Yinan gold (copper) deposit, Shandong Province

Yinan gold (copper) deposit is located on the west side of Yishu fault zone, which is controlled by the Gegou fault and its secondary faults. Gold (copper) mineralization is related to hypabyssal and ultrahypabyssal magmatism in the late Yanshan period. Yinan gold mine consists of Jinchang and Tongjing ore fields.

The exposed strata in this area are gneiss of Taishan Group in Early Precambrian (Ar), marl and shale of Tumen Group in Late Precambrian (Z), carbonate and shale of Cambrian and Ordovician (O), and andesite in Cretaceous. Among them, CAMBRIAN is the most widely distributed in this area, and it is also the main ore-bearing stratum in this area. Its main lithology is limestone, followed by dolomite limestone and marl (Figure 20- 1).

The structure in this area is dominated by faults, and folds are not developed. Faults can be divided into three groups according to the direction (Figure 20- 1): ① NE group, mainly the surface grid fault and small faults parallel to it, the former is the largest fault in this area and runs through the whole area; ② There are two groups in NW direction, namely Majiawo-Tongjing fault and Mamuchi-Jinchang fault; (3) NE-trending strata are found in the Yeguan cemetery section of Jinchang and the tunnel section of Tongjingdui Jinshan.

Magmatic rocks are widely distributed in the late Yanshanian period in this area (Figure 20- 1). It is mainly composed of Ming Sheng complex, Tongjing complex, Jinchang complex, Yinshanzhuang complex, Chaoyang complex and Yanjiazhuang complex. Only Ming Sheng rock mass has basic lithofacies. Some rocks are also produced in the form of multiphase complex.

In addition, there are widely developed but small-scale intermediate-acid dikes and bedrock. Among them, the rocks related to mineralization are Jinchang complex, Tongjing complex and Ming Sheng complex.

1) Ming Sheng complex: Rock mass intrudes into CAMBRIAN and Ordovician carbonate rocks along the southeast end of Mamuchi-Jinchang fault in the form of rock strands. The complex is composed of perilla gabbro, gabbro and late intrusive biotite diorite. Perilla gabbro is the central phase and pyroxene diorite is the marginal phase. There is magnesium skarn magnetite in the contact between rock mass and dolomite limestone.

2) Tongjing Complex: Rock mass intrudes into Cambrian, Ordovician carbonate rocks and early Cretaceous andesite along the dome anticline core at the intersection of Qiangge-Gegou fault and Majiawo-Tongjing fault. The complex is composed of diorite and diorite porphyrite. Diorite is mainly distributed in the west of the complex (Figure 20-2). The main lithology is biotite diorite, followed by biotite diorite, hornblende diorite, quartz diorite and diorite. The diorite porphyrite is mainly distributed in the east of the rock mass (Figure 20-2), and the main lithology is hornblende porphyrite, diorite porphyrite and timely diorite porphyrite. Tongjing complex is one of the important ore-forming rocks in this area, and gold (copper) mineralization is mainly related to diorite porphyrite.

Figure 20- 1 Geological Schematic Diagram of Yinan Gold (Copper) Mine

3) Jinchang Complex: This complex consists of biotite granodiorite and amphibole monzoporphyry (Figure 20- 1), and intrudes into the dome core at the intersection of Mamuchi-Jinchang fault in northwest and Zaolinzhuang fault group in northeast. On the west is biotite granodiorite in Jinchang Village, and on the east is amphibole granite porphyry branch of Yeguan Tomb. There are radial adamellite dikes and amphibole adamellite dikes around the rock mass. This rock mass is one of the important ore-forming rocks in this area, and gold (copper) mineralization is mainly related to amphibole monzoporphyry.

2. Geological characteristics of the deposit

There are two genetic types of deposits in this area, namely skarn type gold (copper, iron) deposits and porphyry type gold (copper, molybdenum) deposits. The former is dominant, while the latter is only found in local areas of Tongjing ore field.

(1) skarn deposit

Skarn ore bodies occur in the contact zone between Jinchang complex, Tongjing complex and Ming Sheng complex and surrounding rocks of carbonate rocks, and their occurrence and shape are strictly controlled by the contact zone (Figure 20-3). It is layered, lenticular or lenticular, with a thickness of 1 ~ 2m and a length of about 50m. Some ore bodies are controlled by faults and are saccular. Metal minerals are distributed in skarn in the form of vein filling or veinlet impregnation. According to the types of useful elements, it can be divided into two kinds of ores:

1) skarn iron ore: It occurs in magnesium skarn in the contact zone of the northwest edge of Mingsheng complex, and the metal minerals mainly include magnetite, molybdenite, pyrite and chalcopyrite. Gangue minerals mainly include montmorillonite, mafic spinel, pumice, tremolite, phlogopite, chlorite and serpentine. The content of metal minerals is 20% ~ 80%, and the content of gangue minerals is 20% ~ 74%.

Figure 20-2 Geological Map of Tongjing Ore Field

2) Skarn-type gold (copper) ore: it occurs in calc-skarn in the contact zone between Tongjing complex and Jinchang complex. The metallic minerals in the ore include pyrite, chalcopyrite, bornite, chalcocite, natural gold, silver gold, tellurite gold and argentite. Gangue minerals mainly include garnet, pyroxene, epidote and chlorite. Natural gold mostly occurs in the crystal gaps or cracks of chalcopyrite in fine grains, veins or strips, and often coexists with argentite and stibnite and interpenetrates with each other. Silver-gold deposits are closely associated with chalcopyrite and bornite, and can also occur in the cracks of chalcopyrite or bornite in the form of microscopic slices. In a word, gold and copper are closely symbiotic.

(2) Porphyry deposit

Porphyry ore bodies mainly occur in diorite porphyry in the east of Tongjing complex. The ore bodies are lenticular, porphyritic or veined in alkaline metasomatic rocks and chalcopyrite sericite in the alteration center zone. Metal minerals are embedded in pyrite sericite in the form of veinlets or disseminated. The metal minerals in the ore mainly include pyrite, galena, chalcopyrite, magnetite, natural gold, molybdenite, bismuthite and argentite. Gangue minerals mainly include sericite, quartz, carbonate, chlorite, kaolinite, potash feldspar, plagioclase, amphibole and biotite. Natural gold is closely related to galena, and it is relatively rich in galena-intensive parts.

3. Ore-bearing metasomatic structure

There are five types of metasomatic formations in this area, namely magnesium skarn formation, calcium skarn formation, pyrite sericite formation and argillaceous rock formation. Each metasomatic formation contains specific metasomatic facies and corresponding metal mineral assemblage, which occur under specific surrounding rock conditions.

Figure 20-3 Geological Profile of Shanzijian Ore Section in Tongjing Ore Field (according to the Eighth Geological Team of Shandong Geological Bureau, 198 1)

(1) Mg skarn formation

Mg-skarn mainly occurs in the contact zone between pyroxene diorite and dolomite limestone in the northwest margin of Mingsheng complex. The mafic skarns in this area include magmatic mafic skarns and post-magmatic mafic skarns. Magmatic magnesium skarn is different from the characteristic mineral assemblage (pyroxene)-mafic spinel-montmorillonite assemblage of post-magmatic magnesium skarn, and can be transformed into serpentine metasomatic rocks. The main metasomatic facies of magnesium skarn after magmatic period are diopside, tremolite, phlogopite and serpentine. The associated metal mineral is magnetite.

(2) Calcareous skarn formation

Calcareous skarn is lenticular, layered and cystic, and occurs in the inner and outer contact zone between Jinchang complex and Tongjing complex and marble. The main metasomatic facies of calc-skarn in the inner contact zone is andalusite-almandine facies, and the main metasomatic facies of calc-skarn in the outer contact zone are pyroxene-andradite facies, garnet facies, wollastonite facies, fushan stone facies, epidote facies and actinolite facies. The associated metallic minerals in calc-skarn mainly include magnetite, chalcopyrite, pyrite, bornite, molybdenite, native gold, silver gold ore and tellurite gold ore.

(3) Alkaline metasomatic rocks

Alkaline metasomatic rocks mainly occur in diorite porphyrite in the east of Tongjing complex and biotite granodiorite in Jinchang complex. The main metasomatic phases are potash feldspar phase, albite phase, potash feldspar-albite phase and chronological-potash feldspar phase, and the associated metal minerals are mainly pyrite, chalcopyrite, bornite, chalcocite, molybdenite and natural gold. Alkaline metasomatic rocks are located in the center of rock mass alteration zone.

(4) Pyrite sericite structure

Chrysotile sericite mainly occurs in diorite porphyrite of Tongjing complex, and a small amount occurs in biotite granodiorite of Jinchang complex. The main metasomatic phases are quartz-sericite phase and quartz-carbonate phase, and the associated metal minerals are pyrite, chalcopyrite, natural gold and argentite. Pyrite sericite is often superimposed on alkaline metasomatic rocks to form porphyry ore bodies, such as Duijinshan ore section in Tongjing ore field. Chrysotile sericite is a host rock for porphyry mineralization.

(5) Construction of argillaceous rocks

The argillaceous rocks are mainly produced in diorite porphyrite of Tongjing complex, and the main metasomatic facies are Yingshi-kaolinite facies and Yingshi-sericite-kaolinite facies, and the associated metal minerals are pyrite, chalcopyrite, native gold, stibnite and argentite. Associated mineralization is generally weak, and mineralization is strong only when argillaceous rocks are superimposed on alkaline metasomatic rocks.

4. metasomatic zoning and its relationship with mineralization zoning

The spatial distribution of the above five ore-bearing metasomatic formations often has certain regularity. The center of the rock mass is alkali metasomatic rock, and the outside is alkali metasomatic rock and argillaceous rock, and skarn is produced in the contact zone. Some mafic skarns were formed in magmatic period, alkaline metasomatic rocks and most skarns were formed in the early alkaline stage after magmatic period, while chrysotile sericite and argillaceous rocks were formed in the acidic leaching stage after magmatic period, which together constituted metasomatic series related to intermediate-acid magmatic rocks.

Skarns and related metasomatic rocks often have obvious zonation. Taking the metasomatic zoning of the Tianye Guanmu ore section in Jinchang Mine as an example, the metasomatic zoning of skarn deposits is illustrated:

00 adamellite porphyry

1 potash feldspar adamellite porphyry

Diopside-almandine skarn is dominated by almandine, diopside is distributed among almandine particles, and contains trace amounts of sphene, apatite and zircon.

3 andradite-diopside skarn, mainly diopside.

0 marble

Gold and metal sulfides are mainly concentrated in andradite-diopside skarn, and a little exists in diopside-almandine skarn. The stronger the chloritization and carbonation of early skarn minerals, the stronger the gold (copper) mineralization.

The metasomatic rocks related to porphyry deposits also have certain zonation. The alteration center zone is generally alkaline metasomatic rocks, and there are pyrite sericite and argillaceous rocks on its outer side. The latter two are often superimposed on alkaline metasomatic rocks, which is the best area for gold (copper) mineralization. The average grade of ore in the mining area is: Au (1 ~ 5) × 10-6, Cu 0.5% ~ 0.8%, TFE 27% ~ 33%.