The genesis of the orogenic gold deposits in the Liaodong Peninsula, North China craton

Abstract

The genesis of the orogenic gold deposits in the Liaodong Peninsula, North China Craton (NCC), is essential for gold exploration in this area and yet remains disputed. Though most researchers link gold mineralization with magmatism and/or metasomatized mantle beneath the NCC, this study here, instead, considers that metamorphic devolatilization of the Paleoproterozoic basement rocks could account for the formation of the Mesozoic orogenic gold deposits in the Liaodong Peninsula, NCC. The representative gold deposits in the Liaodong Peninsula, i.e., Baiyun, Wulong and Xinfang gold deposits, have structurally-controlled orebodies. A systematic study on geological profiles, temperatures, salinities, and O isotopes of the ore-forming fluids at the Wulong gold deposit shows that the lodes have no spatial variations over a 600 m vertical depth interval, which is different from magmatic hydrothermal systems. Fluid inclusion studies have shown that the Wulong and Xinfang gold deposits have middle temperature (200–350 °C), low-salinity (1–15 wt% NaCl eqv.), carbonic-aqueous ore-forming fluids, similar to those of orogenic gold deposits. A combined Fe-S isotopic study at the Baiyun gold deposit shows the sulfur was probably from the pyritic volcanic-sedimentary sequences of the Paleoproterozoic Liaohe Group, rather than from magmas; the calculated δ56Fe values of the ore-forming fluids (-0.78 – -0.37‰) could be modelled in a metamorphic devolatilization model with Fe-species (pyrite & magnetite) of the Paleoproterozoic Liaohe Group as sources. Li isotopic study was conducted at Baiyun to explore the behaviour of Li isotopes during the high-T metasomatism of metapelites. This study reveals that similar high-T metasomatism of metasedimentary rocks in mélange zones could result in low δ7Li sediment inputs. Mixing such low δ7Li sediment inputs with high δ7Li altered oceanic crusts could account for MORB-like Li isotopic signature of arc lavas. Sr isotopes at the Wulong gold deposit identify metabasalts from the Paleoproterozoic Liaohe Group as potential sources. Additional Li and its isotopic modelling further suggests that metamorphic devolatilization of metabasalts from the Paleoproterozoic Liaohe Group (with chain silicates Ch, phyllosilicates Ph, and quartz Qtz as endmembers; Ch, Li ≈ 5 ppm, δ7Li ≈ 8‰; Ph, Li ≈ 63 ppm, δ7Li ≈ -1‰; Qtz, Li ≈ 10 ppm, δ7Li ≈ 30‰) could account for the Li concentrations (5–41 ppm) and δ7Li value of the ore-forming fluids (21.4 ‰) of the Wulong gold deposit. The S isotopic characteristics of sulfides at the Xinfang gold deposit not only record a heterogeneous source including magmatic or gneissic sulfur but also record inter-mineral isotope fractionation. The initial 87Sr/86Sr values of pyrite (0.713480–0.729031) indicate a radiogenic crustal origin for the sources. The metamorphic devolatilization of the underlying basement resulted in the formation of the Xinfang gold deposit. This study integrates geological evidence with multiple isotopic systems to prove that the orogenic gold deposits in the Liaodong Peninsula were formed through metamorphic devolatilization of the Paleoproterozoic basement in the Mesozoic.