The Jianfengling granite complex and the associated polymetallic mineralization, Hunan province, P.R. China

Abstract

The Jianfengling granite (previously known as the `430 granite') is a small, well-exposed stock approximately 4.4. km<sup>2</sup> in area. It was emplaced into a sequence of Devonian and Carboniferous limestones and dolomites during the Yanshan orogeny (Jurassic-Cretaceous) and has a K-Ar age of 160-170 Ma. Evidence from sharp intrusive contacts indicate the stock to be a polystage complex comprising five distinct intrusions which can be divided into two groups on the basis of petrographic and chemical characteristics: the `zinnwaldite granites' (intrusions of stages 2 and 3) which contain zinnwaldite and abundant albite; and the `biotite granites' (intrusions of stages 1, 4 and 5) which contain biotite and abundant K-feldspar. These granites are fine- to medium-grained, equigranular to slightly porphyritic textured, with essential quartz, alkali-feldspars (albitic plagioclase and K- feldspar), biotite, zinnwaldite, and topaz. Chemically, all the granites are weakly peraluminous with a molar ratio Al<sub>2</sub>O<sub>3</sub>/CaO+ Na<sub>2</sub>O+ K<sub>2</sub>O) ranging from 1.10 to 1.58, (normative corundum contents from 1.6-3.5 mol.%). Quartz and feldspar account for more than 95% of the normative compositions. The granites are fluorine-rich (F= 0.55-2.75 wt. % ), high silica (SiO<sub>2=</sub> 72.50-76.40 wt. % ) types with extremely elevated contents of Li, Rb, Cs, Y, Nb, Ta, W, Sn, and U, and depletion of TiO<sub>2</sub>, MgO, CaO, P<sub>2</sub>O<sub>5</sub>, Ba, and Sr. These granites are analogous to the `low-P' subtype of topaz granites of Taylor (1992). Pegmatites and pegmatitic pods were formed at a temperature range of 480-540^oC and are considered to be the results of `liquid segregation' during the late stages of the evolution of the granites. Fluids in equilibrium with the pegmatitic rocks have δ18O values of 8.1-10.7%₀and δD values of -70 to 88%_0.

Various types of mineralisation, including Nb, Ta, W, Sn, Be, Pb, Zn, Ag and F, are spatially and genetically associated with the Jianfengling granites. They can be categorised into two main groups: granite-type and lode-type.

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