西秦岭寒武系硅岩建造喷流沉积作用与矿质聚集

SUBMARINE EXHALATIVE SEDIMENTAION AND MINERALIZA TION OF THE SILICALITE FORMATION IN WESTERN QINLING,CHINA

硅岩建造是西秦岭拉尔玛—邛莫层控金矿床的含矿岩系。含矿层状硅岩呈块状、条带状、层纹状、多孔状和同生角砾状等，单层厚度一般为３０ｍ～２００ｍ。主要成分ＳｉＯ２平均含量高达９５．３０％，其余为ＦｅＯ、Ｆｅ２Ｏ３和有机炭等。硅岩的Ａｌ／（Ａｌ＋Ｆｅ＋Ｍｎ）比值低于０．３５（平均０．１５３）。硅岩中微量元素十分丰富，不仅含具基性、超基性特征的元素群（如Ｃｕ、Ｃｒ、Ｐｔ、Ｐｄ、Ｏｓ等），而且含具酸性特征的元素群（如Ｗ、Ｍｏ、Ｕ等）。硅岩中稀土以总量低（３２．９×１０－６～１００×１０－６）、δＣｅ亏损为特征。经北美页岩标准化后，稀土含量随原子序数的增大而增大。硅岩的δ１８Ｏ值为１７．６０‰～２３．２４‰，其形成温度约为７０℃～１１８℃。硅岩的δ３０Ｓｉ值主要在＋０．４‰～＋０．８‰之间。硅岩的上述特点均显示其与喷流沉积作用有关。喷流沉积作用不仅形成了特殊的硅岩建造，而且也使金等元素在硅岩建造中发生了明显的聚集。

The Cambrian silicalite formation,composed of bedded chert and slate ,is the host rock of Laerma and Qiongmo gold deposits in western Qinling The presence of the typical chert offers an important evidence to evaluate the possible submarine exhalative system and its controls over the formation of gold deposits The ore bearing chert is characterized by massive ,banded ,laminated ,porous, spherulitic and pseudobrecciated structures which changes regularly in space The thickness of a chert bed generally varies from 30 to 200 meters It contains principally SiO 2 (95 30% on average),with FeO,Fe 2 O 3 and organic carbon greater than 1 0% .The ratios of Al/(Al+Fe+Mn)in all chert samples are lower than 0 3 (0 153 on average) The chert is enriched in most trace elements, which is characterized by not only the basic and ultrabasic elements group(e g Ag, Pt,Pd,Cu, Cr) but also the acid elements group (e g W,Mo,U).Rare earth are characterized by a low total contents (ranging between 3 29×10 -6 and 100×10 -6 ),negative Ce anormaly and a gradually increasing NASC—normalized value with increasing atomic number δ 18 O Values of chert mainly vary from 17 60‰ to 23 24‰ (19 17 on average),suggesting that the tempreature for deposition was from 70℃ to 118℃ δ 30 Si values of the chert in the area range mainly from +0 4‰ to +0 8‰ All the geological and geochemical characteristics show that the chert is a product of submarine exhalative sedimentation. During the submarine exhalative sedimentary period ,the silicalite formation and mineralization of some elements were all formed