Geological Structure Study on Caijiaying Pb-Zn-Ag Deposit of Hebei Province.doc

Geological Structure Study on Caijiaying Pb-Zn-Ag Deposit of Hebei ProvinceZhang Yongxing1 Gu Shangyi2(1. 217 Geological Brigade, Shanxi Provincial Bureau of Geology and Mineral Exploration and Exploitation, Datong 037008, China; 2. School of Resources and Environment, Guizhou University of Technology, Gu Shangyi 550003, China)Abstract: Caijiaying PbZnAg Deposit is typically controlled by geological structure. The regional ore-guided structures are characterized by multi-stage movement. The early tensional process of the structures controlled the distribution of magmatic rocks in the area; the late compressive-shear process formed plume structures arranged in parallel beside the regional structures, which provided a proper space for location of thermal fluid and formation of Pb-Zn-Ag deposit. Consistence between isotopic model age of lead ore and intrusion age of quartz porphyry shows close relations among structure movements, magmatism and mineralization in Stage of Yanshan.Key words: Pb-Zn-Ag deposit; geological structure; Caijiaying.Caijiaying Pb-Zn-Ag deposit is a partly blind deposit, which was found in 1959. According to many years work, the results indicate that it is a large-scaled Zn-Ag deposit, a middle-scaled Pb-S deposit and a large scaled associated Au deposit. It is also a new breakthrough in our geologists prospecting work in the northern edge of North China Platform, and some new proposals were suggested as references for ore prospecting afterwards. By now, many researchers have made a large variety of studies on Caijiaying Pb-Zn-Ag deposit, including geochemical characteristics, mineral characteristics, occurrence status of precious metals in mineral, geophysical and geochemical prospecting model, etc., but there are no papers about structures of this district. The authors find out that structures of different stages in this district can control formation or destroy this deposit after its formation. 1. Geological SettingsCaijiaying Pb-Zn-Ag deposit lies on the eastern edge of the Caijiaying convex in the middle of Inner Mongolia Anticline, and it is a paleo-uplift region in the middle of the northern edge of the North China Platform. Although the exposed strata are simple in the district, the tectonic and magmatic activities are frequent and violent, which provide beneficial geological condition for the formation of Pb-Zn-Ag deposit. The exposed strata in the district mainly include Hongqiyingzi Group (Arhn) of Archaeozoic, Baiqi Formation(J3b) and Zhangjiakou Formation(J3zh) of the upper Jurassic of Mesozoic, and Quaternary (Q). The lithology of above-mentioned strata is as follows: Arhn is mainly fine garnet-bearing biotite granulitite sandwiched by a lot of medium-fined light-colored granoblastite and a little feldspar quartzite, and there also are lenticular or banded hornblende plagioclase granulitite and marble at the upper part, while in some local places graphite-bearing leuco granoblastite appears; Baiqi Formation (J3b) is divided into the upper and lower two sections, and the upper is andesite while the lower is rhyolitic welded tuff and tuffaceous sandy conglomerate; Zhangjiakou Formation (J3zh) is trachyte, quartz-trachyte and a little rhyolite and potassic rhyolite; Quaternary (Q) is mainly sandy soil, mild sandy soil, sand, gravel and clays. Moreover, there exists widely quartz-porphyry of Yanshan Stage in the district.The district can be divided into five ore zones, and the scale of the third one is the largest. Main country rocks are granulitite of Hongqiyingzi Group but Baiqi Formation is also a kind of country rock near the surface. There are two kinds of ore type: sulfide one and oxidized one. For the mineral components of sulfide ore, predominant minerals are marmatite, ferrian-sphalerite, sphalerite and pyrite while galena, arsenopyrite, chalcopyrite, hematite, pyrite and marcasite are less. But for the oxidized ore, hematite and limonite are dominated while malachite, marcasite and azurite are less. Au exists in form of natural gold and electrum, and Ag in form of natural silver, electrum, freibergite, argyrose, stephanite and pyrargyrite.The country rocks alteration is characterized by chloritization, sericitization, silicification, pyrophyllitization, carbonatization, and so on. By now, all studies show that Caijiayying Pb-Zn-Ag deposit belongs to the stratabound subvolcanic hydrothermal polymetallic one.2. StructureThe district is located between Kangbao-Weichang and Shangyi-Chicheng Deep Fractures, and located to the southeast side of Zhangbei-Luotuoshan Deep Fracture that strikes northeast. Fracture structures and basement structures are widely developed in the district. The formations of the basement nearly strike southeast, and in the east they are vertical, which are tightly folded; but in the west, their dip is slightly smaller than in the east, which are slightly folded and a series of small folds such as Dongshan Anticline, Xisan Anticline, Center Syncline are formed. The sedimentary cover formation are a set of monocline which strike nearly west-east.According to the temporal relation between fractures and mineralization, the fractures in the district are divided into three types: a. pre-mineral fractures, b. minerogenic fractures, including ore-guided fractures and ore-bearing fractures, and c. post-fractures, which destroyed ore body to some extent.2.1 Pre mineral FracturesThese fractures are located in the ridge of the hill of the middle part of the district, strike to SN, dip to E. and their dip angle wasnt proved conclusively. Because of the effect of the west-east tectonic stress field, all of the formations in the fracture zone are crushed to cataclastic rocks. The crushed rocks include hornblende plagioclase granulitite and biotite plagioclase granulitite. This structural zone is nearly one hundred meters in width and is strongly silicified, so this rock is very hard is projected topographically .2.2 Minerogenic Fractures2.2.1 Ore-guided fractures There are two ore-guided fractures in the district. One is F17, which is located in the north side of the district; another one is F45, which is located in the south side (Fig.1). These two fractures have been subjected to tensile stress followed by shear stress. The main characteristics of F45 are described as follows. F45 is a deeply regional fault, which strikes NEE-SWW, dips to SE with 6570 angle, about 85km in length. The fracture zone is about 20-50m in width in the district, which is mainly composed of faulted breccia. The composition of breccia are granulitite, trachyte, rhyolite and early quartz porphyry. The breccia grain is about 2-20 cm in diameter. The composition of cement of the breccia are mainly siliceous materials such as chert, also partly agate and jasper. In the tension fracture zone, there are compressive fractures, which are composed of fault clay, 3-10m wide and in the regular shape. It is indicated that it has experienced an early NW tension followed by a middle NW compression and a later dextral rotation progress.There are following evidence to prove F17 and F45 to be ore-guided fractures:(1) There is sparse limonite distributed in the fractured zone, in which the content of Pb, Zn and Ag is higher than in the wallrock but there exists few Pb-Zn-Ag deposit. It shows that although F45 is a passage of ore-forming material, no material concentrated to form deposit because of the over-free passage.(2) the contents of Pb, Zn and Ag tend to reduce from F45 to the two sides away from the fracture zone. This shows that ore-forming elements spread form F45 to the two sides away from F45.(3) There are a series of volcanic apparatus in line along F45 and F17 and a lot of ultra-hypabyssal magmatic rocks related to ore-formation such as quartz porphyry along the two sides of F45. This also indicates that the igneous rocks and ore-bodies in the district are controlled by fractures.(4) There exist Pb-Zn-Ag deposits in the plume structures of ore-guided fractures while U-Mo deposits exist around the volcanic apparatus beside ore-guided fractures. This indicates that the different parts of the fracture have controlled the formation of different ore deposits.(5) In the evolution of F45 and F17, especially the stage of shear process, two groups of shearing crannies or fracture cluster have formed beside the main fractures, which formed plumose fracture belt. One group strikes to NNE, the other NWW. The grouped plumose fracture provides favorable host spaces for the formation of five ore-zones.2.2.2 Ore-bearing fracturesOre-bearing fractures are located in the sides of the ore-guided fractures mentioned above, which are plumose fractures that have formed by clockwise shear movement of the F45 and F17. Distribution characteristics of these plume fractures are listed as follows:(1) The ore-bearing plumose fractures between F45 and F17 are very dense, which appears in clusters and belts. And Hongqiyingzi Group is also located in the district.(2) The dense plumose fractures are arranged in echelon shape. (3) The two groups of plumose fractures are conjugated with NWW direction group dominated. These fractures are formed by dextral compress-shear stress. The conjugate relationship in the plumose fracture is obvious in Mineralization Zone , and, because the fractures strike NNE in Mineralization Zone while NWW in Zone and . It is not strange that there are such phenomena as ore-body remification, union, expansion, contraction, pinch-out, recurrent because of the subsequent hydrothermal filling and metasomatisn in the host room resulted from conjugate presence, interweaving with one another and intermittent distribution of the plumose fractures.(4) The plumose fractures is the location where hydrothermal filled and metasomatism acted. Their appearance have been changed completely owing to long and multistage hydrothermal alteration. And the plumose fractures also have been filled by hydrothermal fluid to form ore body or alteration belt. The fracture traces were already blurred.2.3 Post-mineral FracturesThere are a lot of post-mineral, which play a destructive role for ore-bodies and are mainly distributed in the Mineralization Zone, , and . For example, there are many NWW fractures in the Zone which cut the zone into several sections with a ladderlike shape. The F47, which destroyed the Zone , will be introduced as follows: F47 is one of the main NNW fractures. It extends more than several kilometers and pass through the middle part of the district. The main fracture of F47 is a counterclockwise strike-slide reverse fault which deeply dips to NE. It cuts off strata, ore-bodies and quartz-porphyry in the Zone and some ore-guided fractures such as F45, F47, and others. The fault does not expose in the district, but about 70m wide fracture belt made up of fault breccia has been discovered in drilling hole ZK315-8. Branched fractures of F47 in the Zone are mainly F1, F2, F3 and F4. They extend 100-500m along the strike and over 300m along the dip. These faults dip to NE with a higher angle. As a result, the ore bodies are displaced to a distance ranged from 10m to25m.3. Relationship among mineralization, structures and magmatic rocksThe district is located in the middle part of Inner Mongolian Anticline of North China Platform. Since Mesozoic the effects of structural and magmatic activity of Platform activation have led to the corresponding activity in the anticline. This provided beneficial regional structural background for the formation of Pb-Zn-Ag deposit.The regional ore-guided fractures have rejuvenated for multi-stages and subjected to processes of first extension, then compression, and finally shear. So the fractural zone is deep and wide, and also are the typical belts of frequent structural and magmatic activity. The fractures steeply dip towards NNE. In the deep part, they connect to magmatic chamber, which provides good passages for magmas intrusion and flowage of mineralized thermal fluid. In the fracture belt consisted of F45 and F17, there are two superlarge-sized deposits, one medium-large-sized deposit, two medium-sized deposits and several dozens of small-sized deposits. Because of compression and shear, in the two sides of the fracture, especially its inner side, two groups of conjugate plumose fractures which strike NWW and NNE are arranged in the form of echelon, have been formed. So this has constructed a structural system of ore-controlling, ore-guided and ore-bearing structures. Finally, a series of ore-bodies and mineralized belts have been formed in the ore-bearing space of wall rock with multi-stage magmatic activity and hydrothermal elevation after magmatism. According to the analytical results for 12 galena samples in the district, the lead isotopic composition has values of 206Pb/204Pb ranging from 16.700 to 16.825, 207Pb/204Pb ranging from 15.304 to 15.510 and 208Pb/204Pb ranging from 37.267-37.970, which indicate the old lead characteristic with lower radioactivity. By the two stage model age calculation for Stacey and Kramers (1975), one model age value is 2500Ma, another one is 130Ma. The previous one indicates that Hongqiyingzi group of Archeaozoic is the source-bed of the Pb-Zn-Ag deposit in the district, so 2500Ma represents the age of the source-bed and 130Ma represents the age of the deposit formation. K-Ar isotopic age values for the quartz-pophyry is from 102 Ma to 134Ma, which is consistent with the age of deposit formation. It suggests the close relations of mineralization with structure of YanShan stage and magmatism. Fig.2 shows this kind of evolutionary relation.4. ConclusionCaijiaying large-scaled Pb-Zn-Ag deposit is controlled by strata, structure and magmatism. Hongqiyingzi group of Archeaozoic is the source-bed of Pb-Zn-Ag deposit. The magmatism caused by structural activity provides the district a part of the ore-forming material as well as the ore-forming thermal fluid and heat energy. The structural fractures not only led to magmatic intrusion but also controlled the distribution of magmatic intrusion and magmatic rock. The fractural structures also provided good passages for the ore-forming thermal fluid and formed the parallel arranged plumose fractures in the side, and then provided good ore-bearing space for the location of thermal fluid and the formation of ore-bodies.Table 1 Lead isotope composition of galena in Caijiaying Pb-Zn-Ag deposit districtsample No.simple site206Pb/204207Pb/204208Pb/2044Pb953Zone 16.7015.3337.354Pb954Zone 16.7115.3137.274Pb955Zone 16.7115.3637.334Pb956Zone 16.8115.5137.69D(181)12Zone 16.80815.41937.5714Pb964Zone 16.71815.30437.97GT(83)8Zone 16.82115.43437.634GT(83)13Zone 16.76015.38637.477GT(83)3Zone 16.80315.42337.263GT(83)14Zone 16.81815.40537.527GT(83)10Zone 16.79015.39737.556GT(83)98Zone 16.82515.41937.841Acknowlegdement: This paper is written on the basis of “Geological Exploration report on Zinc Ore of Zone of Caijiaying Lead-Zinc-Silver Ore District in Sanhao Township, Zhangb