Zircon U-Pb Age and Geochemistry of Granitoids Within Jinla Pb-Zn-Ag Poly-metallic Ore Field Across China and Myanmar Border

Abstract

Jinla Pb-Zn-Ag poly-metallic ore field across China and Myanma, is tectonically located in the south segment of the Mengtong–Gengma–Ximeng passive continental margin of Proterozoic era-Palaeozoic era, and the eastern margin of the Baoshan–Shantai Massif. Mineralization related granitoids within the ore field include Laochang amphibole admellite, Menglinshan biotite admellite and Nanla K-feldspar granite-porphyry. This paper systematically presents the characteristics of the above granites on their major elements, REE and trace elements (including ore-forming elements) and their isotopic ages determined by zircon SHRIMP dating. The intrinsic connection and genesis of the granites, and the relationships between granites and Pb-Zn-Ag mineralization are discussed. It comes to the conclusions as follows: (1) Among the three types of granites in Jinla ore field, the U-Pb zircon ages for both Laochang porphyraceous amphibole admellite and Menglinshan porphyraceous biotite admellite are (45.41±1) Ma, and Nanla K-feldspar granite porphyry, with a zircon U-Pb age of (43.41±0.78) Ma, formed at the late stage of a syn-collisional mineralization process of Himalayan orogenesis. The magma may have been resulted from partial melting in the crust due to crust and mantle interaction and therefore, is dominated by crustal source materials mixed with some mantle components. Laochang porphyraceous amphibole admellite and Menglinshan porphyraceous biotite admellite contrain significant amount of mantle source materials, whereas Nanla K-feldspar granite porphyry is dominated by crustal materials mixed with minor mantle component. (2) Major and trace elements (including REE and some trace elements (Zr/Hf, Nb/Ta, Rb/Sr, Rb/Ba, K/Rb, (Rb/Yb) N, Sr*, K* and Zr*) suggest that the granitic intrusions in the research area may have formed in locally extensional tectonic environment. Laochang porphyraceous amphibole admellite and Menglinshan porphyraceous biotite admellite may have formed at deep crust during the early stage of magma differentiation. Nanla K-feldspar granite porphyry, on the other hand, may have formed at shallow crust during the late stage of magma differentiation. These three granite intrusions are the products of various stages of magma differentiation during magmatic-tectonic evolution process. (3) Our study on the ore-forming elements in these intrusions suggests that in comparison with granites in China, although Laochang porphyraceous amphibole admellite and Menglinshan porphyraceous biotite admellites are relatively rich in some ore-forming minerals, Nanla K-feldspar granite porphyry has the highest ore-forming potential because, comparing with the other two intrusions, it not only contains more ore-forming minerals, such as W, Cu, Bi, Sb, Mo, Sn, Ag, Pb, Au, etc., the granite is also much richer in F, B, As and other mineralizing elements.