Abstract China is the world’s leading rare earth element (REE) producer and hosts a variety of deposit types. Carbonatite-related REE deposits, the most significant deposit type, include two giant deposits presently being mined in China, Bayan Obo and Maoniuping, the first and third largest deposits of this type in the world, respectively. The carbonatite-related deposits host the majority of China’s REE resource and are the primary supplier of the world’s light REE. The REE-bearing clay deposits, or ion adsorption-type deposits, are second in importance and are the main source in China for heavy REE resources. Other REE resources include those within monazite or xenotime placers, beach placers, alkaline granites, pegmatites, and hydrothermal veins, as well as some additional deposit types in which REE are recovered as by-products. Carbonatite-related REE deposits in China occur along craton margins, both in rifts (e.g., Bayan Obo) and in reactivated transpressional margins (e.g., Maoniuping). They comprise those along the northern, eastern, and southern margins of the North China block, and along the western margin of the Yangtze block. Major structural features along the craton margins provide first-order controls for REE-related Proterozoic to Cenozoic carbonatite alkaline complexes; these are emplaced in continental margin rifts or strike-slip faults. The ion adsorption-type REE deposits, mainly situated in the South China block, are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits, although Caledonian (early Paleozoic) granites are also of local importance. The primary REE enrichment is hosted in various mineral phases in the igneous rocks and, during the weathering process, the REE are released and adsorbed by clay minerals in the weathering profile. Currently, these REE-rich clays are primarily mined from open-pit operations in southern China. The complex geologic evolution of China’s Precambrian blocks, particularly the long-term subduction of ocean crust below the North and South China blocks, enabled recycling of REE-rich pelagic sediments into mantle lithosphere. This resulted in the REE-enriched nature of the mantle below the Precambrian cratons, which were reactivated and thus essentially decratonized during various tectonic episodes throughout the Proterozoic and Phanerozoic. Deep fault zones within and along the edges of the blocks, including continental rifts and strike-slip faults, provided pathways for upwelling of mantle material.

Abstract Iron formations are the most important precursors to economic iron ore deposits. Iron formations originated as chemical sediments rich in iron and silica that accumulated almost exclusively on Archean and early Paleo-proterozoic sea floors. Most are banded and known as banded iron formations (BIF); they originated as thinly layered chemical muds. The less common granular iron formations (GIF) are rich in sand-sized detritus known as granules and generally crossbedded, indicating deposition in shallower, higher energy environments. Banded iron formations display a heterogeneous suite of iron-rich minerals including oxides, silicates, carbonates, and sulfides; iron oxide and silicate minerals dominate GIF, although a few are rich in iron carbonates. The acme of iron sedimentation was reached between ~2.65 and 1.85 Ga when large iron formations were deposited globally due to a unique confluence of (1) a large supply of aqueous iron from oceanic hydrothermal systems, (2) the appearance of large continental shelves to serve as depositional repositories, and (3) a stratified ocean capable of connecting the two. Depositional mechanism(s) are still being debated, but evidence for the involvement of microbes is increasing. Iron formation-hosted iron ore deposits account for the majority of current world iron ore production and consist of three classes: (1) iron-rich primary iron formation with typically 30 to 45 wt percent Fe, (2) martite-goethite ore with abundant hydrous iron oxides containing 56 to 63 wt percent Fe, and (3) high-grade hematite ores with 60 to 68 wt percent Fe. The high-grade hematite ores, which account for the majority of world reserves of high-grade iron ore (>31,000 Mt), can be further subdivided into hematite and microplaty hematite ore types. Individual iron ore deposits range from a few millions of tons to over 2 billion tons at >64 wt percent Fe, although most are within the range of 200 to 500 Mt. Many depositional features of parent BIF and GIF, especially microbanding, have been preserved during ore formation for the martite-goethite and high-grade hematite ores. There is a general consensus that martite-goethite ores from Australia formed as a result of relatively recent supergene enrichment of iron formation through replacement of gangue by goethite beneath Cretaceous to Te rtiary weathering profiles. Likewise, a supergene origin is well supported for the soft, high-grade hematite ores from the Quadrilátero Ferrífero in Brazil, but this involved leaching of iron formation gangue and residual concentration of hematite. The origins of both microplaty hematite and Brazilian hard high-grade hematite deposits are still controversial; alternatives proposed vary from supergene to initial supergene with subsequent burial metamorphism, hypogene, and supergene-modified hypogene-hydrothermal involving warm basinal brines plus ascending or descending heated meteoric fluids. Although the supergene-modified hypogene-hy-drothermal has received widespread support, it is unlikely that a single hypogene model can explain the wide diversity of deposits around the world.

Abstract There are six distinct classes of gold deposits, each represented by metallogenic provinces having hundreds to more than 1,000 tonnes (t) gold production. These deposit classes are as follows: (1) orogenic gold; (2) Carlin and Carlin-like gold deposits; (3) epithermal gold-silver deposits; (4) copper-gold porphyry deposits; (5) iron oxide copper-gold deposits; and (6) gold-rich volcanic-hosted massive sulfide to sedimentary-exhalative (sedex) deposits. This classification is based on ore and alteration mineral assemblages, ore and alteration metal budgets, ore fluid pressure(s) and compositions, crustal depth or depth ranges of formation, relationship to structures and/or magmatic intrusions at a variety of scales, and relationship to the P-T-t evolution of the host terrane. The classes reflect distinct geodynamic settings. Orogenic gold deposits are generated at midcrustal (4–16 km) levels proximal to terrane boundaries, in transpressional subduction-accretion complexes of cordilleran-style orogenic belts; other orogenic gold provinces form inboard by delamination of mantle lithosphere or by plume impingement. Carlin and Carlin-like gold deposits develop at shallow crustal levels (<4 km) in extensional convergent margin continental arcs or back arcs; some provinces may involve asthenosphere plume impingement on the base of the lithosphere. Epithermal gold and copper-gold porphyry deposits are sited at shallow crustal levels in continental margin or intraoceanic arcs. Iron oxide copper-gold deposits form at middle to shallow crustal levels; they are associated with extensional intracratonic anorogenic magmatism. Proterozoic examples are sited at the transition from thick refractory Archean mantle lithosphere to thinner Proterozoic mantle lithosphere. Gold-rich volcanic-hosted massive sulfide deposits are hydrothermal accumulations on or near the sea floor in continental or intraoceanic back arcs. The compressional tectonics of orogenic gold deposits are generated by terrane accretion; high heat flow stems from crustal thickening, delamination of overthickened mantle lithosphere inducing advection of hot asthenosphere, or asthenosphere plume impingement. Ore fluids advect at lithostatic pressures. The extensional settings of Carlin, epithermal, and copper-gold porphyry deposits result from slab rollback driven by negative buoyancy of the subducting plate, and associated induced convection in asthenosphere below the overriding lithospheric plate. Extension thins the lithosphere, advecting asthenosphere heat; promotes advection of mantle lithosphere and crustal magmas to shallow crustal levels; and enhances hydraulic conductivity. Siting of some copper-gold porphyry deposits is controlled by arc-parallel or orthogonal structures that in turn reflect deflections or windows in the slab. Ore fluids in Carlin and epithermal deposits were at near-hydrostatic pressures, with unconstrained magmatic fluid input, whereas ore fluids generating porphyry copper-gold deposits were initially magmatic and lithostatic, evolving to hydrostatic pressures. Fertilization of previously depleted subarc mantle lithosphere by fluids or melts from the subducting plate, or incompatible element-enriched asthenosphere plumes, is likely a factor in generation of these gold deposits. Iron oxide copper-gold deposits involve prior fertilization of Archean mantle lithosphere by incompatible element enriched asthenospheric plume liquids, and subsequent intracontinental anorogenic magmatism driven by decompressional extension from far-field plate forces. Halogen-rich mantle lithosphere and crustal magmas form, and likely are the causative intrusions for the deposits, with a deep crustal proximal to shallow crustal distal association. Gold-rich volcanic-hosted massive sulfide deposits develop in extensional geodynamic settings, where thinned lithosphere extension drives high heat flow and enhanced hydraulic conductivity, as for epithermal deposits. Ore fluids induced hydrostatic convection of modified seawater, with unconstrained magmatic input. Some gold-rich volcanic-hosted massive sulfide deposits with an epithermal metal budget may be submarine counterparts of terrestrial epithermal gold deposits. Real-time analogues for all of these gold deposit classes are known in the geodynamic settings described, excepting iron oxide copper-gold deposits.

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 The Tsagaan Oloom Formation (Fm.) in southwestern Mongolia contains two Neoproterozoic glacial deposits, with diamictite in the Maikhan Ul Member (Mb.) and in the Khongoryn Mb., which are separated by over 500 m of limestone. The Maikhan Ul Mb. ranges in thickness between 5 m and greater than 300 m, expanding in deeper-water sections towards the SW, where it is composed of two massive diamictites separated by over 100 m of sandstone, siltstone and shale. The basal 10 m of the overlying Tayshir Mb. of the Tsagaan Oloom Fm. consists of a fine-laminated, dark grey limestone. The Khongoryn Mb. is composed primarily of limestone clasts in a shale matrix, and is between 0 and 23 m thick. The overlying Ol Mb. contains sedimentary structures characteristic of basal Ediacaran cap carbonates including micropeloids, tubestone stromatolites, giant wave ripples and former aragonite crystal fans. U–Pb evaporation ages from zircons in the underlying Dzabkhan Volcanics constrain the Tsagaan Oloom Fm. to <773 Ma, and tuffs within the Maikhan Ul and Tayshir members testify to the potential for additional geochronology. The Cryogenian organic-rich limestone of the Tayshir Mb., which lies between the two glacial deposits, is ideally suited for geochemical studies and has been the subject of several carbon, strontium and rare earth element investigations. Limited palaeomagnetic studies suggest a mid- to low-latitude position of the Dzabkhan platform during deposition of the glaciogenic strata, and additional studies are in progress.