Pegmatites and aplites: Their genetic and applied ore geology

Pegmatitic rocks are very coarse-grained basement rocks abundant in quartz, feldspar or/and mica, in places, endowed either with mega crystals of the aforementioned rock-forming minerals or rare-element minerals. Pegmatites are treated in this study together with aplitic rocks, which are compositionally similar to pegmatites but strikingly different from them by their fine-grained texture. Rocks of the granitic suite take an intermediate position between the two and, locally, they are transitional into both end-member types, emphasized in the denomination by supplements such as aplite granite or pegmatitic granite. A similar scenario can be reported for syenitic and, less frequently, for granodioritic through dioritic rocks which are found to be associated in time and space with pegmatites and aplites.The approach taken in the current review to explain, how the pegmatites evolved through time closely resembles the way petroleum geologists in search of or during study of a “petroleum system” address this problem. Their “basin analysis” at the beginning is equivalent to the analysis of the geodynamic setting out of which only three – Variscan-, Alpine- and Rift-type – are considered in this study as pegmatite-prone. These geodynamic settings have particular “source rocks” for elements used to be enriched in granites and pegmatites, they provide physical regimes capable of sparking the mobilization of fluids and melts and they are endowed with migration pathways such as deep-seated lineamentary rifts, shallow thrust and shear planes used by A- and S-type granites and pegmatites, alike. Pegmatites sensu stricto are found as immigrations into environments different from their birthplace where they were trapped in structures providing the accommodation space necessary for their emplacement and sealed off by impervious roof rocks. In the petroleum system, there are also “oil shows” close to the source rocks analogous to the in-situ anatectic pegmatoids. “Oil and gas seeps” are comparable to the various types of (auto)hydrothermal alteration common to many pegmatite systems. In principle, granites and pegmatites are two sides of the same coin, both are undergoing mobilization and migration; the granite mirrors diffusion and dissemination, the pegmatite reflects trapping and concentration. Fractionation and separation can be recognized in the petroleum as well as in the pegmatite–granite systems.While the mineralogy of pegmatites has been intensively studied and also backed by experimental work, the (economic) or ore geology of these felsic rocks has not been given adequate attention, particularly when it comes to the classification of the pegmatites. The newly elaborated CMS classification scheme (Chemical composition–Mineral assemblage–Structural geology) pays attention to the three ore-controlling factors of pegmatites, in general, and rare element deposits associated with them, in particular (Sn–W, Be, REE, Zr, Th–U, B, F, P, Li–Cs–Rb, Nb–Ta, Sc, Mo, Bi). The second string to the bow is the wide range of industrial mineral deposits (feldspar, feldspathoids, quartz, alumosilicates–corundum, garnet, mica, graphite, kaolin). The “ore body” of the pegmatite is described by two items—1st order and 2nd order terms, the type of deposit (e.g. metapegmatite, pegmatoid, pseudopegmatite) and by its shape and structure (e.g., stock-like, tabular, miarolitic). The “ore composition” is defined also by two characteristics, labeled as 3rd order and 4th order terms, by a chemical (e.g. Be–Li–Nb pegmatite stock-like) and a mineralogical qualifier (e.g. (andalusite)–quartz–feldspar metapegmatite tabular) added to the 1st order and 2nd order terms. The CMS classification scheme as it stands is purely descriptive and designed for genetic and applied economic geology.In terms of structural geology and geodynamics, pegmatitic deposits primarily occur in ensialic Variscan-type orogens (calc-alkaline) with a thickened crust and a preponderance of thrusting and nappe stacking. In Rift-type settings (alkaline) where a strong subcrustal impact is evident and as reactivated/reworked pseudopegmatites in Alpine-type orogens (calc-alkaline) these deposits developed during the initial stages when the crustal section was still rather thick. Both types pertain to the marginal ensimatic settings. Fully-developed ensimatic Andean- and Arc-type settings are devoid of pegmatitic deposits. There are metals in rare element pegmatites that are typical of Variscan-type, such as U, B, P and Sn. Th, REE, Mo and Zr preferably show up in Rift-type settings whereas Li and Ta are of widespread occurrence in reactivated Alpine-type orogens. The highest economic potential has been observed in pegmatites/aplites sensu stricto, pseudopegmatites and pegmatite–skarns. Granite pegmatites are ranked second in the abundance of rare elements, mainly Sn and W, whereas metapegmatites and pegmatoids are used to concentrate only feldspar, quartz and mica.A similar tripartite subdivision as performed for the geodynamic positioning of the host environment can also be done for the emplacement of pegmatites themselves. They are part of the (1) thrustbound and fold-related metamorphogenic deposits, (2) collision and intrusive-related deposits and (3) deposits originated from deep-seated lineamentary remobilization. With this in mind a direct correlation of pegmatite deposits with non-pegmatitic deposits such as carbonatites or skarn deposits can easily be performed and all physico-chemical processes inherent to these groups of non-pegmatitic deposits can be applied to pegmatitic deposits as well. Based upon this joint chemical–mineralogical–geological approach taken in the classification of pegmatites it becomes evident that pegmatites can no longer be referred to as a simple product of fractionation of a parental granite but have to be placed as an entity of its own hierarchically besides the granite suite. Pegmatitic rocks cannot be put into a category sharply delimited from the adjacent ones. In nature they are often transitional from simple pegmatoids in migmatites to complex pegmatites sensu stricto. They are characterized by a polyphase development with their formation guided by structures, controlled by open access to crustal and subcrustal heat and element sources. Since pegmatites and aplites used to be smaller in size than granites, a more consequent concentration of elements accompanied by a more intensive interaction with their country rocks takes place during their emplacement than in granites (skarn, episyenites, and albitites).Considering the economic part of pegmatites, the primary pegmatite deposits and their clastic aprons with placer deposits from residual to fluvial type will be left unchallenged as far as the exploitation of colored gemstones is concerned, because there is no other choice. The hard rock deposits will still have a say when the requirements for the raw material are very strict (ultra-high quartz) or a shortage of electronic and strategic elements is looming (Ta, Nb, Be). Exploitation of industrial minerals from hardrocks is competitive if no easy-to access deposits (near-surface sedimentary deposits) of similar quality are close-by and the labor costs are moderate in the country of production. Low-grade large tonnage deposits (salars, brines) are a challenge particularly for lithium. The pegmatites will maintain their position as a source for those elements which make up the lion share in the mineral association, quartz and feldspar. Pegmatites fueled from subcrustal sources, and related in time and space with reactive country rocks (ultrabasic, basic igneous rocks and carbonate rocks) have not yet been given the attention they might deserve.