Geology of Needle Hill tungsten mine

Abstract

Needle Hill Tungsten Mine comprises the major Tungsten deposit in Hong Kong which has been closed for several decades. A study of the geology in the mine was carried out because there were a few detailed studies to tell the origin of the ore formation and the paragenesis of the minerals in the mine. The objectives of this study were to reveal the paragenesis of the ore minerals including wolframite and molybdenite. In addition, the possible origin of the wolframite was reconstructed and the distribution of wolframite and molybdenite were identified. Macroscopic and microscopic analyses including several site reconnaissances, thin sections studies, Electron Microprobe (EPMA) and X-Ray Fluorescence Spectroscopy (XRF) were carried out. The images of thin sections were taken by digital camera as well as Scanning Electron Microscope (SEM) in EPMA. The major and trace element data from XRF analyses were tabulated and plotted with dozens of graphs. Our data demonstrate that the genesis of ore minerals was related to hydrothermal alterations (Greisenisation) of granite after the A-type granite emplacement. Ore minerals were deposited in sheeted quartz veins and patches of pegmatites after Greisenisation. The paragenesis of the ore minerals involved late magmatic stage, Greisenisation stage and deposition stage. In Greisenisation stage, feldspars and biotite in granite were altered to sericite (muscovite), quartz and magnetite. Wolframite, fluorite and molybdenite were formed afterwards. Topaz and quartz were formed finally. Ore oxides and sulphides were deposited from quartz vein and pegmatite formations along the contact with Greisen. Meantime, late stage chloritization was evident in Greisen. Moreover, the source of ore minerals was interpreted to be related to A-type granite which was derived from lower crust or mantle in orogenic environment and contaminated by felsic upper crust. Fractional crystallization might have facilitated the concentration of wolframite in the hydrothermal fluid during magmatism. In the ore mineral distribution, wolframite was abundant at the upper level of the mine and it was neither iron nor manganese rich. EPMA analysis indicates that the iron content was within 20% and 80% of the sum of Fe and Mn. Molybdenite was abundant at the lower level of the mine.