Background content of ore-forming elements in Cambrian ore-controlling strata

At present, the understanding of the source of Tongren-Fenghuang mercury mine belt has not been unified. Luo proposed for the first time that the provenance of stratabound mercury deposits in eastern Guizhou may come from Banxi Group, and Hua Yongfeng proposed the diversity of mercury deposits in Guizhou. He Lixian and others believe that the Middle and Lower Cambrian is an important source of mercury in this area, and Wang Huayun and others believe that mercury mainly comes from the Lower Cambrian black rock series.

In order to objectively evaluate the source of mercury in Tongren-Fenghuang mercury mine belt, the author measured the systematic profiles of Huangshaxi in Anhua, a mercury-free area outside Tongren-Fenghuang mercury mine belt, and Sunjiatian in the periphery of Xinhuang Restaurant Tang deposit in Tongfeng mercury mine belt, and made a comparative study on the contents and regional distribution of elements such as Hg, Au and Sb in this set of strata.

1. Content and distribution of mercury in Paleozoic Cambrian strata

The average content of mercury in the Lower Cambrian black rock series in the Huangshaxi section of Anhua, a remote mining area not affected by mineralization, is 294.38× 10-9, with a variation coefficient of 57.9%. Compared with the average value of the upper continental crust, the enrichment of Hg is about 3.7 times, indicating that the Lower Cambrian black shale in this area has obvious contemporaneous enrichment of Hg (Table 4).

Table 4 Average content of ore-forming elements in Cambrian ore-bearing formations in western Hunan

Note: The numerical units of Au and Hg are 10-9, Sb and W are 10-6, and CV is the coefficient of variation; The average value of the upper continental crust is based on Taylor's.

The most striking feature of the Lower Cambrian sedimentary formation in this area is a set of black shale series rich in organic carbon. The study reveals (Table 5) that there is a positive correlation between organic carbon content and trace element content in the strata, indicating that the high content of ore-forming elements in the Lower Cambrian black rock series is closely related to the strong adsorption of organic matter.

The average content of mercury in carbonate rocks of Middle Cambrian in Huangshaxi area of Anhua is only 78.33× 10-9, and the coefficient of variation is 29.3%. The average content of mercury in carbonate rocks of Upper Series is 50.67× 10-9, and the coefficient of variation is 30.4 1%. There is no contemporaneous enrichment of mercury in the Middle and Upper Cambrian in this area.

Table 5 Relationship between Organic Carbon Content and Trace Elements in Strata

Compared with Huangshaxi area in Anhua, which is not affected by mineralization, the mercury content and distribution in Sunjiatian section near Xinhuang Restaurant Tang deposit in Tongren-Fenghuang mercury mine belt have the following characteristics. ① The mercury content in the Lower Cambrian black rock series is obviously low, only 73.63× 10-9. Compared with Huangshaxi area in Anhua, the mercury content in this area is reduced by about 75%, which shows that there is a strong loss of mercury in the Lower Cambrian black rock series in the ore-bearing strata near the mining area (figure 10). ② Although the average mercury content in Sunjiatian profile near the mining area is low, the coefficient of variation is large (CV = 86. 1%), which may be the result of hot water leaching in the later period. ③ The mercury content of the Middle Cambrian in Sunjiatian section of Xinhuang is much higher than that of Huangshaxi in Anhua, which is not affected by mineralization (Figure 10). The average mercury content is 273.57× 10-9, Hg-9 and the coefficient of variation is 148%. In view of the low mercury content of the Middle Cambrian in Huangshaxi area of Anhua, which is not affected by mineralization, the Middle Cambrian in the mercury ore belt is the main occurrence horizon of mercury deposits, and the high mercury content in Sunjiatian section of Xinhuang in Tongren-Fenghuang mercury ore belt is the result of the enrichment of thermal fluid to this horizon in the later period. ④ The mercury content in Huangshaxi of Anhua and Sunjiatian of Xinhuang is similar, but the variation coefficient of mercury content in Sunjiatian of Xinhuang is large.

Figure 10 Comparison of Mercury Content in Cambrian Section of Xiangxi (solid line is Huangshaxi Section and dotted line is Sunjiatian Section)

A- lower series; B- central China; C- upper series

2. The content and distribution of gold, antimony and tungsten in Paleozoic Cambrian.

The average content of gold in lower CAMBRIAN black rock series in Huangshaxi section of Anhua is 10.33× 10-9, and the coefficient of variation is 12%, which indicates that gold is evenly distributed in this area. The average content of Sb is 3.02× 10-6, and the coefficient of variation is 10 1%, which shows discreteness compared with Au. The average content of W is 2.72× 10-6, and the coefficient of variation is 12%, which still shows the uniformity of distribution (see Table 4). Compared with the average value of the upper continental crust, Au, Sb and W in the Lower Cambrian black rock series in this area are enriched by 5.7 times, 15 times and10.4 times respectively, indicating that Au and Sb are obviously enriched contemporaneously, and W is enriched to some extent.

The average content of CAMBRIAN gold in Huangshaxi of Anhua is 9.67× 10-9, and the coefficient of variation is 1 1.5%. The average antimony content is 0.65× 10-6, and the coefficient of variation is 5 1.6%. The average content of W is 2.77× 10-6, and the coefficient of variation is 7.6%. The content distribution of gold, antimony and tungsten is relatively uniform. Compared with the average value of the upper continental crust, Au, Sb and W are enriched by 5.4 times, 3.3 times and 65,438 0.4 times respectively. It shows that gold and antimony are obviously enriched at the same time.

The average content of Upper Cambrian gold in this area is 8.7× 10-9, and the coefficient of variation is 8.9%. The average content of antimony is 1.07× 10-6, and the coefficient of variation is 68.8%. The average content of tungsten is 2.63× 10-6. The enrichment of gold and antimony is remarkable.

Compared with Huangshaxi section in Anhua, Sunjiatian section in Xinhuang has the following characteristics: ① Au and Sb in the black rock series of Lower Cambrian still show great losses, and the variation coefficient of Au and Sb content is large, which further reveals that the Lower Cambrian near the mining area may have experienced a large-scale water-rock reaction; ② The Cambrian antimony content in the mercury mine belt is higher than that in Huangshaxi area outside the mercury mine belt, showing the same rule as mercury, while the Cambrian gold content in the two areas is close, revealing the difference in chemical activity between antimony and gold, but the variation coefficient of Cambrian gold and antimony content in the mercury mine belt is large. ③ The Cambrian antimony content in the mercury mine belt is low, but the gold content is close.

To sum up, Au, Sb and Hg in the Lower Cambrian black shale in western Hunan are remarkably enriched, which can be called Hg-Au-Sb(W) combined (derived) ore-bearing formation, while the Au and Sb in the Middle and Upper Cambrian are obviously enriched, which can constitute Au-Sb(W) combined ore-bearing formation. The comparison of regional mercury content reveals that there is a large-scale mercury loss in the Lower Cambrian black rock series near the mining area, which indicates that it may be the result of hot water leaching. Obviously, the Lower Cambrian black shale is an important mercury source bed, and the Middle Cambrian is of little significance in providing ore sources.