1.0 Tectonic Setting of South China
Hong Kong, which is located near the southeastern margin of the Eurasian Continental Plate (ECP), called the Cathaysia Block, in Southeast China, rests on a stable intraplate region of the ECP some 700 km from its nearest plate boundary, an area that underlies Taiwan and trends southward to the Philippines and northeastward to Japan (Pappin, Koo, Free, & Tsang, 2008). Although stable, this continental intraplate region has been associated with earthquakes in the past as it straddles the northeast-trending Lianhuashan Fault Zone.
There are two major fault zones, which trends in the regional northeast within the Cathaysia Block’s Mesozoic Magmatic Belt and is believed to run along the Southeast China coast (Pappin, Koo, Free, & Tsang, 2008). This magmatic belt extends to Shanghai and towards Nanjing in the northwest within the Yangtze Block and to Guangzhou and the Hainan Island and towards Ji’an and Guilin in the southeast within the Cathaysia Block.
The first fault is the Changle-Nanao Fault Zone (CNFZ), which is offshore northeast of Hong Kong, facing Taiwan and running along the coast from kilometers northwest of Hong Kong to some kilometers southeast of Wenzhou. Meanwhile, the second fault is the Lianhuashan Fault Zone (LFZ), which is running inland and parallel to the coast and connecting Shanghai in the northwest and Hong Kong in the southeast. Evidence indicates that it overlies a major discontinuity in the layers of the crustal plate, which, particularly its middle to lower crusts, are flanked by Proterozoic crust, which are more mafic (Fletcher, et al., 1997). The upper crusts are mostly heterogeneous while the lower to middle crusts primarily an Archaean segment of felsic characteristics. Thus, the LFZ constitutes the major fault zone immediately north of Hong Kong within the Cathaysia Block.
Farther north, within the Yangtze Block, another fault zone exists. The fault zone is called the Tan-Lu Fault Zone (TLFZ), which transects southeasterly the Yangtze Block from the western border of the North China Block and disappears in the intersection between the Yangtze and the Cathaysia blocks.
Pappin, Koo, Free, and Tsang (2008) used a seismic source zone model, which divides the Hong Kong region into seven zones and a Taiwan region. The zones located in the northern side of Hong Kong are located both onshore at the northeast side, the immediately surrounding areas, and the northernmost side, which include the northern border with Guangdong Province.
The northeast side of Hong Kong, which has a 108,303 km2 area, includes Nanjing, Shanghai, and Wenzhou (Pappin, Koo, Free, & Tsang, 2008). It also includes the larger parts of the Tan-Lu, Lianhuashan, and Changle-Nanao fault zones. Historical records from the Guangdong Seismological Bureau (GSB) showed an annual seismic activity of 1.28 times annually with earthquake magnitudes of more than 4.0 and a density level of 11.8 events within a land area of 1 million km2. It has the highest seismic activity levels in the northern Hong Kong zones principally due to the presence of the larger parts of the three fault zones.
Conversely, the area immediately surrounding Hong Kong up to 100 km northward in the eastern side and up to 350 km in the northwest side, which has a land area of 143,938 km2, includes Guangzhou (Pappin, Koo, Free, & Tsang, 2008). It has less than one annual activity with a density of 4.96 per 1 million km2. This higher annual seismic activity can be attributed to the presence of the smaller portion of the two fault zones (CNFZ and LFZ) located within.
Meanwhile, the northernmost area comprises a land area of 128,196 km2 and located from up to 175 km from Hong Kong northward in the eastern side to a maximum of 500 km to the northwest (Pappin, Koo, Free, & Tsang, 2008). This area includes Wuhan and most of the Yangtze Block. It has the lowest annual activity level (0.12) and with a density of 0.97 per 1 million km2. This low but observable seismic source activities indicate the presence of minor faults that remained to be identified and located. It is also the location of the Lin Ma Hang mine.
2.0 Geology of Northern Hong Kong
Hong Kong’s location near the southeastern margin of the Eurasian Continental Plate (ECP) in Southeast China places it within the Mesozoic Magmatic Belt, which is a unique feature of the Cathaysia Block as mentioned in Section 1.0. At least seventy-five percent of its land area rests upon igneous rocks, which are consisting of tuffs and granites of volcanic materials believed to have occurred from the Late Jurassic (140 Ma) to the Early Cretaceous (120 Ma) (Pappin, Koo, Free, & Tsang, 2008).
Moreover, the Late Jurassic-Early Cretaceous igneous rocks are mostly found underlying the Lantau Island’s Lantau Peak, which comprised more than half of the island’s land area; the Hong Kong island, also comprising more than half of the land area; the New Territories; most of the area surrounding Sai Kung, from where sample 16 was obtained; and almost all of Sheung Shu (and nearby islets located in Mirs Bay) where the Lin Ma Hang mine is located (see the Orange arrow).
The majority of the remaining area comprised of Late Paleozoic sedimentary rocks (420-240 Ma) and Late Mesozoic (140 Ma) and Tertiary (2 Ma) sedimentary rocks (Pappin, Koo, Free, & Tsang, 2008). There are only limited areas where these rocks can be found, such as the northern coast and southern patches along the Tolo Channel and small portions of Yuen Long. Nevertheless, majority of the tectonic deformations observable in Hong Kong rocks today resulted from the Yanshanian Orogeny, which was a series of tectonic events in the Late Jurassic (160 Ma) to the Late Cretaceous (90 Ma) (Pappin, Koo, Free, & Tsang, 2008).
However, Late Jurassic to Early Cretaceous granitoids can be found in a large part of Hong Kong, but mostly in the southern half, particularly part of the Lantau Island, most of Tsuon Wan and Sha Tin, and almost half of the Hong Kong Island. In the northern half, these rocks concentrated in the Shan Ma Tsuin, particularly around the coastal and inland areas facing the Deep Bay and in the eastern side, and the Tai Po areas. Overall, the northern half of Hong Kong is dominated by Late Jurassic to Cretaceous volcanic rocks followed by relatively comparative levels of Late Jurassic to Early Cretaceous granitoids and Quaternary deposits.
Near the Hong Kong-Guangdong border, specifically by the Sham Chun River (Williams, 1991), the site of the Lin Ma Hang mine, which is of interest in this paper, is located exclusively within a large area of Late Jurassic to Cretaceous volcanic rocks, which came from the Repulse Bay Volcanic Group (Williams, 1991), with sedimentary rocks of the Upper Jurassic Tai Mo Shan Formation (Williams, 1991).
The geological elements found in the Lin Ma Hang mine provide an important clue on the role of volcanic rocks in the formation of lead and zinc ores observed in this northernmost part of Hong Kong. The dominant rock types are metamorphisms from coarse ash tuff, which transformed into Tai Mo Shan porphyries, and schistosities of silty and sandy-silty sedimentary rocks that formed into Lok Ma Chau schists (Williams, 1991). Apparently hydrothermal events in these periods and thereafter had led to the eventual abundance of lead-rich (galena) and zinc-rich (sphalerite) ores in this mining site. Metallic mineralization apparently resulted to the highest abundance of galena, followed by pyrite, sphalerite, and chalcopyrite (Williams, 1991), as can be observed in the sample shown in Section 3.0.
As historically validated, the presence of lead and zinc ores had been associated with the presence of silver and gold (Leong & Mujumdar, 2009). In the Lin Ma Hang mine, Galenas carry significant silver values, ranging from 10 to 15 ounces per ton (oz/ton) or an equivalent of 300 to 500 grams per tonne (g/tonne) (Williams, 1991). However, silver was believed to occur as a solid solution within the galena instead of distinct silver minerals. Lead minerals also exist in the forms of anglesite (PbSO4) and cerrusite (PbCO3).
3.0 Hydrothermal Events
Abraham, Konka, and Gebreselassie (2015) noted that sulfide minerals (e.g. galena, chalcopyrite, sphalerite, and pyrite) can be found in samples in two generations. The first generation formation follows a replacement process wherein older forms are replaced by younger forms. The process begins with a source of galena and/or pyrite (old form), which, through millennia, are gradually replaced by the younger forms of sphalerite and chalcopyrite before becoming a first generation pyrite. Samples will show remnants of the older pyrites or galena as inclusions in the younger pyrites, filling its interstices and showing clear replacement textures.
Conversely, second generation ores develop through metamorphisms, deformations, orogenies, and even intrusive plutonic rocks (Abraham, Konka, & Gebreselassie, 2015). Pyrites, due to its natural hardness, show clearly defined fractures without resulting to cleavage. Through centuries or even millennia, pyrites are transformed into galena through these processes as the softer minerals continued filling the pyrite fractures and that of other hard minerals.
Many times, though, hydrothermal events (e.g. metamorphic hydrothermal fluid intrusions usually from a core complex or even large-scale invasion of magmatic-meteoric mixed fluids) may take place, facilitating the second generation ore formation processes (Marchev, et al., 2005). According to Williams (1991), these events, usually of volcanic origin, had been evident in the Lin Ma Hang area. In fact, a review on the samples may indicate a massive pyrite (Py) second generation galena (G) formation (Abraham, Konka, & Gebreselassie, 2015). The figure, for instance, indicates a massive replacement event, which was hydrothermal in nature. This hydrothermal event could either be a large-scale invasion of magmatic fluids into the sampled area, which can be verified if other sampling areas also show similar profiles, or a less aggressive intrusion of metamorphic hydrothermal fluids, which can be observed when only samples like this and within a limited and defined area showed this massive galena formation while other samples (especially if obtained from a distant enough source location) showed less dominant galena formation.
References
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