C.R.M. Butt

Naturally occurring gold nanoparticles and nanoplates

During the weathering of gold deposits, exceptionally pure, <200 nm diameter, nanoparticulate gold plates (6 nm thick) are formed. The particles display controlled growth of both size and shape and signs of assembly to form belts and sheets. The gold is associated and intergrown with minerals formed by evaporation and is interpreted to have been deposited rapidly from saline groundwater during a drying event. The size and morphology of the gold nanoparticles and nanoplates are identical to the products of experimentally manufactured gold colloids. This represents the fi rst direct observation of colloidal nanoparticulate gold in nature, confi rming this as an active mechanism of gold transport during the weathering of gold deposits.

Conceptual models in exploration geochemistry: Australia

Abstract This is the fourth in a series of “Conceptual Models” volumes reviewing geochemical exploration practice in different areas of the world. This volume provides an overview of geochemical exploration for base and precious metals in Australia. The subaerial weathering history of most of the continent probably dates from the Permian including, broadly, an early period of widespread lateritization followed by a late trend to aridity. This gave rise to the formation of a deep weathering profile which may subsequently have been partially or wholly eroded. It differs markedly, therefore, from the terrains described in previous volumes in the series. Consequently, emphasis has been placed throughout on the long weathering history, and the influence it has had on element dispersion and on the surface expression of mineralization in a wide range of sample media - ironstones, gossans, weathered bedrock, soils, transported overburden, stream sediments, waters, plants and atmospheres. The nature, use and geochemical characteristics, including multi-element data where possible, are described for each of these sample media. A set of eight idealized models has been derived using data from fifty-two case histories and previously published literature. The models are based on generalized landform settings, classified hierarchically according to: 1. (1) degree of preservation of the deep weathering profile, i.e. complete, partly stripped or absent; 2. (2) relief, i.e. low, moderate or high. A further model describes the landform situation within which calcrete uranium deposits are formed. The case histories present, for the most part, previously unpublished data and include examples not only of successful exploration but also situations where surface anomalies are spurious.

A guide for mineral exploration through the regolith in the Yilgarn Craton, Western Australia

The Yilgarn Craton is one of the worlds principal mineral provinces, with considerable resources of gold, nickel, bauxite and iron ore, as well as lesser amounts of a wide range of other commodities. As such, it is a major target for exploration. However, as most of the Craton has been exposed to sub-aerial conditions since at least the Late Proterozoic, it has had a long history of weathering and erosion under a wide variety of climatic conditions, resulting in a widespread and complex regolith cover. This diverse and extensive regolith poses considerable difficulties for exploration—as well as opportunities in terms of different sample media and the formation of resources within the regolith itself. The development of appropriate geochemical exploration procedures has been the subject of considerable research by government, university and industry for nearly 50 years. This guide is intended to summarise the results of this research in a single, stand-alone document to assist mineral explorers. It provides a synthesis of the characteristics and evolution of the Yilgarn landscape and advice on appropriate strategies and techniques for exploring through different types of cover, based on current knowledge and best practice. Because broadly similar deeply weathered regolith-dominated terrains occur elsewhere in Australia and, indeed, in other continents, many of the principles discussed here can be more widely applied. The guide develops exploration strategies for gold, base metals, nickel and surficial uranium deposits within the context of the evolution of the landscape and regolith of the Yilgarn Craton. The guide commences with a brief introduction to the geology and principal ore deposits of the Craton, followed by a description of the development and characteristics of the regolith, and the relationship between regolith type and landforms. The distribution, characteristics and use of the principal sample media are described. The regolith and landforms provide a framework for empirical models that describe the processes of geochemical dispersion, based on numerous case histories and other investigations, grouped according to the principal commodities. The models anticipate and summarise the surface expression of mineralisation and include recommendations on exploration procedures, such as sample selection, sampling interval and analytical approach. A comprehensive reference list provides a source for further information.

Granite weathering and silcrete formation on the Yilgarn Block, Western Australia

Weathering profiles developed on granitic rocks, exposed in the breakaways of the Barr‐Smith Range in the N of the Yilgarn Block of Western Australia, consist of kaolinitic saprolites merging upwards into silcrete, sandstone and grit. The sandstones and silcretes may also form columns or dykes, penetrating downwards into the saprolite. The silcretes are cemented by quartz and anatase, with zircon (QAZ‐cement), and‐the sandstones are cemented by aluminosilicates, either apparently amorphous (as siliceous allophane) or partly crystalline, as kaolinite and opaline silica. Transitional zones between silcretes and sandstones have all cement types. The profiles are characterized by low concentrations of alkalis and alkaline earths and most metals. The QAZ‐silcrete horizons may contain over 3% TiO2 and 1000 p.p.m. Zr. The profiles evolved through at least four stages: (i) Formation of the deep saprolite‐sand weathering profile by kaolinization of feldspar and mica at depth, and the solution of kaolinite near the...

A basis for geochemical exploration models for tropical terrains

Abstract the principles and mechanisms of geochemical dispersion and the formation of anomalies govern the effective use of exploration geochemistry. In any terrain, the dispersion characteristics can be summarized in the form of geochemical exploration models that illustrate the nature of the surface expression of mineralization and that in turn should aid the planning, execution and interpretation of future surveys. In much of the tropics and sub-tropics, and in some areas in higher latitudes, evidence of weathering under former seasonal tropical conditions is preserved. These regions have similarities in landforms, deeply weathered regoliths and hence geochemical dispersion patterns relating to such weathering conditions. Geochemical models can be based on these common features and their subsequent modification under different climatic regimes. Two model systems are described, one in which the deeply weathered profile is preserved and the other in which the profile has been partially truncated. These models can be used to compare and contrast the surface expression of mineralization under present-day arid, savanna and equatorial rainforest environments, thereby, in effect, extending the data base for exploration of such areas.

Classification of geochemical exploration models for tropically weathered terrains

Abstract Generalized exploration models can be established for deeply weathered terrains, based on the degree of preservation of pre-existing profiles formed under predominantly humid, tropical climates, and the modification due to later climatic and tectonic events. The models thus integrate the full weathering and erosional histories rather than considering only the present morphoclimatic environment. The following factors are important in classifying the models: • - preservation of the pre-existing profile, i.e. whether it is complete, or partly or wholly truncated; • - intermediate and present climates, and related chemical and mineralogical alteration of the former profile; • - presence and nature of the overburden. The proposed models allow, to some extent, prediction of the nature of secondary geochemical haloes, including the dispersion characteristics of given elements or suites of elements. Such information enables the most appropriate operating parameters (e.g. sample media, sample spacing, interpretational criteria) to be selected for geochemical surveys of specific areas and environments.

Helium in soil and overburden gas as an exploration pathfinder — an assessment

Abstract Orientation surveys were conducted over five deposits to test the potential of determining helium in overburden gas as a pathfinder for uranium mineralization and other deposits containing uranium or thorium. Samples were collected via fixed tubes emplaced at depths of 6 m in backfilled holes drilled for this purpose. Compared to the atmospheric background value of 5.24 ppm v/v He, a variable weak anomaly (maximum 5.45–5.65 ppm He) was found over part of the Angela uranium deposit, N.T., in an arid area where mineralization is mostly at a depth of 60–90 m, at or below the water-table. Helium contents were mostly at background levels over a uranium deposit in the Officer Basin, W.A., where mineralization is at the water-table at 30–35 m, although radon gave a marked anomaly. Neither helium nor radon indicated the Manyingee deposit, W.A., which has uranium mineralization in a confined aquifer at 60–110 m. Similarly, no helium anomalies were found over the uranium- and thorium-rich Mt. Weld carbonatite or mineral sands at Eneabba. There appeared to be no correlation between helium distributions shown by groundwater and overburden-gas sampling at Manyingee or Mt. Weld. At Mt. Weld, groundwaters contained 0.06 to 13.60 μ/l He and overburden gases 5.24–5.47 ppm He, with the higher gas concentrations over country rock, where waters had background helium contents. It is presumed that equilibration between overburden gas and the atmosphere is far more rapid than that between overburden gas and groundwater, so that any helium released from the water is quickly dispersed. Overburden-gas helium concentrations were found to vary according to overburden type, being 5.24–5.32 ppm where sandy and porous and 5.30–5.50 ppm where clay-rich and less permeable. These background variations, which are greater than the total background-anomaly contrasts reported in the literature for shallow soil gases, have not been accounted for in most trial surveys, nor has the possibility of similar variations being due to analytical error. From the data obtained, there is little evidence that helium can be considered an effective pathfinder for blind or concealed deposits using soil gas or overburden gas as sample media. Previous work on the use of soil and soil-gas helium determinations in uranium exploration is reviewed in the light of these findings and the concept and techniques assessed.

Aluminosilicate cementation of saprolites, grits and silcretes in Western Australia

Abstract Amorphous to poorly crystalline aluminosilicates have been found as cementing agents within saprolites, hardpans and silcretes, particularly in granitic terrains in the Yilgarn Block, Western Australia. The cements range mineralogically from siliceous allophane to a kaolinite‐opal‐CT assemblage probably derived from the allophane. The allophane is non‐crystalline, with no distinctive X‐ray or electron diffraction patterns, and has close optical similarities to opaline silica. It is characterised by Al/Si ratios of 0.35–1.0, with small amounts of adsorbed iron and titanium. It occurs as colloform void and vein fillings, and permeating the kaolinitic matrix of saprolites. The kaolinite and opal‐CT have a similar occurrence, with the kaolinite oriented parallel to colloform and related banding. The aluminosilicate cements are found in saprolite, usually in the upper zones, and in overlying grits and silcretes. At Gabbin, in the central Yilgarn, the grits are up to 10 m thick and have aluminosilicate...

Groundwater helium surveys in mineral exploration in Australia

Abstract The potential for using groundwater helium surveys in exploration has been evaluated by conducting local and regional surveys in several mineralized and background areas in Australia. Dissolved helium contents decrease markedly upwards, particularly in the top 10–15m, due to degassing in attaining dynamic equilibrium with the atmosphere. Gradients of depth profiles are not uniform but because total concentration variations to 50m or more are commonly less than 10, whereas regional variations are 10 3 –10 5 , samples from the greatest constant depth possible below water-table are adequate for survey purposes. Helium concentrations in excess of the atmosphere equilibrium value of 0.044 μl He/l H 2 O were found to be associated with uranium mineralization at Honeymoon (6.9–44.4 μl/l), Manyingee (0.08–1.68 μl/l), Bennett Well (9.9–29.5 μ/l) and the Stuart Shelf—Roxby Downs (910–2495 μl/l), with a carbonatite at Mt. Weld (0.18–13.6 μl/l), and with kimberlites at Wandagee (0.2–3.25 μl/l). No anomaly was associated with the shallow calcrete uranium deposit at Yeelirrie. However, equivalent or higher concentrations were found in groundwaters from unmineralized areas around these deposits, e.g. up to 85 μl/l at Manyingee—Bennett Well, to 215 μl/l at Yeelirrie and to 1525 μl/l, Stuart Shelf—Mt. Gunson. Similarly, helium in waters from stock wells near Mt. Weld appears to indicate the presence of the carbonatite but in a survey of a geologically similar area near Malcolm, equivalent anomalies were not related to carbonatite. Although the uranium (and thorium) mineralization is a significant source, high helium concentrations can arise by accumulation in groundwaters with long residence times and by leakage from deep sources, particularly granitoid basement. The concentration reached is also a function of the porosity and configuration of the aquifer and the permeability of the aquiclude to helium. These factors are more important in determining dissolved helium concentrations than the uranium and thorium contents of the rocks. However, the effects are not readily quantifiable, particularly in exploration areas where geology and hydrology are relatively unknown, hence helium groundwater data can be difficult if not impossible to interpret.

Possible errors in helium analyses of soil gases by mass spectrometers having constant-pressure inlet systems

Abstract Published data suggest that soil gas helium concentrations of 5.28–5.34 ppm v/v over uranium and hydrocarbon deposits are significantly anomalous compared to the ambient atmospheric background of 5.24 ppm. However, analyses for helium by mass spectrometers having constant-pressure inlet systems, from which most of these data are derived, are subject to errors of equivalent magnitude. These errors arise when the major component composition of unknown and standard gases differ, for the different gases have different flow rates through the inlet system — relative rates being O2