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Origins of the Himalayan Treasure Chest A geologic overview by Bernhard Grasemann Institute for Geological Sciences, University of Vienna, Austria and Erich Draganits Institute for Engineering Geology, Vienna Technical University, Austria Digital Topographic Model (GTOPO 30 - Global Topographic Data, U.S. Geological Survey) of India and the Himalaya mountain range. The Himalayas stretch from Namche Barwa in the east (1), where the Tsangpo River cuts through the range, to Nanga Parbat in the west (2), where the Indus cuts through the mountains. The Karakoram Range runs north of the Indus and is separated from its westward extension in the Hindu Kush by the Ishkoman River (3). The Himalayas and their westward extension into the Karakoram and Hindu Kush are the highest mountains on earth, not to mention the most topographically spectacular. Rising abruptly from the borders of the Indus-Ganges plains in the south, the Himalayas boast all 14 of the world’s 8,000 plus meter peaks (Fig. 1). Ongoing tectonic movement coupled with deep valleys allow scientists insight into otherwise hidden crustal processes, as well as providing outcrops of some spectacular mineral deposits. Those who endure the hardships and privations of a journey into these regions are rewarded with breathtaking landscapes, adventurous experiences, boundless hospitality 22 and rare minerals. The section of the Himalayas in northern Pakistan is especially famous for its mineral riches. This area is relatively easy to reach by way of the Karakoram Highway. In addition to the region’s better-known minerals, such as beryl (var. aquamarine, emerald), corundum, topaz and tourmaline, one also finds interesting and valuable species such as axinite, fluorite and olivine. In general, occurrences of collectible minerals are localized, with the various mineralizations that are limited to certain tectonic and lithologic units; therefore, knowledge of the formation and tectonic events of the Himalayas is essential to the successful discovery of mineral deposits. Origin of the Himalayas The Himalayas and their northern and western extensions in the Karakoram and Hindu Kush are the result of the ongoing collision between the Indian and Asian continents. The two began to collide about 60 million years ago. In the Paleozoic and Mesozoic, India, Africa, South America, Australia and Antarctica were parts of the Gondwana supercontinent. About 130 million years ago, the Indian subcontinent separated from Gondwanaland and pushed northward at a velocity of roughly 20 centimeters per year (Fig. 2). As it moved, the Indian subcontinent pushed the crust of the Tethys Ocean (situated between India and Eurasia), under the southern edge of Eurasia. There the Tethys oceanic crust partially melted. By about 60 million years ago, the oceanic crust of the Tethys had been pushed entirely beneath Eurasia. No longer separated by an ocean, India and Eurasia began to collide along what is known as the Indus-Tsangpo suture zone. This colliding continues today and has created thrust systems that divide the Himalaya, Karakoram and Hindu Kush ranges into tectonic units—units that can be traced for several thousand kilometers. The collision with Eurasia slowed the northward motion of India from its original 20 centimeters per year to its current speed of around 5 centimeters per year, with India’s continental crust being subducted by Eurasia. Parts of the Indian crust reached depths of more than 80 kilometers into the Earth’s mantle and thereby underwent ultra-high-pressure metamorphism creating diamond and coesite. Coesite, recognized petrographically and confirmed by in situ Raman microprobe spectroscopy, is reported from an eclogite (metamorphic rock essentially made up of sodarich pyroxene and magnesium-rich garnet) from the Kaghan Valley, Pakistan. The find represents the first record of ultrahigh-pressure metamorphism in the Himalayas. Formation conditions of greater than 27 kilobars implied by the presence of coesite are supported by garnetpyroxene-phengite barometry (27–29 kilobars, 690–750 °C) (O’Brien et al. 2001). The Himalayas as we know them, with their extreme topographic relief, formed only over the last 8 or 9 million years. Because of their height, these mountains influence global atmospheric circulation and therefore to some extent also the worldwide climate. India-Asia collision Paleogeographic reconstruction of the collision between India and Eurasia from their position 71 million years ago up to the present. Geologic Structure of the Himalaya The Himalaya, Karakoram and Hindu Kush mountains are composed of a series of diverse geologic/tectonic macro-units (Figure 3), which are separated by great fault systems. The northernmost units are the Trans-Himalaya and Karakoram batholiths (granitic plutons). These granites between 130 and 30 million years ago. Such granites are found principally in the Kohistan and Ladakh batholiths as well as in the Karakoram and in the continental suture zone between the Indian and Eurasian continents. Geochemical and isotope data show that these granites were derived from melted oceanic crust of the now extinct Tethys Ocean (Honegger et al., 1982). Current examples of these same processes can be seen in the granites of the Andes, which are derived from the oceanic crust of the Pacific. The Pacific oceanic crust is being subducted beneath the continental crust of South America. The Trans-Himalaya unit is bounded in the south by the Indus-Yarlung-Tsangpo suture zone, which represents the actual collision front 23 Simplified geological map of the major tectonic units in the Himalayas between India and Eurasia. All of the rocks south of this zone were originally part of the Indian subcontinent. The Indus-Yarlung-Tsangpo suture zone is composed of very diverse, strongly deformed rocks from the oceanic crust of the ancient Tethys Ocean. These rocks include deepsea sediments such as flysch and radiolarites, oceanic basalts, and even rocks that were derived from Earth’s mantle! South of the Trans-Himalaya unit is the High Himalaya tectonic unit, which is generally divided into two subunits: the Higher Himalayan Crystalline and the Tethyan Himalaya. The Tethyan Himalaya lies directly south of the Indus-Yarlung-Tsangpo suture zone and is composed of sediments from the previous northern continental margin of India. These sediments were deposited over a period of more than 500 million years. They form a series of sandstones, shales, limestones and dolomites that are more than 10 kilometers thick, and that have 24 been folded by the continental collision. The Higher Himalayan Crystalline subunit is south of the Tethyan Himalaya. It is made up of the sediments of the Tethyan Himalaya, which were in part metamorphosed and were pushed down more than 30 kilometers into the Earth’s crust by the collision between India and Eurasia. The most abundant rocks in the Higher Himalayan Crystalline are mica schists, gneiss, amphibolites, migmatites and two groups of granite intrusions of different ages. The older granites intruded the deeper series of the Tethyan Himalaya about 480 million years ago; whereas the younger granites rose into the already metamorphosed rocks of the Higher Himalayan Crystalline roughly 20 million years ago. Numerous granite and pegmatite dikes formed during the latter phase. The youngest of these veins are only about 4 million years old. They tend to occur in regions that are tectonically active today (eg: Nanga Parbat). The Lesser Himalaya units are south of the Main Central Thrust, this thrust exhibits displacements of more than 140 kilometers! The Lesser Himalaya are composed mainly of weakly metamorphosed Precambrian sedimentary rocks, only minor portions of which can be stratigraphically correlated to the High Himalaya. Beginning about 9 or 10 million years ago, the Lesser Himalaya were pushed over the SubHimalaya along the Main Boundary Thrust. The Sub-Himalaya is the southernmost and youngest tectonic unit in the Himalayas. The unit is composed of sediments—mainly sandstones and conglomerates—that were deposited by erosion from the growing Himalayan ranges to the north. Further south, along the Main Frontal Thrust, the Sub-Himalaya is being pushed above the young, undeformed sediments of the IndusGanges Plains. This fault zone is the youngest overthrust belt of the India-Eurasia collision. It is still active and is responsible for many of the numerous earthquakes in this region. western and eastern extensions. A wedge of these schists, part of the strongly deformed Tethys oceanic crust in the Indus-YarlungTsangpo suture zone, is being pushed over the rocks of the Higher Himalaya along the Main Mantle Overthrust. The combined metamorphism of sedimentary and oceanic volcanic rocks, together with alteration by mineralized water, is likely responsible for the formation of emerald (Kazmi & Snee, 1989). Pink to deep red ruby comes mainly from the metamorphic dolomite marble of the Hunza Valley and from northern border of Pakistan, but are also known from the Neelum Valley in (Indiacontrolled) Kashmir. The world famous cornflower-blue sapphire from Kashmir is found in strongly kaolinized pegmatite dikes, which are hosted in mica schists of the High Himalaya. This deposit is at elevations nearing 5,000 meters, but it seems to be exhausted. Mineral Occurrences Mineral occurrences in the Karakoram and Hindu Kush seem to be directly related to the mountain-building processes. The great variety in local country rocks, variations in element transfers along fault zones, differences in metamorphic alterations and geochemical conditions lead to the diversity of mineralization. Most of the minerals found in northern Pakistan come from pegmatites. Tourmaline, beryl (aquamarine) and apatite are good examples. Corundum (var. ruby and sapphire) and beryl (emerald) owe their formation mainly to metamorphic and hydrothermal processes. The processes of formation are not, however, always sharply defined. Aquamarine and tourmaline tend to be relatively abundant in the young pegmatites in the High Himalaya. They are especially widely distributed in northern Pakistan, where they can reach majestic sizes and are often associated with apatite, topaz and garnet. Yellowish brown to honey-yellow topaz are found in the pegmatites around Skardu. Rare pink to violet-red topaz comes from the Katlang region in the Mardan district, from calcite and quartz veins in carbonate rocks. Pakistani emeralds are mainly found in the talc-carbonate schists of the Swat valley and its View to the south, toward Gumburajum Mountain at the headwaters of the Kurgiakh River in Zanskar (NW Himalayas). The rocks of the more than 1,000 meter high cliff belong to the Higher Himalaya Crystalline. They are composed of numerous granitic veins that intruded into metamorphosed sediments about 22 million years ago. 25