An overview of the pegmatitic landscape from the pole to the equator – Applied geomorphology and ore guides

摘要

Graphical abstractDisplay OmittedHighlights?The pegmatitic landscape and its landforms can be used for exploration.?Pegmatite bodies control the geomorphology, their composition the chemical weathering.?Climate is accountable for the plain- and valley-type supergene deposits.?Wet-and-dry and humid tropical climates are most favorable target areas.?Targets are rare metal pegmatites, kaolin, gemstone-, Nb/Ta, Sn, REE placers.AbstractPegmatites and aplites share the common major constituents with the granitoid suite as well as various gneissic lithologies, e.g., orthogneisses, aplitic gneisses. Not surprising, the pegmatitic landscape has landforms resembling some found in landscapes derived from metamorphic and granitic rocks that genetically next of kin of pegmatites. The wealth of rare minerals, the peculiar shape, the zonation into a rim extremely vulnerable to weathering and a hard silica core renders pegmatites strikingly different from the afore-mentioned crystalline rocks and account for a landscape type of its own. The primary features of pegmatites, shape and composition, the key elements of the CMS classification scheme (Chemical composition-Mineral assemblage-Structural geology) also are critical for the secondary alteration of these rocks and the evolution of a pegmatitic landscape. The 1st order landscape formation, involving geomorphologysensu strictoand weathering contributes to the built-up of five morphological types (erosional type I, alteration type II, mixed type III, composite type IV (erosion-transport-deposition), hidden type V (under an intact roof rock or under clastic overburden)). The minerals produced by chemical weathering are accountable for type II and III, whereas the relic minerals are accountable for type I, III, IV and V. Morphological type IV leads to different placer deposits. The 2nd order landscape formation is governed by the climate giving rise to discrete zones arranged from the pole to the equator and two genetic types, the plain and valley types. Both types can genetically be correlated with the wet-and-dry and the tropical humid climates. Placers developed a clastic apron around pegmatites abundant in relic minerals whereas minerals newly formed during chemical weathering lead to clay deposits resting immediately on top of pegmatites. In terms of applied geomorphology, reading and understanding a pegmatitic landscape means creating an “ore guide” t