Gemstones Web.Indd

OPEN FILE 2004–10 EXPLORATION CRITERIA FOR Coloured Gemstone Deposits in the Yukon by Lori Walton May, 2004 Tigerstar Geoscience Illustrations by Christine Bannerman Energy, Mines and Resources OPEN FILE 2004–10 EXPLORATION CRITERIA FOR Coloured Gemstone Deposits in the Yukon by Lori Walton May, 2004 Tigerstar Geoscience Published under the authority of the Minister of Energy, Mines and Resources, Yukon Government http://www.emr.gov.yk.ca Printed in Whitehorse, Yukon, 2004. © Minister of Energy, Mines and Resources, Yukon Government This, and other Yukon Geological Survey publications, may be obtained from: Geoscience and Information Sales c/o Whitehorse Mining Recorder 102–300 Main Street Box 2703 (K102) Whitehorse, Yukon, Canada Y1A 2C6 phone (867) 667-5200, fax (867) 667-5150 Visit the Yukon Geological Survey web site at www.geology.gov.yk.ca In referring to this publication, please use the following citation: Walton, L., 2004. Exploration criteria for coloured gemstone deposits in the Yukon. Yukon Geological Survey, 184 p. This report is based on the 1996 Open File 1996–2(G), Exploration criteria for gemstone deposits and their application to Yukon geology, by L. Walton, Walton Geological Services, prepared for Exploration and Geological Services Division, Yukon Region, Indian and Northern Affairs Canada, 130 p. Production by K-L Services, Whitehorse, Yukon. Illustrations by Christine Bannerman. Note: All prices in this report are given in U.S. dollars. Contents Preface . .1 Acknowledgements . .2 1. Introduction . .3 2. Field identification of gemstones . .9 3. Ruby and sapphire . .15 4. Emerald . .57 5. Tsavorite garnet and tanzanite . 105 6. Chrysoberyl . .119 7. Gem-bearing pegmatites . .131 8. Gem topaz and red beryl associated with rhyolites and greisens . .157 9. Application to Yukon geology . .161 Appendices A. Summaries of shale-hosted and schist-hosted types of emerald deposits and exploration guides . .177 B. Coloured posters . .182 PREFACE The ancient Greeks believed that four elements — fire, air, water and earth — comprised the universe and that metals in ore deposits were water elements. There is some scientific evidence today that the mysterious fifth element first postulated by the Greeks may in fact exist as ‘dark energy’ or ‘dark matter’ comprising 95% of our universe. Ancient Greek philosophers/naturalists/historians were, like all humans thoughout history, fascinated by geology and gemstones, especially the beautiful coloured gems that came from the island of Serendipity, or Sri Lanka. The physico- chemical controls on gem formation were beautifully described in those days by the philosopher Theophrastus who declared gemstones to be ‘solidified lynx urine’ and in fact valuable gem crystals grew wherever this beautiful feline decided to pee. Before scoffing, we must think about how our present day ideas and models for gemstone formation may be viewed not just 100 years in the future, but 1,000 or even 10,000 years. Some of the best and most useful work on gemstone deposits is not the fluid inclusion and stable isotope laboratory work, but the careful, detailed and precise descriptions, sometimes hand drawn, of the geology, mineralogy and general nature of the deposit. So little is known about some of the best gem deposits on our planet, such as the ruby deposits of Burma or the sapphire deposits of Kashmir, India. We must rely on descriptive geology done many years ago until we can unleash our modern scientific arsenal to decode how and why these fabulous gems formed and if there is potential for finding world-class coloured gemstone deposits in Canada. Exploration criteria for coloured gemstone deposits in the Yukon, by L. Walton, Tigerstar Geoscience 1 ACKNOWLEDGEMENTS I would like to gratefully acknowledge the funding granted for this report by the Yukon Geological Survey, Department of Energy, Mines and Resources, Government of Yukon. Many individuals contributed their expertise and took the time to discuss aspects of gemstone deposits and I thank all of them. I would also like to thank the people who contributed to the editing. Brad Wilson, Robert Simpson, Peter Long and Diane Emond. K-L Services did their usual outstanding job. I would also like to thank my family for their support. 2 1. INTRODUCTION For over 20,000 years and probably longer, humans have looked for beautiful and durable natural materials from which to make personal ornaments and attractive decorative objects. Shell necklaces from graves near Rome have been dated at 22,000 years old and amazingly, would not look out of place around the neck of a woman from the 21st century. Magnificent jewellery sets of gold, lapis lazuli and carnelian agate, belonging to a Sumerian queen in the area we now call Iraq, continue to astonish us by their intricate beauty. Besides being beautiful, gems are an important part of our cultural heritage on this planet. For centuries, gemstones have been a globally recognized and easily transportable form of wealth. Their acquisition can cost lives, as in the conquest of the Indian tribes of South America by the Spaniards in the 16th century, or save lives, as in the efforts of Jewish people to leave Nazi Germany before and during the Second World War. A more recent example is that of emerald mining in Afghanistan, which helped finance the Northern Alliance rebellion against Taliban-ruled Afghanistan in the late 1990s up to 2001. Now, at the beginning of the 21st century, consumers are becoming more and more aware of the manner in which gems are mined and marketed. Canada’s fledgling gem trade has no long tradition or history behind it, but we can market our stones as being mined in a socially and environmentally responsible manner. For decades, Canadian geologists have been at the leading edge of developing exploration guidelines and techniques for exploring for gold, silver, copper, lead, nickel and zinc deposits, and, more recently diamond deposits. Before diamonds were discovered in the Northwest Territories, Canadian geologists and prospectors really had no idea what the kimberlitic hostrock for diamond looked like and how to find it. Since then, there has been a flood of technical special volumes, short courses, seminars and papers outlining diamond exploration guidelines for geologists and prospectors. In fact, in less than 15 years, the geologists of Canada have become world-leaders in the geology of diamond deposits and how to explore for them. When it comes to coloured gemstones like ruby, emerald and sapphire, geologists and prospectors know very little about how to explore for and evaluate occurrences of these very rare minerals. Recent discoveries of emerald mineralization in the Yukon, Northwest Territories and Ontario have generated a flurry of scientific papers and heightened awareness of the coloured gemstone market, but it is still an unfamiliar market, not only to exploration geologists, but also to governments, securities commissions and investors. A main area of concern is that of identification of gem rough in the field. Many, if not most coloured gemstones we see today in jewellery stores are coated, fracture- filled, dyed and/or irradiated in linear accelerators in efforts to enhance their appearance. Heat treatment is the most common technique used. The gemstones Exploration criteria for coloured gemstone deposits in the Yukon, by L. Walton, Tigerstar Geoscience 3 1. Introduction that one sees in the jewellery store, the vivid violet-blue tanzanites, midnight blue sapphires, crimson red rubies and brilliant yellow sapphires, are typically, almost without exception, heat-treated or treated in some other manner to improve their appearance. In some cases this improvement is dramatic, as in the heat treatment of tanzanite, which transforms non-descript greyish crystals to vivid violet-blue stones. Buckets of plain white, cloudy corundum (sapphire) crystals can be heat treated to a clear, sparkly midnight blue. Emeralds are quite easy to identify in the field, although almost all of them have been oiled after mining to improve the clarity of the stone. Most gem crystals are difficult to identify in the field in their rough form and embedded within the hostrock, or within a bucket of placer concentrate. SCOPE OF THIS OPEN FILE This report is an updated version of an Open File report (Walton, 1996) describing the geology and geochemistry of gemstone deposits and applicable exploration criteria. At the time of the 1996 report, emeralds had not yet been reported in the Yukon or NWT and yet it was obvious that Canada’s geology is very prospective for significant coloured gemstone deposits. Exploration efforts in the north are hampered by the short exploration season and harsh climate. The 1996 report and this update are both aimed at exploration geologists and prospectors who are unfamiliar with the geology of gemstone deposits and how to explore for them. Its objective is to provide information on gemstone deposit geology and provide general guidelines for the identification of, and exploration for, gemstone occurrences. The report focuses on a selection of some of the most valuable Educating the Canadian mining industry Listed below are some significant Canadian Gemstones: Old & New, by Northern Gems Short Course, organized papers, seminars and short courses on Willow Wight. (Excerpts from a by L. Walton, H. Falck and S. Craig, the subject of the geology of coloured presentation, Canadian Gemological presented at 2002 Geoscience Forums gemstones. In addition, the Canadian Association Conference in Toronto, in Whitehorse and Yellowknife. Institute of Gemology and the Canadian 1994.) Canadian Gemmologist, 1995, Gem Materials and Mineralogy, special Gemological Association sponsor XVI (3) p. 82–87. session organized by L. Groat, gemstone talks and seminars throughout University of British Columbia, Sessions and short courses the year, and there are articles on Geological Association of Canada Coloured Gemstones and Ornamental gemstones on the Canadian Rockhound – Mineralogical Association of Canada Stones II Session, Chaired by website.

Load more