Major types and Metallogenic model of Early Cretaceous Pb-Zn and associated metal deposits in the southern Great Xing’an Range, NE China

The southern Great Xing’an Range (SGXR) is one of the most important non-ferrous metal ore concentrating areas in China, and a large number of Pb-Zn and associated metal deposits have been found and mined in this area. The Early Cretaceous deposits of SGXR can be divided into three principal types according to their geological characteristics: skarn type deposits, porphyry type deposits and hydrothermal vein type deposits. In this contribution, we list some important Early Cretaceous deposits in the SGXR and summarize their geological characteristics. Research of stable isotope and fluid inclusion reveal that the sources and properties of ore-forming fluids varied between different types of mineral deposits, while the sources of ore-forming materials of different deposits are similar(characterized by deep-seated magmatic activities). We therefore conclude that the Early Cretaceous porphyry, skarn and hydrothermal vein type deposits in SGXR belong to a unified metallogenic series and developed a synthetical model for these deposits.


1.Introduction
As one of the important non-ferrous metal ore concentrating areas in northeastern China, the southern Great Xing'an Range (SGXR) has been listed as one of the 19 key belts that are often targets for mineral exploration in China [1][2][3][4] . Apart from the newly discovered deposits, the number of mineral resources from known deposits has also increased in recent years, making this belt a continuous hot area for ore deposit research in northern China.
During the past years, geologists have carried out a lot of work on zircon U-Pb, molybdenite Re-OS, cassiterite U-Pb, sphalerite Rb-Sr, and muscovite Ar-Ar dating to accurately constrain the mineralization ages of these deposits [5][6][7][8][9][10] . And the results reveal that Early Cretaceous deposits constitute the most important part of hydrothermal mineralization in SGXR. On the basis of their mineral assemblages, host rocks and major ore controlling factors, the major deposits of Early Cretaceous from SGXR can be divided into the following three types: (1) skarn type deposits, (2) porphyry type deposits, and (3) hydrothermal vein type deposits [11] . During the past years, we have successively carried out studies on geology, fluid inclusion, stable isotope, lithogeochemistry, and geochronology metallogenic prediction of the major Early Cretaceous deposit in the area [12][13][14][15] . In this paper, we reviewed the geology, fluid inclusion and isotope characteristics of the Early Cretaceous hydrothermal Pb-Zn and associated metal deposits, and the temporal, spatial and genetic relationships between different types of ore deposits.  [7,8,10,11] (zircon LA-ICP-MS U-Pb dating). Therefore, we can conclude that the Haobugao and Huanggang deposits are genetically related to Early Cretaceous magmatism. While the formation age of Baiyinnuoer deposit is still controversial between scholars till the present moment, and no accurate metallogenic age has been obtained yet.
The Huanggang and Haobugao deposit generally occurred along the contact between Mesozoic intrusions and Permian carbonate rocks. Magnetite, hematite, cassiterite, varlamoffite, sphalerite, scheelite, loellingite, chalcopyrite constitute the main ore minerals of the ores, which display a variety of textures. Gangue minerals of the deposit are dominated by skarn minerals such as garnet, pyroxene, amphibole, fluorite, calcite, quartz, epidote, chlorite and phlogopite. The ores of the deposit show mainly massive, banded, breccia, veinlike structures and disseminated structures. The ore-forming process of the two deposits both can be divided into the four stages based: (1) prograde skarn stage. Mineral assemblage of this stage is mainly composed of pyroxene, garnet, wollastonite; (2) retrograde alteration stage. This stage is characterized by occurrence of hydrous skarn mineral,including hornblende, actinolite, epidote, and chlorite; (3) quartz-sulfide stage; and (4) carbonate stage ( Fig. 1 3 mineralization that developed in the granitic rocks (Fig. 2), and show characteristics of alterationmineralization zoning. Ma and 135.0Ma, respectively [18][19] . It can be concluded that the formation of hydrothermal vein type deposits in SGXR also mainly concentrate in the Early Cretaceous. forming fluids of late stage are affected by materials of Permian strata. Fluid inclusion assemblages of the hydrothermal vein type deposits are quite different, which represent the complexities of metallogenic process and formation mechanism. Liquid-rich, aqueous-carbonic inclusions, vapor-rich inclusions, and CO2-bearing three-phase inclusions have been identified in main-ore stage of vein type Weilasituo and Bairendaba depostit, indicating that the main-ore stage fluids are characterized by a complex NaCl-H2O-CH4-CO2 system. Daughter minerals can also be recognized in fluid inclusions of Dajing deposit, and the fluid inclusions from Shuangjianshan deposit are characterized by simple liquid-rich two-phase inclusions. Stable isotope data from ore minerals and associated gangue minerals indicate that the initial ore fluids were dominated by magmatic waters, some of which had clearly exchanged oxygen with wall rocks during their passage through the strata. Sulfur isotope values reported from SGXR span a narrow range, generally ranging from -6 to + 5‰. The narrow range for the δ 34 S values presumably reflects the corresponding uniformity of the ore forming fluids, and these δ 34 S values have been interpreted to reflect a magmatic source for the sulfur. The comparation of lead isotope ratios between ores and different geological units also reveals that deeply seated magma has been a significant source of lead in the ores. Fig. 4. Metallogenic model and genetic-spatial relationships for skarn, porphyry and hydrothermal vein type deposits in SGXR The results of zircon U-Pb geochronology and major-, trace-and rare earth-element geochemistry reveal that the Early Cretaceous granitoids of SGXR show characteristics of A-type granites. And it is broadly accepted that this kind of granites are formed in an extensional within plate tectonic environment [10][11][12] .
Therefore, the formation process of the Early Cretaceous deposits from SGXR can be summarized as follows: during the Early Cretaceous, the A-type granites originated from the partial melting of the upper crust with some input of mantle material. It appears plausible that the lithospheric thinning not only resulted in emplacement of magmatic rocks and related Pb-Zn polymetallic mineralization, but also caused outward migration of mineralizing fluids in a regional thermal gradient. These magmas emplaced at a shallow level and finally formed porphyry-skarn-hydrothermal vein Pb-Zn and associated metal deposits. The spatial distribution of the deposits usually shows an obvious regularity, that is, from the internal to outer zone of the intrusion, porphyry→skarn→vein deposits distribute in order.

Conclusions
(1) Mineralization of skarn type deposits occur in the contact zone between Early Cretaceous granites and Permian formations. The main characteristics of these deposits are a wide variety of calc-silicate and associate minerals, which is dominated by garnet, pyroxene, tremolite and actinolite. (2) Porphyry type mineralization occurs as veinlets and disseminations in the phyllic and K-silicate alteration zones developed in Early Cretaceous granitic intrusions.
(3) The hydrothermal vein type deposits are mainly hosted in various terranes or different lithologies, and orebodies of this type of deposits are strictly controlled by faults. Alteration assemblages of this type of deposit are dominated by silicification, sericitization, chlorinification and carbonation.
(4) A close genetic relationship exist between porphyry, skarn and hydrothernmal vein type deposits, and these deposits belong to a unified metallogenic series.