The Hot-Spring Genesis of the Shimen Realgar Deposit, Northwest Hunan

The Shimen realgar deposit is characterized by the pipi-shaped orebody and the development of silica sinter and hydrothermal explosive breccia which are typical of hot spring activity.Very similar trace-element associations are noticed between the silica cap and the breccia and modern hot spring waters in the area.The chemistr of ore-forming solutions is also well comparable with that of modern hot spring.,The spring system that gave rise to the mineralization was charged by ground waters heated through thermal conducting systems in the deep crust and,to a lesser extent,by geothermal gradient.ΔD,δ^18O,δ^13CCH4andδ^13CH4andδ^13CCO2values and ^40Ar/^36Ar and 3^He/^4He ratios indicate that the spring system is of crustal derivation.The ore-forming metals were supplied by surrounding strata,particularly those underlying the ore deposits.The mechanim of ore deposition is thought to be hydrothermal explosion and accompanying boiling and abrupt changes in pH and Eh.Located in northwest Hunan,the Shimen realgar deposit is the leading arsenic producer in the country,However,regardless of its long mining history,the genesis of this deposit has long been a puzzle.It was considered to be postmagmatic epithermal in the leading arsenic producer in the puzzle.It Was considered to be postmagmatic epithermal in origin,but this is trongly challenged by filling(metasomatism)in karst environment proposed later by Zhou Zhiquan(1986)also encounters a number of difficulties.For example,why can the pipi-shaped orebody vertically extend up to several hundreds meters without any compatible development in the lateral dimension? A hot spring genesis is suggested in the present paper based on geological observations and laboratory studies conducted by the authors in recent years.

The Shimen realgar deposit is characterized by the pipe-shaped orebody and the development of silica sinter and hydrothermal explosive breccia which are typical of hot spring activity. Very similar trace- element associations are noticed between the silica cap and the breccia and modern hot spring waters in the area. The chemistry of ore-forming solutions is also well comparable with that of mod- ern hot springs. The spring system that gave rise to the mineralization was charged by ground waters heated through thermal conducting systems in the deep crust and, to a lesser extent, by geothermal gradient. δD, δ^(18)O, δ^(13)C_(CH_4)and δ^(13)C_(CH_4)and δ^(13)C_(CO_2) values and ^(40)Ar/^(36)Ar and ~3He/~4He ratios indi- cate that the spring system is of crustal derivation. The ore-forming metals were supplied by sur- rounding strata, particularly those underlying the ore deposits. The mechanism of ore deposition is thought to be hydrothermal explosion and accompanying boiling and abrupt changes in pH and Eh. Located in northwest Hunan, the Shimen realgar deposit is the leading arsenic producer in the country. However, regardless of its long mining history, the genesis of this deposit has long been a puzzle. It was considered to be postmagmatic epithermal in origin, but this is strongly challenged by the fact that no igneous rocks have ever been found in the mining district. The model of epithermal filling (metasomatism) in karst environment proposed later by Zhou Zhiquan (1986)also encounters a number of difficulties. For example, why can the pipe-shaped orebody vertically extend up to several hundreds meters without any compatible development in the lateral dimension? A hot spring genesis is suggested in the present paper based on geological observations and laboratory studies conducted by the authors in recent years.