Classification
Rock alterationtakes severalforms in the rocks of ChekLap Kok. Hydrothermalalterationby heatedwater passingthroughthe rock results in kaolinizationof the granite(Plate 19). Rockscanalso be altered by
weatheringprocessescloseto the surface.Chemicalalterationand progressivemechanicaldisintegration
create a weatheredprofile that ultimatelyprovidesthe material for the formation of superficialdeposits
throughmasswasting,surfaceerosionand subsequent
deposition.
KaolinizationThe
altered rocks were fonned dominantly by the process ofhydrothennal
activjty, which takes the fonn
of kaolinization of the granite (Plate 19). Kaolinization is confmed to an area of about II ha to the east ofthe
test embankment and an area of about 8 ha east of Sham Wan.
The area of kaolinization
grained
granite
exposed
east of the test embankment
in a stream
bed
(1112
is sharply
1901)
bounded
to the east, with
less than 50 m from the zone
unaltered
of alteration.
fmeDeeply
Plate 19 -Deeply
Weathered
Kaolinized
Granite
with Thin
Quartz
Veins,in the Old Kaolin Mine East of the
Test
Embankment
(11041901)
38
The
weatheredgranite occurs on the ridges of Fu Tau Shanto the southand to the north, but is not accompanied by kaolinization. The kaolinizationis not readily apparentin the borrow area,althoughthe granite is
deeplyweathered.The westernboundaryof the kaolinizationis unclear,asthe weatheredgranitehas been
extensivelyexcavatedfor kaolin eventhoughthe mineralmay not be presentin significantamounts.
Kaolinization has beenrecordednearthe northerntip of the island,the areato the eastof ShamWan having been commerciallymined in the 60s. The areais strewnwith quartz vein debris and is deeplyweathered(Plate20).
Weathering
An extensivemantle of weatheredrock overlies freshrock in most of the island (Plate21, Figure 9). The
weatheredmantle usuallyconsistsof materialthatretainsthe originaltexture of the rock, and consist$of a
mixture of Grades IV-VI (GeotechnicalControl Office, 1988b)materialand relatively fresh corestones.
The estimatedvolume of the weatheredmantleis 21 million m3,but could be as low as 14 million m3or as
high as 28 million m3.Comparedwith the volume of the island,this gives an averageof 20% weathered
mantleabove sea-levelor 25% above8 mPD.
deeply weatheredhilltops and ridges are prone to gullying, a processoften exacerbatedby miningactivity.
This is well illustrated in the aerialphotographsfrom the early 1960s(Plate22), which contrasts
with the more recenttrend towardsrevegetationover mostareas(Plate23). However,certaindeeplygullied areas,outside those areasaffected by mining, remainsusceptibleto further erosion.Plate 24 illus-trates
the regrowth of dense,low vegetationbelow steep,gullied weatheredgranite with quartz veins tothe
north ofFu Tau Shan(188 111).
Details
Granite. The granite has weatheringfeaturestypical of those observedelsewherein the Territory, for
example in the nearbyhills of Tsing Shanand Tai Lam (Langford et ai, 1989).The ridge tops are commonly erodedto exposea pale granularsoil. The bestexposuresof this phenomenonare nearthe summit
ofFu Tau Shan(Ill 183). Deepgullies in brownishto pinkishwhite completelyweatheredfine-grained
granite have exposedresistantquartzveins.Gullying also occurson the flanks of Fu Tau Shanand in the
areasofkaolinization.
Plate 20 -Abandoned Kaolin and Quartz Mine below the InstrumentApproach System,East of Sham
Wan(122199)
39
41
The northernpeninsula,on the eastside of ShamWan,has a history of stone quarrying,and weathering
depthsare mostly lessthan 10m. This increasesoverthe old kaolin mine,whereweatheringandmineralization probably extendto sea-level.To the west of ShamWan the rock quality is very good, with very
little weatheringand fewjoints. The hillsidesarecharacterizedby exposedsheetsof rock andcorestones.
The main range of granite hills on the eastside of the island are cappedby around lOin of weatheredrock,
but extensivesheetsof exposedrock canbe seenon the flanks,particularlyeastofFu Tau Shan(112
188). The coastalaspectof thesehills is characterizedby abundantcorestonessitting in a thin mantle ofsoil.
Further south,the hills are dominatedby the wide dykes,andthe granite is thoughtto be cappedby
only a thin weatheredzone.
The corestonesexposedin the kaolin mine and borrow are usuallyweathered,and concentric exfoliation
aroundthe corestonesis well displayedin the borrow area(109 191).The weatheredrock disintegratesto
form a uniform pale silty sand.
A fine example of lamellar fracturing can be seenin highly weatheredmegacrysticfme-grainedgranite
nearthe northerntip of the island(12482022). The fracturingdips gently, andhasa similar orientationto
sheetingjoints seenin adjacentsolid exposure.
Dykes and Veins. The basic dykes in particular often exhibit a distinctiveweatheringpattern. Being rich
in ferro-magnesianminerals,they weatherdeeply,but can exhibit small-scalespheroidalweathering,for
examplenearKwo Lo Wan (1156 1827)(Plate7). Coastalexposuresof basicdykesare oftenmarkedby a
slight hollow wherethe more easilyweathereddyke is eroded.This is well displayedin aO.4 m wide dyke
at Fui Yiu Wan (1203 1964)(Plate 12).Thesedykescan also exhibit a markedlycarioussurface,accentuated by a mineral boxwork in the rock, for examplein a coastalexposurenear Fu Tei Wan (10761819).
Quartzveins often fonn upstandingribs in the surroundingweatheredgranite. In coastalexposures,they
usually lie along lines of weaknessin the graniteexhibiting deepererosion.Thereare excellentexamples
of quartz vein ribs and sheetsboth in and aroundthe kaolin mineseastof the test embankmentand eastof
ShamWan. Plate 19, takenwithin the fonner mine (1104 1901),illustrateshow a narrowquartz vein pro-tects
the adjacentweatheredand kaolinizedgranite from erosion.Plate 9 (1111 1912)illustratesa quartz
Plate21 -Deeply Weatheredand Eroded Granite on Fu Tau Shan,with the DisusedKaolin Mine in the
Background
Plate 24 -Vegetation RegrowthEncroachingon Gullied WeatheredGranite with Quartz Veins,North of
Fu Tau Shan(185111)
vein exposedin a gully in weatheredgranite eastof the test embankmentbroken down into tablets. The
disintegrationof the vein gives a mosaic-likeappearance
before erosioncausescompletecollapse.A similar feature southof CheungShaLan (1101 1930)showsprogressiveoverturningof the mosaic-likewall of
vein quartzdueto soil creep.
The rhyolite dykes have little compositionalcontrastwith the surroundinggranite and the only readily
observableweatheringcontrastis in the abundanceof hard, resistantcorestones.The soil derived from
weatheringof the aphaniticgroundmassis muchmore clayeythan that of granite,but this feature is not
well seen in the limited exposures.The dykes form good corestones,on the southernpeninsula for
example.
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