Geology, geochronology and geochemistry of large Duobaoshan Cu–Mo–Au orefield in NE China: Magma genesis and regional tectonic implications

Duobaoshan is the largest porphyry-related Cu–Mo–Au orefield in northeastern (NE) Asia, and hosts a number of large-medium porphyry Cu (PCDs), epithermal Au and Fe–Cu skarn deposits. Formation ages of these deposits, from the oldest (Ordovician) to youngest (Jurassic), have spanned across over 300 Ma. No similar orefields of such size and geological complexity are found in NE Asia, which reflects its metallogenic uniqueness in forming and preserving porphyry-related deposits. In this study, we explore the actual number and timing of magmatic/ mineralization phases, their respective magma genesis, fertility, and regional tectonic connection, together with the preservation of PCDs. We present new data on the magmatic/mineralization ages (LA–ICP–MS zircon U–Pb, pyrite and molybdenite Re–Os dating), whole-rock geochemistry, and zircon trace element compositions on four representative deposits in the Duobaoshan orefield, i.e., Duobaoshan PCD, Tongshan PCD, Sankuanggou Fe–Cu skarn, and Zhengguang epithermal Au deposits, and compiled published ones from these and other mineral occurrences in the orefield.
In terms of geochronology, we have newly summarized seven magmatic phases in the orefield: (1) Middle–Late Cambrian (506–491 Ma), (2) Early and Middle Ordovician (485–471 Ma and ~462 Ma), (3) Late Ordovician (450–447 Ma), (4) Early Carboniferous and Late-Carboniferous to Early Permian (351–345 and 323–291 Ma), (5) Middle–Late Triassic (244–223 Ma), (6) Early–Middle and Late Jurassic (178–168 Ma and ~150 Ma), and (7) Early Cretaceous (~112 Ma). Three of these seven major magmatic phases were coeval with ore formation, including (1) Early Ordovician (485–473 Ma) porphyry-type Cu–Mo-(Au), (2) Early–Middle Triassic (246–229 Ma) porphyry-related epithermal Au-(Cu–Mo), and (3) Early Jurassic (177–173 Ma) Fe–Cu skarn mineralization. Some deposits in the orefield, notably Tongshan and Zhengguang, were likely formed by more than one mineralization events.
In terms of geochemistry, ore-causative granitoids in the orefield exhibit adakite-like or adakite-normal arc transitional signatures, but those forming the porphyry-/epithermal-type Cu–Mo–Au mineralization are largely confined to the former. The varying but high Sr/Y, Sm/Yb and La/Yb ratios suggest that the ore-forming magmas were mainly crustal sourced and formed at different depths (clinopyroxene-/amphibole-/garnet-stability fields). The adakite-like suites may have formed by partial melting of the thickened lower crust at 35–40 km (for the Early Ordovician arc) and >40 km (for the Middle–Late Triassic arc) depths. The Early Jurassic Fe–Cu skarn orecausative granitoids show an adakitic-normal arc transitional geochemical affinity. These granitoids were likely formed by partial melting of the juvenile lower crust (35–40 km depth), and subsequently modified by assimilation and fractional crystallization (AFC) processes.
In light of the geological, geochronological and geochemical information, we proposed the following tectonometallogenic model for the Duobaoshan orefield. The Ordovician Duobaoshan may have been in a continental arc setting during the subduction of the Paleo-Asian Ocean, and formed the porphyry-related deposits at Duobaoshan, Tongshan and Zhengguang. Subduction may have ceased in the latest Ordovician, and the regional tectonics passed into long subsidence and extension till the latest Carboniferous. This extensional tectonic regime and the Silurian terrestrial-shallow marine sedimentation had likely buried and preserved the Ordovician Duobaoshan magmatic-hydrothermal system. The south-dipping Mongol-Okhotsk Ocean subduction from north of the orefield had generated the Middle–Late Triassic continental arc magmatism and the associated Tongshan PCD and Zhengguang epithermal Au mineralization (which superimposed on the Ordovician PCD system). The Middle Jurassic closure of Mongol-Okhotsk Ocean in the northwestern Amuria block (Erguna terrane), and the accompanying Siberia-Amuria collision, may have placed the Paleo-Pacific subduction system in NE China (including the orefield) under compression, and formed the granodiorite-tonalite and Fe–Cu skarn deposits at Sankuanggou and Xiaoduobaoshan. From the Middle Jurassic, the consecutive accretion of Paleo-Pacific arc terranes (e.g., Sikhote-Alin and Nadanhada) onto the NE Asian continental margin may have gradually distant the Duobaoshan orefield from the subduction front, and consequently arc-type magmatism and the related mineralization faded. The minor Late Jurassic and Cretaceous unmineralized magmatism in the orefield may have triggered mainly by the far-field extension led by the post-collisional (Siberia-Amuria) gravitational collapse and/or Paleo-Pacific backarc-basin opening.