Applied Earth Science: Transactions of the Institutions of Mining and Metallurgy

The United States Geological Survey (USGS) Three-part Undiscovered Mineral Resource Assessment provides a framework for estimating undiscovered mineral endowment. Previous studies that applied the Three-part Assessment to estimate the undiscovered orogenic gold endowment of the Sandstone Greenstone Belt, Western Australia, have relied upon dated or expert-derived grade-tonnage models. Here, several assessments are conducted using local grade-tonnage models, comprising known orogenic gold deposits within the entire Yilgarn Block and several individual terranes with contrasting lithosphere- to terrane-scale characteristics. These models are generated through comprehensive review of historical exploration, resource and production data. Based on these models, the Sandstone Greenstone Belt is estimated to contain significant undiscovered gold mineralisation, with a median total endowment of between 166 and 298 t gold, and mean of 167–319 t gold. Although these updated grade-tonnage models provide an approximately 80 per cent variation in predicted gold endowment, it is still evident that the belt remains an under-explored region within the Yilgarn Block, Western Australia.

Geochemical sampling media, be they rock chip, soil or stream sediment samples, routinely collected in mineral exploration, are key to detecting mineralisation-related geochemical dispersion patterns. Notwithstanding the diversity of methods applied to geochemical anomaly detection, they can be broadly divided into two major groups, namely structural and non-structural techniques. The former group covers those methods in which threshold values are assigned based on sample location while the sample locations are not required in the application of the non-structural techniques. In this study, rock chip samples from the Shadan porphyry copper–gold deposit are used to address the question as to how structural and non-structural methods can separate geochemical populations for the purpose of a deposit-scale study. It was revealed that the two structural techniques used in this study, concentration-area (C-A) fractal and U-spatial statistic methods, outperformed the non-structural techniques employed in this study.

Petrophysical evaluation of a formation using petrophysical logs and core data plays an important role in determining the quality of the formation and the quantitative and qualitative characteristics of the reservoir. By zoning the reservoir layers, the focus is more effectively on areas with higher potential for hydrocarbon production. This research was conducted to interpret petrophysical data to identify reservoir zoning in one of Iran's oil reservoirs. The petrophysical core data and well logging charts were integrated and adapted for the reservoir zone. For the vertical samples, four flow units were identified and the formation was divided into four reservoir zones, with most of the samples taken from the second and third zones. Six hydraulic flow units were identified for horizontal samples. In the second and third zones, the samples were closely spaced and had the petrophysical properties that are similar, but superior, to those of the other zones.

Geophysical data processing further constrained inversion is evolving progressively prevalent in geoscience domains for three-dimensional modelling and resources evaluation. The process is based on the magnetic and gravity data processing further constrained Cartesian cut cell inversion to discern the maximum of information about HAJJAR deposit in order to calculate its tonnage. This article exhibits data and inversion processing technique for tonnage calculation based on an important geophysical magnetic and gravity surveys of defined extent of HAJJAR region, which presents a great benefit to save time and have accurate and realistic results to a same case. Otherwise, the potential-field signatures of what are regarded to be geologically expressive features are sought within the magnetic and gravity data. The preliminary stage for tonnage calculation was residual anomaly processing and depth estimation of the orebody using spectral analysis method. However, progressing towards extracting the deposit signature, the used method leads to invest the gravity signature of the orebody in adequacy with the magnetic signature. Finally, the tonnage calculation was developed by constrained Cartesian cut cell inversion using Voxi Earth Modelling^TM. Obtained results were very important, given their qualitative and quantitative accuracy, which give an added value for the governmental geological and geophysical survey.

Geoscientific datasets can contain individual data for more than 50 different chemical elements. The association between these variables is as important as their individual values. However, it is commonly overlooked that the observed covariance may be overestimated due to correlated errors. Dependent errors arise from many sources, such as the segregation process of minerals associated with these variables during delimitation, extraction, and preparation steps. This study extends a classical model composed of grade-independent (additive) and grade-proportional (multiplicative) errors to a generalised multivariate model that can estimate the real variance, covariance, and correlation from observations affected by shared errors. The use of estimates of the real covariance is recommended when the study objective is to evaluate or estimate the association between processes instead of the association between observations. A numerical example illustrates the bias in statistics and discusses the relevance of considering shared errors in linear regression and kriging.

Gibbsite, typically a clay-sized mineral in lateritic profiles, occurs as crystals between 50 and 150 µm across in druse-like cavities that characterise veins hosted in lateritised Cenozoic sediments, exposed along a road cut in the Quadrilátero Ferrífero of Minas Gerais, Brazil. Both the veins and the host rock, a red diamictite, contain gibbsite, but the drusy gibbsite in the former is remarkably larger than the matrix gibbsite in the latter. It is suggested that the coarser gibbsite is a low-temperature hydrothermal mineral. The coarse grain size of gibbsite and its occurrence in veins comprise important criteria to recognise hydrothermal overprint of low temperature on lateritic profiles.