10 Economic Geology

	Introduction Introduction Despite its small size (1104 km2), Hong Kong has a relatively large number of mineral occurrences. Some mineral deposits have been exploited commercially (Figure 10.1). Mesozoic igneous activity is largely responsible for this diversity of mineral deposits and the mineral concentrations have been variably enhanced by hydrothermal activity associated with faulting. There are currently no commercial mining or prospecting licences operative in Hong Kong. General descriptions of the mineral resources of Hong Kong have been given by Uglow (1926), Davis (1952, 1964), Ruxton (1960) and Allen & Stephens (1971). More site-specific descriptions are provided in the Hong Kong Geological Survey Memoirs (see Chapter 1) and in various publications referred to below. The most recent detailed account of mineralization in Hong Kong is contained in Sewell (1999).
	Historical records Historical records It has long been accepted that local deposits of clay suitable for pottery have been exploited for centuries. However, only recently have archaeological finds suggested that locally derived iron was smelted in kilns during the Tang dynasty, approximately 900 years ago (Plate 10.1). Organized mining operations seem only to have been active since the late 19th century and initially, these were concerned with two lead deposits. Ormsby (1898) commented that deposits of silver and alluvial tin had been reported by the local population and in 1906, an iron mine at Ma On Shan was opened. However, based on the large number of abandoned small surface workings, especially in the granitic areas, it seems that the region has been heavily prospected in the past. Prior to regulation, there may also have been many small uncontrolled mining operations. The natural resources of Hong Kong fall into three main categories: metalliferous veins and non-metalliferous industrial minerals in the onshore area; quarried rock and building stone; and offshore sand deposits. Offshore sand resources are described in the companion volume on Quaternary deposits (Fyfe et al., 2000).
	Metalliferous minerals Metalliferous minerals Sn–W–Mo mineralization Sporadic Sn–W–Mo mineralization is present in many parts of Hong Kong but is confined mostly to pegmatites, veins and greisen in contact zones of granites, and along major northwest-trending faults. Many of the fine-grained granites provide hosts for Sn–W–Mo mineralization. The richest deposits are associated with the Needle Hill Granite, and the Chek Lap Kok Granite. At the Needle Hill mine, west of Sha Tin, quartz veins, 0.2 to 0.4 m wide and dipping steeply to the southwest, were worked for their wolframite ((Fe,Mn)WO4) content. The veins intrude Needle Hill Granite (Chapter 6). The mine yielded 1000 tonnes of wolframite concentrate between 1938 and 1967. Roberts and Strange (1991) have described the mine in detail. Cassiterite [SnO2] has been noted from several areas (e.g. Sheung Tong, Needle Hill, Devil's Peak; Davis, 1964), but has never been commercially exploited. Wolframite [(Fe,Mn)WO4)] has been mined at Needle Hill, Lin Fa Shan and Sha Lo Wan, with minor workings at Castle Peak and Devil's Peak. Molybdenite is commonly present with wolframite, but even as a by-product of wolframite mining has never been exploited commercially. Pb–Zn–Cu mineralization Pb–Zn–Cu mineralization is present mainly in association with epithermal veins in coarse ash crystal tuff in the New Territories and on Lantau Island. The richest epithermal mineral deposits occur close to major northeast-trending shear zones. Pb–Zn ore was once mined at Lin Ma Hang (Plate 10.2), and on a small scale at Tai Mo Shan, Silver Mine Bay, and southeast Lantau Island (Figure 10.1). At the Lin Ma Hang mine, moderately steeply northeast-dipping quartz veins contain galena [PbS], pyrite, sphalerite [ZnS], and chalcopyrite [CuFeS2] in order of decreasing abundance. The galena is argentiferous and traces of gold occur in the chalcopyrite (Davis & Snelgrove, 1956). The veins intrude the Tai Mo Shan Formation, are up to several metres wide and extend laterally for up to 2 km. The mine operated intermittently between 1915 and 1958, producing 16 000 tonnes of lead metal and 360 000 ounces of silver. A detailed description of the mine is contained in Williams (1991). Silver also occurs within galena at Silver Mine Bay on Lantau Island (Peng, 1978). In general, chalcopyrite is present in small quantities wherever deposits of galena and sphalerite [ZnS] occur. Sphalerite and galena have been reported as minor accessory phases in some wolframite-bearing quartz veins at Needle Hill, Lin Fa Shan and Devil's Peak (Peng, 1978). Fe mineralization Fe mineralization is present in many areas of Hong Kong and in a variety of mineral occurrences. The largest such deposit is found at Ma On Shan where magnetite [(Fe,Mg)Fe2O4] has been mined from a granite-related calc-silicate skarn deposit. The skarn has a rich and varied mineralogy (Peng, 1978), and surrounds dolomitic marble at the contact between granite and Palaeozoic sedimentary rocks (Devonian and Carboniferous). Between 1906 and 1976, the mine produced more than 3 million tons of processed ore and remaining ore reserves have been estimated at 4 million tons (Strange & Woods, 1991). A small magnetite-bearing skarn, like that at Ma On Shan mine, has been encountered in boreholes beneath the Ma On Shan reclamation (Sewell, 1996). The skarn developed at the contact between the Sha Tin Granite and the Ma On Shan Formation. Other magnetite-bearing skarn deposits have been reported at Sha Lo Wan, west of Tung Chung and east of The Brothers islands. These and other skarn-related deposits in Hong Kong generally occur in pre-Jurassic sedimentary rocks, and especially Carboniferous and Permian marble. Their formation was related to intrusion of Middle Jurassic to Early Cretaceous granites. Pyrite [FeS2] has been reported in association with: Pb–Zn–Cu deposits at Lin Ma Hang, Mui Wo, and Tai Mo Shan; wolframite-bearing quartz veins at Needle Hill, Sha Lo Wan and Devil's Peak; and as concretions in sedimentary rocks on Ma Shi Chau, A Chau, and Ping Chau (Peng, 1978). Pyrrhotite [FeS] and haematite [Fe2O3] have also been reported from the Ma On Shan Mine and as concretions in sedimentary rocks.
	Non-metalliferous minerals Non-metalliferous minerals Feldspar High quality feldspar used for ceramics, tile and glass manufacture was once mined from a large pegmatite at Tung Lo Wan, Sha Tin. Mining operations did not last long and although a mining licence was granted in 1976, the mine was abandoned in 1984 when the deposit became uneconomic. Feldspar was also mined at Cha Kwo Ling from a weathered dyke within granite (see Fyfe et al., 2000). Quartz For many years, quartz was mined from residual granitic soils and thick quartz veins at several places across Hong Kong. On Chek Lap Kok, silica sand was produced as a by-product of kaolin mining. Smaller operations working quartz veins have existed at Pak Kok and Mong Hau Shek, Cheung Shue Tau, Mong Tung Hang on Lantau Island , Siu Lam, Mai Po, Tung Lo Wan, Needle Hill, Sheung Kwai Chung and Lai Chi Kok. Beryl In the Devil's Peak area, high-grade beryl [Be3A12(SiO18)] has been reported from wolframite-bearing quartz veins within hydrothermally altered fine-grained granite (Ruxton, 1958). However, the deposit has not been commercially exploited. Minor quantities of beryl have also been reported from the D'Aguilar Peak area by Ruxton, (1958) and Peng, (1978). Graphite Graphite [C], interbedded with quartzite, sandstone, siltstone, and mudstone of the Mai Po Member of the Lok Ma Chau Formation (Langford et al., 1989), was once mined on West Brother island. The graphite occurs in steeply (60o) northeast-dipping seams, up to 4.5 m thick. The graphite may have originated from coal seams within the Carboniferous sedimentary rocks (Plate 10.3), that were later metamorphosed by intrusion of granite (Ruxton, 1957). The mine operated between 1952 and 1971. A full description of the history of the mine is contained in Woods & Langford (1978).
	Quarried rock and sand resources Quarried rock and sand resources Building stone and aggregate Granite derived from local quarries was used extensively in the past for dressed stone but is no longer used for that purpose. This is because the granite is subject to rapid discolouration and exfoliation in the hot, humid conditions of Hong Kong, and is especially vulnerable to attack by acids produced by carbon dioxide and sulphur dioxide emissions. The local volcanic rocks are much more resistant to weathering but are not as decorative as granite, so are not used for ornamental purposes. For many years, granite and volcanic rocks have been quarried locally for road base metal, pell mell, armour stone and asphalt although the main purpose now is for concrete aggregate. At present, there are only four quarries operating in Hong Kong. These are principally in fine-grained granite and are located at Lam Tei, Shek O, Lamma, and Anderson Road. All except the Lam Tei Quarry are in the process of rehabilitation and have a life expectancy of between three and fifteen years. Texturally, the granite lithologies are mostly fine grained and equigranular although porphyritic fine-grained lithologies are common at Lam Tei Quarry, and medium-grained lithologies are dominant at Shek O Quarry. A small proportion of volcanic rock, mainly fine ash crystal tuff, is quarried at Anderson Road.