Metallogenic stage

The metallogenic stages of Yangshan gold deposit can be roughly divided into four stages: diagenesis stage, regional metamorphism stage, hydrothermal stage and supergene stage, and hydrothermal stage can be divided into several different metallogenic stages.

4.5. 1 diagenetic stage

Sulfide is mainly represented by strawberry pyrite, which is mostly produced in the form of raspberry ball, and only occurs in argillaceous siltstone in sedimentary strata, and is well preserved between S0 schists in original sedimentary strata. The particle size of pyrite berry balls is 5 ~ 30μ m. Under the electron microscope, it is found that the middle of strawberry pyrite is actually composed of many tiny cubic pyrite, and the particle size of cubic pyrite is 0.5 ~ 1.5 μ m (Figure 4.7). Strawberry pyrite does not exist in altered plagioclase granite porphyry and limestone strata. There are many carbonate minerals developed during diagenesis, mainly dolomite and ankerite, and dolomite developed first. Ankerite grows around the early dolomite, forming a good area (Figure 4. 12). In addition, some iron carbonate minerals are produced from argillaceous siltstone in the form of cement (Figure 4. 12). A large number of oolitic clastic rocks with grain size of 100 ~ 400μ m were found in the sedimentary strata of Guanyinba, Anba and Getiaowan ore sections in Yangshan mining area, and most of them were crushed and cemented by late iron carbonation. There are suture lines between some oolites, which indicates that large-scale tectonic movement occurred in this area after the oolites cemented and solidified into rocks. Most oolites are mineralized to varying degrees (Figure 4. 1 1). These oolitic clastic rocks indicate that the Devonian sedimentary environment in Yangshan mining area should be turbulent coastal shallow sea environment. The timely particles in clastic rocks have low roundness and sorting degree, which indicates that the debris comes from the environment near the continental margin.

4.5.2 Regional metamorphic period

In the Middle and Late Triassic, with the collision between the North China Plate and the Yangtze Plate, the Paleo-Tethys Ocean on the west side of the Qinling Mountains closed, forming the Mianlue suture zone (Zhang Guowei et al., 2003). However, under the action of SN- directional compressive stress, the strata and rock mass in Motianling-Longmenshan area were strongly deformed in Triassic and early stage, forming strong deformation folds and regional shear cleavage with excellent permeability, accompanied by regional thermodynamic metamorphism, forming Indosinian metamorphic rocks (regions). Due to the transformation of metamorphic fluid, timely recrystallization occurred in the timely sandstone in this area, and timely fine network veins appeared. Timely and pyrite recrystallize in phyllite, forming timely veinlets (Figure 4. 12f). However, the gold content of pyrite-quartz veinlets in this period is generally low, indicating that the metamorphic hydrothermal solution in this period did not cause large-scale gold enrichment.

Hydrothermal stage

(1) hydrothermal stage Ⅰ (pyrite-sericite-isochronous stage)

Pyrite is mainly composed of {100} crystal aggregates (about 60%) and {2 10} crystal aggregates (about 30%), and a few are {100}+{1. The grain size of pyrite is 0. 1 ~ 1 mm, and a small amount of pyrite with smaller grain size (10 ~ 30μ m) is superimposed around strawberry pyrite, forming a secondary expanded edge of pyrite (Figure 4. 12). At the later stage of this stage, a small amount of fine-grained pyrite {100} with a particle size of 50 ~100 micron was generated. Pyrite in this stage mainly occurs in strongly sericitized siltstone, argillaceous siltstone, limestone and altered plagioclase granite porphyry in sparse disseminated form and veinlet disseminated form, and often coexists with silicification time pulse. The granular structure of pyrite is developed, and most of it has been broken, which becomes the core of pyrite crystal growth in the next stage (Figure 4.7). Pyrite at this stage does not have banded structure.

(2) The second stage hydrothermal solution (pyrite-arsenopyrite-timely stage).

At this stage, pyrite is the main sulfide (about 70%) and arsenopyrite is less (about 20%). Pyrite is mainly produced in autogenous {2 10} and {100}+{2 10} aggregates, with a particle size of 50 ~ 200 microns. The core of {2 10} pyrite is often wrapped with {/kloc of the second-stage crushed hydrothermal solution. A small amount of authigenic arsenopyrite is distributed around {2 10} pyrite, and some arsenopyrite fragments are wrapped in {2 10} pyrite (Figure 4.7), indicating that arsenopyrite and other minerals began to be generated at the later stage. At this stage, pyrite is also wrapped with a small amount of tiny galena particles (Figure 4.7) and mica, rutile and apatite fragments. Muscovite and apatite are wrapped in pyrite in tiny fragments, which may be the products of magmatic diagenesis and hydrothermal alteration. Galena often occurs along pyrite cracks, and rutile usually cuts through pyrite particles in the form of emulsion drops and veinlets, which proves that galena and rutile in these occurrences are formed later than pyrite. In addition, there are still a small amount of chalcopyrite in this stage, most of which are semi-automorphic-heteromorphic.

One of the characteristics of Carlin-type gold deposits is that arsenic-bearing pyrite rich in gold in the middle and late stage is banded around early ore-free pyrite (Hofstra et al., 2000). The above-mentioned low-temperature element combination and the discovery of arsenic-bearing banded pyrite indicate that Yangshan gold deposit has undergone low-temperature fluid mineralization and has mineralogical characteristics of Carlin-type gold deposit.

(3) The third stage of hydrothermal period (arsenopyrite-pyrite-timely stage).

At this stage, the gold-bearing minerals are mainly authigenic banded and acicular arsenopyrite, accounting for more than 60%, and some fine pyrite is produced, accounting for about 30%. At this stage, arsenopyrite is mainly produced in the form of {10 1}+{230}, with a length of100 ~ 300 μ m, a rhombic cross-section and a width of 50 ~100 ~ 300 μ m. Fine particles of galena are usually wrapped in needle-shaped arsenopyrite. The pyrite particle size at this stage is generally very small, ranging from 20 to 100 micron, and some {100} pyrite is wrapped by arsenopyrite. At this stage, pyrite and arsenopyrite are mainly distributed in the physical and chemical zone of altered phyllite and around the matrix of altered porphyry and plagiogranite.

In the later stage of this stage, the content of sulfide poisoned sand increased greatly, reaching about 90%. The arsenopyrite is mostly short columnar, fine granular and needle-like crystals with a length of 50 ~100μ m, and the cracks formed along the late brittle deformation are mainly produced in semi-autogenous-special-shaped veins, colloidal structures and venules-veinlets disseminated structures, which are easily confused with stibnite veinlets by naked eyes. In altered plagioclase granite porphyry, this kind of arsenopyrite is mainly distributed on the contact surface between the brittle fracture time pulse of granite porphyry and porphyry fragments, which is obviously different from the early arsenopyrite (Figure 4.7). This stage only contains very little {100} pyrite (

Combined with mineralogical research results, the first and second mineralization times of hydrothermal period mainly occurred after the intrusion of plagioclase granite porphyry and before the brittle deformation of porphyry. The early intrusive porphyry in Yangshan mining area experienced ductile and ductile-brittle deformation after intrusion. In this process, the biotite, syenite and other porphyries have plastic deformation in different degrees, while pyrite formed in the early hydrothermal period has brittle fracture. In addition, a large number of sulfide scratches are often seen on the surface of porphyry in the field, indicating that ductile-brittle deformation occurred at the lower limit of these two metallogenic stages.

The third hydrothermal arsenopyrite mineralization occurred after sericitization alteration and ductile-brittle deformation of plagioclase granite porphyry, and the formed arsenopyrite veinlets were disseminated in porphyry breccia and debris and timely lining. In Guanyinba and Getiaowan ore blocks in Yangshan mining area, it is usually seen that the third stage of hydrothermal arsenopyrite mineralization is injected into altered plagioclase granite porphyry and its surrounding rock altered phyllite in the form of veinlets, making them all black. Therefore, the formation of some black ores in Yangshan mining area is caused by the mineralization of arsenopyrite at this stage.

(4) The fourth hydrothermal period (natural gold-stibnite-tetrahedrite-yingshi period).

The main minerals are natural gold, Yingshi and stibnite. , accompanied by a small amount of stibnite, jamesonite, jamesonite, tetrahedrite, galena and so on. At this stage, the ore-bearing hydrothermal solution was poured along the cracks of the altered porphyry, and the bright gold minerals formed wrapped the previously formed pyrite, arsenopyrite and other minerals (Figure 4. 14). Antimony mainly occurs in the fracture of plagioclase porphyry with strong alteration and fracture, and most of it grows with the lining of time pulse. The development of kneading twins in stibnite shows that it experienced late tectonism after its formation. Galena mainly runs through the corresponding veins in the main metallogenic period. In the later stage of this stage, realgar and orpiment appeared. The appearance of these two minerals indicates that the physical and chemical characteristics of ore-forming hydrothermal solution have changed obviously in the later stage of this stage. As in solution mainly exists in positive valence state, the oxygen fugacity of fluid increases and the Eh value decreases.

Fig. 4. 14 Relationship between natural gold and other minerals in Yangshan Gold Mine

Microscopically, natural gold (Au) in the fourth hydrothermal mineralization of A- Yingshi (Qz) vein and pyrite (Py) and arsenopyrite (Asp) formed in the early stage; Natural gold and jamesonite in the fourth hydrothermal solution of B- Yingshi (Qz) vein under microscope.

As for the genesis of natural gold in this stage, Hofstra et al. (2000) thought that the main reason was that after pyrite dissolved and leached in the main metallogenic stage, gold gathered into natural gold particles, and pyrite was produced as grapes. At this stage, mineral combinations such as stibnite and Yingshi appear, indicating that the temperature has decreased, because the solubility of these minerals will decrease significantly with the decrease of temperature (Rytuba,1985; Zhang et al., 1994). However, for natural gold, in the temperature range of 175 ~ 220℃, the change of temperature has little effect on its solubility (Schenberg et al., 1989), and the change of fluid oxidation degree will lead to large-scale precipitation of natural gold (Rhomberg et al.,/kloc-).

(5) The fifth stage of hydrothermal period (late calcite-synchronous stage).

Under the background of extensional structure, a large number of extensional cracks have been formed. In the later period, calcite was poured along the lining to cut the veins and ore bodies formed in the early period.

4.5.4 Supergene period

Large-scale oxidation occurred, and pyrite, arsenopyrite and other sulfides were oxidized. Oxidized minerals such as limonite, scorpion stone and sky blue are formed on the surface. Bronze blue is also developed on the surface outcrop of porphyry dikes. In the supergene stage, late clay veinlets passing through plagioclase granite porphyry and isochron-calcite veinlets formed in the late hydrothermal period are also developed.

In a word, the diagenetic period laid a certain material foundation for the formation of Yangshan gold deposit, and the gold in the regional metamorphic strata was enriched to some extent, but no gold body was formed. The main metallogenic stages of Yangshan gold deposit are the second, third and fourth stages of hydrothermal period, which laid the foundation of Yangshan gold deposit and constituted the main body of Yangshan gold deposit. The mineralization in the first and second stages of hydrothermal period is closely related to the intrusion of plagioclase granite porphyry veins in space and time, while the mineralization in the third stage of hydrothermal period occurred after the condensation of porphyry veins. In the fourth and fifth stages of hydrothermal period, the ore bodies formed in the early stage of mineralization were enriched and transformed, forming visible natural gold and calcite veins. Supergene stage is mainly the oxidation and secondary enrichment process of ore body. The division of these metallogenic stages is also consistent with the precipitation sequence of metal minerals in hydrothermal deposits, that is, Fe-As-Zn-Cu-Pb-Te-Au-Sb, which is consistent with the decrease of temperature and the increase of solution alkalinity (Chen Guangyuan et al., 1988). See Table 4.22 for the characteristic minerals formed in different metallogenic stages of Yangshan Gold Mine.

Table 4.22 Characteristics of Yangshan Gold Belt in Different Metallogenic Stages