3. The Geology of the Hac Sa Archaeological Sites

The oldest rocks exposed on Coloane are metasediments of possible Devonian Age. During the Jurassic and Cretaceous Periods, the metasediments were altered by heat from a granitic batholith into tough metamorphic rocks, with abundant biotite mica. . The textures include megacrystic, porphyritic and equigranular. These grey-coloured metamorphic rocks are rarely exposed, and their junction with the granite is unclear in the field. The rock which crops out adjacent to the archaeological sites is mainly granite, with occasional suites of small quartz monzonite dykes. A more detailed petrographic description lies outside the scope of this article. Dykes are variously infilled with feldsparphyric rock, which ranges in composition from rhyolitic to dacitic; and in texture from pegmatite and microgranite. Massive quartz occurs as the main veinstone, and as strings associated with some dykes. Minor occurrences of apatite, Fe-As-Cu sulphide minerals and calcite were seen, in hand specimens, associated with quartz in some pegmatites. Deposits of fine-grained sediments (clay and muds), which may have covered the area, were considerably eroded during the Quaternary Period.

The two granitic outcrops, which form prominent hills to the north and south of Hac Sa beach, are faulted and jointed. The major and minor faults alignments correspond broadly with the Euler gravity anomalies reported for Hong Kong (Sewell et al. 2000). Most major faults show evidence of lateral or vertical displacement, and are mineralised. Normally, the eroded joints in the granitic rocks, which are mainly associated with stresses developed during the final cooling stage of the intrusion, are not mineralised. Mineralised en echelon tension gashes sometimes occur.

In comparison with Macau, a much greater range of rock types is exposed in Hong Kong, including rocks known to have been favoured by stone-tool makers throughout prehistory, worldwide. These rock types are generally absent from Macau, and do not outcrop adjacent to the archaeological sites on Coloane. For example, the Palaeozoic sedimentary rocks and metasediments are absent, the Teuen Wan Volcanic Group of Lower Jurassic age, and the Lantau Volcanic Group of Upper Jurassic age are not represented in Coloane. For comparison, the Coloane granites are tentatively matched to the biotite granites in the Lion Rock Granitic Suite of middle Cretaceous age. Rocks that do not outcrop in Macau, Coloane or Taipa may, however, be represented in beach and river deposits found locally.

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