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A typical, finer-grained sample of basalt (HK9237, 1156 1827) from the peninsula south of KwoLo Wan'is a dark green, very fine grained, and speckled with mafic phenocrysts. Thephenocrysts, seen in thin section, are composed of secondary chlorite and calcite, and thegroundmass contains abundant hornblende, with plagioclase and epidote.Laniprophyre dykes in hand specimen can display prominent, dark green, mafic phenocrysts, andoverall are mafic-rich relative to the basaltic dykes. A thin section (HK10859, 1117 1986) of therock contains abundant augite and hornblende, with some plagioclase which is mostly less than 1mm long (Plate 37). Chlorite replacement of lath-like mafics is common, and there are nofeldspar phenocrysts. The composition of the lamprophyre is probably vogesite.Age RelationsAllen & Stephens (1971) considered that the basalt dykes were emplaced during separate UpperCretaceous and Tertiary phases. They cut the granite (Late Jurassic-Early Cretaceous) countryrock, but are themselves cut by small, northwest-trending faults. The lamprophyre dykes also cutrhyolite dykes and granite, and are considered to be of a similar age to the basalt dykes. Atone locality (1051 1920), north of the test embankment, a lamprophyre is cut by a quartz veinabout 0.25 m wide.Aplite and PegmatiteDistribution andLithologySmall icnpersistent dykes of aplite are occasionally found in the granites. For example, they occurin small groups at a number of places around the coast of Chek Lap Kok, where the dykes areleucocratic, with a markedly equigranular grain size of around 0.5 mm. They can be from 25 mmto 5 m wide, but are commonly around 0.25 m, The aplite and granite are closely related in ageand genesis. Similarly, apiite dykes occur in medium-grained granite on Cheung Chau (Plate 38).Pegmatite, or very coarse-grained granite, with crystals over 20 mm across, is rare, and generallyoccurs as pods or lenses too small to be shown on the published map of the district. However, anirregular, roughly east-west trending pegmatite is exposed west of Fu Tei Wan (1075 1824).Pegmatite is only found within granite, and formed at a late stage during the cooling of theplutons.Quartz veinsQuartz veins occur in rocks of all types and all ages in the district, but were clearly emplacedduring several phases. A vein chronology has not been established in the district, and only themost substantial quartz veins are shown on the published maps,DetailsChek Lap Kok (prior to development). Quartz veins are widely distributed throughout the island, and mostlytrend east to eastnortheast They cut the granite country rock and most minor intrusions, and are generally narrow,ranging from stringers a few millimetres wide, to veins up to 03 m wide. Quartz veins are especially common inand around the kaolinized granite east of the test embankment. The quartz veins commonly form upstanding ribs inthe deeply weathered granite. The thin veins tend to separate along cracks on joint faces to produce mosaic-likesheets of quartz. Soil creep bends and displaces these sheets to form debris trails downslope,As well as discreteveins, there are places where the granite country rock has been silicified. To the south of the test embankment, in acoastal exposure (1072 1835), the fine-grained granite country rock has been finely veined and silicified in a zone,at least 1 m wide, which trends southsoutheastPetrographically, the veins typically comprise massive white crystalline quartz. Euhedxal crystals are rare, althoughcrystal aggregates up to 0.2 m across occur, with individual crystals up to 45 mm across (HK9555,1091 1912).90
Plate 33 - Core stone Development in Basalt Dyke, Intruded into Feldsparphyic Rhyolite (2025 1414),at Man Kok on the Northeastern Side of Silver Mine BayPlate 34 - Thin Section of Basalt (HK8351) (1998 1009), from Chi Ma Wan; XPL
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UNIVERSITY OF HONG KONGLIBRARIES
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© Government of Hong KongPublished
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CONTENTSTitle pagePage1ForewordCont
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Distribution and Lithology 73Detail
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Chapter 10 Economic Geology 149Intr
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(Plates)Plate 22 - Porphyritic Medi
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Table 1 — Grain Size Description
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0 1 2 3 4 5 k mCONTOURS AT 100m INT
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properties of the marine deposits b
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Archival DataAll records from the s
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Chapter 2Outline of GeologyThe soli
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The Mesozoic volcanic rocks are div
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Chapter 3Palaeozoic Sedimentary Roc
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Carboniferous metasedimentaiy rocks
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Plate 1 - Fining-Upward Sequence of
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Plate 3 - Well-bedded Sandstones an
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Reef Island. The northern end of th
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Table 3. Evolution of Nomenclature
Page 41 and 42: are crudely stratified and welded i
Page 43 and 44: Shing Mun FormationThe type localit
Page 45 and 46: Tai Che Tung. Crystal tuff, interpr
Page 47 and 48: Plate 7 • Lapilli-Ash Crystal Tuf
Page 49 and 50: Plate 11 - Pyroclastic Breccia (086
Page 51 and 52: appears fragmental, and contains mu
Page 53 and 54: exposures in the stream bed consist
Page 55 and 56: Lin Fa Shan. The steep eastern flan
Page 57 and 58: dipping northwest at 46°, is prese
Page 59 and 60: Sunset Peak MemberThe Sunset Peak M
Page 61 and 62: Plate 16- Block and Lapilli-bearing
Page 63 and 64: Chapter 5Major IntrusionsClassifica
Page 65 and 66: in grain size from 0.4 to 1.5 mm, a
Page 67 and 68: 20 - Megacrystic Medium-grained Gra
Page 69 and 70: Pui O Wan. Porphyritic medium-grain
Page 71 and 72: minimum age of c. 145 Ma (see earli
Page 73 and 74: The granite on Chek Lap Kok is most
Page 75 and 76: Southeast of Tin Sam, on the wester
Page 77 and 78: About 1 km east of Nam Shan, exposu
Page 79 and 80: Plate 25 - Thin Section of Porphyri
Page 81 and 82: Chapter 6Minor IntrusionsIntroducti
Page 83 and 84: The microgranite grades southwards
Page 85 and 86: the Lantau Formation, however, and
Page 87 and 88: Plate 29 - Flow-banded Quartzphyric
Page 89 and 90: also cut volcanic rocks of the Tsue
Page 91: Silver Mine Bay. On the headland so
Page 95: Plate 37 - Thin Section ofLamprophy
Page 98 and 99: Pre-Yensfaanlan StructureWithin the
Page 100 and 101: separates areas of different lithol
Page 102 and 103: NeotectonicsFault activity in the r
Page 105 and 106: Chapter 8Metamorphism and Alteratio
Page 107 and 108: the majority of rocks in the distri
Page 109: Chapter 9Superficial GeologyIntrodu
Page 112 and 113: Detailstal O. Alluvial deposits for
Page 114 and 115: trains. The regolith on most steep
Page 116 and 117: Plate 44 - Boulder Debris on the So
Page 118 and 119: gazetted for recreational purposes.
Page 120 and 121: DetailsTai O. Extensive intertidal
Page 122 and 123: Plate 48 - Sandy Beach at Fan Lau S
Page 124 and 125: Pre-Chek Lap Kok Formation Deposits
Page 126 and 127: formation. The sequence is usually
Page 128 and 129: BOREHOLE NO: ESC 17GRID REFERENCE:
Page 130 and 131: BOREHOLE NO: A5/1GRID REFERENCE: 80
Page 132 and 133: formation, thinly laminated clays o
Page 134 and 135: Quinqueloculina seminulum, which is
Page 136 and 137: Channel and Transgressive Deposits.
Page 138 and 139: Figure 20 - Areas of Acoustic Turbi
Page 140 and 141: West Lamma Channel These may be att
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Figure 23 - Depth-Averaged, Maximum
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The formation is Holocene in age. T
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Boreholes (A5/1, A5/2, B2/1, ESC17
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Plate 51 - Deep Weathering Profile
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Chapter 10Economic GeologyIntroduct
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Non-metalliferous MineralsFissure v
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REFERENCESAddison, R. (1986). Geolo
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James, J.W.C. (1993). The offshore
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Sewell, R.J., Strange, PJ., Langfor
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Appendix 1Microfossils identified f
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Appendix 2 (continued)Sample HK9913
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Appendix 2 (continued)Sample HK1054
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Appendix 4aSelective normalized pal
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,Appendix 5 -Radiocarbon and Other
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construction materials 151contact m
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molybdenite 149Mong Tung Hang 151Mo
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talus 107,115Tarn Tsui Wan 71,87Tan
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LB 555.125 G34 LGeology of Lantaii
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