from OMEGA images of the Nili Fossae area at two different spatial resolutions, of 0.95 and 2.2 km, and five CRISM images in targeted mode, at 18 m spatial resolution. The wavelength positions and their spatial patterns in the two OMEGA images were generally similar, except that the higher spatial resolution OMEGA image showed a larger diversity of wavelength positions and more spatial detail than the lower resolution OMEGA image. Patterns formed by groups of pixels with relatively deep absorption features between 2.250 and 2.350 µm in the OMEGA imagery were in agreement with the patterns calculated from the CRISM imagery. The wavelength positions of clusters of similar pixels in the wavelength position maps are consistent with groups of minerals that have been described elsewhere in the literature. We conclude that mapping the wavelength position of the deepest absorption features between 2.1 and 2.4 µm provides a useful method for exploratory analysis of the surface mineralogy of Mars with hyperspectral OMEGA imagery. The method provides a synoptic spatial view of the spectral diversity in one single image. It is complementary to the use of summary products, which many researchers have been using for assessment of the information content of OMEGA imagery. The results of the exploratory analysis can be used as input for the construction of surface mineralogical maps. The wavelength position mapping method itself is equally applicable to other terrestrial and planetary data sets and will be particular useful in areas where field validation is sparse and with imagery containing shallow spectral features