The giant tin polymetallic mineralization in southwest China: Integrated geochemical and isotopic constraints and implications for Cretaceous tectonomagmatic event

摘要

The Gejiu-Bozushan-Laojunshan W–Sn polymetallic metallogenic belt (GBLB) in southeast Yunnan Province is an important part of the southwestern Yangtze Block in South China. Tin polymetallic mineralization in this belt includes the Niusipo, Malage, Songshujiao, Laochang and Kafang ore fields in the Gejiu area which are spatially and temporally associated with the Kafang - Laochang and Songshujiao granite plutons. These granites are characterized by variable A/CNK values (mostly??>??1.1, except for two samples with 1.09), high contents of SiO2(74.38–76.84??wt.%) and Al2O3(12.46–14.05??wt.%) and variable CaO/Na2O ratios (0.2–0.65) as well as high zircon δ18O values (7.74‰–9.86‰), indicative of S-type affinities. These rocks are depleted in Rb, Th, U, Ti, LREE [(La/Yb)N??=??1.4–20.51], Ba, Nb, Sr, and Ti and display strong negative Eu and Ba anomalies. The rocks possess high Rb/Sr and Rb/Ba ratios, relatively low initial87Sr/86Sr ratios (0.6917–0.7101), and less radiogenicεNd(t) values (?8.0 to ?9.1). The zircon grains from these rocks show negativeεHf(t) values in the range of ?3.7 to ?9.9 with meanTDM2(Nd) andTDM2(Hf) values of 1.57??Ga and 1.55??Ga. They show initial207Pb/204Pb ranging from 15.69 to 15.71 and206Pb/204Pb from 18.36 to 18.70. Monazite from Songshujiao granites exhibits higher U and lower Th/U ratios, lower δ18O values and higherεHf(t) values than those of the zircon grains in the Kafang - Laochang granites. The geochemical and isotopic features indicate that the Laochang - Kafang granites originated by partial melting of Mesoproterozoic crustal components including biotite-rich metapelite and metagraywacke, whereas the Songshujiao granites were derived from Mesoproterozoic muscovite-rich metapelite crustal source. Most zircon grains from the Songshujiao, Laochang and Kafang granites have high-U concentrations and their SIMS U–Pb ages show age scatter from 81.6 Ma to 88.6??Ma, 80.7 Ma to 86.1??Ma and 82.3 Ma to 87.0??Ma, suggesting formation earlier than the monazite and cassiterite. Monazite SIMS U–Pb ages and Th–Pb ages of three same granite samples are consistent and show yielded206Pb/238U ages of 83.7??±??0.6 Ma, 83.7??±??0.6 Ma, and 83.4??±??0.6??Ma, and208Pb/232Th ages of 83.2??±??0.5 Ma, 83.8??±??0.4 Ma, and 83.5??±??0.9??Ma, which are within the range of the SIMS zircon U–Pb ages from these rocks. The data constrain the crystallization of the granites at ca. 83 Ma. In situ U–Pb dating of two cassiterite samples from the cassiterite-sulfide ore in the Songshujiao ore field and Kafang ore field, and two from the cassiterite - oxide??+??cassiterite bearing dolomite in the Laochang ore field yielded weighted mean206Pb/238U ages of 83.5??±??0.4 Ma (MSWD??=??0.6), 83.5??±??0.4 Ma (MSWD??=??0.5), 83.6??±??0.4 Ma (MSWD??=??0.6) and 83.2??±??0.7??Ma (MSWD??=??0.6), respectively. Combined with geological characteristics, the new geochronological data indicate that the formation of the granites and Sn polymetallic deposits are coeval. We correlate the magmatic and metallogenic event with lithospheric thinning and asthenosphere upwelling in continental extension setting in relation to the eastward subduction of the Neo-Tethys beneath the Sanjiang tectonic domain during Late Cretaceous.