Xiaosigou copper-molybdenum deposit, Pingquan County, Hebei Province

The deposit is located on the side of the rift zone at the junction of the uplift zone on the northern margin of North China shelf and Liaoyan Mesoproterozoic rift zone. Liugou-Pingquan-Yang Shuling fault passes through the north side of the mining area and becomes the dividing line between Chengde uplift and Heping Spring-Xinglong syncline. The deposit is located in the northeast section of syncline.

Second, the mining area geology

(1) stratum

The exposed strata in the mining area are relatively simple. To the north of Liugou-Pingquan-Yang Shuling fault is Archaean Qianxi Group Paomachang Formation, and to the south is Yuanguyu Jixian Wumishan Formation, with a small amount of early Cretaceous volcanic rocks and Quaternary loose deposits (Figure 2-86). Paomachang Formation consists of amphibolite plagioclase gneiss, amphibolite and granulite. Wumishan Formation mainly exposes dolomite limestone and calcareous dolomite in its upper part, which is in fault contact with Qianxi Group. Dolomitic limestone is influenced by intrusive rocks, but marble, serpentine and ore bodies exist in altered marble and dolomite. The Upper Jurassic is mainly volcanic breccia, breccia tuff and rhyolite, which are scattered in the mining area.

Figure 2-86 Geological Map of Xiaosigou Copper-molybdenum Deposit

1- four yuan; 2- Upper Jurassic volcanic rocks; 3- dolomite of Wumishan Formation in Jixian system; 4- metamorphic rocks of Qianxi Group; 5- marble; 6- serpentine marble; 7-breaking rocks; 8- granodiorite porphyry (stage I, stage I, stage I); 9- sericitized rocks; 10-gabbro diorite; 11-diorite porphyrite; 12- adamellite porphyry; 13- granite porphyry; 14 —— Inferring and measuring geological boundaries; 15- lithofacies boundary; 16- normal fault and reverse fault; 17- event

(2) Structure

Faults are developed and folds are not obvious in the mining area. Faults are mainly divided into three groups: east-west, northwest and northeast. The east-west fault (F 1) runs across the northern part of the mining area, which is a part of regional deep fault and a thrust fault, making the Archaean metamorphic rock series in the northern part thrust on the dolomite of the Mesoproterozoic Wumishan Formation. F 1 fault is the oldest and largest fault in this area, and it was revived in Mesozoic. Regionally, it controls the distribution of Middle and Late Proterozoic and Paleozoic, and also controls the northwest boundary of Xiaosigou rock mass. NEE fault (F2) is mainly a translational normal fault, which intercepts the east-west fault. The NW direction is the most developed group of faults in the area, which controls the boundary, alteration zone, ore body distribution, dike and volcanic rock distribution of Xiaosigou rock mass. It is mainly divided into northeast boundary fault and southwest boundary fault of rock mass. The northern section of the northeast boundary fault controls the northeast boundary of Xiaosigou rock mass, and the southwest boundary fault is often filled with breccia, granite porphyry or rhyolite veins along the fault.

(3) intrusive rocks

Magmatic rocks are developed in the mining area. Most of them occur in the form of porphyry and dikes, followed by volcanic lava and pyroclastic rocks. It can be divided into three stages: gabbro in the early stage, Xiaosigou granodiorite porphyry in the middle stage, and acid volcanic lava and pyroclastic rocks in the late stage. The magma composition in the area is mainly intermediate acid, and there is an evolutionary trend from neutral to intermediate acid and then to acid from morning till night. SiO2 _ 2 and K _ 2O in petrochemical products also tend to increase from morning till night. Mineralization of rock mass is common. Xiaosigou granodiorite porphyry is the main ore-bearing rock mass in the mining area, which is rich in copper and molybdenum. Rock mass is mainly distributed in Tongdongzigou, Shangzhangzi and Xiazhangzi areas. The K-Ar age of the rock mass is 129Ma. The rock mass intrudes into Wumishan Formation of Jixian system along the NW-trending fault, and it is an irregular oval small rock, with its long axis in the NW direction, partially overlapping with dolomite, with an exposed area of about 7km2. There are many xenoliths around the edge of the rock mass, ranging from several meters to tens of meters to tens of centimeters. They are composed of dolomite or dolomite limestone, which have been recrystallized and arranged along the contact zone of the rock mass, and the long axis direction is consistent with the long axis of the rock mass.

Xiaosigou rock mass can be divided into three lithofacies zones from the center to the edge, and the relationship between the lithofacies zones is gradually transitional. Phase ⅱ is the most extensive exposed zone in the mining area, which is closely related to copper and molybdenum mineralization. The main rock-forming mineral components and phenocrysts in each facies belt are plagioclase, and its brand gradually increases from the center to the outside, but they all belong to acid plagioclase. The response time gradually decreases from the inside out, while amphibole is the opposite. The average content of silica is 66.35%, K2O is 3.55%, Na2O is 4. 15%, and Na2O > K2O；; ; It is moderately acidic and belongs to the calc-alkaline series. According to the pyrometer and the oxygen isotope fractionation equation of Yingshi-magnetite, the formation temperature of Xiaosigou rock mass is calculated to be 630 ~ 670℃, and the fugacity of water during rock mass crystallization is calculated to be FH2O = (1300 ~1955) ×105 Pa according to the composition of biotite.

According to W (Na2O) > W (K2O), W (Al2O3)/W (Na2O+K2O+Cao) <1.1in Xiaosigou rock mass, the mineral assemblage is intermediate acid plagioclase+magnesium-rich biotite+hornblende+sphene+magnetite; Magnetite content is 0.5% ~ 1.2%, excluding lead-rich minerals and ilmenite. A combination of high-content trace elements vanadium, chromium, cobalt and nickel; The biotite is above the Ni-NiO buffer line; Whole rock oxygen isotope δ 18O = 7 ‰ ~ 8.2 ‰, indicating that Xiaosigou pluton is formed by syntectic granite magma in the lower crust.

Volcanic rocks in the mining area are mainly distributed in Longtangou, Xigou and Xiazhangzi on the southeast edge of Xiaosigou rock mass, with pyroclastic rocks and a small amount of lava. The whole rock age of rhyolite measured by K-Ar method is 1 12Ma. Veins are widely distributed in this area, which are the last products of the late magmatic period and are mainly moderately acidic. Granite porphyry is distributed in rock mass and occurs in dense veins, which strike northwest, followed by volcanic breccia and dolomite. Diorite porphyrite dikes are distributed in the edge of rock mass and surrounding rock, mostly in northeast and near east-west direction. Adamellite porphyry is distributed in rock mass, surrounding rock and volcanic rock. Vein rocks contain no ore and are the product of mineralization.

Three. Copper mine geology

(a) the size, shape and spatial distribution of ore bodies

Xiaosigou deposit is a nonferrous metal deposit with copper and molybdenum as the main components, accompanied by lead-zinc ore, iron ore and associated elements such as gold, silver, sulfur, rare earth and selenium.

Copper ore bodies: copper mineralization is mainly distributed in serpentine altered rocks and serpentine dolomite of mafic skarn in the outer contact zone of Xiaosigou rock, followed by garnet skarn of calc-skarn. The former has strong mineralization and large scope, which constitutes the main copper ore body; The distribution of the latter is limited, and copper mostly exists in the form of associated elements. Copper ore bodies are mainly distributed from Xiazhangzi to Tongdongzigou. Ore bodies are generally 300 ~ 500m long, with a maximum of 1000m, an average thickness of 40m and a buried depth of 250~350m, with a maximum of 570m. The shape of ore body is layered and banded, which is consistent with the occurrence of contact zone. The general trend is NW3 15, with a dip of SW and a dip angle of 60 ~ 80. Copper ore bodies composed of xenoliths in the contact zone vary in size, generally 200 ~ 300m long,1~10m thick and 40m thick. Lenticular, vein-shaped and lentil-shaped, generally trending northwest with an inclination of 30 ~ 60 (Figure 2-87).

Molybdenum ore body: Molybdenum mineralization is mainly distributed in pyrite sericite in alteration zone of rock mass, followed by garnet skarn in calc-skarn. Molybdenum mineralization is intense and extensive. Part of molybdenum mineralization occurred at the edge of the inner contact zone and met with copper ore bodies to form molybdenum copper ore. Molybdenum ore bodies are distributed in the area from Shangzhangzi to Nangounao. The ore body is 300 ~ 500m long, 50 ~100m thick and 300 ~ 500m long. They are thick lenses with sharp contraction and thinning at both ends and branches.

In addition, there are small molybdenum ore bodies with great changes in shape and scale, mostly in the form of veins and lentils.

Lead-zinc ore body: Lead-zinc mineralization or lead mineralization is mainly distributed in weakly altered (qingpanlithized) crystalline dolomite far away from Xiaosigou rock body, which is obviously controlled by rock fractures and forms vein-like ore bodies. High grade, but small scale.

Iron ore bodies: mainly distributed in metamorphic skarns, with small scale and discontinuous ore bodies. Late copper mineralization is superimposed in local mineralized areas, forming magnetite-bearing copper ore bodies.

The above mineralization and ore body distribution are controlled by the contact zone and alteration zone of rock mass, and an annular mineralization halo is formed around Xiaosigou rock mass. From the contact zone in the rock mass to the outside, it is molybdenum mineralization → molybdenum copper mineralization → iron copper mineralization → copper mineralization → lead-zinc mineralization → lead mineralization.

(2) Mineral composition

There are three types of copper ore, namely copper ore, iron copper ore and molybdenum copper ore.

Copper ore is mainly composed of pyrite and chalcopyrite, and associated minerals are pyrrhotite, sphalerite, galena and magnetite. Gangue minerals include serpentine, dolomite, calcite, diopside, phlogopite and sillimanite. This ore accounts for more than 90% of copper reserves.

Iron copper ore is mainly composed of pyrite and chalcopyrite. Gangue minerals include diopside, magnesia and dolomite.

Molybdenum-copper ore is mainly composed of molybdenite and chalcopyrite, and the molybdenum content is 0.0 1% ~ 0.08%. This kind of ore has the same characteristics as copper ore except molybdenum.

(3) Ore texture and structure

Ore texture: mainly irregular granular and metasomatic texture, with occasional melting-off texture and broken texture.

Ore structure: disseminated, veined and massive structure. Copper ore is often veinlet disseminated structure.

(4) Wall rock alteration

The alteration of surrounding rocks in the mining area is mainly K-feldspar (K-feldspar, biotite), chrysotile sericitization, argillization and skarnization (including serpentine). Altered rocks are regularly distributed in bands in space. From the rock mass to the outside, it is granodiorite porphyry-potash feldspar, biotite alteration zone-pyrite, Yingshi, sericite alteration zone-(clay rock zone)-calcium skarn zone-magnesium skarn, metamorphic magnesium skarn zone-serpentine dolomite zone. Among them, the main ore-bearing altered rocks are molybdenite sericite, garnet calcium skarn, copper-bearing magnesium skarn and serpentine dolomite.

The formation of altered rocks in the deposit has the characteristics of multi-stage and multi-stage According to the mineral assemblage, rock texture and structural characteristics of altered rocks and the temperature measurement of altered mineral inclusions, the ore-forming stages are divided into: early alkaline stage, formation of potassium altered rocks and skarn, and the formation temperature is above 400℃; In the acid leaching stage, carbonate-sulfide combination veinlets are formed at 350 ~ 280℃.

Figure 2-87 Geological Profile of No.9 Exploration Line of Xiaosigou Copper-Molybdenum Mine

1-serpentine marble; 2- granodiorite porphyry; 3- granite porphyry; 4- copper ore body

Four. minerogenetic condition

(1) sulfur isotope

The δ34S value of sulfur isotope data of metal sulfide is 65438 0.8 ‰ ~ 3.3 ‰. The sulfur isotope characteristics and macroscopic mineralization characteristics of Xiaosigou pluton indicate that the ore-forming materials come from the lower crust.

(2) Fluid inclusion

Yingshi and calcite in the alteration zone of Xiaosigou rock mass contain a large number of fluid inclusions, and the gas content in these inclusions is high, accounting for about 45%, and coexist with liquid and solid phases. According to the occurrence and content statistics of neutron minerals in inclusions, the occurrence area of high salinity inclusions is consistent with that of copper mineralization. Salt minerals in inclusions are mainly NaCl and KCl. It is estimated that the total salinity of the ore-bearing solution is 45.4%. The results of temperature measurement of a large number of inclusions by homogenization method show that the temperature is concentrated in 320 ~ 360℃, which reflects that the hot liquid activity is the strongest in this temperature range, and it is also the formation temperature range of molybdenite. The homogeneous temperature of chalcopyrite-pyrite-isochron pulse is 280 ~ 320℃, which represents the main metallogenic temperature of copper. The water pressure at this salinity and temperature is (300 ~ 500) × 105 Pa.

(III) Metallogenic stage

Mineralization has experienced contact metasomatism, sulfide and serpentine carbonate formation, which can be divided into the following stages:

Magnesium skarn formation stage: diopside, Labrador, tremolite, olivine and spinel are mainly formed;

The formation stage of calc-skarn: garnet and pumice are mainly formed until part of diopside is formed at this stage;

The formation stage of water-bearing skarn: the formation of phlogopite and serpentine is dominant, followed by the formation of epidote, pyrite, magnetite and some carbonate;

Yanshi-sulfide formation stage: Yanshi, molybdenite, pyrite, chalcopyrite and sericite are mainly formed, and some carbonate minerals are also formed at this stage;

Serpentine-quartz-carbonate formation stage: pyrite and carbonate minerals are mainly formed, followed by chlorite.

Generally speaking, the acidic evolution of rock alteration in this area can be from early alkaline stage (skarn stage) to intermediate acidic stage (late hydrated skarn stage) and then to late alkaline stage (serpentine-carbonate stage). However, the copper-molybdenum ore was formed in the late stage of acidity, that is, in the process of transformation to alkalinity, which further shows that copper-molybdenum mineralization is closely related to the change of acidity. The later hydration stage is a prerequisite for the development of mineralization intensity.

In a word, the good differentiation of Xiaosigou granodiorite porphyry makes the fluid alkalinity, silica and minerals well differentiated. In the process of magma crystallization, with the boiling of saturated water, volatile components and ore-forming materials gather. At the same time, potassium metasomatism makes the minerals dispersed in rock-forming minerals move out. When the surrounding rock was leached by strong acid, it entered the contact metasomatism stage, and the magma mixed with Tianshui, which changed the acidity of the solution. With the increase of rock porosity, ore-forming elements gather and precipitate, forming copper-molybdenum deposits. The genesis of Xiaosigou deposit can be attributed to porphyry copper deposit and contact metasomatic copper deposit.

Five, prospecting indicators

Magmatic conditions: Xiaosigou molybdenum-bearing granodiorite porphyry is a compound rock mass, which can be divided into biotite granodiorite porphyry, granodiorite porphyry and potash feldspar quartz porphyry. Mineralization occurs in the granodiorite porphyry stage, which is the main sign of prospecting.

Surrounding rock conditions: the dolomite of Wumishan Formation in Jixian system belongs to the direct surrounding rock of mineralized body in space. In the process of mineralization, it is the basis of skarn formation and is directly related to mineralization. Skarn in the contact zone between granodiorite porphyry and dolomite in Wumishan Formation is an intuitive sign for prospecting.

Tectonic conditions: NW-trending fault structures not only control rocks, but also control ore and alteration range. The combination of fracture, rock mass and alteration is the condition of spatial positioning of ore bodies, which is of guiding significance for prospecting.

Geochemical anomalies: comprehensive anomalies of copper, molybdenum, gold, silver, nickel and cobalt appear on the geochemical scanning map of1∶ 200,000, indicating the prospecting potential.

The above signs are effective signs for finding similar deposits in this area.