Abstract
In our previous study on petrogenesis of quartz syenite and granite porphyry, the host rocks of the Late Mesozoic Shapinggou Mo deposit in the Qinling–Dabie orogenic belt, we found that the initial Sr isotopic composition of the host rocks is strongly affected by the degree of K-alteration. Here, we provide further isotopic evidence of the host rocks and their minerals to investigate the geochemical behaviour of trace elements and isotopes during the alteration and to explain the phenomenon of decoupling of Sr–Nd isotopic composition. The quartz syenite and granite porphyry are altered by K-alteration in varying degrees and have high K2O and Rb contents and low Na2O, CaO, Sr, and Ba contents. Rock samples of both quartz syenite and granite porphyry have variable Rb/Sr ratios and initial 87Sr/86Sr values (even < 0.70) but contain quite homogeneous εNd(t) values (−12.8 to −14.8). Minerals from the rocks of moderate to intense K-alteration have very low initial 87Sr/86Sr values (even < −17), while those from the weakly altered rocks have 87Sr/86Sr(t) values of 0.7044 to 0.7084. The same phenomenon of the decoupling in Sr–Nd isotopic composition can be observed from several Mo deposits within the eastern Qinling–Dabie orogenic belt. This fact suggests similar hydrothermal features and a comparable origin for both the magmatic rocks and hydrothermal fluids in this belt. A comparison between porphyry Mo and porphyry Cu deposits shows that elements and the Rb–Sr isotope system have different behaviours during the K-alteration, implying distinct material sources and igneous rocks for porphyry Mo and porphyry Cu deposits, respectively.
Highlights
Radiogenic isotopic inhomogeneity of granitoid rocks derived by crustal anatexis is commonly attributed to inhomogeneity in the source, incomplete magma mixing, assimilation and contamination, and incongruent melting [1,2]
We focused on the major Mo ore-bearing intrusions, including quartz syenite, and granite porphyry, and their minerals, especially those Sr-hosting minerals, e.g., plagioclase, K-feldspar, quartz, apatite, and biotite, in the Shapinggou porphyry Mo deposit
Anomalous Sr isotopic compositions, observed in granite porphyry rocks associated with the Late Mesozoic Shapinggou porphyry Mo deposit and their minerals, especially biotite and K-feldspar of high potassic contents, were caused mainly by extensive alteration of K-rich fluids
Summary
Radiogenic isotopic inhomogeneity of granitoid rocks derived by crustal anatexis is commonly attributed to inhomogeneity in the source, incomplete magma mixing, assimilation and contamination, and incongruent melting [1,2]. In the last decade, Sr isotopic inhomogeneity of the ore-bearing granite porphyries, previously reported in several studies [3,4,5,6,7] on molybdenum (Mo) deposits within the eastern Qinling–Dabie orogenic belt, is hard to explain through the abovementioned causes, as many of them have very low initial 87Sr/86Sr values, even lower than the recommended value for Basaltic Achondrite Best Initial (0.69899) [8,9]. They have a consistent Nd isotopic composition. In order to systematically characterise the nature and extent of hydrothermal alteration and evaluate the cause of anomalously low initial 87Sr/86Sr values and the nature and origin of Mo ore-forming hydrothermal fluids, geochemical and Sr–Nd isotopic data of whole rocks and minerals with various degrees of alteration are reported
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