I= MINERALOGY AND GEOCHEMISTRY OF THE GEMSTONES AND THE GEMSTONE-BEARING PEGMATITES IN SHIGAR VALLY OF SKARDU NORTHERN AREAS OF PAKISTAN
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Title of Thesis
MINERALOGY AND GEOCHEMISTRY OF THE GEMSTONES AND THE GEMSTONE-BEARING PEGMATITES IN SHIGAR VALLY OF SKARDU NORTHERN AREAS OF PAKISTAN

Author(s)
MUHAMMAD HASSAN
Institute/University/Department Details
National Centre of Excellence in Geology/ University of Peshawar
Session
Subject
Geology
Number of Pages
384
Keywords (Extracted from title, table of contents and abstract of thesis)
gemstones, gemstone-bearing pegmatites, shigar, skardu, northern areas, aquamarine, topaz, tourmalines, apatite fluorite, tourmaline, gamet, zircon, apatite, fluorite, chlorite, biotite, epidote, allanite, alkali feldspar, muscovite

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.

Download Full Thesis
10285.78 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
242.88 KB
2 1 Introduction
104.09 KB
  1.1 General Statement 1
  1.2 Pegmatites In Northern Areas Of Pakistan 3
  1.3 Location And Accessibility 4
  1.4 Aims And Objectives 5
  1.5 Previous Work 5
3 2 Regional Geology
927.03 KB
  2.1 General Statement 7
  2.2 Regional Tectonic Setting Of The Karakoram Mountains 10
  2.3 Divisions Of The Karakoram Block 12
  2.4 Karakoram Axial Batholith 13
  2.5 Western Karakoram Batholith 14
  2.6 Hunza Karakoram Batholith 15
  2.7 Karakoram Batholith : (The Baltoro-Biafo-Hushc Areas) 19
  2.8 Eastern Karakoram Batholith 22
  2.9 Tirich Mir Zone 23
  2.10 Southern Sedimentary Belt 23
  2.11 Northern Sedimentary Belt 24
  2.12 Kohistan-Ladakh Island Arc 25
  2.13 Intraoceanic Stage Of Crustal Growth 26
  2.14 Yasin Group 27
  2.15 Chait Volcanic Group 28
  2.16 Gilgit Formation 30
  2.17 Kamila Amphibolite Belt 31
  2.18 Intrusive Rock Units 33
  2.19 Mafic And Ultramafic Complexes 33
  2.30 Sapat And Jijal Complexes 33
  2.31 Chilas Complex 34
  2.32 Intermediate To Felsic Intrusions 36
  2.33 Continental-Margin Stage Of Crustal Growth 37
  2.34 Kohistan Batholith 37
  2.35 Dir Group 39
  2.36 Main Karakoram Thrust 41
  2.37 Main Mantle Thrust 42
  2.38 Indian Plate 44
  2.39 Tethyan Himalayas 45
  2.40 High Himalayas 46
  2.41 Lesser Himalaya 47
  2.42 Sub-Himalayas 48
  2.43 Himalayan Fore Deep 49
4 3 Geology Of The Shigar V Alley (Study Area)
682.87 KB
  3.1 Major Stratigraphic Units 54
  3.2 Rock Units South Of The Main Karakoram Thrust 55
  3.3 Katzarah Formation 55
  3.4 Bauma-Harel Formation 56
  3.5 Rock Units North Of The Main Karakoram Thrust 57
  3.6 Daltumbore Formation 57
  3.7 Dassu Gneiss 58
  3.8 Other Igneous Rocks Of The Study Area 59
  3.9 Igneous Intrusions North Of Main Karakoram Thrust 60
  3.10 Igneous Intrusions South Of Main Karakoram Thrust 60
  3.11 Deformed And Metamorphosed Intrusions Of First Stage 61
  3.12 Undeformed Intrusion Of Second Stage 61
  3.13 Mafic Dikes Of Third Stage 62
  3.14 Metamorphism In The Shigar Valley 62
  3.15 Metamorphism South Of The Main Karakoram Thrust 63
  3.16 Metamorphism North Octhe Main Karakoram Thrust 64
  3.17 Pegmatites Of The Shigar Valley Area 65
5 4 Methodology
194.65 KB
  4.1 Field Methodology 75
  4.2 Collection Of Rock Samples 75
  4.3 Laboratory Methodology 77
  4.4 Preparation Of Thin-Sections 77
  4.5 Crushing And Pulverizing Of Rock Samples 78
  4.6 Preparation Of Stock Solutions 78
  4.7 Stock Solution A: Using Acid Digestion Method 78
  4.8 Stock Solution B: Using Sodium Hydroxide Fusion Method 79
  4.9 Determination Of Major And Minor Oxides 80
  4.10 Detennination Of Si0 2 80
  4.11 Detennination Of A1 2 0 3 81
  4.12 Detennination Of Ti0 2 82
  4.13 Determination Of P 2 0 5 82
  4.14 Determination Of Total Iron As Fe 2 0 3 83
  4.15 Determination Of CaO And MgO 84
  4.16 Determination Of Na 2 0 And K 2 0 85
  4.17 Determination Of MnO 86
  4.18 Determination Of Loss On Ignition By Using Gravimetric Method 86
  4.19 Qualitative Analysis Of Gemstones 87
  4.20 Preparation Of the Sample 87
  4.21 Mineral Analysis By Electron Probe Microanalyzer 87
  4.22 Scanning Electron Microscope And Preparation Of Samples 89
6 5 Mega And Microscopic Features Of Pegmatites
5486.24 KB
  5.1 General Statement 91
  5.2 gem-Bearing Pegmatites 91
  5.3 Gcm-Barrcn Pcgmatitcs 82
  5.4 gcm-Bearing Pegmatites 92
  5.5 m Uscovitc-Schorl-Bcryl-Garnct Pcgmatitcs 92
  5.6 Megascopic Features 92
  5.7 Microscopic Features 92
  5.8 muscovite-Schorl Pegmatitcs 93
  5.9 Megascopic Features 95
  5.10 microscopic Features 95
  5.11 gem-Barren Pegmatites 96
  5.12 Biotite L: Garnet :: I: Muscovite Pegmatites 97
  5.13 Megascopic Features 97
  5.14 Microscopic Features 97
  5.15 Muscovite-Biotite L: Garnet Pegmatites 97
  5.16 Megascopic Features 98
  5.17 Microscopic Features 98
  5.18 Shengus And Indus- Gilgit Confluence Pegmatites 99
  5.19 Megascopic Features 100
  5.20 Microscopic Features 100
  5.21 Host Rocks 101
  5.22 Megascopic Features Of The Dassu Biotite Gneiss 103
  5.23 Microscopic Features Of The Dassu Biotite Gneiss 103
7 6 Mineral Chemisrty
1737.18 KB
  6.1 general Statement 153
  6.2 mineral Phases Of The Shigar Valley Pegmatites 153
  6.3 plagioclase 153
  6.4 alkali Feldspar 156
  6.5 Muscovite 156
  6.6 biotite 158
  6.7 tourmaline 160
  6.8 Gamet 162
  6.9 Zircon 164
  6.10 Apatite 166
  6.1 1fluorite 168
  6.12 chlorite 168
  6.13 Mineral Phases Of The Indus- Gilgit Confluence And Shengus Pegmatite Plagioclase 169
  6.14 Alkali Feldspar 170
  6.15 biotite1 71
  6.16 Epidote 172
  6.17 allanite 173
  6.18 apatite 174
  6.19 chlorite 176
  6.20 zirRcon 176
  6.21 Illmenite 178
  6.22 magnetite 179
  6.23 gemstones Of The Shigar Vahey Pegmatites And Their Mineral Chemistry 180
  6.24 Topaz 180
  6.25 Gamet 182
  6.26 Beryl 183
  6.27 Schorl-Foitite 186
  6.28 Zoisite 188
  6.29 axinite 189
  6.30 fluorite 190
  6.31 apatite 191
  6.32 Clinozoisite 192
  6.33 Economics Of The Gemstones Of The Shigar Valley Area 193
  6.34 Exploration And Evaluation Of Gemstones 195
  6.35 Mining Of The Gemstones 195
  6.36 lapidary Work 196
8 7 whole Rock Geochemistry
412.61 KB
  7.1 major Element Characteristics And Classification 265
  7.2 trace Element Characteristics And Tectonic Environments Characteristics Of The Rare 270
  7.3 earth Elements 273
  7.4 possible Source Of Magma For The Shigar Valley Pegmatites 274
  7.5 collisionalleucogranites And The Shigar Valley Pegmatites 277
  7.6 chemical Characteristics Of indus-Gilgit Confluence Pegmatites 278
9 8 discussions
567 KB
  8.1 leucogranites 313
  8.2 himalayan Leucogranites 314
  8.3 models And Causes For The Generation Of The Himalayan Leucogranites 315
  8.4 classification Of The Himalayan Ieucogranites 316
  8.5 shigar Valley Pegmatites And Other Post- Eollisionalleucogranites And Pegmatites 317
  8.6 bahoro Plutonic Unit: The Possible Parent Magma Of The Shigar Valley Pegmatites 318
  8.7 comparison Of The Shigar Valley Pegmatites With The Hunza Valley Leucogranite 321
  8.8 classification Of The Shigar Valley Pegmatites 325
  8.9 causes Of Zonation And Shigar Valley Pegmatites 329
  8.10 trace Element Fractionation 335
  8.11 petrogenesis Of Shigar Valley Pegmatites 336
  8.12 li-Bearing Source Rock And Shigar Valley Pegmatites 345
  8.13 mechanics Of Pegmatite Intrusion 347
10 9 conclusions 358
388.7 KB
  9.1 references 362