拉尔玛-邛莫金矿床的地质特征及成矿模式

拉尔玛-邛莫金矿床产于西秦岭南亚带寒武系硅岩建造中,受地层、岩性、构造控制十分明显.但与其它层控金矿床相比较,该矿床不仅含矿主岩特殊,而且矿石矿物和元素组合十分复杂.矿床中除Au外,Cu、U、Mo、Sb、V、Zn等元素在局部地段亦可圈出独立矿体.同时,矿床中存在铂族元素(PGE)、铟的高异常以及硒的矿化富集体,从而构成了Au-Cu-U-Mo-Se-PGE建造矿床.研究表明,金矿床的形成,至少经历了喷流沉积和地下水热液活动改造两个成矿期.前者表现为硅岩建造中出现了众多成矿元素的高异常和黄铁矿等条带状、层纹状构造的形成,显示出矿质的初步聚集;后者则促使矿质富集层中的成矿物质组分发生活化、迁移和再聚集,并最终导致工业矿床的形成.矿床属与海底喷流作用有关的喷流型层控金矿床.

GEOLOGICAL FEATURE AND METALLOGENIC MODEL OF LAERMA AND QIONGMO GOLD DEPOSITS IN WESTERN QINLING

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 middle low temperature(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 hydrothermal solution with cold groundwater 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 sedimentation