铜同位素示踪覆盖区地表土壤的异常来源初探:以福建罗卜岭隐伏铜钼矿床为例

地表微细粒土壤地球化学测量是国内研究较多的深穿透地球化学方法之一,判断地表异常与深部隐伏矿体的关系是未来该方法广泛应用的理论基础。以已知的隐伏矿床——紫金山罗卜岭铜钼矿床为研究对象,利用铜同位素示踪覆盖区地表土壤的异常来源。结果显示:(1)矿石单矿物的δ⁶⁵Cu变化范围较小(-0.04‰~0.56‰),指示较高的成矿温度(250~300 ℃);钻孔由深至浅,矿石的δ⁶⁵Cu值具有明显升高的趋势,符合斑岩型成矿系统早期至晚期铜同位素的变化特征,流体分馏是造成不同阶段形成的矿石铜同位素存在差异的主要原因。(2)背景区地表土壤铜含量的平均值(59.4 μg/g)显著低于异常区(131.0 μg/g)。背景区土壤δ⁶⁵Cu变化范围为-1.94‰~-0.82‰,平均值为-1.38‰;异常区土壤δ⁶⁵Cu变化范围为-5.01‰~1.05‰,平均值为-1.40‰,背景区和异常区铜同位素组成相差不大。(3)本次利用铜同位素组成判别地表介质铜异常来源的效果不理想,可能因为矿石本身的铜同位素组成存在差异,地表土壤本身的铜同位素信息覆盖了来自深部与隐伏铜矿体有关的信息,或从深部原生环境迁移至地表氧化环境的过程中,因氧化还原反应发生了铜同位素的分馏。

Preliminary Study on Anomalous Source of Surface Soils in Covered Area Traced by Cu Isotope: A Case Study in the Luobuling Concealed Cu-Mo Deposit,Fujian Province

Surface fine-grained soils have been sampled to explore concealed deposits in the covered areas in China. Determining the relationship between the surface anomalies and concealed orebodies is critical for the extensive application of this method in the future. In this paper, the Cu isotope is applied to trace the anomalous source of the surface fine-grained soils in the known concealed deposit, Luobuling Cu-Mo deposit, Fujian pro-vince. The following results have been obtained. (1) The δ⁶⁵Cu range of minerals in ores is small (-0.04‰ to 0.56‰), indicating a high mineralization temperature (250 to 300 ℃). The δ⁶⁵Cu value of ores increases obviously from deep to shallow, which is consistent with the variation of Cu isotopic composition in the porphyry metallogenic system from the early to the late stage. The difference in Cu isotopic composition among different stages is mainly caused by fluid fractionation. (2) The average Cu content of surface soils in the background area (59.4 μg/g) is significantly lower than that in the anomalous area (131.0 μg/g). However, the δ⁶⁵Cu range of surface soils in the background area is -1.94‰ to -0.82‰, with an average of -1.38‰, and -5.01‰ to 1.05‰ in the anomalous area, with an average of -1.40‰. There is little difference between the background and the anomalous area. (3) Using Cu isotope to distinguish the origin of the Cu anomaly in surface fine-grained soils is not ideal in this deposit, which might be partly caused by the difference of Cu isotope composition in the ores. Meanwhile, the Cu isotope information of the surface soils could overlay that of hidden Cu deposits, or the Cu isotope fractionation occurs during the migration from the deep primary environment to the surface oxidation environment.