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

Portable X-ray fluorescence (pXRF) technology is fast becoming an important part of the geochemical toolkit for exploration geology. However, the relatively new development of this technology means that the awareness of the issues and pitfalls in pXRF use is lacking somewhat in both industry and academia. For some, pXRF analyses are becoming a panacea and involve an instrument that can be used with little consideration of the data quality and calibration issues that might be associated with the data. This study aims to highlight key considerations in using pXRF during the early phases of exploration within a geologically and geochemically poorly constrained gold deposit. In this scenario, pre-existing manufactured standards may not be adequate to ensure the accuracy of the pXRF analyses. Instead, the use of laboratory-based whole rock analyses of representative samples from the project area act as orientation samples designed to establish reference compositions and used to derive correction factors to apply to pXRF data. These data also highlight the issues in the common industry practice of taking pXRF measurements of pulverised samples through paper packets, an approach that introduces bias towards underestimating the concentrations of many elements of interest and must be mitigated against in order to ensure pXRF data are both accurate and precise.

Accumulation and lag concentration, together with dispersion, are important processes in offshore and coastal placer development. Mineral grains and sediment particles are sorted by flowing water according to their hydraulic equivalence. Distinct placer types are developed. Marine regression and transgression result in reworking leading to modification or destruction. Some placers are stranded on the coast, others are drowned. A vertical grade profile (VGP) graphically depicts the grade variations in the pay section of a placer. A VGP may be plotted for all heavy minerals and every placer type, and two basic patterns are evident. An upward or downward decline in grade indicates that accumulation and lag processes, respectively, were dominant. A reversal in decline direction marks a scour horizon or stillstand. Combinations of the basic patterns exist and some are complex with three grade maxima, revealing multiple mineralisation units. The history of a placer is revealed by its VGP(s) unless part is erased by reworking. Plotting the grade to a common logarithmic scale may assist interpretation. Different placer types have been developed in favoured locations visible on model cross-sections through the coast to the offshore. With minor variations, the VGP pattern displayed by a specific placer type is common to all minerals. Sampling protocol and resource estimation procedures should be guided by the VGP.

Carbon capture and storage by mineralisation (CCSM) is a method proposed for capturing CO₂ by reacting it with magnesium in ultramafic rocks to form carbonate minerals and silica. Large quantities of magnesium silicate rocks are required for this process and to demonstrate the feasibility, and adequately plan for the development and supply of mineral resources, their locations and quantities must be known. This study attempts to globally define the spatial extent and quantity of resources that could be used for the CCSM processes and to assess, if based on resources, this could be a viable, widely applicable CO₂ sequestration process. It has been estimated that around 90 teratonnes of material is available. This is sufficient to capture global CO₂ emissions for over 700 years at current levels of output and highlights the enormous resource. Even if only a small part is utilised, it could make a significant impact on CO₂ reduction. The majority of the resource is contained within ophiolitic rocks. The study further attempts to split CCSM resources into altered (serpentine-rich rocks) and unaltered (olivine-rich rocks) due to the different processing requirements for these rock types. Carbon capture and storage by mineralisation is likely to be of most use in areas with no access to underground geological CO₂ storage or for small operations where underground storage is not practical. This study demonstrates that substantial resources are available and their supply is unlikely to be a constraint.

Mineral resource and ore reserve estimates are founded on two sources of data: tonnage and grade. The tonnage is a product of volume and density; both of which are estimates. Density impacts numerous operational factors, which include, but are not limited to, mine design, mine planning, equipment selection and operational performance. Hence, density is a significant parameter and its determination requires similar care as the measurement of grade. This paper provides an overview of methods used to determine density within the Anglo American Group. It is not the purpose of this paper to identify a preferred method, but to highlight the importance of choosing the best suited practice for a project or mine site. In addition, a case study comparing two different density determination methods applied to the same rock samples from the Los Bronces Copper mine in Chile was undertaken and the results of that study are presented here. Selecting the most appropriate method to determine density and comparing results from two or more techniques against each other, together with other suitable quality control procedures, is considered to be essential for mining operations and exploration projects in order to reduce risk and to improve operational performance, which in turn increases profit margin.