Geochemical mechanism of mercury deposit formation in Tongren-Fenghuang mercury deposit belt

The geochemical study of ore-bearing formations shows that the sedimentary formations of Lower Cambrian black rock series in western Hunan have obvious simultaneous enrichment of mercury. Compared with the Lower Cambrian in Huangshaxi section outside Tongren-Fenghuang mercury mine belt, the Hg content of the latter is reduced by about 75%. The matching relationship between the depletion and enrichment of mercury in this area and the geochemical yoke of carbon and oxygen isotopes in the ore belt reveals that the Lower Cambrian near the mining area suffered from large-scale water-rock leaching.

Janasson and Boyle( 1972) think that the abundance of Hg in most rock types is low (< 0.1×10-6 ~ 0.2×10-6). Black shale rich in organic matter often has high mercury content and can be used as the main source rock of mercury mines, which is consistent with the actual source rock types of most mercury mines in the world (Krupp R., 1988). The research shows that (Nerhey M.C and Buseck,1973; Roehsler et al.,1977; Beuge, 1982), mercury is mainly fixed on the surface of clay minerals and organic matter in an easily activated adsorption state; The adsorbed mercury starts to activate at 60 ~ 65℃ and can be completely activated at 150℃. The water-rock leaching experiments of gold, antimony, mercury and arsenic in the hydrothermal system of sulfur-bearing and chlorine-bearing strata show that the formation process of Tongren-Fenghuang mercury ore belt is mainly the water-rock reaction process at low temperature.

Second, the source of ore-forming fluids

The fluid properties rich in C 1 and high salinity reveal that the ore-forming fluid in Tongren-Fenghuang mercury deposit belt has obvious characteristics of underground hot brine. The isotopic composition of hydrogen and oxygen is close to the atmospheric precipitation line and similar to the hot springs in eastern Guizhou, which clearly shows that the ore-forming fluid in the mercury ore belt is mainly atmospheric precipitation.

Thirdly, hydrothermal cryptoexplosive breccia and hydrothermal cryptoexplosive metallogenic model in Tongren-Fenghuang mercury deposit belt.

In the author's opinion, the understanding of the causes of the widely developed "breccia" in Tongren-Fenghuang mercury ore belt is the key to solve the formation mechanism of the mercury ore belt.

In the previous research work, most researchers attributed this kind of breccia to structural breccia, but Wang Huayun and others (1987) once thought that "the characteristics of this kind of chaotic breccia are similar to the blasting breccia in porphyry copper mine to some extent". The latter understanding is a leap.

The author thinks that "breccia" is hydrothermal breccia, not structural breccia. The reasons are as follows.

1) This breccia is only distributed in the core of the anticline produced by the ore body, but disappears to the two wings, and dolomite-time pulse appears in the surrounding rock (Figure 50). "Breccia" is steeply inclined columnar, cystic and veined, and obviously cuts through the bedding of surrounding rock. It is very similar to the characteristics of breccia formed in the center of hot water explosion.

Fig. 50 Schematic diagram of distribution and morphology of breccia (Chatian mercury mine)

1-dolomite; 2- timely dolomite veins; 3 breccia and its ore

2) The breccia of breccia is angular and disordered, and there is no obvious separation and dissolution. This is obviously the result of rapid cementation of gangue minerals under sudden environmental changes.

3) The breccia has obvious characteristics of cementation and re-crushing. The Yanshi-dolomite vein is a kind of cemented breccia with net filling (Figure 5 1), which has the characteristics of breccia formed by many hidden explosions of hot water.

Fig. 5 1 breccia fragmentation and cementation (hand-made specimen, collected from hotel pond)

1-dolomite breccia; 2- Dolomite veins and minerals in them; 3- Ore

4) The mercury deposits in the mercury mine belt are all located to the east of the Baotou Copper Deep Fault, with obvious directionality and equidistance, and have similar structural background and distribution characteristics to the typical hydrothermal breccia deposits in the world (such as Waiotapu, New Zealand).

5) The distribution range of the breccia and its disharmony with surrounding rock bedding are obviously different from gravity flow carbonate deposits widely developed in the Middle Cambrian. The latter generally has regional relative continuity, horizon stability and coordination with surrounding rock bedding, and the cement composition is mainly micrite calcite.

6) There is no trace of dissolution and collapse of carbonate rocks in the area, and the breccia blocks of breccia are not concentrated at the bottom of the vein area, which does not have the general characteristics of dissolved breccia (Wang Huayun et al., 1987).

Based on the above understanding of the genesis of breccia, the author puts forward the explosion metallogenic model of hot water hidden danger in Tongren-Fenghuang mercury mine belt (Figure 52).

The model emphasizes the distributary action of copper-clad deep fault, the change of geometric shape of ore-forming fluid migration channel and the dissolution of gas in ore-forming fluid.

When the precipitation seeps along the copper-clad deep fault, due to the heat energy provided by geothermal warming and tectonic movement, the precipitation heats up and has a wide water-rock exchange reaction with the black rock series of the Lower Cambrian, so that a large amount of easily activated mercury in the adsorption state is activated and transferred to the ore-forming fluid to form a thermal reservoir. Induced by tectonic movement, this high salinity mercury-bearing ore-forming fluid is rich in Cl, CO2, CH4, N2, H2 and so on. It rises along the secondary fault related to copper-clad fault and converges to the core of anticline. Because there is a space for storing fluid in the core of anticline, the fluid migration channel suddenly opens. In an instant, ore-forming fluids will converge to the core of anticline with the decrease of temperature, thus producing a large number of CO2, CH4, N2, H2, etc. It will dissolve in the ore-forming fluid and rapidly expand to the cracks at the top and wings of the anticline at the limit speed until a "gas cap" with greater pressure is formed at the top of the anticline. Due to the dissolution of gas, the fluid in the core of anticline is fractionated into gas phase and liquid phase, and the gas phase is dominant in volume. The high-pressure gas-liquid two-phase fluid in the core will inevitably prevent the upwelling and confluence of the deep fluid, which will eventually lead to the instantaneous overpressure state of the anticline core, trigger the hidden explosion of hot water and form hydrothermal breccia. The intensity and frequency of this action will depend on the replenishment speed and quantity of deep high-density fluids and the silicification sealing speed of upper fractures until the internal pressure of deep fluids is insufficient to maintain rapid convergence. If a hydraulic fracture reaches the surface, the deep fluid will reach the surface and appear in the form of hot springs.

Fig. 52 Block diagram of metallogenic geochemical model of Tongren-Fenghuang mercury ore belt

According to Wang Huayun's estimation, the maximum mineralization depth of Tongren-Fenghuang mercury mine belt is less than 2500m (mineralization pressure is 250× 105Pa, which is converted into mineralization depth according to hydrostatic pressure). The metallogenic pressure estimated by Yan Junping is130×105pa ~150x105pa, and the metallogenic depth is about 1000 ~ 1500m.

After hot water erupts, due to the dissolution and escape of CO2, CH4, H2S and other gases, as well as the decrease of temperature, a large number of ore-forming fluids saturated in the breccia space will be unloaded to form dolomite and timely, and cemented breccia will be rapidly accumulated and mineralized, forming breccia-type ore bodies. Due to the supplement of deep fluid, ore-forming fluid is injected into the interlayer cracks and interlayer peeling parts on the two wings of anticline and the cracks induced by hydrothermal explosion, and precipitated into vein-like and interlayer vein-like ore bodies.