应用氢氧同位素研究矿床成因的一些问题探讨

成矿热液的氢、氧同位素组成与其水的类型、水/岩交换的岩石成分和同位素组成、水/岩交换时的温度及水/岩交换程度(W/R比值大小)等诸多因素有关,微生物和有机质也对其有一定的影响。因此,仅通过简单投影的方法将成矿热液的氢、氧同位素值与一些所谓的标准值进行类比,由此就推断出热液中水的来源,这种方法是不可取的;尤其当成矿热液的氢、氧同位素值介于大气降水和岩浆水的值之间时,切忌滥用两种水混合成矿模式,因为实际情况往往并不是这样。本文以胶东乳山金矿床为例,展开了这方面的讨论。

DISCUSSIONS ON THE APPLICATIONS OF HYDROGEN AND OXYGEN ISOTOPES TO THE PROBLEMS OF ORE GENESIS

The hydrogen and oxygen isotopic compositions of water in the ore fluids are related with the initial isotopic compositions of waters and rocks, chemical compositions of rocks, and the ratio of exchangeable oxygen (or hydrogen) atoms in the water to those in the rock (W/R) and the exchanging temperature, Some organic matters may also have some efferts on the isotopic fractionations. A discussion on these problems through the case of Jiaod}ng Rushan gold deposit is presented in this paper.The Rushan gold deposit one of the important quartz-vein gold deposits in the Jiaodong district, is located in the Kunyushan granitic pluton. Initial data on the hydrogen and oxygen isotopic compositions of water in fluid inclusions in quartz from the gold deposit fall within a wide range:δD=-128 to-71 per mil and δ¹⁸O=-7.70 to 5.97 per mil, relative to SMOVN. These data compared with those in primary magmatic water and metamorphic water and meteoric waters are shown in figure 1. They have the isotopic compositions ranging from magmatic water to meteoric water, thus suggesting that magmatic and meteoric water should have been mixed in the ore fluids. However that is not the case Fig.2. shows the changes in the δ¹⁸O and δD values of magmatic and meteoric waters through equilibnum isotopic exchange with granitic rocks at various temperatures and with various water/rock weight ratios, and suggests that meteoric water alone can evolve into the observed isotopic compositions of the Rushan gold ore fluids by isotopic exchange reactions with granitic racks at temreratmes hetween 200℃ and 350℃.In ether words,a genetic model in which the meteoric water reacts with granitic rocks in different tempcrature and W/R ratio can be used to explain the mmerogenetic mechanism of the Rushan gold deposit. This model is also supported by other geological and geochemical studies of the Rushan gold deposit.As mentioned above, it is not the best method to compare she δD and δ¹⁸O values of the ore fluids with those of standard only by means of plotting figure and then determining the origin of the H₂O involved in ore mineralizations. Therefore, gread care has to be taken particalarly in the ease to explain the genesis of a deposit using mixing water model because there arc differences in hydrogen and oxygen isotopic compositions between the water of ore fluids and initial water attcr water-rock exchanged. No simple mixing model for origin is likely to explain all or even most metalliferous ore deposits.