Abstract Petroleum systems (conventional and unconventional) and hydrothermal sedimentary rock-hosted copper, lead-zinc (clastic-dominated and Mississippi Valley-type), and uranium systems can be described in a common system framework comprising the critical processes of (1) establishing the fertility of source(s) of the commodity of interest and the transporting fluid, (2) geodynamic triggers for commodity movement and accumulation, (3) establishing an architecture for fluid movement, (4) accumulation by deposition of the commodity, and (5) preservation. To translate these commodity system models to effective exploration targeting models, they must correspond to business decisions. Exploration is an exercise in scale reduction and has a number of natural business decision points that map to scale: Regional-scale targeting—what basin has the potential of hosting a substantial mineral or petroleum system? Play-scale targeting—where within the basin could a number of deposits be clustered? Prospect scale targeting—where is there a deposit of sufficient quality within the play? Marrying the systems to the decision points involves identifying (1) constituent processes relevant at each scale, (2) the geology that can map the evidence of the processes occurring, and (3) the data or interpretative products that are best used as spatial proxies to map the evidence and guide area selection at the appropriate scale. A common change in focus is noted across spatial scales for all commodities: in basin selection, fertility is key, with lesser focus on other aspects of the system; in play analysis within a basin, all elements of the mineral system are fully considered; in prospect delineation the focus shifts toward accumulation and preservation. The similarity in the targeting workflow highlights that similar key data sets, tools, and interpretative products are required to assess each mineral system across scale, albeit looking for different features within those products, dependent upon the system being targeted. There are several key differences between mineral and petroleum systems. First, petroleum systems involve a mass trapping process with the transporting fluid as the commodity, whereas mineral systems involve mass scrubbing processes, with the transporting fluid having low concentrations of the commodity, thus requiring much fluid throughput. Second, petroleum systems require the entire system to remain reduced to maintain high-quality hydrocarbon, whereas most copper, lead-zinc, and uranium systems require the systems to remain oxidized until the site of deposition. Consideration of these commodity systems in the context of the Earth’s evolving atmosphere-hydrosphere-biosphere-lithosphere highlights the power of paleotectonic, paleogeographic, and paleoenvironmental reconstructions in the critical step of basin selection. Such consideration also highlights common gaps in understanding the commodity systems. These knowledge gaps constitute high-value research paths that would provide greatest leverage in area selection at the basin and play scales. These include improved knowledge of paleogeographic and paleoenvironmental reconstructions, basin hydrodynamics, and timelines of mass and energy flow through basins. For metal systems, better understanding is required of how metal extraction efficiency, solubility, mineral precipitation, permeability, and pressure and temperature gradients dynamically interact along flow paths during the evolution of basins.

Abstract Zircon composition has great potential as a pathfinder for porphyry Cu ± Mo ± Au systems. The present study used a large integrated laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb age and trace element dataset for both infertile and fertile magmatic suites in order to elucidate distinctive zircon signatures diagnostic of metallogenic fertility of the parent magma. The infertile suites are defined as magmatic rocks that are absent of alteration and mineralization at any grade, whereas fertile suites refer to the causative intrusions leading to porphyry-type ore formation. The infertile suites are relatively reduced S- and A-type and relatively dry A- and I-type magmas, including the Yellowstone rhyolite (Wyoming), Bandelier rhyolite (New Mexico), Bishop tuff rhyolite (California), Lucerne reduced granite (Maine), and Hawkins S-type dacite and Kadoona I-type dacite (Lachlan belt, Australia). The fertile suites are more oxidized and hydrous and are selected from representative causative I-type intrusions from porphyry and high-sulfidation epithermal Cu-Au deposits (Batu Hijau, Indonesia, and Tampakan, Philippines), porphyry Cu-Mo-Au deposits (Sar Cheshmeh, Iran; Dexing, eastern China; and Jiama, southern Tibet), porphyry Cu-Mo deposits (Sungun, Iran, and Qulong, southern Tibet), and porphyry Mo deposits (Nannihu and Yuchiling, central China). The best fertility indicators are zircon Eu/Eu* and (Eu/Eu*)/Y ratios, whereas zircon (Ce/Nd)/Y and Dy/Yb ratios are moderately useful. In particular, fertile magmatic suites have collectively higher zircon Eu/Eu* ratios (>0.3), 10,000*(Eu/Eu*)/Y (>1), (Ce/Nd)/Y (>0.01), and lower Dy/Yb (<0.3) ratios than infertile suites. In fertile suites, zircon (Eu/Eu*)/Y ratios are positively correlated with (Ce/Nd)/Y ratios, but this correlation is lacking in the infertile suites. The distinctive zircon ratios in the fertile suites are interpreted to indicate extremely high magmatic water content, which induces early and prolific hornblende fractionation and suppresses early plagioclase crystallization. In addition, we found that Mo is able to substitute for Zr in the zircon lattice. The Mo-rich porphyry systems that were analyzed as part of this study tend to produce some zircons with a higher Mo content (>1-9 ppm) than Mo-poor porphyry systems and infertile suites, indicating that Mo content in zircon is a potential pathfinder to porphyry Mo ore deposits. The zircon Mo/Ti ratio has a broad positive correlation with the oxygen fugacity of the magma, indicating that this ratio may be potentially used as a proxy for the oxidation state of the melt. Analyzing the compositions of detrital zircons from an area with little geologic information or poor outcrop could efficiently and cheaply discriminate whether the drainage source area is dominated by unprospective A-, S-, and I-type granitoids or by prospective I-type granitoids, which could help focus exploration on prospective areas.

Abstract This review paper examines banded iron formation-hosted higher-grade (>58 wt% Fe) iron ore types present in the two main metallogenic districts of Western Australia, the Yilgarn Craton and the Hamersley Province. The principal iron ore deposits from both districts exhibit variation in ore properties and genesis within and across districts, but also striking similarities. There are five critical elements involved in iron ore formation and preservation: (a) BIF iron fertility defined by stratigraphic and geodynamic setting; (b) Si-dissolving fluid flow; (c) high permeability at a range of scales; (d) exhumation and supergene modification; and (e) preservation of BIF-hosted iron ore bodies by surficial modification, cover or structures (downdrop, overthrust). Several subsidiary or constituent processes are important for the formation of distinct iron ore types and have expressions as (mappable) targeting elements. Deposits in the Hamersley Province record the presence of basinal brines and meteoric fluids, whereas deposits in the Yilgarn Craton, while less well constrained, suggest the influence of metamorphic/magmatic and meteoric fluids. A scheme for BIF alteration related to ore formation in a crustal depth continuum is presented, which integrates pressure-/temperature-dependency of assemblages, fluid–rock ratios and Si-dissolution capability and is a conceptual guide to prospective zones for iron ore.

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 Fault zones that cut Paleoproterozoic Birimian Supergroup sedimentary and mafic volcanic rocks in southwestern Ghana, west Africa, host numerous gold deposits that form one of the richest mesothermal lode gold provinces in the world. The Ashanti gold deposit is the largest discovered to date in west Africa, with past production and current reserves exceeding ~1,200 tonnes (t) of gold. A complex multiphase deformation history is evident in the Birimian sedimentary rocks that host the deposit. The prominent northeast-striking structural grain and fold-thrust belt architecture that characterizes the Paleoprotero zoic rocks of southwestern Ghana was established during regional-scale southeast-directed shortening (D 2 ) after development of a widespread bedding-parallel cleavage (S 1 ). A further minor episode of southeast-directed shortening (D 3 ) overprints D 2 . Structures associated with D 1 -D 3 are folded around 300- to 500-m- scale upright folds (F 4 ) that plunge to the northeast and have axial planes that strike ~east-west and dip 50° to 80° N. Upright folding was followed by development of north-striking, small-displacement, sinistral strike-slip faults (D 5 ) and local sinistral reactivation of some older D 2 thrust faults. Disseminated auriferous arsenopyrite grains in rocks adjacent to the mineralized faults are either localized on or cut the crenulation cleavage associated with the F 4 folds, which implies that gold mineralization occurred towards the end of, or after, F 4 . Mineralization along the faults themselves is hosted in quartz vein arrays that commonly have sinistral asymmetries at scales ranging from a few centimeters to several hundred meters, implying that the main gold event occurred during D 5 . Mineralized faults locally cut across F 4 folds without deflection, again implying that ore deposition occurred after F 4 folding. Ore shoots within the Ashanti deposit and adjacent satellite deposits are predominantly structurally controlled and are located in the following: Dilatant and subordinate compressional sites where mineralized shear zones step left and right, respectively, across F 4 kink folds and reactivated D 2 transfer faults; In pressure shadows associated with volcanic units, felsic and granitoid intrusions within the sedimentary sequence; At the intersections of major structures that were active during mineralization. The Ashanti deposit as a whole occupies an ~8-km-long segment of an otherwise unmineralized northeast-striking D 2 thrust fault known as the Obuasi/Main Reef fissure. Sinistral reactivation of this specific fault segment during the D 5 mineralization event occurred in response to movement on the younger north-striking Ashanti fissure, which merges with the Obuasi/Main Reef fissure at the northern end of the Ashanti deposit. The southern end of the mine is marked by a sharp right-hand flexure in the Obuasi fissure where it steps across a D 2 transfer zone. Recognition of these structural controls on mineralization allowed extensions to ore shoots within the Ashanti deposit to be targeted with a greater degree of confidence and has led to delineation of significant additional resources. Similar structural sites were targeted during exploration of the surrounding area using “integrated” geologic maps that combined the results of geologic mapping, airborne geophysical surveys, soil geochemical data, aerial and satellite photography, and local costeaning. Detection of mineralized faults was best achieved with a combination of geologic mapping, soil geochemical surveys, and costeaning. Routine recognition of structural sites similar to those noted above is probably only possible with geologic mapping at scales larger than 1:50,000. Attempting to remotely detect 200- to 400-m-long bends in poorly exposed faults was the most difficult aspect of this program. However, the detailed understanding of the timing and structural controls on mineralization gained in the mine area is a powerful exploration tool in its own right, which allows the significance of scattered structural observations to be appreciated and incorporated into a robust targeting strategy.

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