Chapter 6 Skarn Deposits of China

Abstract

Abstract 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.