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

The deposits of Rio Tinto are located in the Spanish segment of the Iberian Pyrite Belt and are hosted within felsic porphyritic volcanic rocks and tuffs. The orebodies comprise a spectrum from sedimentary exhalative (San Antonio) to sub-sea floor replacement of the volcanic host rocks (Filon Norte); the two largest masses (San Dionisio and Filon Sur) are closely associated with black shale and probably formed by partial replacement of these units in an anoxic setting. Alteration associated with ore deposition is typified by marginal sericitic (white mica) alteration, and central chloritisation and silicification in a multi-phase alteration history. Structures formed during initial plate convergence may have acted as controls on ore deposition, and evidence of such controls is retained in the distribution of various elements in the sulphide deposits. Tectonism followed the mineralisation, and overprinted previous extensional events and resulted in the development of slaty cleavage in the pelitic rocks with partial remobilisation of sulphides, and tight folds with associated shearing of the southern limb.

Rock density information is needed for the estimation of ore tonnage, and also for determining the amount of contained metal; this is because mineralisation grades are usually reported as a ratio of the economically viable components, metals or minerals, to weight units (e.g. grams/tonne). Despite the obvious importance of rock density for the accurate estimation of tonnage and grade of mineral resources, this parameter is often overlooked and receives significantly less attention than assayed metal grades. In particular, the quantity and spatial distribution of the rock density measurements may be chosen without considering the impact of the rock density measurements on the mineral resource estimation. This paper attempts to address several issues associated with current practices of rock density modelling. The first part of the paper overviews the methods most commonly used for measuring the dry bulk density (DBD) of rocks for the estimation of mineral resources. This is followed by a second part proposing the methodology of geostatistical modelling of the rock densities which is suggested as a mathematically supported approach for choosing optimal sampling grids for density samples. It is supported by several case studies of the deposits where geostatistically optimal DBD sampling grids were estimated and proposed for definition of the Measured and Indicated resources and for the grade control purpose. It was noted that the exploration team needs to assure that DBD data collected during the drilling campaigns are not only sufficient for definition of resources and reserves but are also closely enough spaced to allow accurate grade control. The optimal for grade control DBD grids, according to the geostatistical analysis, is similar to the chemical assay grids required for definition of the Measured resources or, less commonly, Indicated resources of the studied deposits.

A standard has been developed to improve and upgrade the collection and coding of geotechnical data as part of geological exploration activity. CoalLog was developed by industry-based geoscientists and geotechnical specialists in response to outdated terminologies, non-translatable data coding, and non-conformances with relevant Australian and international standards for geotechnical investigation, testing, and reporting. The new standard will allow the industry to use geotechnical information efficiently and accurately, thereby minimising potential legal liabilities associated with non-conformance to recognised standards that are already in place in the non-coal geotechnical sector. Adoption of this standard will also promote opportunities for a wider range of geotechnical specialists to provide services to the coal industry.

The Late Devonian Strathbogie batholith is a 1500 km², semi-contiguous mass of undeformed, peraluminous monzogranite and syenogranite in SE Australia. The rocks are S-type and contain abundant igneous cordierite. Internal variations within the batholith have been investigated here by systematic fieldwork guided by modern concepts of granite batholith structure, combined with geophysical and geomorphological data. Important field parameters include the outcrop character and abundance of tors and pavements, feldspar phenocryst size and abundance, groundmass grain size, the textures, grain sizes and abundances of quartz, cordierite, garnet and biotite, and the presence or absence of tourmaline, vugs and enclaves. Mapping has been supplemented with aeromagnetic data to define the external boundaries of this essentially non-magnetic batholith, but these data cannot be used to delineate internal structures. The K-Th-U distribution, from airborne radiometric surveys, was found to assist in sub-division of the batholith, and the digital elevation model formed the basis of an interpretation of batholith structure, including lineaments. Some parts of the batholith have quite distinctive field characteristics and are interpreted as discrete plutons (e.g. the Kerrisdale and Lightning Ridge plutons). Broad trends in feldspar phenocryst abundance and the presence of tourmaline and vugs (miarolitic cavities) indicate an upper contact (roof) of the batholith in the northwest. Cordierite is ubiquitous, most rocks contain a few percent of biotite, there is widespread accessory garnet, and tourmaline is present within the inferred near-roof zones. The field-based approach described here offers a method for mapping large batholiths without resorting to grid sampling and the accumulation of vast sets of whole-rock analyses. This field-based method is especially applicable in cases where there are no obvious pluton boundaries, such as internal wall-rock screens or contacts exposed on glacial pavements. The field approach records outcrop- and district-scale variations, and many textural and mineralogical features that would be omitted in any map based only on geochemistry.