Large Paleozoic and Mesozoic porphyry deposits in the Central Asian Orogenic Belt: geodynamic setting, magmatic sources, and genetic models

The Central Asian Orogenic Belt (CAOB) or Altaids contains several large porphyry Cu(-Au-Mo) and Au(-Cu) deposits. These deposits are genetically associated with equigranular and porphyritic tonalite, granodiorite and diorite. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that these hydrous, mineralized calc-alkaline intrusions were emplaced from the Cambrian to the Triassic.

Trace element geochemistry and Sr–Nd–Hf–O isotopic compositions show two different signatures of the mineralizing intrusions. Mineralizing intrusions at Bozshakol, Aktogai, Nurkazghan, and Tuwu-Yandong have depleted Sr–Nd–Hf–O isotopic compositions (Isr = 0.7033 to 0.7047, εNd(t) = +3.6 to +6.6, εHf(t) = +9.6 to +16.4, δ18O = 6.8 to 4.7‰) and variable high Nb/Ta (11.6 to 16.4) ratios, implying a dominantly juvenile lower crustal source with mantle involvement at Bozshakol and Aktogai and sedimentary rocks contamination at Nurkazghan. Mineralizing intrusions at Erdenet, Koksai and Yubileinoe have slightly depleted Sr–Nd–Hf isotopic compositions (Isr = 0.70403 to 0.7043, εNd(t) = +2.9 to +3.8, εHf(t) = +6.6 to +11.6) and low δ¹⁸O (3.1 to 2.5‰), slightly enriched Sr–Nd–Hf–O isotopic compositions (Isr = 0.7044 to 0.7049, εNd(t) = +0.04 to +1.9, εHf(t) = 3.2 to 12.0, δ¹⁸O = 5.2 to 6.4‰) and enriched Sr–Nd isotopic compositions (Isr = 0.7041 to 0.7081, εNd(t) = −2.6 to −2.9), respectively, as well as intermediate to low Nb/Ta ratios (13.4–8.1), implying a juvenile lower crustal source with wall rocks contamination at Erdenet and ancient crust contamination at Koksai and Yubileinoe. Therefore, Juvenile sources played a significant role in the generation of fertile magmas for porphyry-Cu and -Au style mineralization in the CAOB.

Mineralizing intrusions include the adakitic, transitional, and normal arc rocks. Mineralizing intrusions in some porphyry Cu(-Mo) deposits (e.g., Aktogai, Erdenet) have adakitic affinities based on their high Sr/Y (30–193) and La/Yb (17–49) ratios coupled with low Y (1.5–10.3 ppm) and Yb (0.2–1.0 ppm) concentrations. Variable high Sr/Y, Sm/Yb and La/Yb ratios suggest that the lower crustal source is probably a garnet-bearing amphibolite. These adakitic rocks were formed by partial melting of thickened lower crust at depths of N40 km. Mineralizing intrusions at most porphyry Cu(-Au) and Au deposits, including Bozshakol, Nurkazghan, Koksai, Tuwu-Yandong, and Yubileinoe, have variable intermediate Sr/Y (13–84) and La/Yb (7–23) ratios, intermediate to low Y (4.7–16.2 ppm) and Yb (0.5–1.3 ppm) concentrations, showing a transitional signature between adakitic and normal arc rocks. Variable moderate Sr/Y, Sm/Yb and La/Yb ratios suggest that the lower crustal source is probably a hydrous amphibole. These transitional rocks were formed by the MASH (melting, assimilation, storage, and homogenization) processes at depths of b40 km and AFC (assimilation and fractional crystallization) processes in a midcrustal magma chamber. One large porphyry Cu-Au deposit (e.g., Nurkazghan) is associated with intrusions that have calc-alkaline geochemical signatures, with enrichment of light rare earth elements (LREE), depletion of heavy REE, Ba, Nb, Sr and Ti and low Sr/Y, Sm/Yb and La/Yb ratios, suggesting that the lower crustal source could be a clinopyroxenite. They are formed by partial melting of lower crust at depths of 30–35 km with AFC important to magma evolution. Here, three genetical models are suggested for these mineralizing magmas in the CAOB: (1) thick (N 40 km) lower crust melting for adakitic magmas; (2) MASH and AFC for transitional magmas; and (3) thin (30–35 km) lower crust melting followed by AFC for normal arc magmas. A majority of transitional and normal arc magmas suggest that thinner crust was important to the formation of porphyry Cu(-Au) and Au deposits that dominate in the CAOB.

Our geochronology and geochemistry results indicate that most of the mineralizing intrusions from the porphyry deposits in the CAOB formed in island arc settings from the Cambrian to Triassic. The intrusions at Erdenet, Koksai and Yubileinoe are exceptions, forming in continental arcs in the Permian, Silurian and Devonian, respectively. Paleozoic and minor Mesozoic porphyry deposits in the CAOB formed during six periods. The greatest metal endowment is associated with the Devonian porphyry deposits (374 to 372 Ma, e.g. Oyu Tolgoi). The largest number of porphyry deposits were emplaced in the Carboniferous (334 to 317 Ma, e.g., Aktogai, Kounrad, TuwuYandong, Almalyk). The CAOB is a complex collage of Paleozoic and Mesozoic tectonic elements and has considerable potential for further discoveries of large porphyry Cu(-Au-Mo) and Au deposits.