Economic Geology – Minerals and Mining in Hong Kong

Mineral Occurrences

Despite its small size, Hong Kong has a relatively large number of mineral occurrences (Figure 1). Some mineral deposits have been exploited commercially. Mesozoic igneous activity was 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 operating in Hong Kong.

Figure 1: Location of economic mineral occurrences in Hong Kong.

Mining History in Hong Kong

The Lin Ma Hang Lead Mine

Lead was discovered in the Lin Ma Hang area in the 1860s. The lower, or Portuguese, workings were operated in the nineteenth century, and the main vein was discovered in 1915. A mining company was formed in 1917, but only operated for three years. A 75-year mining lease was issued in 1925, backdated to 1922. The mine changed hands in 1932, and again in 1937, at which stage about 2,100 metres of tunnels had been developed. Further development work was undertaken by the new owners, but mining was suspended in 1940 with the outbreak of war. Small-scale working was carried out by the occupying Japanese from 1941 to 1945, mostly by robbing pillars in the eastern section of the mine, which resulted in caving of the roof. The mine remained abandoned until 1951, when working by various contractors resumed. Labour disputes, strikes, typhoon damage, and falling lead prices led to closure of the mine on the 30th June 1958, with about 60% of the reserves mined. The mining lease expired in April 1962 (Figure 2).

The Needle Hill Tungsten Mine

Wolframite deposits were discovered in 1935, and a mining licence issued in the same year. Mine development commenced in 1938 and continued throughout the Japanese occupation. Mining activity, including unlicenced surface excavations, increased during the Korean War period of 1949-1951 when tungsten prices rose sharply. By 1967, declining tungsten prices and increasing labour costs prompted the suspension of mining operations.

The Ma On Shan Iron Mine

The mine operated for 70 years between 1906 -1976. Small-scale opencast mining was carried out between 1906 - 1949, increasing in scale after 1949, with underground mining beginning in 1953 (Figure 3). Mining had moved entirely underground by 1959. Major development work occurred in the 1960s, after all the reserves in the upper levels had been exhausted. In 1963, 5,458 metres of main tunnels and shafts and 3,000 metres of sub-levels, including 5 main ore passes, were constructed. A 2.2-kilometre long haulage drive was constructed at the 110 metre level, with a new portal near the processing plant only 200 metres from the coast. During the mid-1970s, a worldwide decline in the demand for steel, the opening-up of large iron deposits in Australia, and the termination of a contract to supply Japan, led to mining being suspended in March 1976. The workforce of 400 was laid off and the mining lease expired in March 1981.

The West Brother Graphite Mine

Underground mining commenced in 1952. Manual mining methods were employed, using picks and chisels to extract the ore, and candles to light the underground workings. The ore was hauled to the surface in baskets, and then carried to the two drying grounds where it was hand-sorted and dried in the open. Natural ventilation was good at first because of the numerous tunnels and shafts in the complex. Later, electric lighting was introduced, along with electric dewatering pumps, ore hoists, and compressors. However, by 1971, the cost of pumping and ventilation in the deepening mine was deemed to be uneconomic, so mining ceased. The mining licence expired in January 1973 (Figure 4).

Figure 2: Abandoned mine adits at the former Lin Ma Hang Mine.

Figure 3: Historic photograph of blast hole drillingin the mining tunnel, Ma On Shan Mine.

Figure 4: Graphite seams on West Brother Island.

Metalliferous Minerals

Metalliferous mineral occurrences are grouped into four broad categories:

1. Tin - tungsten - molybdenum (Sn-W-Mo) mineralisation

Sporadic Sn-W-Mo mineralisation is mostly concentrated along major NW-trending structures, such as swarms of quartz veins, and in areas underlain mainly by fine-grained granites. These include areas such as Needle Hill, Sha Lo Wan and Lin Fa Shan.

• Wolframite [(Fe,Mn)WO₄] (Figure 5) was mined at Needle Hill, Lin Fa Shan and Sha Lo Wan, with minor workings at Castle Peak and Devil's Peak.
• Cassiterite [SnO₂] (Figure 6) has been noted in several areas, such as Sheung Tong, Needle Hill, and Devil's Peak, but has never been exploited commercially.
• Molybdenite [MoS₂] (Figure 7) commonly occurs in association with wolframite, but has never been exploited commercially.

Figure 5: Wolframite.

Figure 6: Cassiterite.

Figure 7: Molybdenite.

2. Copper – lead – zinc (Cu-Pb-Zn) mineralisation

Cu-Pb-Zn mineralisation is concentrated mainly in veins along NE-trending fault zones within areas underlain by coarse ash crystal tuffs in the New Territories. These include areas such as Lin Ma Hang and Tai Mo Shan. Pb–Zn ore was once mined at Lin Ma Hang, and on a small scale at Tai Mo Shan, Silver Mine Bay, and southeastern Lantau Island.

• Galena [PbS] (Figure 8), chalcopyrite [CuFeS₂] (Figure 9), and sphalerite [ZnS] (Figure 10), with traces of gold (Au) (Figure 11), were mined at the Lin Ma Hang mine
• Silver (Ag) (Figure 12) also occurs within galena at Silver Mine Bay on Lantau Island.

Figure 8: Galena.

Figure 9: Chalcopyrite.

Figure 10: Sphalerite.

Figure 11: Gold.

Figure 12: Silver.

3. Iron (Fe) mineralisation

Iron mineralisation is associated with skarn deposits where granite has come into contact with marble. The largest known mineral deposit is magnetite at Ma On Shan.

• The largest iron deposit is found at Ma On Shan where magnetite [(Fe,Mg)Fe₂O₄] (Figure 13) has been mined from a granite-related calc-silicate skarn deposit.
• Pyrrhotite [FeS] (Figure 14) and haematite [Fe₂O₃] (Figure 15) have also been reported from the Ma On Shan Mine.
• Pyrite [FeS₂] (Figure 16) has been reported at Lin Ma Hang, Mui Wo, and Tai Mo Shan, and as concretions in sedimentary rocks on Ma Shi Chau, A Chau, and Ping Chau.

Figure 13: Magnetite.

Figure 14: Pyrrhotite.

Figure 15: Haematite.

Figure 16: Pyrite.

4. Placer deposits of tin (Sn) and gold (Au).

Traces of tin and gold have been reported in alluvial deposits in the Sheung Tong area and these are thought to be derived from veins within nearby fine-grained granite. The minerals have been concentrated naturally by gravity separation in flowing water to produce alluvial placer deposits.

Non-metallilferous Minerals

Concentrations of non-metalliferous minerals that have been commercially exploited include kaolin clay, feldspar, quartz, beryl and graphite.

Kaolin clay has been mined at Cha Kwo Ling, Chek Lap Kok (Figure 17), and Tsing Yi, but there have also been numerous other small occurrences mainly in the northwest of Hong Kong.

Feldspar was once mined from a large vein at Cha Kwo Ling, whereas quartz has been produced by numerous mining operations exploiting mainly weathered granite and quartz veins.

Beryl, associated with tungsten veins in granite in the Devil's Peak area, was never commercially exploited, whereas graphite was mined extensively from seams in metasedimentary rocks on The Brothers.

Feldspar

High quality alkali feldspar [K(AISi₃O₈)] (Figure 18) used for ceramics, tile and glass manufacture was once mined from a large pegmatite at Tung Lo Wan, Sha Tin. Alkali feldspar and plagioclase feldspar [Ca(Si₃O₈)] (Figure 19) were also mined at Cha Kwo Ling from a weathered dyke within granite and on The Brothers and Chek Lap Kok.

Figure 17: Kaolin mine on the former island of Chek Lap Kok.

Figure 18: Alkali feldspar.

Figure 19: Plagioclase feldspar.

Quartz

Quartz [SiO₂] (Figure 20) was mined from weathered granitic soils and thick quartz veins at several localities 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. Offshore sand, predominantly from the Pleistocene alluvial deposits, was an important source of fill materials for major reclamation projects, such as the Chek Lap Kok, West Kowloon and Kwai Chung Container Terminals.

Beryl

In the Devil's Peak area, high-grade beryl [Be₃Al₂(SiO₁₈)] (Figure 21) has been reported from wolframite-bearing quartz veins within hydrothermally altered fine-grained granite. However, the deposit has not been exploited commercially. Minor quantities of beryl have also been reported from the D'Aguilar Peak area.

Graphite

Graphite [C] (Figure 22), interbedded with quartzite, meta-sandstone and meta-siltstone was once mined on West Brother Island. The graphite occurs in steeply dipping seams, up to 4.5 m thick.

Figure 20: Quartz.

Figure 21: Beryl.

Figure 22: Graphite.

Quarrying

Quarrying is an important activity, providing both dressed building stones and aggregates for concrete. However, quarrying, which is an accelerated form of artificial erosion, dramatically changes the appearance of the landscape, both by producing "erosion scars' (quarries), and by redistributing and "depositing' vast quantities of, largely granitic, material.

Prior to 1966, there were numerous small "permit' quarries scattered around Hong Kong, largely producing dressed building stone. The last "permit' quarry closed in 1974 in favour of larger, licenced quarries.

Until 1978, all processed stone was obtained from quarries in Hong Kong. Subsequently, importation of stone began, so that by 1987 about 44% of the demand was met by imports from the Shenzhen and Zhuhai Special Economic Zones. This change reduced the pressures on the limited land area of Hong Kong, and many quarries were closed.

In 1980, two large Government quarries and seven contract quarries were operating, which produced 15 million tonnes of aggregates. By 1988 the number was reduced to one Government quarry and five contract quarries.

As the population of Hong Kong grew, the demand for aggregates grew. There was a significant increase in the consumption of aggregates between 1960 and 1990, from 3 million tonnes to 18 million tonnes. Despite a 50% increase in population over the period, this represented an annual increase from 0.75 tonnes per head to 3.4 tonnes per head of population.

To express the removal of this amount of rock material in terms of erosion rates, the quarrying of 18 million tonnes of granite is equivalent to removing a layer 6.3 mm thick from the whole land area of Hong Kong (1,105 km²).

Today, under a plan formulated in 1989, the three remaining operational quarries in Hong Kong are being rehabilitated to form green areas for future development. Shek O (completion in 2009), Anderson Road (completion in 2013) and Lam Tei (in 2015) quarries are being rehabilitated under contracts that involve major recontouring to soften the appearance of the angular quarry profile, tree and shrub planting, and erosion control. A similar rehabilitation contract at Lamma Quarry was completed in 2002 to form a green site of 0.49 km².

Rehabilitation contracts involve the landscaping of the quarries, and the production of saleable rock products. Thus, Anderson Road Quarry (Figure 23) will produce 50 million tonnes of rock up to 2013 (2.6 million tonnes in 2006), Shek O Quarry will produce 23 million tonnes, and Lam Tei Quarry will produce 6.5 million tonnes.

Quarries, both large and small, are a feature of the Hong Kong landscape and can be identified at many localities, indicating the scale of the human contribution to "erosion'.

Figure 23: Anderson Road Quarry,
eastern Kowloon

Offshore Sand Deposits

The demand for aggregate sand and reclamation fill in Hong Kong has grown as the rate of urban development has increased. As early as in the 1920s and 1930s, fine aggregate sand was extensively extracted from beaches around Hong Kong. Over-exploitation led to the enactment of the Sand Ordinance in 1935, which was designed to regulate the removal of natural sand. The 1950s saw the first use of sea bed sand for major engineering projects. During the 1960s, the government carried out a sand survey and compiled an inventory of some 30M m³ of marine sand that was considered to be suitable for engineering use. However, by the 1980s, the scale of reclamation had increased and the demand for sand outstripped the available sand resources in Hong Kong.

Beginning in 1982, the systematic geological surveying and mapping of Hong Kong provided a geological framework that assisted the exploration for offshore sand resources. In 1988, the Geotechnical Control Office (now Geotechnical Engineering Office) embarked on a preliminary prospecting survey in western and central waters to locate 100M m³ of offshore sand and gravel deposits that could be used as both fill and aggregate. Following the success of this initial survey, the search was extended into eastern waters in the Seamat (Seabed Materials) Study. The surveys were based on the prevailing understanding of the offshore superficial stratigraphy and palaeogeography of Hong Kong, from which occurrences of sand deposits in several discrete geological environments were predicted. As the survey progressed, the offshore geological model was continuously refined. The Seamat study located 14 major sand bodies that contained a total volume of 588M m³ of identified resources (Figure 24).

Figure 24. Seamat resources, sand borrow areas and mud disposal grounds.