Judging the Sedimentary Environment of the Silicalite Formation on the Chemical Characteristics of Rocks in Western Qinling
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摘要: 西秦岭寒武系硅岩建造是拉尔玛、邛莫金矿床和牙相金矿点的赋矿岩系。其主要由炭质硅岩和炭质板岩组成。其中的含矿硅岩构造十分丰富，有块状、条带状、层纹状、多孔状和同生角砾状等。硅岩单层厚度一般为30～200 m。其主要成分除SiO2( 平均含量 95. 30%) 外，其它氧化物含量能达1% 者仅有 FeO 、Fe2O3和 P2O5,且 FeO/Fe2O3比值大于 1, 而 Sr/Ba 比值小于 1 。硅岩建造中有机炭含量相当高，一般为(0.12～8.14) %，高者达 22 %。硅岩中稀土以总量低(3.29×10-6～100×10-6)、铈亏损为特征。硅岩建造中沉积黄铁矿的δ34S值变化在-10‰～ +46.9‰之 间。 硅岩的δ300Si值主要变化在+0.4 ‰～+0.8‰之间。硅岩建造的上述特点反映其主要是在深海—半深海环境下形成的。Abstract: The Cambrian silicalite formation, composed of black chert and black slate, is the host-rock of Laerma and Qiongmo gold deposits in western Qinling.The ore-bearing chert is characterized by bedded, laminated, mas- sive and pseudobrecciated structures which change regularly in space.The thickness of a chert bed generally varies from 30 to 200 meters.Host elements are simple and concentrated in the chert.Besides SiO2(95.30 % on average), only FeO, Fe2O3 and P2O5 reach or are more than 1.0 %.The ratios of FeO/Fe2O3 in all chert samples are mainly greater than 1, but the ratios of Sr/Ba are less than 1.The organic carbon concentrations of silicalite formation range mostly from 0.12 % to 8.14 %, the highest concentration being 22 %.REEs(rare- earth elements) are characterized by a low total content (ranging between 3.29×10-6～100×10-6), nega- tive Ce anormal and a gradually increasing NASC —normalized value with increasing atomic number of REEs. The δ30Si values of the chert in the area range mainly from + 0.4 ‰to 0.8 ‰.All the geochemical characteris- tics of the silicalite formation manifest that the silicalite formation deposited in the deep and half- deep sea en- vironment.
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Key words:
- silicalite formation /
- sedimentary environment /
- western Qinling
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