Preliminary Study On Geological Characteristics And Genesis Of The Xinliaodong Cu Polymetallic Deposit In Eastern Guangdong Province, China

The Xinliaodong Cu polymetallic deposit is a newly-discovered copper ore deposit in eastern Guangdong Province.The study degree of the deposit is relatively low, and there has been no report on the interrelated research achievements. Based on detailed field investigation of the ore deposit, we got some conclusions about geological characteristics, petrogenic and metallogenic epoch, characteristics of metallogenic rock, ore-forming material source and ore genesis through element geochemistry, petrological geochemistry, U-Pb chronology, Hf isotope test and stable isotope study.The mineralization process of the Xinliaodong deposit can be divided into hydrothermal stage and supergene stage, and the hydrothermal stage can be further divided into arsenopyrite-pyrite-quartz stage, quartz-chalcopyrite-pyrite stage and quartz-calcite stage. The main metallogenic stage is the second phase, which formed quartz, chalcopyrite, pyrite, chlorite and so on.By means of electron microprobe analysis, the author investigated chemical composition of the main metal sulfides in the deposit and concluded that they have the following characteristics: ① Fe and S content in pyrite are lower than theoretical values, but atomic ratios of S/Fe are higher than theoretical values, besides, the ratios of Co/Ni are all higher than 1, suggesting a hydrothermal environment of relatively high sulfur fugacity; ② In sphalerite, the atomic ratios of S/Zn and the S content are higher than theoretical values while the Zn content is lower than theoretical values, furthermore, the iron-rich sphalerite show a relatively reducing hydrothermal environment and a relatively high formation temperature; ③ The cassiterite are relatively rich in Ta, suggesting they are formed in a weak alkaline environment; ④ Some elements in arsenopyriye, chalcopyrite and other sulfides occur in ismorphism.The zircon LA-ICP-MS dating yields concordant ages of 157±1Ma, 161±1Ma and 160±1Ma, which correspond to the petrogenic ages of the biotite granite, quartze-diorite and granite-porphyry respectively. The quartz diorite is considered as the ore-forming mother rock which is closely related to mineralization. The dating results of the granitoid intrusions indicate that there had happened a magma intrusion event in late Jurassic in this ore district.Geochemical data shows that the quartze-diorite is magnesium enriched in composition with moderate content of alkali. It is a metaluminous granite and belongs to the calc-alkaline series. Their REE and trace elements are enriched in LREE and large ion lithophile elements(K, Rb, Ba, Th and U), and depleted in HREE and high-field strength elements(Nb, Ta, Ti and P). The mineral chemistry characteristics of the biotites from the quartze-diorite show that the rock formed in the deep source environment and the rock-forming material source is the crust-mantle mixed source. The Hf isotope shows that the εHf(t) value of the quartze-diorite range from-5.8 to 2.7, with tDM2 ages between 1.03 Ga and 1.58 Ga. Zircon Hf isotopic compositions indicate that the parental magmas of the quartze-diorite are rather complex. The diagenetic mass of the pluton derived from the sourec of crust-mantle mixed and it may origin from partial melting of a subducted slab, which reacted with mantle wedge peridotites. Besides, the magma were intermingled with ancient crustal material during magmatic ascent. Accordiing to petrogeochemistry, mineral chemistry and Hf isotope, combining with the tectonic evolution of the eastern Guangdong Province and former achievements, we speculate that the quartze-diorite was generated in a active continental margin setting triggered by slab subduction.δ34S of the ore mineral ranges from-1.7‰ to 1.2‰, which closes to the mantle sulfur isotopic composition, suggesting that the ore-forming materials come from the deep magma system, maybe a few from the surrounding sedimentary rock.The Rb-Sr isotopic dating is carried out on pyrite and its intergrowth mineral assemblage samples(chalcopyrite) from Xinliaodong Cu polymetallic deposit. The results of Rb-Sr isotopic dating shows that the metallogenic epoch is about 150 Ma and the ore deposit formed in late Jurassic, which is in accord with the Mesozoic large-scale mineralization(150-170Ma) in South China. Study of the geodynamical setting indicates that the ore deposit was formed in the margin of activitive continental which is related to the subduction of Pacific plate during late Jurassic.The Xinliaodong Cu polymetallic deposit are consistent with the basic characteristics of typical porphyry copper deposit in geological characteristics, mineral typomorphism, sulfur isotope composition and metallogenic rock characteristics, so we think that the genetic type of Xinliaodong Cu polymetallic deposit is porphyry copper deposit according to the comprehensive analysis. Most of the ore bodies have vanished because of weathering and denudation, with the result that only the root of the ore deposit are left. So the prospecting potential of the deep of the ore district is limited.