Genesis Of The Laodou Gold Deposit, Xiahe-Hezuo Area, West Qinling Orogen:Constraints From The Geochemistry And Isotopic Geochronology

Gold veins of the Laodou gold deposit are typically controlled by the N-S striking directions fracture structure in Laodou quartz diorite porphyry, and generally dip-down with a stabilized mineralization in vein or lenticular. The ore types consist of altered quartz diorite porphyry, quartz-tourmaline veins, and quartz-stibnite veins, the later two overprinting the former. The altered quartz diorite porphyry is marked by intense sericitization and highest gold grade. The chemical analysis of ores indicates that all Au、As、Sb、Bi (Sn、Cu、Pb、Zn) enriched, but only Au attained sufficient ore grades. The common metallic mineral include native gold, pyrite, arsenopyrite, stibnite, chalcopyrite, galena, sphalerite, etc. Non-metallic minerals are mainly quartz, sericite, tourmaline, calcite, chlorite, kaolinite, and dickite, etc. On the basis of detailed field geological survey, the ore-forming process can be divided into hydrothermal replacement period and open-fracture filling period, the former include potassium, beresitization and early tourmalinization stage of quartz diorite porphyry, and the latter include quartz-tourmalinization, quartz-stibnite and last carbonatization stage.Scanning electron microscopy combnined with energy dispersive spectrometry (SEM-EDS) and electron microprobe analysis (EMPA) indicate that gold occurs both as visible and invisible gold. The visible gold is present as irregular grains between sericite and quartz minerals or as inclusions enclosed in quartz; The invisible gold may consist of nano-sized inclusion gold and solid solutions in pyrite and arsenopyrite.Two distinct occurrences of tourmaline have been recognized in Laodou gold deposit. The first type of tourmaline occurs as disseminated gains, nodular assemblages or veins in host rock. The second type of tourmaline mainly from hydrothermal veins host in altered quartz diorite porphyry is typically intergrown with arsenopyrite and other sulfide minerals. EMPA data shows that two types of tourmaline are belong to the alkaline subgroup of tourmaline. The tourmalines from quartz diorite porphyry transform from black tourmaline to dravite because of the isomorphism; while tourmalines from hydrothermal veins are belong to dravite. Field geological relations and the analysis of the zone grains shows that tourmalines host in the wall rock are formed earlier than tourmaline in hydrothermal vein host in quartz diorite porphyry, suggesting that ore-forming fluid of Laodou gold deposit transform from early Fe rich fluid to late magnesium rich fluid.H-O-S-B stable isotope data show that H-O datas of sericite in early stage are mostly plotted in the fields of magmatic-hydrothermal fluid, while H-O data of tourmaline in the diagram have a trend from magmatic-hydrothermal fluid to meteoric water. Sulfur isotope data(-5.9～5.1‰, with the mean value of-1.15‰) of pyrite and other sulfide minerals suggests a magmatic input to the hydrothermal system. Tourmaline of different types has the similar δ11B value between-0.89to-11.19‰which indicted from the magmatic source (δ11B:0～-30‰). Moreover,δ11B values have three obviously peaks from-7～-9‰,-3.5～-5.5‰to-0.5～-2.5‰, respectively which revels that the fluid composition of Laodou gold deposits has at least three significant change during the ore-forming processes.Laser ablation ICP-MS zircon U-Pb dating shows that the Dewulu quartz diorite and Laodou quartz diorite porphyry emplaced at238.6±1.5Ma and247.6±1.3Ma, respectively. The age datas indicate that the Dewulu intrusive complex eventually formed through early Laodou quartz diorite porphyry and late Dewulu quartz diorite intrusions, separated nearly10Ma. The40Ar/39Ar dating of hydrothermal sericites indicate that the gold mineralization of Laodou gold deposit occurred at～249Ma, absolutely consistent with the emplacement age of quartz diorite porphyry in the error range, and belongs to the early indosinian magmatic metallogenic events.Whole-rock geochemical and Sr-Nd isotopic compositions of Dewulu complex shows that the Dewulu intrusive complex has narrow SiO2range, with64.21～66.22wt.%for the quartz diorite,56.28～58.18wt.%for the MMEs therein and62.33～64.58wt.%for the quartz diorite porphyry. They have relatively high K contents with Na2O/K2O ratios close to or more than1. The quartz diorite and MMEs are metaluminous with A/CNK=0.77～0.99, whereas the quartz diorite porphyry is metaluminous to weak peraluminous with A/CNK=0.93～1.11. Rocks from the Dewulu complex are of medium-high potassium calc-alkaline series and have high Mg#values (61～70). Both the Dewulu and Laodou intrusive units share similar trace element and chondrite-normalized REE patterns with (La/Yb)N=6.88～16.55and Eu*=0.43～0.79. In addition, the quartz diorite, MMEs and quartz diorite porphyry have similar whole-rock initial Sr isotopic ratios0.7073～0.7078, εNd(t) values-6.7～-7.6. The geochemical and Sr-Nd isotopic compositions indicate that the Dewulu intrusive complex was generated by partial melting of K-rich meta-basaltic rocks in the lower crust, with addition of mantle-derived magmas. The MMEs show geochemical features akin to typical high-Mg#andesites, indicating that they were derived from partial melting of an enriched lithospheric mantle.Our study demonstrates that the Laodou gold deposit is a typically orogenic gold deposit controlled by fracture structure. This type of deposit was commonly found worldwide, and also widely development in Fengxian-Taibai and Lixian basins, West Qinling Orogenic Middle belt. Relationship with magmatic activity is an important question in the studying ore genesis of orogenic gold deposit. In this study, we ascertain that the Laodou quartz diorite porphyry is the parent of the Laodou gold deposit based on the mineral element geochemical, H-O-S-B stable isotope, and age-compared of the diagenetic and mineralization. namely, the ore-forming fluid and ore-forming metal source is closely related to the magmatic activity. Compared with the regional typically orogenic gold deposit and Calin-type gold deposit, the information of the sources of ore metals and fluids is more directly. Combined with the geochemistry study of the Dewulu complex and regional geological data, our results suggest that the Laodou gold deposit and the Dwulu intrusive complex formed in an active continental margin setting, possibly related to the northward subduction of the Paleo-Tethys oceanic crust during the early Triassic.The Xiahe-Hezuo metallogenic district is located in the northwestern part of the WQOB, the tectonic position across the Middle belt and the South belt of WQOB. The mineralization types is difference along the Xiahe-Hezuo faults as the dividing line, the Carlin type gold deposits mainly development in the southern belt (e. g., Zaozigou gold deposit), and the Orogenic gold deposits mainly development in the northern belt (e. g., Laodou gold deposit). Compared with gold mineralization between the southern belt and northern belt in host rock, ore-controlling structure, wall rock alteration, and main mineral assemblages, this article suggest that it’s the different mineralization types of the same tectonic-magmatic-hydrothermal system in different depth.