Geochronology and metallogeny of the Hunjiang Basin, northeastern North China block, with a focus on the genesis of the White Mountain sedimentary rock-hosted gold deposit

Abstract

The Hunjiang basin is a sedimentary basin located along the northeastern margin of the North China block in southern Jilin Province, China. The basin is well-endowed in many types of mineral deposits, particularly magmatic-hydrothermal deposits, that have been exploited by over 100 past and producing mines. The largest deposit in the basin is the White Mountain gold deposit, which is hosted in sedimentary rocks and contains 1.86 Moz of gold. Despite the large mineral endowment of the basin, the tectonic and metallogenic history has been poorly constrained. A geochronological study of igneous rocks in the basin was conducted using laser ablation-inductively coupled plasma-mass spectrometry on magmatic zircons to obtain crystallization ages for a suite of igneous rocks. All of the rocks studied, with the exception of a Neoarchean granite dike, have Mesozoic crystallization ages between 180 ± 1 Ma and 90 ± 2 Ma. The rocks are felsic to intermediate in composition and of calc-alkaline affinity, and the granitic and porphyritic rocks show adakite-like geochemistry. It is suggested here that igneous activity in the Hunjiang basin resulted from partial melting of a delaminated eclogitic lower crust. The results of the study indicate that extensive lithospheric delamination beneath the eastern half of the North China block began at least in the Middle Jurassic in the Hunjiang basin area. In addition, a study of the geology of the White Mountain sedimentary rock-hosted gold deposit was undertaken. Mineralization at White Mountain occurs in silicified, polymict breccias along the contact between Proterozoic dolomite and overlying sandstone. Gold at White Mountain deposit predominantly occurs as native gold formed through supergene oxidation of gold-bearing marcasite and as electrum in late-stage barite. Fluid inclusions indicate that the mineralizing fluids were low temperature (<200°C) and of low salinity (1‒4 wt. % NaCl equiv). Alteration of siliciclastic rocks is dominated by kaolinite, indicating that the mineralizing fluids were of moderate acidities. Sulfur isotopic data of marcasite from the White Mountain deposit are consistent with a magmatic source of the sulfur. The White Mountain deposit is proposed to represent a distal sedimentary rock-hosted gold deposit that formed in genetic association with the Mesozoic intrusive activity within the Hunjiang basin. Field and laboratory investigations of the other deposits in the Hunjiang basin allowed the development of a regional metallogenic model. In addition to basement-hosted and stratabound deposits in sedimentary rocks, skarn occurrences are widespread in the basin. More distal to the intrusions, carbonate replacement base metal and distal sedimentary rock-hosted gold deposits occur. A regional metallogenic model is proposed in which these different magmatic-hydrothermal deposits are associated with postsubduction porphyries formed during extensional processes related to the lithospheric delamination beneath the eastern North China block. Emplacement of at least some of the intrusions occurred at a large depth (>4 km) compared to average porphyry deposits, as indicated by fluid inclusion studies. This depth of formation may have prohibited the formation of typical shallow epithermal deposits in the basin, instead forming skarns, carbonate replacement deposits, and sedimentary rock-hosted gold.