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

The South Mathiatis deposit in the Troodos ophiolite of Cyprus is located within Lower Pillow Lavas in the central part of the Mitsero (Ayios Epiphanios) graben, and is characterised by elevated Zn contents, and a hydrothermal mineral assemblage in which galena is locally an important component and barite is a common gangue mineral, features which place it apart from other Cyprus-type deposits. The mineralisation is hosted within a gently dipping sheet flow-dominated sequence of lavas with common autobrecciation textures and associated hyaloclastites, and is stratigraphically controlled between lava flows, locally floored by unmineralised hyaloclastites. The environment of extrusion is interpreted as a ponded sequence, similar to the adjacent Agrokipia deposit. The Zn-rich mineralisation appears to be controlled by local structures which allowed access to the hydrothermal fluids, which then migrated laterally along lithologically favourable concordant zones. The sulphide assemblage is dominated by sphalerite together with ubiquitous pyrite; however, both chalcopyrite and galena are locally important; quartz and barite comprise the gangue-mineral assemblage. Red jasper is spatially associated with the sulphide mineralisation and displays abundant filamentous structures similar to forms identified in other VMS environments. Wall rock alteration associated with the mineralisation is weak, and this is reflected in the absence of a magnetic signature, in contrast to typical Cyprus deposits. The Zn mineralisation is also transparent to electrical methods suggesting that other similar deposits may be present in the ophiolite but remain as yet undetected.

This study considers realistic data for a planned open-pit iron ore mine, but is applicable to any open pit situation. By interpolating drill hole data, grades are generated for a rectangular block model. Each block's grade vector has components for each analyte (chemical element or compound) influencing ore value. Deriving the average (E-type) over many simulations leads to each block being assigned its expected value, and thus underestimates the overall grade variability. Alternatively, interpolation by means of conditional simulation is a method that implements random sampling from an infinite population of solutions. Each conditional simulation has appropriate overall grade variability, but estimating any block's mean and variance requires sampling from multiple conditional simulations. For a block model, an ore/waste selection criterion maximises the expected tonnage at a target grade. This criterion is a linear composite of the grade components, with positive coefficients for the beneficial analyte (Fe) and negative coefficients for the deleterious analytes (such as SiO₂, Al₂O₃ and P). Although conditional simulation often leads to a similar expected grade as kriging for each block, the expected maximum tonnage of ore selectable at a target grade may differ from that obtainable from the E-type solution. We apply the linear composite selection criterion to each of 25 conditional simulations, as well as to the E-type block model. Simulation confirms the distribution of product tonnage to have an expected tonnage that is over 20% greater than that of the E-type model. The method also enables a selection probability to be computed for each block, and thus a probabilistic pit boundary distribution to be identified and used in mine planning. Proposed extensions to this method will consider risk-based scheduling of the multiple selection solutions through minimisation of a derived stress factor and treating the mining process as an iterative system with actual or artificial depletions modelled in line with the mine plan, using the updated state (with new information) to re-evaluate the mine plan for subsequent periods.

In order to develop downstream processing routines for iron ore and to understand the behaviour of the ore during processing, extensive mineralogical characterisation is required. Microscopic analysis of polished sections is effective to determine mineral associations, mineral liberation and grain size distribution. There are two main imaging techniques used for the characterisation of iron ore, i.e. optical image analysis (OIA) and scanning electron microscopy (SEM). In this article, a QEMSCAN system is used as an example of SEM methodology and results obtained from it are compared against results obtained by the CSIRO Recognition3/Mineral3 OIA system. Both OIA and SEM systems have advantages and drawbacks. Even though the latest SEM systems can distinguish between major iron oxides and oxyhydroxides, it is still problematic for SEM systems to distinguish between iron ore minerals very close in oxygen content, e.g. hematite and hydrohematite, or between different types of goethite. Scanning electron microscopy systems also can misidentify minerals with close chemical composition, i.e. hematite as magnetite and vitreous goethite as hematite. In OIA, iron minerals with slight differences in their oxidation or hydration state are more easily and directly recognisable by correlation with their reflectivity. In both methods, the presence of microporosity can result in some misidentification, but in SEM methods misidentifications due to microporosity can be critical. Low resolution during QEMSCAN analysis can significantly affect the textural classification of particle sections. The main conclusion of this study is that, for low iron content ores or tailings, SEM systems can provide much more detailed information on the gangue minerals than OIA. However, for routine characterisation of iron ores with high iron content and containing a variety of iron oxides and oxyhydroxides, OIA is a faster, more cost effective and more reliable method of iron ore characterisation. A combined approach using both techniques will provide the most detailed understanding of iron ore samples being characterised.

Early models for the origin of banded iron formations (BIFs) assumed that under a reducing atmosphere the supergene anoxic hydrolysis of mafic silicate minerals would provide an abundance of ferrous iron in solution to be transported to the oceans, there to be oxidised and precipitated by locally produced oxygen in shallow seas. The sparse distribution of BIFs in the Archaean era, its greater abundance during the Palaeoproterozoic era and perceived absence thereafter was considered to be essentially linked to the concentration of O₂ in the atmosphere and this was a key factor in hypotheses of atmospheric evolution from anoxic to oxidising. Several assumptions regarding the deposition of BIFs were used to infer atmospheric evolution; however, chemical considerations indicate that the deposition of BIF is independent of atmospheric oxygen levels. Ferrous iron is only soluble in acid solution and in anoxic conditions in natural waters is precipitated as ferrous hydroxide or by carbon dioxide as ferrous carbonate. Silica in solution typically exists in equilibrium between ionic solution and colloidal suspension. Flocculation of colloidal silica is catalysed by the presence of cations and polymerises to a hydrophobic gel, thus removing silica from solution. It is highly unlikely that the oceans could ever have been the reservoir of iron and silica for the deposition of BIFs.

Modern interpretations consider BIFs as deep sea sediments with the source of the iron and silica derived from reactions between circulating sea water and hot mafic to ultramafic rocks producing hydrothermal systems venting onto the sea floor. The solubility of ferrous and ferric iron and silica is greatly increased at elevated temperatures and hydrothermal solutions would immediately precipitate iron hydroxide and iron silicates on quenching by cold seawater, even in the absence of ambient oxygen, to form hydrothermal plumes and mound deposits, subsequently resedimented by turbidity and density currents across the ocean floor. No transportation of ferrous iron in solution at ambient temperatures and no external source of O₂ or Fe²⁺ is necessary and the deposition of BIFs was independent of atmospheric oxygen, biogenic processes and continental sources of dissolved ferrous iron and silica, although any or all of these may have been present during deposition.

A similar process occurs today with black smoker hydrothermal vents depositing large quantities of iron hydroxide and iron silicates on the ocean floor that will eventually be subducted beneath the continents by plate tectonic processes. During the Archaean era, shallow oceans and immature continents, preserved sea floor deposits as greenstone belts and in marginal sedimentary basins until the formation of large buoyant continents caused the subduction of deep sea ocean crust including BIF deposits. The temporal distribution of BIFs is thus related to the preservation of deep ocean sedimentary rocks that since the onset of modern style plate tectonics in the late Archaean to Proterozoic eras have largely been destroyed by subduction of the ocean floor beneath the continents.

Pre-crusher stockpiles are designed principally as buffers to decouple the mining and processing operations. They are usually paddock dumped or dumped over a face to form fingers by dumping haul truckloads and reclaimed by front-end loader in an ad hoc manner. In addition, they are often not built and reclaimed to completion but continually added to and extracted from. Whereas this design suits the mining process and is operationally simple, there can be little confidence in the grade of the ore that is fed to the crusher. This makes short-term grade control difficult and reconciliation back to the mine face imprecise. Well-designed and operated pre-crusher stockpiles can overcome these deficiencies. The different types of grade variability in mining are reviewed. The role of pre-crusher stockpiles in reducing short-term variability is discussed and their ineffectiveness in addressing long-term variability is highlighted. Their role in removing the serial correlation of the extracted ore is explained. Pre-crusher stockpiles carry out the four, at times competing, objectives of storing, buffering, blending and grade separation. Their actual design and operation result from a compromise between these four competing roles. These roles are explained in detail along with the advantages and disadvantages of the different types of pre-crusher stockpiles. The recommended blended-in blended-out stockpile (BIBO) design is discussed in detail. Matched pairs of BIBO stockpiles of limited tonnage built and reclaimed to completion retain the best knowledge of grade and facilitate the reconciliation process. Simulation studies have been carried out on BIBO stockpiles to understand the blending opportunities of the different options for building and reclaiming, and to identify the methods that achieve maximum blending. It was found that laying down rows in one direction and reclaiming across these rows gives the best blending. Matching the width of the reclaim face with the daily crushing requirements is also important. The length of the rows, and building in either one or two layers has little effect on the extent of blending. Appropriate design can provide a reasonable compromise between the four objectives.