Abstract Cripple Creek is among the largest epithermal districts in the world, with more than 800 metric tons (t) Au (>26.4 Moz). The ores are associated spatially, temporally, and genetically with ~34 to 28 Ma alkaline igneous rocks that were emplaced into an 18-km 2 diatreme complex and surrounding Proterozoic rocks. Gold occurs in high-grade veins, as bulk tonnage relatively low-grade ores, and in hydrothermal breccias. Pervasive alteration in the form of potassic metasomatism is extensive and is intimately associated with gold mineralization. Based on dating of intrusions and molybdenite and gangue minerals (primarily using 40 Ar/ 39 Ar and Re-Os techniques), the region experienced a protracted but intermittent history of magmatism (over a period of at least 5 m.y.) and hydrothermal activity (intermittent over the final ~3 m.y. of magmatic activity). Key factors that likely played a role in the size and grade of the deposit were (1) the generation of alkaline magmas during a transition between subduction and extension that tapped a chemically enriched mantle source; (2) a long history of structural preparation, beginning in the Proterozoic, which created deep-seated structures to allow the magmas and ore fluids to reach shallow levels in the crust, and which produced a fracture network that increased permeability; and (3) an efficient hydrothermal system, including effective gold transport mechanisms, and multiple over-printed hydrothermal events.

Abstract Alkaline igneous rock-related gold deposits, primarily of Mesozoic to Neogene age, are among the largest epithermal gold deposits in the world. These deposits are a subset of low-sulfidation epithermal deposits and are spatially and genetically linked to small stocks or clusters of intrusions possessing high alkali-element contents. Critical-, near-critical, or energy-critical elements associated with these deposits are F, platinum-group elements (PGEs), rare earth elements (REEs), Te, V, and W. Fluorine and tungsten have been locally recovered in the past, and some other elements could be considered as future by-products depending on trends in demand and supply. The Jamestown district in Boulder County, Colorado, historically produced F from large lenticular fluoritebearing breccia bodies and Au-Te veins in and adjacent to the Jamestown monzonite stock. Several hundred thousand metric tons (t) of fluorspar were produced. Some alkalic epithermal gold deposits contain tungstenbearing minerals, such as scheelite, ferberite, or wolframite. Small tungsten orebodies adjacent to and/or overlapping the belt of Au telluride epithermal deposits in Boulder County were mined historically, but it is unclear in all cases how the tungsten mineralization is related genetically to the Au-Te stage. Micron-sized gold within deposits in the Ortiz Mountains in New Mexico contain scheelite but no record of tungsten production from these deposits exists. The most common critical element in alkaline igneous-rock related gold deposits is tellurium, which is enriched (>0.5%) in many deposits and could be considered a future commodity as global demand increases and if developments are made in the processing of Au-Te ores. It occurs as precious metal telluride minerals, although native Te and tetradymite (Bi2Te2S) have been reported in a few localities. Assuming that the Dashigou and adjacent Majiagou deposits in Sichuan province, China, are correctly classified as alkalic-related epithermal gold deposits (exact origin remains unclear), they represent the only primary producers of Te (as tetradymite) from this deposit type. It is worth noting that some epithermal veins (and spatially or genetically related porphyry deposits) contain high contents of Pt or Pd, or both. The Mount Milligan deposit typically contains >100 ppb Pd, and some values exceed 1,000 ppb. However, owing to the presence of other large known PGE resources in deposits in which PGEs are the primary commodities, it is unlikely that alkaline-related epithermal gold deposits will become a major source of PGEs. Similarly, many epithermal gold deposits related to alkaline rocks have high vanadium contents, but are unlikely to be considered vanadium resources in the future. Roscoelite (V-rich mica) is a characteristic mineral of alkalic-related epithermal deposits and is particularly abundant in deposits in Fiji where it occurs with other V-rich minerals, such as karelianite, Ti-free nolanite, vanadium rutile, schreyerite, and an unnamed vanadium silicate. A few alkaline intrusive complexes that contain anomalous concentrations of gold or were prospected for gold in the past are also host to REE occurrences.The best examples are the Bear Lodge Mountains in Wyoming and Cu-REE-F (±Ag, Au) vein deposits in the Gallinas Mountains in New Mexico, which have REE contents ranging up to 5.6% in addition to anomalous Au.

Abstract Analyses of temporal and geographic distributions of the minerals of beryllium, boron, copper, mercury, and molybdenum reveal episodic deposition and diversification. We observe statistically significant increases in the number of reported mineral localities and/or the appearance of new mineral species at ~2800 to 2500, ~1900 to 1700, ~1200 to 1000, ~600 to 500, and ~430 to 250 Ma. These intervals roughly correlate with presumed episodes of supercontinent assembly and associated collisional orogenies of Kenorland (which included Superia), Nuna (a part of Columbia), Rodinia, Pannotia (which included Gondwana), and Pangea, respectively. In constrast, fewer deposits or new mineral species containing these elements have been reported from the intervals at ~2500 to 1900, ~1700 to 1200, 1000 to 600, and 500 to 430 Ma. Metallogenesis is thus relatively sparse during periods of presumed supercontinent stability, breakup, and maximum dispersion. Variations in the details of these trends, such as comparatively limited Hg metallogenesis during the assumed period of Rodinia assembly; Proterozoic Be and B mineralization associated with extensional environments; Proterozoic Cu, Zn, and U deposits at ~1600 and 830 Ma; and Cenozoic peaks in B, Cu, and Hg mineral diversity, reveal complexities in the relationship between episodes of mineral deposition and diversification on the one hand, and supercontinent assembly and preservational biases on the other. Temporal patterns of metallogenesis also reflect changing near-surface environments, including differing degrees of production and preservation of continental crust; the shallowing geotherm; changing ocean chemistry; and biological influences, especially those associated with atmospheric oxygenation, biomineralization, and the rise of the terrestrial biosphere. A significant unresolved question is the extent to which these peaks in metallogenesis reflect true episodicity, as opposed to preservational bias.

Abstract The role of biological processes in the formation of sediment-hosted ore deposits has long been recognized. In this review, we focus on the biogeochemical cycling of C, Mn, Fe, and S as they relate to the formation of sediment-hosted Mn and Fe deposits, metalliferous black shales, clastic-dominated (CD) Pb-Zn deposits, and phosphorites. Biological mediation of ore-forming processes occurs over large spans of space and time. The most important step is oxygenic photosynthesis, a biological innovation dating from the Archean Eon that releases free oxygen into the surface oceans and atmosphere and delivers chemical potential, in the form of reduced carbon, to the seafloor. Photosynthetic oxygen is available to precipitate dissolved Fe 2 + and Mn 2 +, and therefore it augments the formation of sedimentary Mn and Fe deposits, and drives oxidative weathering of exposed crust, thereby delivering sulfate and transition metals to the ocean. Where reduced carbon accumulates in the deep oceans and on the seafloor, bacterial sulfate reduction produces hydrogen sulfide thereby facilitating the formation of metalliferous black shales, sediment-hosted Pb and Zn sulfide deposits, and phosphorites. Thus, an understanding of major biogeochemical processes and how they have evolved over time is required in order to refine genetic models for sediment-hosted ore deposits and to guide future mineral exploration. A close secular relationship between deposit formation and trends in major biogeochemical cycles provides a potentially powerful tool for mineral resource assessment. Sedimentary basins that formed during a time that is known to lack deposits of a particular metal can be eliminated during exploration programs, whereas others of permissive ages should be considered priorities. For example, sedimentary basins older than ca. 1.8 Ga are unlikely to contain large CD Pb-Zn deposits, and basins that formed between 1.6 and 0.6 Ga are not prospective for phosphorites. Recent technological advances in the application of nanometer-, micron-, and bulk-scale analytical techniques allow for imaging of complex biological structures and have provided new insights into the role of bacteria, not only in direct formation of mineral deposits, but also in leaching of metals from ore and mineralized rocks. Future exploration for, and exploitation of, mineral deposits may include offshore or land-based, low-grade, high-tonnage targets; understanding the role of bacteria in mineral growth, mineral dissolution, and redox transformations will aid in predicting where such deposits exist, and how metal extraction from ores can be enhanced.

Abstract The establishment of accurate time scales of mineral systems is essential to construct reliable genetic models about their formation. Time scales of fossil mineral systems are directly determined through radiometric dating of different stages of development of the mineral system. In theory, porphyry systems are, among mineral systems, those whose duration can be bracketed with most accuracy and precision, because of the universal occurrence of ore and gangue minerals that can be dated with the high precision U-Pb zircon, Re-Os molybdenite, and 40 Ar/ 39 Ar dating techniques. Time scales of fossil porphyry systems reported in the literature range between <0.1 to >4 Ma. The long durations (>1 Ma) of magmatic-hydrothermal activity measured in several porphyry systems are likely the result of multiple magmatic pulses in agreement with field observations indicating that porphyry systems are associated with several intrusive events. Nonetheless, estimated long durations could also be affected by methodological problems. One methodological problem is the accuracy of the intercalibration among the three different methods. It has become evident during the last 15 years that 40 Ar/ 39 Ar dates are systematically younger compared to U-Pb dates. This has been attributed to incorrect values of the secondary standard (Fish Canyon Tuff sanidine), most commonly used to calculate 40 Ar/ 39 Ar ages, and/or of the 40 K decay constant. Systematic cross calibrations to check the consistency between Re-Os and U-Pb dates are lacking and should also be carried out. Another possible cause of erroneous long durations of porphyry systems concerns the way to determine the emplacement age of the causative intrusion. The current high precision (≤0.1%) of single zircon U-Pb dating by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) shows that zircon grains extracted from a single sample of intermediate/felsic magmatic rocks do not overlap in age. This is so because zircon grains record a protracted evolution of magmas within the crust lasting several hundreds of thousands of years. Under these conditions, the emplacement age of a magmatic intrusion is best approximated by the youngest ID-TIMS age measured from a population of zircon grains. In contrast, spot ages measured with in situ techniques, due to their lower precisions (1-3%), are not able to discriminate such protracted magmatic evolution recorded by different zircon grains. This allows pooling together spot ages of different zircons, resulting in a statistically significant mean age with a low uncertainty. In reality this is a mixed age that is characteristically older (by up to a few hundreds of thousands of years) than the age of the youngest single zircon grain measured by ID-TIMS. A further problem in estimating the duration of magmatic-hydrothermal activity in porphyry systems derives from the widespread use of 40 Ar/ 39 Ar dating. Because this method does not date the crystallization of a mineral but rather its cooling below its closure temperature, 40 Ar/ 39 Ar dates may be affected by (hydro-)thermal activity that postdates the mineralization.

Abstract Classifications of magmatic-hydrothermal ore deposits are largely geochemical, based on metal associations and characteristic alteration types, but the process of metal enrichment is primarily controlled by the physical hydrology of fluids flowing through rocks. Physical hydrology plays a decisive role in forming distinct ore deposit types, including volcanogenic massive sulfide deposits at mid-ocean ridges or submarine arc volcanos, porphyry-style ore deposits in continental collisional arcs, and epithermal vein and replacement deposits. Results from simulations of magmatic-hydrothermal systems using a new numerical modeling platform for thermohaline convection are used to determine the implications for ore formation in light of the different structural styles, timing, and igneous characteristics of major magma-related ore deposit types. Thermal convection, volatile expulsion, and salt water dynamics are shown to be the first-order hydrologic components, and different combinations or successions of general hydrologic patterns characterize particular oreforming systems. Due to the nonlinear properties of fluids and rocks as a function of pressure, temperature, and composition, the physical behavior of hydrothermal systems can be counterintuitive, and understanding their self-organization requires numerically rigorous models. Thus, mid-ocean ridge hydrothermal systems do not involve broadscale lateral infiltration of seawater; instead, focused warm downflow in the immediate vicinity of hot upflow zones provides a more efficient mechanism for metal leaching and ore formation in Cyprus-type massive sulfide deposits. Phase separation in submarine magmatic-hydrothermal systems can lead to a decoupling of vapordominated venting, which is expected to favor sulfur complexing of some metals leading to the formation of Au-rich chimneys, whereas chloride-complexing metals may precipitate during the waning stages, favoring the formation of base metal-rich sulfide deposits from negatively buoyant brines. Porphyry copper mineralization is localized by a self-stabilizing hydrologic front, located at the transition from brittle to ductile rock behavior and controlled by the heat balance between an external convective cooling engine and an overpressured magmatic fluid plume. This hydrologic divide also provides a mechanism for the transition to epithermal-style deposits where magmatic and meteoric fluids mix on ascent to the surface.

Abstract The capacity of hydrothermal fluids to transport metals in concentrations sufficient to form ore deposits is due in large part to the polar nature of the water molecule and the ability of metals to form strong aqueous complexes with a number of ligands commonly found in nature. In this paper, we review the properties of hydrothermal liquids and vapors, show how the hard/soft acid/base (HSAB) principle can be used to predict why certain metals form strong complexes with particular ligands, and review the experimental data on the aqueous speciation of a selection of base, precious, and critical metals in high- and low-density hydrothermal fluids. Based on these data, we identify the important complexes for each metal and determine the physicochemical conditions under which they may predominate and thereby control hydrothermal metal transport. This information is used to quantitatively determine the solubility of the main ore minerals in hydrothermal liquids and vapors, and evaluate the mechanisms of metallic mineral deposition (cooling, fluid mixing, boiling, and fluid-rock interaction) in selected ore-forming systems.

Abstract As mineral exploration moves into regions dominated by transported cover, conventional techniques (e.g., lag gravel) may not be applicable and thus, increasingly, there is a need for new, innovative approaches. To develop these approaches, potential mechanisms that transfer metals from buried mineral deposits through cover to the surface need to be identified. This paper presents an overview of some of the experimental and field trials conducted in Australia as part of an industry-supported three-year CSIRO/AMIRA project. The objective was to define vadose zone processes that might form elemental anomalies at surface over buried deposits in semiarid and arid terrains, and to compare methods that detect these anomalies. Studies were conducted at seven sites representing orogenic Au, volcanogenic massive sulfide (VMS; Cu-Zn-Ag), and magmatic Ni mineralization with transported cover ranging in thickness from 2 to 30 m. Three vertical metal migration mechanisms are important in vadose environments: (1) biological, (2) gaseous, and (3) capillary. An integrated approach, combining different mechanisms with the nature and evolution of transported regolith and climatic settings, was considered to obtain the best prediction of metal transfer. Upward element transfer by vegetation (Acacia aneura and Eucalyptus spp.) occurs in areas of transported cover up to 30 m thick, but not in environments which lack supergene enrichment and have hypersaline acid groundwater. Microbial populations are different in soil over mineral deposits than in those from background sites. Metals, detected by gas collectors, are transferred to surface as gases. Soil pit experiments show that strong geochemical anomalies can form rapidly (over 7 months) through 2 m of transported cover, and assist in understanding the genesis of natural geochemical anomalies. Seasonal variations suggest that migration of elements from source to surface may vary in time and intensity. Anomaly formation in the pit experiments is an episodic process largely driven by capillarity, in which batches of metals in water-soluble form are translocated. Soil-forming processes may form false anomalies and the data need to be interpreted with care.

Abstract Propylitic alteration halos to porphyry deposits are characterized by low- to moderate-intensity replacements of primary feldspars and mafic minerals by epidote, chlorite, calcite ± actinolite, pyrite, prehnite, and zeolites. The pyrite halo that surrounds porphyry deposits typically extends part way through the propylitic halo and provides strong responses to conventional geochemical and geophysical exploration techniques. When exploring outside of the pyrite halo, porphyry deposits have proven to be difficult to detect based simply on the presence of weak epidote-chlorite alteration. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of epidote from propylitic alteration zones around porphyry and skarn deposits in the central Baguio district, Philippines, have shown that low-level hypogene geochemical dispersion halos can be detected at considerably greater distances than can be achieved by conventional rock chip sampling of altered rocks. Epidote chemistry can provide vectoring information to the deposit center and potentially provides insights into the potential metal endowment of the porphyry system, providing explorers with both vectoring and fertility assessment tools. Epidote chemistry varies with respect to distance from porphyry deposit centers, with the highest concentrations of proximal pathfinder elements (e.g., Cu, Mo, Au, Sn) detected in epidote from close to the potassic alteration zone. Distal pathfinder elements (e.g., As, Sb, Pb, Zn, Mn) are most enriched in epidote more than 1.5 km from the deposit center. Rare earth elements and Zr are most enriched in epidote from the edge of the pyrite halo. The lateral zonation in epidote chemistry implies that at Baguio the geochemical dispersion patterns were produced by lateral outflow of spent fluids from the porphyry center, rather than from ingress of peripheral, nonmagmatic waters.

Abstract To aid conceptual targeting, the past two decades have seen the emergence of the mineral systems concept, whereby ore deposits are viewed as small-scale expressions of a range of earth processes that take place at different temporal and spatial scales. The mineral systems approach has been spurred by three main drivers: the recognition of patterns of mineralization in increasingly available large geoscience datasets; advances in geographic information system (GIS) technologies to spatially query these datasets; and marked advances in understanding the evolution of earth systems and geodynamics that provide context for mineralization patterns. An understanding of mineral systems and the scaledependent processes that form them is important for guiding exploration strategies and further research efforts. Giant ore deposits are zones of focused mass and energy flux. Advances in understanding of the physics of complex systems—self organized critical systems—leads to a new understanding of how fluid flow is organized in the crust and how high-quality orebodies are formed. Key elements for exploration targeting include understanding and mapping threshold barriers to fluid flow that form extreme pressure gradients, and mapping the transient exit pathways in which orebodies form. It is proposed that all mineral systems comprise four critical elements that must combine in nested scales in space and time. These include whole lithosphere architecture, transient favorable geodynamics, fertility, and preservation of the primary depositional zone. Giant mineral deposits have an association with large, longlived deeply penetrating and steeply dipping structures that commonly juxtapose distinctly different basement domains. These structures are vertically accretive in nature, often having limited or subtle expressions at or above the level of ore deposition. Three transient geodynamic scenarios are recognized that are common to many mineral systems: anomalous compression, initial stages of extension, and switches in the prevailing far-field stress. In each of these scenarios, “threshold barriers” are established which produce extreme energy and fluid/magma pressure gradients that trigger self-organized critical behavior and ore formation. Fertility is defined as the tendency for a particular geologic region or time period to be better endowed than otherwise equivalent geologic regions. Fertility comprises four major components: secular Earth evolution (variations in the Earth’s atmosphere-hydrosphere-biosphere-lithosphere through geologic history that result in formation of deposits), lithospheric enrichment, geodynamic context, and paleolatitude (in specific mineral systems). The primary depositional zone is usually within the upper 10 km of the Earth’s surface, where large P-T-X gradients can be established over short distances and time scales. The variable preservation of this zone through subsequent orogeny explains the secular distribution of many ore deposit types. The mineral system approach has advantages in exploration targeting compared to approaches that use deposit models. Emphasizing common ore-forming processes, it links many large ore systems (e.g., VMS-epithermal, porphyry-orogenic gold) that are currently considered disparate deposit models and relates these ore systems in a predictable way to their large-scale geodynamic context. Moreover, it focuses mineral exploration strategies on incorporating primary datasets that can map the critical elements of mineral systems at a variety of scales, and particularly the regional to camp scales needed to make exploration decisions.

Abstract Mineral exploration is the primary means to define new mineral resources. Following the end of World War II, there was a global economic boom which required the identification and mining of vast numbers of new deposits in order to provide the needed raw materials to sustain the demand. By and large, shallow easy-to-define orebodies were recognized first and developed. In the past 20 years, the discovery performance across virtually all mineral sectors has fallen, resulting in growing concern that if unchecked, there could be shortfalls in a number of commodities within the next 20 years. The collective sense is that there are more deposits to be found, but these are expected to be at greater depths than those that have been typical targets in the past. To operate in this environment, new approaches for identifying deposits are required and the concept of a mineral systems approach, first suggested 20 years ago, has emerged as a powerful means going forward to build strategies and capabilities. In terms of geophysical exploration, the major change that will be required is a shift from a focus almost entirely on direct targeting with geophysical surveys of deposits, to a staged process where geophysical approaches are used initially to help define the pathways in the earth that carried the mineralizing solutions, which formed the target deposit. These pathways would provide a much larger target and if detected and mapped, should allow explorers to follow the pathway to the location of potential deposits. This task is different from most geophysical studies, where the focus has typically been on improving the direct targeting capabilities and not the larger scale mapping problem that a mineral systems approach requires. A review of the current state-of-play for a number of major deposit styles shows how geophysical data are being used at present to explore for the larger scale mapping problem. The assessment overall is encouraging but major challenges remain outside of the technical issues of defining a mineral systems strategy that relate primarily to human resources and the commercial environment. With regard to the human resources issue, are there going to be a sufficient number of the right people to develop and implement the required programs? Universities play a critical role in producing new geoscientists but the industry then must take responsibility to train and mentor these people to become functioning professionals. In the commercial environment, at present there is little interest for long-term, strategic programs, either in terms of the needed fiscal support or commitment to undertake the implementation of outcomes. Although governments likely have a greater sense of urgency with regard to this problem, it may be difficult to unilaterally and successfully deal with this complex issue.

Abstract Comparison of conglomerate-hosted, Witwatersrand-type gold deposits and/or occurrences worldwide reveals that this deposit type is by no means unique to the Kaapvaal craton but common to most Archean and/or Paleoproterozoic cratons. The age of the variably mineralized fluvial to fluvio-deltaic conglomerates ranges from 3.1 to 1.9 Ga. They were deposited in tectonic settings ranging from continental rifts to passive margins and synorogenic foreland basins, and all of them are paleoplacers. Although several of them show evidence of local mobilization of ore components by postdepositional hydrothermal fluids, purely epigenetic hydrothermal models fail to explain the geometry of the orebodies as well as available lithogeochemical, mineral chemical, and isotope data. Conglomerates older than 2.4 Ga are characterized by an abundance of detrital (and secondary) pyrite, and in most cases also detrital uraninite, whereas most of the younger examples (<2.2 Ga) contain Fe oxides instead. A common denominator of Witwatersrand-type deposits is the stratigraphic position above erosional unconformities adjacent to an Archean to Paleoproterozoic hinterland. The Witwatersrand deposits themselves differ from all other examples of this type by a gold endowment that is two to three orders of magnitude greater, an abundance of gold-rich “carbon” seams that reflect former microbial mats, a scarcity of gold nuggets, and orders of magnitude higher Os contents in the gold. For the Witwatersrand gold, a genetic model is proposed that involves the following requirements: (1) an anomalous mantle domain as the ultimate source, strongly enriched in siderophile elements, caused by inhomogeneous mixing with cosmic material that was added during intense meteorite bombardment of the Hadean to Paleoarchean Earth, plume-like ascent of relics from inefficient core formation, or plumes from the core-mantle boundary; (2) elevated gold extraction into juvenile crust when mantle temperature reached its maximum in the Mesoarchean; (3) several orders of magnitude higher run-off of gold from the Mesoarchean land surface due to intense weathering under an aggressive, reducing atmosphere and high gold solubility in coeval river water; (4) trapping of gold from river water on the surface of local photosynthesizing microbial (cyanobacterial) mats; and (5) reworking of these mats into erosion channels during flooding events (and by eolian deflation) and redeposition of gold as placer particles. Postdepositional hydrothermal and/or metamorphic overprints explain why much of the gold is now located in texturally late positions but had little significance on the macroscale distribution of the gold. Elsewhere in the world, a less fertile hinterland and/or less reworking of older sediments led to correspondingly lower gold endowment. Most of the Archean sedimentary rocks were affected by crustal reworking in the course of later tectonic overprints. The multitude of fluids and melts involved in these reworking processes gave rise to the great variety of gold deposit types known in post-Archean crustal sections. The probability of discovering a new supergiant cluster of Witwatersrand-type deposits is considered very low. However, considerable potential exists for finding new smaller economic deposits of this type in Mesoarchean to Paleoproterozoic fluvial to fluviodeltaic basal conglomerates, deposited especially in foreland basins next to Mesoarchean hinterland and/or auriferous sediment successions that could be reworked.

Abstract Muon geotomography, a novel geophysical exploration and imaging technology, uses cosmic rays to create three-dimensional (3-D) images of subsurface density distributions. The first controlled field test confirming the capability of muon geotomography for imaging a dense orebody in a complex geologic environment was conducted at the Price volcanic-hosted massive sulfide (VHMS) deposit, Vancouver Island, British Columbia, Canada. The semimassive and massive polymetallic mineralization of the Price deposit is situated in a Paleozoic stratigraphic package of rocks known as the Sicker Group including the Price, Myra, Thelwood, and Flower Ridge Formations, indicative of volcanic rocks formed in a rifted oceanic island-arc system. The field application involved placing a sensor with an active area of 1 m 2 beneath the massive sulfide orebody in an underground tunnel for exposures of about two weeks at several locations. Muon flux data were inverted to recover a 3-D density image of the deposit. The inverted data were in good agreement with drill core data. However, some distortions of the image were observed due to the limitations imposed by the available tunnel which restricted the angular views available to the sensors. Muon geotomography works best when sensors are placed such that they can view the region under study from a range of different angles. The demonstrated ability to perform accurate forward model simulations makes the sensitivity of the technique predictable for specific survey situations. The results demonstrate the potential of muon geotomography for identification and characterization of orebodies located in complex geologic environments. Three-dimensional images from muon geotomography surveys may be used to guide drilling operations toward regions of high-density contrast, thereby significantly reducing costs and environmental impact associated with locating orebodies.

Abstract Tier 1 mineral resource discoveries are critical to maintaining Australia’s, and indeed the world’s, mineral resource inventory without continuing decline in the grade of mined resources. Such discoveries are becoming less common because, increasingly, remaining prospective, underexplored areas are obscured by deep, barren cover. We argue that improving the rate of Tier 1 discoveries obscured by deep, barren cover requires a step change in mineral exploration techniques that may be provided by “prospecting drilling,” i.e., extensive drilling programs that map mineral systems beneath cover, enabling geophysical and geochemical vectoring toward deposits. The technological platform for prospecting drilling must include low-cost drilling due to the dense subsurface sampling required. Low-cost drilling may be provided by transferring coiled tubing drilling technology, with its continuous drill pipe on a reel, from the oil and gas sector. Key challenges to the deployment of coiled tubing drilling in mineral exploration, i.e., its rate of penetration in hard rocks, the durability of coiled tubing, and the recovery of cuttings, are being assessed and addressed by researchers of the Deep Exploration Technologies Cooperative Research Centre (DET CRC). The optimum technology platform for prospecting drilling would be coiled tubing drilling complemented by downhole and top-of-hole sensing, providing realtime petrophysics, structure/rock fabric, geochemistry, and mineralogy. The first manifestation of real-time, downhole sensing is our newly developed autonomous sonde that is deployed by the driller and logs natural gamma radiation as the dill rods are pulled. Our experimentation on real-time, top-of-hole sensing (on drill cuttings from diamond cored holes) has demonstrated cost-effective, rapid, repeatable, and accurate determination of geochemistry and mineralogy with the necessary depth fidelity. The rationale for prospecting drilling is provided by two examples: (1) a dataset of antimony from the Kalgoorlie district of Western Australia, which shows that subsampling at a 2-km spacing would map the mineral system and enable vectoring toward the contained deposits, and (2) analysis of hypogene alteration systems of iron oxide-copper-gold (IOCG) deposits in South Australia that presents the possibility of vectoring toward the deposits within such systems starting from >10 km distant. At the target cost of $50/m, coiled tubing drilling could cost effectively undertake prospecting drilling in large, covered provinces, such as the IOCG prospective Gawler craton of South Australia.