Abstract:

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.

Key words: Luobuling Cu-Mo deposit, concealed deposit, surface fine-grained soils, anomalous source, Cuisotope