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Chapter 10Economic GeologyIntroduction and classificationThere are no active mineral workings in the district, but lead, silver, tungsten and graphite haveall been mined in the past and there are occurrences of zinc, tin, fluorite and beryl. Quartz andclay, which occur within intrusive, and interleaved volcanic rocks, have also been mined on asmall-scale at numerous locations. Although there are no active quarries, there are large borrowareas on Tsing Yi, and smaller borrows in northeast Lantau Island. As part of the new airportdevelopment, Chek Lap Kok was largely levelled and during the preparation of this memoir itbecame the biggest borrow area in the district. The Brothers islands were similarly levelled andexploited.The economic geology is divided into three sections; metalliferous minerals, non-metalliferousminerals, and construction materials. The mineral deposits (Figure 24) include those mined, orprospected for, and other significant mineral occurrences 1 . The construction materials includefreestone and aggregate quarries, and borrow areas.Metalliferous MineralsMetalliferous mineral deposits usually occur as narrow fissure veins, infilled by precipitates fromhigh temperature, mineralized fluids. These fluids were probably mainly derived from Mesozoicgranite bodies at a late stage during their emplacement. Two abandoned mines are known in thedistrict; one at Silver Mine Bay, and another at Sha Lo Wan.The mine which lends its name to Silver Mine Bay is about 1km northwest of Mui Wo. It wasactive until 1896, and produced galena, from which lead and small amounts of silver wereextracted. The only visible remnants of the mine, are a large tailings fan, adjacent to an aditwhich is sealed with a concrete wall (17085 14965). The adit was driven along aneasterly-trending fault, which separates a silicified feldsparphyric rhyolite dyke, with traces ofgalena along the northern wall of the adit, from medium-grained granite, seen in the roof.The wolframite (tungsten oxide) mine at Sha Lo Wan (0901 1584, 0896 1564, 0885 1570) wasactive until the 1960s. It exploited extensive mineralization in a stockwork of quartz veins andshears, commonly cut by thicker, typically southsouthwest-dipping, quartz-filled, and, or,pegmatitic, shear-veins, joints and fractures. The mineralization is located at, or near, the locallywestnorthwestcrly-striking contact zone between granitic intrusions to the north, and volcanicrocks infilling the Lantau Caldera to the south. Fine-grained quartz syenite is intruded along thiscontact zone near the mine workings. Ruxton (1957) recorded sporadic ore minerals in thestockwork, including molybdenite, chaicopyrite, arsenopyrite, wolframite and pyrite. He alsonoted very minor occurrences of ore minerals in the cross-cutting quartz veins.The area was extensively trenched prior to the 1950s, although there are no records thatwolframite was mined, However, wolframite ore was extracted in 1952 and 1953, and in the late1950s, the Far Eastern Prospecting and Development Corporation started a large-scale working1Metallifereous minerals, together with associated non-metalliferous minerals, are subject to theprovisions of the Mining Ordinance, Permission to prospect for, or work these deposits, is granted in theform of prospecting or mining licences respectively.149