Abstract: Laerma and Qiongmo gold deposits,occurring in the south subzone of Western Qinling,are typical and important stratabound gold deposits,closely associated with submarine exhalative-sedimentation.The gold deposits exist in the Cambrian silicalite formation composed of black chert and slate.The presence of the typical chert offers an important evidence to evaluate the possible submarine exhalative system and its control over the forming of the gold deposits.
Bedded-quasibedded and lens-like orebodies occur in the silicalite formation,showing approximately comfortable attitudes with the host rocks and their development in interbedded fractures or breccias there.More than 80% of the ore bodies are present in the chert.Element paragenesis and mineral assemblage of the ores are quite complicated.More than 80 kinds of minerals were identified in the ores.In addition to ordinary minerals such as pyrite,marcasite,stibnite and native gold,there are a number of minerals of copper,uranium and selenium.In some parts,independent Cu,Sb,U,Mo and Se ore bodies could be delimited.
It is particularly interesting that there are abnormally high concentration of platinum group element (PGE) and disperse elements (e.g.,Se and In).With such complex element paragenesis of Au-Cu-U-Mo-Se-PGE,the gold deposits in the Cambrian silicalite formation are really unusual.As a matter of fact,this type of deposit was known for the first time in China and is expected to be studied.
In the light of geological features in the deposits,the authors conclude that they underwent 2 ore-forming periods at least,namely a submarine exhalative sedimentary period and a hydrothermal minerlization period of hot circulating groundwater (meteoric water).During the former period,a high concentration of ore-forming elements and pyrite-rich silicalite formation were formed.This was a stage of preliminary enrichments of ore materials and source bed formation.During the latter period,ore materials in the silicalite formation were remobilized, migrated and re-enriched. The ore-forming thermal solution is characterized by middlelow tempera ture (142~270℃), low pressure (less then 30MPa), weak acidity and neutrality (pH=3.69~5.88), reduction and redox transition (Eh=-0.5~+0.082). The decrease in sulfer fugacity, the increase in oxygen fugacity and the mixing of hydro thermal solution with cold groundw ater are the direct reasons for promoting the sediment action of gold from the ore-forming thermal solution. So the gold deposits belong to stratabound gold deposit associated with submarine exhalative sedimenta tion.