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Compared with the fast and intermediate spreading mid-ocean ridges, the hydrothermal fields forming at slow and ultra-slow mid-ocean ridges usually contain abundant metal sulfide resources. Previously statistical results suggested the gold concentrations in massive sulfide deposits decreases with the increase of spreading rate, and the hydrothermal fields, located in ultra-slow mid-ocean ridges, have the highest gold concentrations. Previous studies carried out detailed research on the tectonic environment and sulfide assemblage of the Longqi hydrothermal field, but the occurrence and precipitation mechanism of gold in the Longqi hydrothermal field still need further research. In this paper, the texture and trace element concentration of sulfides in the Longqi hydrothermal field are analyzed in order to investigate the occurrence and precipitation mechanism of gold. The sulfides in the Longqi hydrothermal field are mainly pyrite, along with chalcopyrite (isocubanite) and sphalerite. Minerals such as ferronatrite and native gold have also been observed. According to the mineral texture and morphology, pyrite is divided into two types the first type (Py1) is fine-grained or colloidal, while the other type (Py2) shows subhedral-euhedral with coarse grain. Py1 usually exists in Py2 or is surrounded by Py2 as inclusions, and Py2 coexists with euhedral-subhedral chalcopyrite and sphalerite. Native gold mainly exists in the internal pores of Py1, and minor grains exist between Py2 and other sulfides. Compared with Py2, Py1 contains higher trace element contents of Ni, Zn, Pb, Ba, Mn, V, Mg, U, Au, Ag, Cd, and lower contents of Co, Se, As, Sb. Under the physico-chemical conditions of the Longqi hydrothermal field, Au(HS) is the main existing form of Au. The decrease of HS− concentration and the increase of pH value will promote the precipitation of gold. The mixing of hydrothermal fluids with seawater in the Longqi hydrothermal field results in an increase in pH values and a decrease in temperature, which can lead to pyrite crystallization and consequently decrease of sulfur fugacity of hydrothermal fluid. An increase in pH and decrease of sulfur fugacity both contributed to the precipitation of gold.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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