Geological and geochemical characteristics of the Wulong gold deposit, Liaodong Peninsula: Implications for gold mineralization

Abstract

As a typical hydrothermal vein-type gold deposit in the eastern Liaodong Peninsula, northeast of North China Craton, the Wulong gold deposit is associated with the Jurassic gneissic two-mica granites with gold reserves over 80 t. In this paper, the gold precipitation and enrichment mechanism of the Wulong deposit is investigated by studying the morphological structure and geochemical characteristics of pyrite. Pyrites are divided into four types based on their microstructural characteristics: Py1 is represented by disseminated pyrite, which was developed in granites; Py2a is dominated by porous pyrite, which developed in the early quartz-pyrite vein; Py2b commonly wraps Py2a, distinguished from Py2a by its “smooth” surface, and developed in the quartz-polymetallic sulfide stage; Py3 was observed in quartz-calcite-fluorite stage. X-Ray Fluorescence Spectrometer (XRF) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) analysis revealed the enrichment of trace elements in pyrite at different stages: 1) In an overall view, the strong positive correlation among chalcophile elements was detected; 2) The total amount of trace elements in Py1 and Py3 was significantly lower than that in Py2a and Py2b; 3) In Py2a, the content of chalcophile elements represented by Ag, Te, Bi and Pb is higher than that of other types of pyrite, and gold mainly coexists with those elements in the form of nanoscale micro-inclusions (Au0); 4) The lattice gold (Au+) was positively correlated with As in Py2b. In contrast to the extremely low invisible gold content (Py1: 0.94 ppm; Py2a: 2.23 ppm; Py2b: 4.45 ppm; Py3: 0.28 ppm) in pyrite, there is a large amount of visible gold in the deposit in the form of scattered natural gold and electrum particles (silver content 4.38–36.01%). In addition, these visible gold is often associated with mineral aggregates consisting of galena, native bismuth, joséite-B, etc. Combined with the research results of previous studies, we proposed that visible gold is the main source of gold in the Wulong gold deposit. It is concluded that the occurrence of a large amount of visible gold in this deposit is closely related to the thinning and destruction event of the North China Craton during the Mesozoic. Different from the deposition characteristic of gold in gold-rich pyrite, in the high arsenic (>2000 ppm) area of pyrite, only very low gold content less than 5 ppm was detected. Meanwhile, the complex dyke intrusion and cutting phenomena observed in the mining area indicate the multi-stage tectonic activities in the metallogenic stage. Therefore, a reasonable explanation is that the pressure of the ore-forming fluid decreased sharply during tectonic activity, which promoted the separation of fluid phases and resulted in the massive precipitation of natural gold and chalcophile elements. This paper is of great significance to the gold prospecting in the Liaodong Peninsula, and also provides an important reference for the comparative study of metallogenic mechanisms in the Liaodong Peninsula and Jiaodong Peninsula.