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Lesson 22 - Lesson 22 Geology and Geography of Gem Bearing...

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Lesson 22: Geology and Geography of Gem Bearing Pegmatites Introduction Pegmatite - the word itself sends shivers of excitement up my spine! Pegmatite is the premier rock type for finding large high quality gemstones, and with the exception of diamond, can produce basically the whole range of the most sought after coloured gems. Not all pegmatite gems occur in all pegmatites - most are restricted to specific varieties of pegmatites and some even more restricted to the type of host rock these igneous rocks intrude. Further, pegmatite gemstones tend to occur in specific locations within a pegmatite called pockets. We touched briefly on pegmatites during the beryl section, but here we'll talk a bit more about the internal structure of pegmatite and the global distribution of pegmatites (with a focus on Canada). We'll read two articles on pegmatites from Colorado and California. I also recommend reading a third paper on pegmatites in BC (reading not required). These articles are accessible for download below along with the EOSC118 guides to help you focus on the important aspects of each article. You may want to hunt these journal articles down yourself with your UBC Library access. The journals Gems & Gemology and Rocks & Minerals have loads of other interesting articles that you might also be interested in! For instructions on how to access these articles from the UBC Library, go to Help: FAQs Course Basics on the Home Page. Essential Readings Outside the Textbook Lees, B.K. (2005). " A Spectacular New Aquamarine Find on Mount Antero, Chaffee County, Colorado ". Rocks & Minerals , Vol. 80, No. 3, p198-199. Use the Lees-2005 Reading Guide to help you with the article. Mauthner, M. (2008). " The 49er Pocket: Oceanview Mine, Pala District, San Diego County, California ". Rock & Minerals , Vol. 83, No. 4, p292-297. Use the Mauthner-2008 Reading Guide to help you with the article. Optional Reading Brown, J.A. (2003). " Mineralogy And Geochemistry Of Beryl and Rare-Metal-bearing
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Granitic Pegmatites in the Kootenay Region of Southeastern British Columbia ". 2003 BC Geological Survey Annual Mineral Exploration Report, p167-183. Pegmatite Genesis Recall the definition by Prof. Simmons, "pegmatite is a textural term used to describe very coarse to gigantic sized textures in intrusive igneous rocks". In addition, most pegmatites are genetically associated with larger igneous bodies and will have a base geochemical signature similar to their parental pluton. The parental pluton, commonly granite, is a key factor in the genesis of most pegmatites in that it gives rise to, or feeds, a pegmatite. During the magmatic history of a granite body it may undergo significant fractionation. Fractionation is a process that involves the sequential crystallization of minerals as granitic magma cools. As certain minerals crystallize, they essentially remove the elements required for it from the molten magma. As the magma cools further, it becomes more depleted in the elements which make up the minerals that have crystallized.
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