The Central Asian Orogenic Belt ( c . 1000–250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic–Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian–Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with formation of the Solonker suture. We evaluate contrasting tectonic models for the evolution of the orogenic belt. Current information provides little support for the main tenets of the one- or three-arc Kipchak model; current data suggest that an archipelago-type (Indonesian) model is more viable. Some diagnostic features of ridge–trench interaction are present in the Central Asian orogen (e.g. granites, adakites, boninites, near-trench magmatism, Alaskan-type mafic–ultramafic complexes, high-temperature metamorphic belts that prograde rapidly from low-grade belts, rhyolitic ash-fall tuffs). They offer a promising perspective for future investigations.

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Tectonic models for accretion of the Central Asian Orogenic Belt

Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment-hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer.

The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems.

Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace.

Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization.

Porphyry Copper Systems

Orogenic gold deposits : A proposed classification in the context of their crustal distribution and relationship to other gold deposit types

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Global continental and ocean basin reconstructions since 200 Ma

Orogenic gold and geologic time: a global synthesis

\n Recent exploration has led to definition of a Middle–Late Jurassic copper belt with an extent of ∼2000 km along the southeast China coast. The 171–153 Ma magmatic-hydrothermal copper systems consist of porphyry, skarn, and vein-style deposits. These systems developed along several northeast-trending transpressive fault zones formed at the margins of Jurassic volcanic basins, although the world-class 171 Ma Dexing porphyry copper system was controlled by a major reactivated Neoproterozoic suture zone in the South China block. The southeast China coastal porphyry belt is parallel to the northeast-trending, temporally overlapping, 165–150 Ma tin-tungsten province, which developed in the Nanling region in a back-arc transtensional setting several hundred kilometers inboard. A new geodynamic-metallogenic model linking the two parallel belts is proposed, which is similar to that characterizing the Cenozoic metallogenic evolution of the Central Andes.

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Recognition of a Middle–Late Jurassic arc-related porphyry copper belt along the southeast China coast: Geological characteristics and metallogenic implications

Tectonics and metallogeny of gold deposits in China

Thermochronologic constraints on evolution of the Linglong Metamorphic Core Complex and implications for gold mineralization: a case study from the Xiadian gold deposit, Jiaodong Peninsula, eastern China

The Tombstone, Mayo and Tungsten plutonic suites of granitic intrusions, collectively termed the Tombstone-Tungsten Belt, form three geographically, mineralogically, geochemically and metallogenically distinct plutonic suites. The granites (sensu lato) intruded the ancient North American continental margin of the northern Canadian Cordillera as part of a single magmatic episode in the mid-Cretaceous (96–90 Ma). The Tombstone Suite is alkalic, variably fractionated, slightly oxidised, contains magnetite and titanite, and has primary, but no xenocrystic, zircon. The Mayo Suite is sub-alkalic, metaluminous to weakly peraluminous, fractionated, but with early felsic and late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten Suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The Tombstone Suite was derived from an enriched, previously depleted lithospheric mantle, the Tungsten Suite is from the continental crust including, but not dominated by, carbonaceous pelitic rocks, and the Mayo Suite is from a similar sedimentary crustal source, but is mixed with a distinct mafic component from an enriched mantle source.Each suite has a distinctive metallogeny that is related to the source and redox characteristics of the magma. The Tombstone Suite has a Au-Cu-Bi association that is characteristic of most oxidised and alkalic magmas, but also has associated, and enigmatic, U-Th-F mineralisation. The reduced Tungsten Suite intrusions are characterised by world-class tungsten skarn deposits with less significant Cu, Zn, Sn and Mo anomalies. The Mayo Suite intrusions are characteristically gold-enriched, with associated As, Bi, Te and W associations. All suites also have associated, but distal and lower temperature Ag-Pb- and Sb-rich mineral occurrences. Although processes such as fractionation, volatile enrichment and phase separation are ultimately required to produce economic concentrations of ore elements from crystallising magmas, the nature of the source materials and their redox state play an important role in determining which elements are effectively concentrated by magmatic processes

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0263593300001115

Richard J. Goldfarb

Papers

Recognition of a Middle–Late Jurassic arc-related porphyry copper belt along the southeast China coast: Geological characteristics and metallogenic implications

Tectonics and metallogeny of gold deposits in China

Thermochronologic constraints on evolution of the Linglong Metamorphic Core Complex and implications for gold mineralization: a case study from the Xiadian gold deposit, Jiaodong Peninsula, eastern China

Source and redox controls on metallogenic variations in intrusion-related ore systems, Tombstone-Tungsten Belt, Yukon Territory, Canada

Geology, Exploration, and Discovery in the Tintina Gold Province, Alaska and Yukon