Leon Bagas

Gold and silver metallogeny of the South China Fold Belt: a consequence of multiple mineralizing events?

Abstract The South China Fold Belt is part of the South China Block that is interpreted to be the result of multiple tectonic and magmatic events that formed a collage of accreted Proterozoic and Phanerozoic terranes. The Jurassic to early Cretaceous Yanshanian period (180–90 Ma), a time of major tectono-thermal events that affected much of eastern and southeastern China, is of great metallogenic importance in the fold belt. This period is linked to subduction of the Pacific plate beneath the Eurasian continent, and is manifested by voluminous volcano-plutonic activity of predominantly calc-alkaline affinity. The distribution of gold and silver deposits in the South China Fold Belt indicates the presence of two distinct metallogenic provinces. A region of basement uplifts, which are controlled by shear zones and form Neoproterozoic inliers of metamorphosed iron-rich rock types, defines the first province. In this province, orogenic lodes and volcanic-related epithermal deposits represent the more significant precious-metal mineralization. The second province is essentially confined to a belt of Yanshanian felsic–intermediate volcanic and subvolcanic rocks that extends along most of the southeastern China coast in an area known as the Coastal Volcanic Belt. Deposits in the Coastal Volcanic Belt are silver- and/or copper-rich, volcanic-hosted and epithermal in character. The precious-metal metallogeny of the South China Fold Belt is interpreted to have developed in at least three stages: one as a result of collision events, during the Caledonian Orogeny (ca. 400 Ma), the second during the Indosinian Orogeny (ca. 200 Ma) and the third during or soon after the formation of the Yanshanian magmatic belt (Yanshanian Orogeny; 180–90 Ma). The latter was responsible for a hydrothermal event that affected large sections of the belt and its Proterozoic substrate. This may have resulted in the redistribution and enrichment of precious metals from preexisting orogenic gold lodes in Neoproterozoic basement rocks, which are now exposed as windows in zones of tectonic uplift. The Yanshanian hydrothermal activity was particularly widespread in the Coastal Volcanic Belt and resulted in the formation of both low- and high-sulfidation epithermal gold and silver, and locally copper and other base-metal mineralization. It is suggested that the Coastal Volcanic Belt has greater potential for world-class epithermal and porphyry deposits than previously realised.

High pressure amphibolite-granulite facies metamorphism in the Paleoproterozoic Rudall Complex, central Western Australia

Abstract The Rudall Complex forms the basement of the Paleoproterozoic to Neoproterozoic Paterson Orogen, which was deformed during at least three major orogenies. The complex comprises a sequence of metamorphosed clastic and chemical sediments, and basalts, deposited in a marginal basin environment, prior to extensive felsic intrusion. Deformation of the sequence during the first “Yapungku” orogeny between 2000 and 1760 Ma, included extensive thrust stacking from ENE to WSW. The thermal peak of the associated high-pressure metamorphism was late- to post-tectonic with respect to this deformation. Thermobarometry on amphibolites and mafic granulites that contain various combinations of the minerals hornblende, plagioclase, quartz, garnet, othopyroxene and clinopyroxene, indicates that temperatures peaked at around 800°C, while pressures reached 1200 MPa. The late timing of peak temperatures together with the presence, in some amphibolites, of amphibole-plagioclase symplectitic coronas around garnet, indicates a steeply decompressive clockwise P-T-t path. Peak metamorphic pressures indicate that the crust was locally thickened by at least 40 km, during an event that showed similarities to Phanerozoic collisional tectonics. It is suggested that this deformational and metamorphic event records the collision between the Pilbara Craton and a continent to the northeast. The Paleoproterozoic evolution of Australia has previously been attributed to ensialic processes characterised by low-pressure metamorphism and anti-clockwise P-T-t paths. The metamorphic history of the Rudall Complex is not consistent with this view, and the documented high-pressure metamorphism is broadly synchronous with a medium-pressure metamorphic event in central Australia (the Strangways Orogeny), and possibly also in northwestern Australia (the Capricorn Orogeny). Collisional plate-tectonic processes played a significant role in the Paleoproterozoic evolution of northern Australia.

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.

Gold mineralization of the Chencai-Suichang uplift and tectonic evolution of Zhejiang Province, southeast China

Abstract A metallogenic model is proposed in which gold and base metal mineralization in SE China is related to plate interactions along the Pacific margin with Southeast Asia. From about 900 to 70 Ma, all depositional, orogenic, and metallogenic processes can be explained by subduction events along northeast-trending zones. With progressive accretion and cratonization along the continental margin, these subduction zones moved southeast for a distance of less than 150 km. As a result, the area is one of crustal reworking in which the rocks and mineral deposits of earlier events have been repeatedly modified. The Chencai-Suichang Uplift is an uplifted crustal block in central Zhejiang Province of mainland China, in which the Proterozoic basement is exposed as tectonic windows in overlying Mesozoic volcanics. Several gold and base metal deposits are located in these Proterozoic windows as well as in the younger volcanics. Most economic deposits were formed during the last major tectono-magmatic event affecting the area, the Yanshanian (160–170 Ma) event, although there is evidence for earlier mineralization that has been remobilized during the Yanshanian. It is suggested that crustal reworking was particularly important in remobilising and upgrading gold mineralization. The model presented is supported not only by local geology and isotopic data, but also by the regional distribution of gold deposits, which are arranged in a 50-km-wide belt parallel to the Yanshanian subduction zone. Three major episodes of mineralization occurred during: (1) a subduction-related period of metamorphism, partial melting, and mesothermal fluid generation and mineralization in the lower crust during the Caledonian; (2) subduction-related volcano-plutonic activity with epithermal mineralization during the early Yanshanian; and (3) hydrothermal mineralization during late Yanshanian tectono-magmatic event. In the third, two main categories of metallogenic control are recognized, namely: (1) hydrothermal fluid generation in the lower crust and (2) upper crust structural and lithological controls on mineralization.

Paleoproterozoic stratigraphy and gold mineralisation in the Granites-Tanami Orogen, North Australian Craton

The Granites-Tanami Orogen (GTO) is a significant auriferous province located in the poorly exposed southwestern part of the North Australian Craton. The stratigraphic succession of the Paleoproterozoic Tanami Group in the orogen is hard to define conclusively, owing to the general lack of outcrop, and is best understood by studying orientated diamond drill holes, such as those at the Dead Bullock Soak and Tanami goldfields, and the Groundrush and Coyote gold deposits. The group is broadly a succession of turbiditic sedimentary rocks included in the Killi Killi Formation conformably overlying shale, mafic volcanic units associated with abundant dolerite and diorite sills, and chert nodules and bands in the Stubbins and Dead Bullock formations. The Mount Charles Formation, previously inferred to be younger than the Killi Killi Formation, is lithologically similar to the Stubbins Formation and also overlain by the Killi Killi Formation. Based on whole-rock and trace-element geochemistry, geochronology, lithostratigraphy and the principles of superposition, the ca 1864 Ma Stubbins, Dead Bullock and Mount Charles formations are here correlated and assigned to the Dead Bullock Formation of the Tanami Group. The deposition of the now extended Dead Bullock Formation was in a back-arc basin setting to the east of the Halls Creek Orogen. Deposition in the basin changed with the introduction of sand-dominated turbiditic successions forming the Killi Killi Formation before ca 1850 Ma when collisional tectonics between the GTO and Halls Creek Orogen resulted in the development of northerly trending isoclinal (FGTO1) folds and associated layer-parallel faults. A second (DGTO2) major collisional event followed at ca 1795 Ma, which is associated with orogenic lode-gold deposits at Callie, Groundrush, Old Pirate and Coyote, and the intrusion-hosted Buccaneer gold deposit in the GTO. This was a period in the history of the GTO when granitic rocks were emplaced shortly before or synchronously with gold mineralisation at ca 1795 Ma.

Metallogeny of the North Atlantic Craton in Greenland

Abstract The North Atlantic Craton (NAC) extends along the coasts of southern Greenland. At its northern and southern margins, Archaean rocks are overprinted by Palaeoproterozoic orogeny or overlain by younger rocks. Typical granite-greenstone and granite-gneiss complexes represent the entire Archaean, with a hiatus from ~3.55-3.20 Ga. In the granulite- and amphibolite-facies terranes, the metallogeny comprises hypozonal orogenic gold and Ni-PGE-Cr-Ti-V in mafic-ultramafic magmatic systems. Gold occurrences are widespread around and south of the capital, Nuuk. Nickel mineralization in the Maniitsoq Ni project is hosted in the Norite belt; Cr and PGE in Qeqertarssuatsiaq, and Ti-V in Sinarsuk in the Fiskenæsset complex. The lower-grade metamorphic Isua greenstone belt hosts the >1000 Mt Isua iron deposit in an Eoarchaean banded iron formation. Major Neoarchaean shear zones host mesozonal orogenic gold mineralization over considerable strike length in South-West Greenland. The current metallogenic model of the NAC is based on low-resolution data and variable geological understanding, and prospecting has been the main exploration method. In order to generate a robust understanding of the metal endowment, it is necessary to apply an integrated and collective approach. The NAC is similar to other well-endowed Archaean terranes but is underexplored, and is therefore likely to host numerous targets for greenfields exploration.

40Ar/39Ar evidence for the timing of Paleoproterozoic gold mineralisation at the Sandpiper Deposit, Tanami region, northern Australia

At the Sandpiper gold deposit in the Tanami region of northern Australia sericite is intimately intergrown with arsenopyrite in gold-bearing quartz veins and breccias, suggesting sericite crystallisation synchronous with gold-bearing fluid flow. This ore-stage sericite yields a 40Ar/39Ar plateau age of 1785 ± 32 Ma (2σ including both analytical and systematic uncertainties). Recalculation using revised and more precise values for the 40K decay constants and the age of the Fish Canyon Sanidine standard shifts the age to 1794 ±12 Ma (2σ including all known uncertainties). Given the possibility of post-mineralisation isotopic resetting this age can be conservatively interpreted as a minimum constraint on the timing of gold deposition although, given local geological relationships and estimates for the argon retentivity of white mica, we consider complete isotopic resetting to be unlikely. The preferred interpretation is, therefore, that the sericite 40Ar/39Ar age indicates the timing of gold mineralisation. Thesericite age accords with a limited dataset of 207Pb/206Pb xenotime ages of ca 1800 Ma from other gold deposits in the Tanami region, interpreted as mineralisation ages. The agreement between independently derived ages from several gold deposits lends support for a widespread gold-mineralising event at ca 1800 Ma in the Tanami region.

An irregular triangle mesh buffer analysis method for boundary representation geological object in three-dimension

Three-dimensional (3D) buffer analysis is a basic function of spatial analysis used widely in 3D Geographic Information Systems (3DGIS). Current buffer analysis methods for spatial points and curves generally function well. One exception is buffer zone of surface. Previous researchers in this field have used voxel models to overcome this limitation; however, defects with voxel model buffer analysis include redundancies, approximations, and poor visualization characteristics. In this contribution, a surface buffer analysis method is presented for the boundary representation of geological objects. Exact geometric representation is achieved via the construction of irregular triangle meshes in 3D. The results can be used for 3D structural modeling and then form the basis for spatial analysis or model-based quantitative assessment in mineral potential mapping and resource evaluation. Three comparisons between existing voxel methods and our new method, evaluating visualization, precision and redundancy, are conducted. The comparisons show that our proposed method is robust and provides a higher quality output than voxel modeling. Finally, uncertainty analysis of buffer distance in different geological objects was discussed.

Research on GIS-Based 3D Prospectivity Mapping and a Case Study of Jiama Copper-Polymetallic Deposit in Tibet, China

This paper reports a deposit-scale GIS-based 3D mineral potential assessment for Jiama copper-Polymetallic deposit area in Tibet, China. The assessment is achieved through the combined use of a metallogenic model, and 3D geological, geochemical modelling and Prospectivity modelling. In this contribution, a metallogenic model for the Jiama deposit and a 3D modelling flow chart are used to construct multiple 3D layers of volumetric and triangular mesh models to represent geology, geochemistry and ore-controlling features in the study area. GIS-based 3D weights-of-evidence analysis is used to quantify and target the subsurface Prospectivity for Cu(-Mo) ore-bodies in the area, which defined three prospective deep-seated exploration targets. This contribution ends with a discussion on the potential fluid flow pathways based on the 3D zonation of major geochemical elements and their ratios, such as Zn / Pb ratios. The discussion confirms the GIS-based 3D quantitative assessment of the Prospectivity of the Jiama copper-Polymetallic deposit.

A geophysically constrained multi-scale litho-structural analysis of the Trans-Tanami Fault, Granites-Tanami Orogen, Western Australia

The Trans-Tanami Fault in the poorly exposed Paleoproterozoic Granites-Tanami Orogen of Western Australia is an ∼100 km long curvilinear structure with ∼6 km right lateral displacement. Multi-scale integration and analysis of aeromagnetic, gravimetric, reflection seismic and remote sensing data have constrained the relative timing and architectural relationship of this structure. Interpretation of regional scale long-wavelength potential field (gravity and magnetic) anomalies, which are commonly used to define first-order structures, show that the fault is not a terrane boundary. Structural interpretation of short-wavelength potential field data illustrates that the structural domains on either side of the fault represent the products of a non-homogeneous stress regime developed between rigid granitic plutons. Additionally, 2D joint forward modelling of gravity and magnetic data and interpretation of reflection seismic data confirms the vertical displacement across this fault to be negligible indicating a predominant lateral displacement. The lateral displacement along a portion of this structure has exploited a pre-existing plane of a north-dipping thrust fault. Where this early thrust fault terminates, the Trans-Tanami Fault displaces previously unfaulted rock as a wrench fault step-over. These observations differ from previous findings in the area by constraining the absolute displacement of this structure and through the recognition of a wrench fault system that includes lateral step-overs between re-activated early thrust fault planes.