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ocks adjacent to the caldera margin. Hydrothermal alteration has not been recognised within thePalaeozoic sedimentary rocks northwest of the caldera.Around Pok To Van (1360 1592) and Por Kai Shan (1397 1633), in the northeastern corner ofthe Lantau Caldera, an area of alteration up to 2 km across affects all lithologies in the LantauFormation, including rhyolite lavas, crystal and other tuffs and tuffites and siltstones. The mainarea of alteration transgresses stratification and faults within the caldera, but has not beenrecognised in the feldsparphyric rhyolite dykes or in the medium-grained granites adjoining thenortheast-, and northwest-trending, caldera-related, faulted contacts of the Lantau Formation.The alteration is therefore broadly controlled by northeast- and northwest-trending faults.Adjacent, subsidiary, northeasterly-trending narrow zones of alteration (141 154, 143 155) mayalso be crudely stratabound.A relatively narrow, elongate corridor of alteration (080 157 to 102 153) borders the southerlymargin of a syenite dyke along a westnorthwest-trending section of the northern margin of theLantau Caldera. In addition to alteration in rocks of the Lantau Formation, a small area ofmedium-grained granite is also affected on the southern side of the syenite. However, the area ofalteration does not extend into similar, or other granitic rocks, or feldsparphyric rhyolite dykesnorth of the syenite. Hence, it is unlikely that the alteration is contact-related to the emplacementof the syenite dyke. A subsidiary, narrow zone of selective alteration (087 157) strikes parallel tothe main zone, and to flow-banding within lavas of the Lantau Formation.Another major zone of hydrothermal alteration extends from Fan Lau 7 castnortheastwards alongthe southern coast of Lantau Island. It splits into discrete, northeasterly-trending lenses,developed within a 3 km wide corridor, and these can be traced towards Nci Lak Shan (088 138),although not beyond its lower, southwestern slopes, and to the southern flank of Lantau Peak(098 123). In both instances, the zones of alteration arc truncated by eastnortheast- ornortheast-trending faults.In summary, the hydrothermal alteration is preferentially developed towards the margins of thecaldera, most notably in the north and southwest, and along northeast-trending zones within thecaldera in the southwest. This distribution can be interpreted in terms of silica-rich hydrothermalfluids being preferentially channelled along the fault system created, or enhanced by developmentof the caldera. The steep, northeast-trending faults within the caldera, and the caldcra-boundingstructures, appear to have been the dominant fluid pathways.KaolinizationBefore reclamation, an area of kaolinized granite occurred east of the former test embankment atChek Lap Kok, and is sharply bounded to the east by unaltered fine-grained granite, exposed in astream bed (1112 1901) within 50 m of the kaolinized zone (Langford, 1994). The westernboundary of the kaolinization is unclear due to surface workings (sec also Chapter 10).Kaolinization also occurred near the northern tip of Chek Lap Kok, cast of Sham Wat, where itwas mined in association with extensively developed quartz. The white, clay mineral kaolin is analteration product of feldspar and mica, and formed as a result of the passage of silica-richhydrothermal fluids through the granite, rather than by surface weathering (Langford, 1994). Thefluids were probably derived from the granite itself, during its later stages of cooling (Plate 41).Contact MetamorphismContact metamorphism related to emplacement of granitoid intrusions and dykes is common, butrarely sufficiently extensive to be shown on the published maps of the district. The main reasonfor this is that104