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MINERALOGY AND GEOCHEMISTRY OF THE GEMSTONES AND THE GEMSTONE-BEARING PEGMATITES IN SHIGAR VALLY OF SKARDU NORTHERN AREAS OF PAKISTAN

Muhammad, Hassan (2008) MINERALOGY AND GEOCHEMISTRY OF THE GEMSTONES AND THE GEMSTONE-BEARING PEGMATITES IN SHIGAR VALLY OF SKARDU NORTHERN AREAS OF PAKISTAN. PhD thesis, University of Peshawar, Peshawar .

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Abstract

The granite-pegamatites of the northern areas of Pakistan are well-known for hosting of various types of gemstones such as the aquamarine, topaz, bi-and tri-colored tourmalines, apatite fluorite etc. The Shigar valley pegmatites are also one of these that hold a variety of gemstones, This valley is about 32 km northeast of Skardu, the headquarter of Baltistan, northern Pakistan and it provides a route to K2 mountains. Both meta-igneous and meta-sedimentary rocks of the Asian plate and Kohistan-Ladakh island arc, are exposed in the Shigar valley. The Main Karakoram Thrust is passing through the center of this valley, separating the meta-sediments of the Asian plate from the meta-igneous and meta-sediments of Kohistan-Ladakh island arc Multiphase metamorphism and deformation has occurred in this area, therefore, the meta-sediments to the north of the Northern Suture are collectively known as the "Karakoram Metamorphic Complex". The grade of metamorphism varies from greenschist to amphibolite grade facies. Besides, the multi phase metamorphism and deformation, igneous intrusions of both the pre-and post-collisions are also exposed in the valley. The last phase of post-collisional igneous intrusion is present in the form of leucogranites and gem-bearing pegmatites (the focus of present study). The pegmatitic rocks need special attention to understand their petrogenesis because these are of dual nature. They occur either the off shoots of near by plutonic bodies or separate magmatic pulses due to direct anataxis. There are series off different processes involving the emplacement of plutonic units, the formation of pegmatile, and their host gemstones and the rare element-hearing minerals. The present study is being carried out to understand the petrogenetic evolution of the Shigar valley pegmatites and the formation different types of gemstones in these pegmatites. A reconnaissance field work was carried out in the study area and the samples were collected from different pegmatite bodies for petrographic, geochemical studies and mineral chemistry. On the basis of field features and petrographic studies, the pegmatites of the Shigar valley are broadly classified into two types: gem-bearing and gem-barren pegmatites. The gem-bearing pegmatites are generally zoned having three or four zones, while the gem-barren pegmatites are simple both in mineralogy and internal structure. Symmetrical zoning is the common feature of the gem-bearing zoned pegmatites, while some of the dykes exhibit asymmetrical zoning. Besides this, on the basis of presence or absence or the relative proportion of the accessory mineral phases, these pegmatites are further classified into four suh-classes. These four suh-classes are: 1. muscovite-schorl-beyl-garnet pegmatites,2. muscovite-schorl pegmatites. 3. biotite ± garnet± muscovite pegmatite and 4. muscovite-biotite ± garnet pegmatite." The petrographic and mineral chemistry data of the studied rocks and gemstones indicate that no rare element-bearing mineral phases have been identified in these pegmatites. This suggests that these pegmatites belong to the miarolitic pegmatite class instead of rare element pegmatite class. The gemstones found in the Shigar valley are mainly aquamarine, topaz, fluorite, apatite, tourmaline, axinite, zoisite and clinozoisite etc. Field study suggests that all the gemstones are not of pegmatitic origin but some of them, such as axinites, zoisites, and clinozoisites are either metasomatic or metamorphic in origin. It is also noticed that the gemstones of pegmatitic origin are mostly found either at the core-margin zone or in the intermediate zone of the zoned pegmatites. In case of internal evolution of the Shigar valley pegmatites, grain size increases from the margin towards core, which indicates that the crystallization starts from margin toward,' core. The presence of vugs/cavities of the gemstones near the core zone also supports the above crystallization sequence. The geochemical data of the studied pegmatites shows that these pegmatites are mostly peraluminous and granitic in character. The mqjor, trace and rare earth element characteristics of these pegmatite." exhibit that these belong to the post-collisional tectonic settings. Keeping 'in view the results obtained for the studied pegmatites and the prevailed models for the generation and propagation of granitic pegmatites, it is evident that these pegmatites have been formed by the direct ana taxis of a meta-sedimentary protolith instead of heing the off shoots either of Baltoro or Mango Gusar plutons, which are exposed to the northeast of the Shigar valley. On the basis of radiometric dates for the host rocks of the Shigar valley pegmatite, it is concluded that the Shigar valley pegmatite, are not older than 5 Ma. This is in correspondence with the youngest leucogranites of the High Himalayas, which are generally of 5-JO Ma, The Shigar valley pegmatites , share many characteristics with the other post-collisional leucogranite of rthe High Himalayan and Trans-Himalayan regions. Like the post-collisional leucogranites of High Himalayas, the Shigar valley pegmatites also have tourmaline as a ubiquitous mineral phase, suggesting a similar source rock for these pegmatites. In addition, the presence of topaz, fluorite and apatite in the Shigar valley pegmatites indicates that the source rock was rich in boron. fluorine, chlorine, water and other volatiles. Moreover, the absence of lithium and rare-metal bearing mineral phases in the pegmatites of Shigar valley indicates that the source rock might have been depleted in these metals.

Item Type:Thesis (PhD)
Subjects:Physical Sciences (f) > Earth Sciences(f4) > Geology, (f4.1)
ID Code:2596
Deposited By:Mr. Javed Memon
Deposited On:08 Jul 2009 12:09
Last Modified:12 Dec 2009 14:30

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