Genesis of the Jiuqu gold deposit, Jiaodong gold province, China: Constraints on the texture, mineral geochemistry, and sulfur isotope of pyrite

Abstract

The Linglong gold ore field is situated in the northwestern region of the Jiaodong gold province, China, with over 1000 tons of gold resources. Although the metallogenic mechanism and fluid sources of the Linglong gold deposit have been the subject of extensive discussion and analyzed by numerous scholars, a definitive conclusion remains elusive. The Jiuqu Au deposit is a significant metallogenic area in the eastern part of the Linglong gold ore field, characterized as a quartz vein-type gold deposit. Gold mineralization in the Jiuqu gold deposit is classified into four stages, primarily occurring within the Late Jurassic Linglong granite and Early Cretaceous Guojialing granodiorite. Pyrite, the primary gold-bearing mineral at the Jiuqu Gold Mine, has been present throughout the metallogenic period, is rich in a variety of trace elements and is closely related to the formation of gold ore. This study focused on evaluating pyrite from the essential metallogenic stages (Stages I–III) of the Jiuqu gold deposit using scanning electron microscopy (SEM), electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) sulfur isotope analysis and detection. The work aimed to reconstruct the origin of metallogenic materials and the evolution of ore-forming fluids and to restrict the genesis of the deposit. Pyrite exhibits discernible alterations in both shape and crystal structure as it progresses through its ore-forming stages. The aforementioned stages can be delineated as follows: type 1 pyrite (PyI), developed in the initial ore stage, is characterized by coarse-to-medium-grained euhedral pyrite in Stage I; type 2 pyrite (PyII), developed in the second ore stage, appears during the middle stage of mineralization and is characterized by subhedral or anhedral pyrite; type 3 pyrite (PyIII), developed in the third ore stage, primarily occurred in the late-middle stages of mineralization. It is fragmented and coexists with various metal sulfides, including chalcopyrite, sphalerite and galena. The in situ major and elemental compositions of pyrites at Jiuqu indicate that Au, in the form of visible gold, exists in pyrites with low Au and Ag contents throughout all stages. PyI had lower Pb, Bi, Co and Ni contents than those of PyII. PyIII displayed decreased Pb, Bi, Co and Ni levels compared with PyII. The δ³⁴S values of the PyI, PyII and PyIII pyrite stages ranged from 7.19% to 8.71%, 6.24% to 7.68% and 7.66% to 8.07%, respectively. According to the structural and geochemical analysis of pyrite, the ore-forming fluid of the Jiuqu gold deposit was derived from a magmatic-hydrothermal formation created by enriched lithospheric mantle-derived magma, mixing S from Precambrian metamorphic rocks. Previous studies on the H–O isotopes of quartz in the region's ores have indicated the presence of primary magma water. Additionally, studies concerning the C-O isotopes of carbonate minerals in ores have suggested that C may have originated from the mantle. Fluid migration and water-rock interaction resulted in sulfide and gold precipitation. In the Late Meosozoic, lithospheric thinning of eastern North China Craton led to upwelling of asthenospheric mantle and partial melting of lithospheric mantle in the Jiaodong area. Under tectonic changes, magmatic-hydrothermal fluid migrated upward along fault structures to form a gold province.