Geology of the Mongolian Altai
The Mongolian Altai were formed during orogenic thrusting, which occurred between 500 and 300 million years ago, and have since eroded into an undulating plateau. Beginning in the Quaternary Period new upheavals have formed magnificent peaks of considerable size. Earthquakes are still common in the region along live fault lines.
Figure 1 Map of the Mongolian Altai Mountain Range (Encyclopedia Britannica)
The extreme deformation suffered by the Altai over a long period of time has formed a variety of rock types; many of them have been altered by magmatic and volcanic activity. Palaeozoic rocks including marine and continental conglomerates, sandstone, mudstone, limestone, and volcanics, some of which have undergone low-grade metamorphism. Granite largely dominates the higher mountain peaks. The rocks were folded and faulted as the Chinese and Siberian cratons collided in the Palaeozoic and Mesozoic eras. Thrust and dextral strike-slip faults dominate the foothills between the Kharkhiraa and Turgen Mountains to the west and the Uvs Nuur basin to the east. The pegmatite mineralisation in the Chinese Altai appear to be associate to Palaeozoic granite formed in this collision. The lithium pegmatites represent the most highly differentiated and last to crystallize components of these granitic melts. (Tong et al 2014)
Most Lithium pegmatite bodies show some sort of structural control; this will be a function of depth of emplacement and can vary from district to district. At shallower crustal depths, pegmatites tend to be intruded along faults, fractures, foliation, and bedding. In higher- grade metamorphic host rocks, pegmatites are typically concordant with the regional foliation, and form lenticular, or ellipsoidal bodies (Fetherston, 2004). Most pegmatite bodies are concentrically, but irregularly, zoned. Zoning is both mineralogical and textural. Cameron et al (1949) identified four main zones: the border, wall, intermediate, and core zones:
- The outermost, or border zone, is a chilled margin just inside the sharp intrusive contact between pegmatite and country rock. Typically, the border zone is a few centimeters thick, fine-grained, and composed of quartz, muscovite, and albite.
- The wall zone is typically less than about 3-m thick. The largest crystals seldom exceed about 30 cm, and in general, the grain size is somewhere between that of the fine-grained border and that of the intermediate zone, where the largest crystals are to be found. The essential minerals are albite, perthite, quartz, and muscovite. Tourmaline and beryl may be present.
- The intermediate zone comprises everything between the wall and the core. These may be discontinuous rather than complete shells, there may be more than one, or there may be none at all. The essential minerals are plagioclase and potassium feldspars, micas, and quartz. In more evolved pegmatites, various rare-element phases such as beryl, spodumene, elbaite, columbite-tantalite, pollucite, and lithium phosphates are present. Overall grain-size is coarser than in the wall zone.
- The core zone in many zoned pegmatites is composed only of quartz. In some core zones, quartz is joined by perthite, albite, spodumene or other lithium aluminosilicates.
Figure 2 Exploration model of an idealized Granitic Pegmatite, redrafted from Bradley et al 2016
Identification of suitable parent granites is an important first step in looking for a potential area of pegmatites. From previous mapping, it is clear that the granites associated with the pegmatites in the Chinese Altai continue into Mongolia following the same orientation. They also follow the regional faults. (See attached map).
As a first stage of exploration the metamorphosed country rock around the granites would be the most prospective areas. Lithium bearing pegmatites have been known to form up to 10km away from the parent granite in metamorphosed country rock. In areas of good bedrock exposure pegmatites are often clearly visible due to their light colour and large crystals. The White Picacho District in Arizona hosts many pegmatites that are visible from Google Earth. (Bradley & McCauley, 2016) Granitic pegmatites are relatively resistant to weathering and erosion, so often tend to stand above the surrounding host rocks. In some deposits, such as Tabba Tabba in Australia only quartz may be exposed.
A useful feature of large pegmatites is that the wallrocks around these pegmatites are metasomatized and the dispersion of alkali rare elements in the metasomatic aureoles around pegmatites can be used as an exploration tool. Lithium anomalies define the widest halos adjacent to pegmatites, which can be in excess of 100 m. Biotite is an abundant metamorphic / metasomatic mineral in the country rocks that surround pegmatites. Because Lithium substitutes for Manganese in biotite, and Rubidium and Caesium for Potassium, the formation of a biotite metamorphic / metasomatic aureole results in well-developed Li–Rb– Cs dispersion patterns around the pegmatites (Linnen et al 2012). The following mineralogical features are useful identifiers for exploration in the field and could be used in this area:
- Mineralogical identifiers of fertile and prospective peraluminious granites include greenish muscovite rather then the usual silver colour, potassium feldspars that are white rather than the normal pink. Accessory minerals are often garnet, tourmaline, fluorite and/or corderite.
- Garnets change in colour as well as composition depending on the potential of the host granite. Fertile granite contains red iron rich alamandine, and the most evolved pegmatites contain orange manganese rich spessartine.
- Tourmalines in fertile host granites and the outer zones of lithium pegmatites are often black and low in lithium and manganese. Tourmalines become pink to green elbaite high in lithium and manganese in lithium rich pegmatites.
- Beryl also changes colour being greenish brown is less evolved pegmatites becoming pale white and pink and more euhedral in evolved lithium bearing pegmatites.
Granitic Pegmatite deposits are favourable for first stage exploration via remote sensing.
The target area is mountainous and at a high altitude, thus the there is very little vegetation that makes using aerial images to look for potential pegmatite outcrops relatively easy.
Pegmatites are likely to form ridges, and follow regional geological structures, lineations, and foliations. Remote sensing with aerial photos, and to begin with freely available data such as Google Earth may identify possible granitic pegmatite out crops. Granitic-pegmatites can be associated with alteration zones ranging from a few meters to 150m accompanied by a mass of kaolinitic clay produced through weathering processes. Kaolinite along with associated iron oxide minerals can be easily identified using ASTER images.
A basic geological map based on Tong et al (2014) of the Chinese and Mongolia Altai Mountains has been prepared (see attached). This highlights the known pegmatitic regions in China, the mapped granites, regional faults, and prospective regions on the Mongolian side of the border. The layers from this map have also been imported into Google Earth to aid identifying prospective regions from satellite images.
BRADLEY, D., AND MCCAULEY, A., (2016). A Preliminary Deposit Model for Lithium-Cesium-Tantalum (LCT) Pegmatites. United States Geological Survey.
CAMERON, E.N., JAHNS, R.H., MCNAIR, A.H., AND PAGE, L.R., 1949, Internal structure of granitic pegmatites: Economic Geology Monograph 2, 115 p.
FETHERSTON, J.M., 2004, Tantalum in Western Australia: Western Australia Geological Survey, Mineral Resources Bulletin 22, 162 p.
LINNEN, R, L., VAN, LICHTERVELDE, M., CERNEY P., (2012) Granitic Pegmatites as Sources of Strategic Metals. Elements.
MIKHAYLOV, N, I., OWEN, L,. (2009) Altai Mountains. Encyclopaedia Britannica. https://www.britannica.com/place/Altai-Mountains
TONG, Y., WANG, T., JAHN, B., HONG, D., SUN, M., GAO, J,. (2014) Post-accretionary Permian granitoids in the Chinese Altai orogen: geochronology, petrogenesis and tectonic implications. American Journal of Science. 314(1):80–109.