Academic literature on the topic 'Geophysics Queensland Mt Isa Region'

SUMMARY We present an inversion algorithm tailored for point gravity data. As the data are from multiple surveys, it is inconsistent with regards to spacing and accuracy. An algorithm design objective is the exact placement of gravity observations to ensure no interpolation of the data is needed prior to any inversion. This is accommodated by discretization using an unstructured tetrahedral finite-element mesh for both gravity and density with mesh nodes located at all observation points and a first-order system least-squares (FOSLS) formulation for the gravity modelling equations. Regularization follows the Bayesian framework where we use a differential operator approximation of an exponential covariance kernel, avoiding the usual requirement of inverting large dense covariance matrices. Rather than using higher order basis functions with continuous derivatives across element faces, regularization is also implemented with a FOSLS formulation using vector-valued property function (density and its gradient). Minimization of the cost function, comprised of data misfit and regularization, is achieved via a Lagrange multiplier method with the minimum of the gravity FOSLS functional as a constraint. The Lagrange variations are combined into a single equation for the property function and solved using an integral form of the pre-conditioned conjugate gradient method (I-PCG). The diagonal entries of the regularization operator are used as the pre-conditioner to minimize computational costs and memory requirements. Discretization of the differential operators with the finite-element method (FEM) results in matrix systems that are solved with smoothed aggregation algebraic multigrid pre-conditioned conjugate gradient (AMG-PCG). After their initial setup, the AMG-PCG operators and coarse grid solvers are reused in each iteration step, further reducing computation time. The algorithm is tested on data from 23 surveys with a total of 6519 observation points in the Mt Isa–Cloncurry region in north–west Queensland, Australia. The mesh had about 2.5 million vertices and 16.5 million cells. A synthetic case was also tested using the same mesh and error measures for localized concentrations of high and low densities. The inversion results for different parameters are compared to each other as well as to lower order smoothing. Final inversion results are shown with and without depth weighting and compared to previous geological studies for the Mt Isa–Cloncurry region.

Surface gravity and magnetic surveys around the giant copper‐silver‐lead‐zinc mine at Mt. Isa in northwest Queensland have shown that each type of mineralization presents characteristic responses which are anomalous within the perspective of the host rocks. The responses reflect the entire mineralization‐alteration package and not simply the economic ore. Copper mineralization and associated alteration products, including “silica dolomite,” are not significantly magnetized, but the mineralization across a depth range of 1000 m produces a positive gravity anomaly of more than 2.5 mGal. The related alteration products are associated with large negative anomalies of up to 8 mGal in both hanging wall and footwall host rocks. Silver‐lead‐zinc mineralization is moderately magnetic (about 0.001 cgs). It is also denser and shallower and generates larger gravity anomalies. Negative secondary alteration effects, while significant, are equivalent in magnitude to the mineralization effect. Sites mineralized comparably to Mt. Isa would be located by semiregional surveys with a station spacing of 200 to 400 m and survey precision of better than 0.4 mGal, although there are limits to resolution of individual parts of the mineralization.

An airborne survey was undertaken on the Mount Isa inlier in 1990–1992. During this survey, both airborne magnetic and gamma‐ray spectrometric data were recorded over 639 170 line-km. Because of perceived value of the radiometric data, stringent calibration procedures, including the creation of a test range, were adopted. In addition to the data from the newly‐flown areas, 76 760 line‐km of existing data were acquired from other companies, and were reprocessed and merged with the Mount Isa survey. The total area covered by the Mount Isa airborne survey was 151 300 km2. Over the last five years, several studies have been undertaken that seek to exploit the Mount Isa region gamma‐ray database and maximise the use of radiometrics for mineral exploration. This paper highlights the results of these studies by focussing on radiometric signatures of major mines in the Mount Isa Inlier, radioelement contour maps, geomagnetic/radiometric interpretation maps, lithological mapping, regolith mapping, geochemical sampling, and spatial modeling using geographical information systems (GIS). Due to the recent introduction of GIS technology and better techniques for handling high quality digital data, there has been a revived interest in making more use of image data sets. The integration of raster and vector data sets for both spectral and spatial modeling has maximized the potential of this approach.

Mineral deposits can be described in terms of their mineral systems, i.e., fluid source, migration pathway, and trap. Source regions are difficult to recognize in seismic images. Many orebodies lie on or adjacent to major fault systems, suggesting that the faults acted as fluid migration pathways through the crust. Large faults often have broad internal zones of deformation fabric, which is anisotropic. This, coupled with the metasomatic effects of fluids moving along faults while they are active, can make the faults seismically reflective. For example, major gold deposits in the Archaean Eastern Goldfields province of Western Australia lie in the hanging‐wall block of regional‐scale faults that differ from other nearby faults by being highly reflective and penetrating to greater depths in the lower crust. Coupled thermal, mechanical, and fluid‐flow modeling supports the theory that these faults were fluid migration pathways from the lower to the upper crust. Strong reflections are also recorded from two deeply penetrating faults in the Proterozoic Mt. Isa province in northeastern Australia. Both are closely related spatially to copper and copper‐gold deposits. One, the Adelheid fault, is also adjacent to the large Mt. Isa silver‐lead‐zinc deposit. In contrast, other deeply penetrating faults that are not intrinsically reflective but are mapped in the seismic section on the basis of truncating reflections have no known mineralization. Regional seismic profiles can therefore be applied in the precompetitive area selection stage of exploration. Applying seismic techniques at the orebody scale can be difficult. Orebodies often have complex shapes and reflecting surfaces that are small compared to the diameter of the Fresnel zone for practical seismic frequencies. However, if the structures and alteration haloes around the orebodies themselves, seismic techniques may be more successful. Strong bedding‐parallel reflections were observed from the region of alteration around the Mt. Isa silver‐lead‐zinc orebodies using high‐resolution profiling. In addition, a profile in Tasmania imaged an internally nonreflective bulge within the Que Hellyer volcanics, suggesting a good location to explore for a volcanic hosted massive sulfide deposit. These case studies provide a pointer to how seismic techniques could be applied during mineral exploration, especially at depths greater than those being explored with other techniques.

The Alta Floresta Gold Province (AFGP) is located in the central-southern portion of the Amazon Craton. In its eastern segment there are more than one hundred gold deposits hosted in volcanic and plutonic rocks of medium to high content of potassium. In many of these deposits the gold ore is correlated spatially and genetically hydrothermal systems and mainly hosted into the silicification potassic and sericitic. In this context, this work focuses on understanding the geological context of the region and the possibility of expanding new exploratory frontiers through techniques further traditional geological mapping. The processing and interpretation of airborne magnetic and gamma-ray spectrometry data of the Juruena-Teles Pires Project (Phase 1) allowed the production of a new regional geological map of the eastern portion of the AFGP. Additionally, theoretical models showed that despite the limitations of the reduction to the pole at low latitudes, the magnetic enhancements proposed can be applied to real data through the use of directional filtering to eliminate the features generated in the magnetic declination direction. The Total Horizontal Derivative of the Tilt Derivative (THDR-TDR) method proved the best enhancement for the interpretation of the structures, showing a structural complexity similar to that proposed for the evolution of the province. The interpretation of the products based on the gamma-ray spectrometry channels allowed the reinterpretation of different geological units, including the individualization of a new high-potassium intrusive unit, which in the context of AFGP may have metallogenetic potential. RESUMO: A Província Aurífera de Alta Floresta (PAAF) situa-se na porção centro-sul do cráton Amazônico. Em seu segmento leste, ocorrem mais de uma centena de depósitos auríferos hospedados em rochas plutônicas e vulcânicas de médio a alto potássio. Além disso, em muitos desses depósitos o minério aurífero apresenta relação espacial e genética com alterações hidrotermais, preferencialmente alterações do tipo potássica e sericítica. Nesse contexto, o trabalho enfoca o entendimento do contexto geológico dessa região e a possibilidade de expansão de novas fronteiras exploratórias por meio de técnicas que vão além do mapeamento geológico tradicional. Assim, através do processamento e interpretação de dados aeromagnetométricos e aerogamaespectrométricos do projeto Juruena-Teles Pires (Fase 1) gerou-se um novo mapa geológico regional da porção leste da PAAF. Adicionalmente, por meio de modelos teóricos observou-se que apesar das limitações da redução ao polo em baixas latitudes, os realces magnéticos propostos podem ser aplicados aos dados reais mediante o uso de filtros direcionais que eliminem os artefatos gerados na direção da declinação magnética. O método Inclinação do Sinal Analítico do Gradiente Horizontal Total (ISA-GHT) mostrou-se como o realce mais adequado para a interpretação de lineamentos, revelando uma complexidade estrutural similar àquela proposta para a evolução da província. A interpretação dos produtos gerados com base nos canais gamaespectrométricos permitiu a reinterpretação de diversas unidades geológicas, inclusive a individualização de uma nova unidade intrusiva de alto potássio, que nocontexto da PAAF pode possuir potencial metalogenético.Palavras-chave: Província Aurífera de Alta Floresta; dados aerogeofísicos; interpretação geológica