Northeastern China is composed of the eastern part of the Central Asian Orogenic Belt and the northeastern margin of the North China Craton. It underwent two major sets of orogenic events, including the pre-Mesozoic amalgamation of several micro-continents and the Mesozoic subduction of the Paleo-Pacific plate. It hosts numerous ore deposits of dominantly porphyry and skarn types, most of which have Mesozoic ages. In this study, both mineralization types were studied, aiming to improve the understanding of ore genesis, hydrothermal evolution, mineralization mechanism, regional metallogeny as well as the geodynamic setting.
Systematic zircon U-Pb and/or molybdenite Re-Os dating on five porphyry deposits (i.e., Aolunhua, Haisugou, Shabutai, Banlashan, and Yangchang) in the northern Xilamulun district indicates that the timing of the magmatism and the Mo mineralization is broadly coeval, mainly at 130-140 Ma. Major and trace element geochemistry reveals the intrusions hosting Mo-only deposits (e.g., Haisugou) have stronger crystal fractionation than intrusions hosting porphyry Cu and Cu-Mo deposits (e.g., Aolunhua), indicating that such a process may have played a role in selective enrichment of Mo. A comparison of zircon Ce/Nd ratios as a proxy for the oxidation state of magmas between mineralized and barren intrusions shows that the mineralized intrusions are associated with more oxidized magmas than the cospatial barren granites, and therefore it is proposed that higher oxygen fugacity may also be important to produce economic Mo mineralization. Whole rock Sr-Nd-Pb and zircon Hf isotopes show that these mineralized granites in Xilamulun are associated with magmas generated from three different source regions (i.e., remelting of old crust material, mixing of old crust material with depleted mantle component, and juvenile mantle-derived magmas). The variation in the origin of the magmas from which the porphyry Mo systems were generated suggests that the composition of magma sources is unlikely to have played a major role in the formation of Mo deposits.
The compilation of existing geochronological data on Mo deposits in NE China, including the newly obtained data from this study, shows that Mesozoic Mo deposits (~250 to 90 Ma) widely occur in this region and are linked to three tectonic-magmatic events: (1) Triassic Mo deposits (250–220 Ma) are mainly distributed along the east-west Xilamulun fault and are related to post-collisional crustal extension following the final closure of the Paleo-Asian ocean; (2) Jurassic to Early Cretaceous Mo mineralization (200–130 Ma) displays a clear younging trend from southeast to northwest and is interpreted to be related to the northwestward flat-slab subduction of the Paleo-Pacific plate beneath the Eurasian continent that started from Early Jurassic (ca. 200 Ma); (3) Cretaceous Mo mineralization (130–90 Ma) shows a distinctly reversed migration trend from northwest to southeast, and can be explained by the coastward migration of slab rollback related lower crust delamination, asthenospheric upwelling and lithospheric thinning.
For skarns, the Baiyinnuo'er Zn-Pb deposit was selected as a representative example for detailed study in this study. It is one of the largest Zn-Pb deposits in China, with 32.74 Mt resources averaging 5.44% Zn, 2.02% Pb and 31.36 g/t Ag. Several phases of igneous rocks, including Permian, Triassic and Early Cretaceous intrusions, are exposed in the mining areas, and among them the Early Cretaceous granites, which intruded into limestone of the early Permian Huanggangliang Formation, are interpreted to be the source of ore, since their Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.25–18.35, ²⁰⁷Pb/²⁰⁴Pb = 15.50–15.56 and ²⁰⁸Pb/²⁰⁴Pb = 38.14–38.32) are highly consistent with the sulfides including sphalerite, galena and chalcopyrite (²⁰⁶Pb/²⁰⁴Pb = 18.23–18.37, ²⁰⁷Pb/²⁰⁴Pb = 15.47–15.62 and ²⁰⁸Pb/²⁰⁴Pb = 37.93–38.44). Sulfur isotope values of the sulfides fall in a narrow δ³⁴S interval of -6.1 to -4.6‰ (mean = -5.4‰, n = 15), suggesting the ore-forming fluid is of magmatic origin.
The deposit formed in three stages: the pre-ore stage (prograde skarn minerals with minor magnetite), the syn-ore stage (sulfides and retrograde skarn minerals including calcite and minor quartz), and the post-ore stage (late veins composed of calcite, quartz, fluorite and chlorite; cutting the above mineral assemblages).The pre-ore stage fluids trapped in pyroxene have higher temperatures (471 ± 31 °C), higher salinity (43.0 ± 3.1 wt. % NaCl eq.), and higher concentrations of Zn (~1.1 wt. %), Pb (~1.7 wt. %), and other elements (e.g., Na, K, Li, As, Rb, Sr, Cs, Ba, Cl and Br) than syn-ore mineralizing fluids (<400 °C, <12 wt. % NaCl eq., ~0.05 wt. % Zn and ~0.03 wt. % Pb). The post-ore fluids are much cooler (<270 °C; averaging ~210°C), with much lower salinity (<5.1 wt. % NaCl eq.), Zn (~38 ppm) and Pb (~19 ppm). Geochemically, the fluids of all paragenetic stages in Baiyinnuo'er are characterized by magmatic signatures based on the element ratios, which are distinctively different from basin brines. The inclusion fluids in pre-ore stage show little variation in composition between ~520 °C and ~420 °C, indicative of a closed cooling system. In contrast, the major components of the syn- and post-ore stage fluids including Cl, Na and K decrease with the temperature dropping from ~350 to <200 °C, indicating a dilution by mixing with groundwater. The metal contents in pre-ore fluid are significantly higher than in syn-ore fluid, but no mineralization occurred. This confirms that the early fluid was, although enriched in metal elements, not responsible for ore precipitation, likely due to its high temperature high salinity nature. The metal deposition was mostly due to mixing with groundwater, which caused temperature decrease and dilution that significantly reduced the metal solubility, thereby promoting metal deposition. The deposition was probably accompanied and facilitated by carbonate dissolution that buffered the acidity generated during the breakdown of Zn (Pb)-Cl complexes and the formation of sulfides. Boiling occurred in both pre-ore and early part of the syn-ore stages, but no evidence indicates that it was related to metal deposition. The current Baiyinnuo'er massive skarns contain both prograde and retrograde minerals (including ore minerals). Paragenetically, they were not formed at the same time, but could be attributed to two (or more) successive pluses of hydrothermal fluids released episodically from residual melts of a progressively downward crystallizing magma. The prograde alteration increased the permeability and porosity, and created sufficient spaces, which was essential for later metal deposition.
Skarn deposits are one of the most common deposit types in China. The 386 skarns summarized in this review contain ~8.9 million tonnes (Mt) Sn (87% of China’s Sn resources), 6.6 Mt W (71%), 42 Mt Cu (32%), 81 Mt Zn-Pb (25%), 5.4 Mt Mo (17%), 1,871 tonnes (t) Au (11%), 42,212 t Ag (10%), and ~8,500 Mt Fe ore (~9%; major source of high-grade Fe ore). Some of the largest Sn, W, Mo, and Zn-Pb skarns are world-class.
The abundance of skarns in China is related to a unique tectonic evolution that resulted in extensive hydrous magmas and widespread belts of carbonate country rocks. The landmass of China is composed of multiple blocks, some with Archean basements, and oceanic terranes that have amalgamated and rifted apart several times. Subduction and collisional events generated abundant hydrous fertile magmas. The events include subduction along the Rodinian margins, closures of the Proto-Tethys, Paleo-Asian, Paleo-Tethys, and Neo-Tethys Oceans, and subduction of the Paleo-Pacific plate. Extensive carbonate platforms developed on the passive margins of the cratonic blocks during multiple periods from Neoarchean to Holocene also facilitated skarn formation.
There are 231 Ca skarns replacing limestone, 15 Ca skarns replacing igneous rocks, siliciclastic sedimentary rocks, or metamorphic silicate rocks, 113 Ca-Mg skarns replacing dolomitic limestone or interlayered dolomite and limestone, and 28 Mg skarns replacing dolomite in China. The Ca and Ca-Mg skarns host all types of metals, as do Mg skarns, except for major Cu and W mineralization. Boron mineralization only occurs in Mg skarns. The skarns typically include a high-temperature prograde stage, iron oxide-rich higher-temperature retrograde stage, sulfide-rich lower-temperature retrograde stage, and a latest barren carbonate stage. The zoning of garnet/pyroxene ratios depends on the redox state of both the causative magma and the wall rocks. In an oxidized magma-reduced wall-rock skarn system, such as is typical of Cu skarns in China, the garnet/pyroxene ratio decreases, and garnet color becomes lighter away from the intrusion. In a reduced intrusion-reduced wall-rock skarn system, such as a cassiterite- and sulfide-rich Sn skarn, the skarn is dominated by pyroxene with minor to no garnet. Manganese-rich skarn minerals may be abundant in distal skarns.
Metal associations and endowment are largely controlled by the magma redox state and degree of fractionation and, in general, can be grouped into four categories. Within each category there is spatial zonation. The first category of deposits is associated with reduced and highly fractionated magma. They comprise (1) greisen with Sn ± W in intrusions, grading outward to (2) Sn ± Cu ± Fe at the contact zone, and farther out to (3) Sn (distal) and Zn-Pb (more distal) in veins, mantos, and chimneys. The second category is associated with oxidized and poorly to moderately fractionated magma. Ores include minor porphyry-style Mo and/or porphyry-style Cu mineralization ± Cu skarns replacing xenoliths or roof pendants inside intrusions, zoned outward to major zones of Cu and/or Fe ± Au ± Mo mineralization at the contact with and in adjacent country rocks, and farther out to local Cu (distal) + Zn-Pb (more distal) in veins, mantos, and chimneys. Oxidized and highly fractionated magma is associated with porphyry Mo or greisen W inside an intrusion, outward to Mo and/or W ± Fe ± Cu skarns at the contact zone, and farther to Mo or W ± Cu in distal veins, mantos, and chimneys. The final category is associated with reduced and poorly to moderately fractionated magma. No major skarns of this type have been recognized in China, but outside China there are many examples of such intrusions related to Au-only skarns at the contact zone. Reduced Zn-Au skarns in China are inferred to be distal parts of such systems. Tungsten and Sn do not occur together as commonly as was previously thought.
The distal part of a skarn ore system may transition to carbonate replacement deposits. Distal stratabound mantos and crosscutting veins/chimneys may contain not only Zn-Pb but also major Sn, W, Cu, Mo, and Au mineralization. The Zn-Pb mineralization may be part of either an oxidized system (e.g., Cu, Mo, Fe) or a reduced system (e.g., Sn). In China, distal Zn-Pb is more commonly related to reduced magmas. Gold and W may also be related to both oxidized and reduced magmas, although in China they are more typically related to oxidized magma. There are numerous examples of distal mantos/chimneys that continuously transition to proximal skarns at intrusion-wall-rock contact zones, and this relationship strongly supports the magmatic affiliation of such deposits and suggests that distal skarns/carbonate replacement deposits systems should be explored to find more proximal mineralization. Carbonate xenoliths or roof pendants may host the majority of mineralization in some deposits. In contact zones, skarns are better developed where the intrusion shape is complicated. The above two skarn positions imply that there may be multiple skarn bodies below drill interceptions of intrusive rocks. Many of the largest skarns for all commodities in China are related to small or subsurface intrusions (except for Sn skarns), have multiple mineralization centers, are young (<~160 Ma), and have the full system from causative intrusion(s) to distal skarns or carbonate replacement extensions discovered.
Chinese skarn deposits fall in several age groups: ~830, ~480 to 420, ~383 to 371, ~324 to 314, ~263 to 210, ~200 to 83, ~80 to 72, and ~65 to 15 Ma. They are typically associated with convergent plate boundaries, mostly in subduction settings but also in collisional settings. Seven major skarn metallogenic belts are recognized based on skarn geographic location and geodynamic background. In subduction settings, skarns may form in a belt up to 4,000 km long and 1,000 km inland, with skarns continuously forming for up to 120 m.y., e.g., the eastern China belt. In most other belts, skarns form in 5- to 20-m.y. episodes similar to the situation in South America. In collisional settings, skarns may form up to 50 m.y. after an ocean closure, and the distance to the collisional/accretionary boundary may extend to ~150 km inland. The size of collision-related skarns may be as large as the largest skarns related to oceanic crust subduction. Older suture zones may be favorable sites for younger mineralization, for example, the Triassic Paleo-Tethys suture between the North and South China blocks for the younger and largest skarn cluster of the Middle-Lower Yangtze belt in the eastern China belt, and the Triassic sutures in southwestern China for Cretaceous to Tertiary mineralization.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
