T. Campbell McCuaig

Archean komatiite volcanism controlled by the evolution of early continents

Significance Komatiites are rare, ultra-high-temperature (∼1,600 °C) lavas that were erupted in large volumes 3.5–1.5 bya but only very rarely since. They are the signature rock type of a hotter early Earth. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. This study used a combination of different radiogenic isotopes to map the boundaries of Archean microcontinents in space and time, identifying the microplates that form the building blocks of Precambrian cratons. Isotopic mapping demonstrates that the major komatiite belts are located along these crustal boundaries. Subsequently, the evolution of the early continents controlled the location and extent of major volcanic events, crustal heat flow, and major ore deposit provinces. The generation and evolution of Earth’s continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that in the Archean, the formation and stabilization of continents also controlled the location, geochemistry, and volcanology of the hottest preserved lavas on Earth: komatiites. These magmas typically represent 50–30% partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the Archean–Proterozoic Earth. As a result, it is vital to constrain and understand the processes that govern their localization and emplacement. Here, we combined Lu-Hf isotopes and U-Pb geochronology to map the four-dimensional evolution of the Yilgarn Craton, Western Australia, and reveal the progressive development of an Archean microcontinent. Our results show that in the early Earth, relatively small crustal blocks, analogous to modern microplates, progressively amalgamated to form larger continental masses, and eventually the first cratons. This cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks. The dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmosphere, while also providing a fundamental control on the distribution of major magmatic ore deposits.

The mineral systems concept: the key to exploration targeting

P-T conditions, involving SO2 gas and liquid water, such as those close to volcanic vents (Fegley & Prinn 1989), or in water-free, high-T systems (Henley et al. 2015). However, release of magmatic volatiles can occur at depth where hot magmatic gas meets crustal rocks +/− pore fluids. We have investigated experimentally the viability of such reactions by reacting SO2 gas with a mixture of calcite and a metal-bearing saline fluid at 1.0– 1.5 kbar pressure and 400–800°C. SO2 gas is supplied via thermal decomposition of sodium bisulphite in an unsealed inner capsule (Figure 1). At all temperatures, the calcite-bearing experiments produced anhydrite and sulphide through reaction of SO2 with calcite (Figure 1). In an experiment where calcite was replaced by quartz, no sulphide was produced, establishing that Ca is a crucial component of this reaction. Our experiments show that production of anhydrite and sulphide through calcite-mediated SO2 disproportionation takes place on timescales of just a few hours. This could occur where hot magmatic SO2 gas encounters magmatic brines in igneous rocks, basinal brines within sedimentary rocks, or calcium-bearing saline groundwaters. We suggest that reaction of hot magmatic SO2 with Ca-bearing crustal rocks containing saline, metal-bearing fluids may be a potent mechanism for scrubbing volcanic gas and precipitating metal sulphides, as previously suggested on the basis of andesite-dacite reaction experiments involving S-bearing magmatic volatiles and brines (Blundy et al. 2015).

Late Permian-Triassic magmatic evolution in the Jinshajiang orogenic belt, SW China and implications for orogenic processes following closure of the Paleo-Tethys

Growth of continental crust involves the complex interplay of subduction zone magmatism, to generate the crust, followed by stabilization through crustal thickening, magmatism and ultimately isolation from the active plate margin. The Jinshajiang orogenic belt, SW China, provides an exceptional record of continental development as the result of closure of the Paleo-Tethys seaway and ensuing collision. A compilation of U-Pb age and geochemical data for the plutonic and volcanic rocks within the southern part of the Jinshajiang orogenic belt, including new high-precision ages for the Ludian granitoid batholith of 231 to 220 Ma, enables us to explore the interaction between magmatism and orogeny in the context of the Paleo-Tethys closure and continental amalgamation. These age and geochemical constraints, in conjunction with other geologic evidence, suggest that subduction of the Paleo-Tethys ocean dominated local tectonics prior to the Triassic, creating a volcano–plutonic arc along the eastern margin of the Qamdo-Simao terrane. Following consumption of the ocean, collision zone magmatism, dated at 247 to 237 Ma, was manifested by eruption of voluminous volcanic rocks in a suture-parallel zone. Crustal anatexis was contemporaneous with the earliest phases of collision, producing high-silica rhyolites of Early Triassic age (ca. 247-246 Ma). Between 245 and 237 Ma, the local tectonic regime switched from compression to extension, probably due to strain partitioning caused by oblique convergence, which led to the development of rift-basins and extensive syn-tectonic bimodal volcanism associated with deep-water sediments. From 234 Ma to 214 Ma, the emplacement of high-K, calc-alkaline granodiorites-monzogranites occurred prior to, or during, isostatic uplift and extension, probably caused by breakoff of the subducted slab. The resultant exhumation brought deep-seated granitoid batholiths to the surface, and was contemporaneous with intrusion of ultramafic-mafic melts. Ophiolitic mélange (ca. 362-294 Ma) and collision-related magmatic suites (247-214 Ma) are unconformably overlain by a Late Triassic (229-217 Ma) conglomerate-rich sequence that represents an overlap assemblage, across the Qamdo-Simao terrane (Indochina) and Yangtze Block of South China.

Age, nature, and origin of Ordovician Zhibenshan granite from the Baoshan terrane in the Sanjiang region and its significance for understanding Proto-Tethys evolution

This contribution reports on a better understanding of the Proto-Tethys evolution in the Sanjiang Tethyan region of China. The manuscript presents laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon ages, Sr–Nd–Hf isotope systematics, and whole-rock major and trace element of Ordovician magmatic rocks from the calc-alkaline Zhibenshan granite, which formed along the northern margin of East Gondwana. The U–Pb zircon dating of monzogranite from the Zhibenshan granite yields crystallization ages of 466–457 Ma. The monzogranite has Cross–Iddings–Pirsson–Washington normative corundum (3.34%) and is peraluminous with Al2O3/(Na2O+ K2O+CaO) molar ratio of 1.26, similar to S-type granites. All samples are enriched in large ion lithophile elements (LILEs, such as Rb, K, U, and Th) and depleted in high field strength elements (HFSEs, e.g. Nb and Ti). These granites are enriched in light rare earth elements (REEs) and depleted in heavy REEs with strongly negative Eu anomalies (δEu = 0.19–0.24). The initial 87Sr/86Sr ratios range from 0.7118 to 0.7176 and εNd(t) values from −11.3 to −10.3 with Nd model ages of 2114–2037 Ma. Magmatic zircons with early Palaeozoic dates have εHf(t) values ranging from −13.3 to −1.8 and Hf model ages from 2258 to 1537 Ma. These geochemical and isotopic features suggest that the Zhibenshan granite originated from an ancient crustal source. Ordovician granites in the Baoshan terrane represent the southward continuation of the early Palaeozoic granitic belt that extended along the northern margin of East Gondwana, providing important evidence for the evolution of the Proto-Tethyan Ocean.

A mineral system approach to iron ore in Archaean and Palaeoproterozoic BIF of Western Australia

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.

Probabilistic Fuzzy Logic Modeling: Quantifying Uncertainty of Mineral Prospectivity Models Using Monte Carlo Simulations

Significant uncertainties are associated with the definition of both the exploration targeting criteria and computational algorithms used to generate mineral prospectivity maps. In prospectivity modeling, the input and computational uncertainties are generally made implicit, by making a series of best-guess or best-fit decisions, on the basis of incomplete and imprecise information. The individual uncertainties are then compounded and propagated into the final prospectivity map as an implicit combined uncertainty which is impossible to directly analyze and use for decision making. This paper proposes a new approach to explicitly define uncertainties of individual targeting criteria and propagate them through a computational algorithm to evaluate the combined uncertainty of a prospectivity map. Applied to fuzzy logic prospectivity models, this approach involves replacing point estimates of fuzzy membership values by statistical distributions deemed representative of likely variability of the corresponding fuzzy membership values. Uncertainty is then propagated through a fuzzy logic inference system by applying Monte Carlo simulations. A final prospectivity map is represented by a grid of statistical distributions of fuzzy prospectivity. Such modeling of uncertainty in prospectivity analyses allows better definition of exploration target quality, as understanding of uncertainty is consistently captured, propagated and visualized in a transparent manner. The explicit uncertainty information of prospectivity maps can support further risk analysis and decision making. The proposed probabilistic fuzzy logic approach can be used in any area of geosciences to model uncertainty of complex fuzzy systems.

New Zircon U-Pb and Hf-isotope data of the Birimian Terrane of the West African craton

Oceanic anoxic events (OAEs) were a frequent occurrence in the Cretaceous greenhouse ocean. Based on a variety of paleoredox indicators, euxinic water column conditions are commonly invoked for these OAEs. However, in a high resolution study of OAE3 deep sea sediments [1], revised paleoredox indicators suggest that euxinic conditions fluctuated with anoxic ferruginous conditions on orbital timescales. Building upon this, we here present new data for a continental shelf setting at Tarfaya, Morocco, that spans a period prior to, and during, the onset of OAE2. We again find strong evidence for orbital transitions from euxinic to ferruginous conditions. The presence of this distinct cyclicity during OAE2 and OAE3 in shallow and deep water settings, coupled with its occurrence on the anoxic shelf prior to the global onset of anoxia, suggests that these fluctuations were a fundamental feature of anoxia in the Cretaceous ocean. The observed redox cyclicity has major implications for the cycling of phosphorus, and hence the maintenance and longevity of OAEs. However, despite this significance, controls on the observed redox cyclicity are essentially unknown. Here, we utilize S isotope measurements (pyrite S and carbonate-associated S) from the deep sea and shelf settings to model oceanic sulphate concentrations across the redox transitions. Perhaps surprisingly, we find no evidence to suggest that ferruginous conditions arose due to extensive drawdown of seawater sulphate (as pyrite-S and organic-S) under euxinic conditions. Instead, S isotope systematics in the deep sea imply increased sulphate concentrations during ferruginous intervals. Based on these observations and other major element data, we infer that the redox cyclicity instead relates to orbitally-paced fluctuations in continental hydrology and weathering, linking the redox state of the global ocean to climate-driven processes on land. [1] Marz et al (2008) GCA, 72, 3703-3717.

Quantitative study of geological target spotting with the use of eye tracking

In this paper we describe the use of eye tracking to quantitatively evaluate and analyse the variations in data interpretation performed by various geoscientists, measuring this against their ability to spot geological targets. We also describe an approach to evaluate the impact data preprocessing (i.e. enhancement) has on ones ability to perform the interpretation task. We adapted a mobile eye tracker to enable it to accurately map the point of gaze to the actual image coordinate instead of the forward facing eye tracker camera allowing the user to move their head as they view. Several visual interpretation tasks were performed by six geoscientists and the results are described in this paper.

Miocene Ultrapotassic, High-Mg Dioritic, and Adakite-like Rocks from Zhunuo in Southern Tibet: Implications for Mantle Metasomatism and Porphyry Copper Mineralization in Collisional Orogens

&NA; High‐Mg diorites and/or ultrapotassic volcanic rocks are generally associated with postcollisional porphyry copper deposits, but their contribution to the formation of the mineralization remains unclear. A suite of Miocene postcollisional ultrapotassic‐potassic lamprophyres, high‐Mg diorites, and adakite‐like intrusions have been recognized in the Zhunuo porphyry Cu deposit, located in a continental collisional zone within the Gangdese belt, southern Tibet. The post‐mineralization ultrapotassic‐potassic lamprophyres have zircon U‐Pb ages of 12·2 ± 0·1 Ma and contain abundant Proterozoic to Miocene inherited zircons. The ultrapotassic lamprophyres have high K2O (>8·5 wt %) and MgO (>8·8 wt %) contents, are enriched in light rare earth elements (LREE; La = 123 ppm) and large ion lithophile elements (LILE; e.g. Ba = 3102 ppm, Th = 116·6 ppm, and Pb = 140 ppm), and display high Th/Yb and Rb/Sr, and low Ba/Rb and Hf/Sm ratios. They have zircon &egr;Hf(t) values of ‐2·8 to 1·3, &dgr;18O values of 6·5 to 7·4‰, and enriched bulk‐rock Sr‐Nd‐Pb isotope compositions ((87Sr/86Sr)i =0·73134, &egr;Nd(t) = ‐13·7, (206Pb/204Pb)i = 19·20). Their parental magmas were derived from partial melting of an enriched mantle source that had been metasomatized by fluids and sediment‐derived melts associated with Neo‐Tethyan oceanic subduction and subsequent Indian continental lithosphere subduction. The potassic lamprophyres have lower contents of K2O, MgO, REE and LILE than the ultrapotassic lamprophyres and (87Sr/86Sr)i of 0·710993 to 0·711139, &egr;Nd(t) of ‐12·3 to ‐12·4, and (206Pb/204Pb)i of 18·59 to 18·72. Taken together with observations of a negative trend between &egr;Nd(t) and MgO content; positive trends between (87Sr/86Sr)i, (206Pb/204Pb)i and MgO content from ultrapotassic lamprophyres to potassic lamprophyres; the existence of abundant Miocene inherited zircons showing similar ages and &egr;Hf(t) values to the adakite‐like intrusions; and variable Hf/Sm ratios with some Hf/Sm ratios similar to adakite‐like intrusions, we propose that the potassic lamprophyres were formed by mixing of ultrapotassic lamprophyre magmas with adakite‐like magmas. The syn‐mineralization high‐Mg diorites including diorite porphyry and enclaves hosted by the adakite‐like intrusions at Zhunuo have zircon U‐Pb ages of 13·0 ± 0·2 Ma and 13·1 ± 0·2 Ma. They show negative correlations between Y, Yb, Dy/Yb and SiO2, and positive correlations between Sr, Sr/Y and SiO2, among which some more evolved samples (such as diorite porphyry) show adakite‐like geochemical signatures. The high‐Mg diorites are enriched in LREE and LILE, depleted in high‐field‐strength elements (HFSE), and have (87Sr/86Sr)i of 0·709401 to 0·710362, &egr;Nd(t) of ‐11·1 to ‐9·9, and (206Pb/204Pb)i of 18·62 to 18·71. Taken together with petrographic observations that show magma mixing, we argue that the high‐Mg diorites were derived from previously subduction‐modified Tibetan lithospheric mantle with little or no input from Indian continental sediment. Mixing with adakite‐like magmas and fractional crystallization of hornblende and/or titanite are also responsible for the differentiation of the high‐Mg diorites. The ore‐hosting, adakite‐like granitic rocks at Zhunuo with zircon U‐Pb ages of 14·7 ± 0·3 Ma and 14·6 ± 0·2 Ma have lower concentrations of REE, LILE and HFSE, much higher &egr;Nd(t) (‐6·1 to ‐6·9) and lower (87Sr/86Sr)i (0·707325–0·707663) values than the ultrapotassic lamprophyres and the high‐Mg diorites. They were derived from remelting of previously subduction‐modified Tibetan lower crust with some involvement of hydrous high‐Mg dioritic magmas during magma mixing. The postcollisional adakite‐like intrusions in the Gangdese belt could be generated by remelting of previously subduction‐modified lower crust and mixing with hydrous high‐Mg dioritic magmas in a lower crustal MASH zone and/or in an upper‐crustal adakite‐like magma chamber. The metallogenic potential of postcollisional adakite‐like intrusions largely depends on rejuvenation of subduction‐modified lower crust by previous arc magmas, differentiation of hydrous high‐Mg dioritic magmas, and magma mixing of high‐Mg dioritic magmas with lower crustal magmas.

CET exSim: mineral exploration experience via simulation

Undercover mineral exploration is a challenging task as it requires understanding of subsurface geology by relying heavily on remotely sensed (i.e. geophysical) data. Cost-effective exploration is essential in order to increase the chance of success using finite budgets. This requires effective decision-making in both the process of selecting the optimum data collection methods and in the process of achieving accuracy during subsequent interpretation. Traditionally, developing the skills, behaviour and practices of exploration decision-making requires many years of experience through working on exploration projects under various geological settings, commodities and levels of available resources. This implies long periods of sub-optimal exploration decision-making, before the necessary experience has been successfully obtained. To address this critical industry issue, our ongoing research focuses on the development of the unique and novel e-learning environment, exSim, which simulates exploration scenarios where users can test their strategies and learn the consequences of their choices. This simulator provides an engaging platform for self-learning and experimentation in exploration decision strategies, providing a means to build experience more effectively. The exSim environment also provides a unique platform on which numerous scenarios and situations (e.g. deposit styles) can be simulated, potentially allowing the user to become virtually familiarised with a broader scope of exploration practices. Harnessing the power of computer simulation, visualisation and an intuitive graphical user interface, the simulator provides a way to assess the user’s exploration decisions and subsequent interpretations. In this paper, we present the prototype functionalities in exSim including: simulation of geophysical surveys, follow-up drill testing and interpretation assistive tools.