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Nitrogen Isotope Characteristics of Orogenic Lode Gold Deposits and Terrestrial Reservoirs

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Nitrogen Isotope Characteristics of Orogenic Lode Gold Deposits and Terrestrial Reservoirs NITROGEN ISOTOPE CHARACTERISTICS OF OROGENIC LODE GOLD DEPOSITS AND TERRESTRIAL RESERVOIRS A thesis submitted to the College ofGraduate Studies and Research in partial fulfillment ofthe requirements for the degree of Doctor ofPhilosophy in the Department ofGeological Sciences University ofSaskatchewan by Yiefei Jia Spring 2002 © The author claims copyright. Use shall not be made ofthe material contained herein without proper acknowledgment, as indicated on the following page. In presenting this thesis in partial fulfillment of the requirements for a Postgraduate degree from the University of Saskatchewan, I agree that the Libraries of this University may make it freely available for inspection. I further agree that permission for copying of this thesis in any manner, in whole or in part, for scholarly purposes may be granted by the professor who supervised my thesis work or, in their absence, by the Head of the Department or the Dean ofthe College in which my thesis work was done. It is understood that any copying or publication or use of this thesis or parts thereof for financial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of Saskatchewan in any scholarly use which may be made ofany material in my thesis. Requests for permission to copy or to make other use of material in this thesis in whole or part should be addressed to: Head ofthe Department ofGeological Sciences University ofSaskatchewan Saskatoon, Saskatchewan S7N 5E2 ABSTRACT This doctoral research reports the first N-isotope characteristics of micas from orogenic gold deposits utilizing continuous flow isotope ratio mass spectrometer (CF­ IRMS), and the first systematic study of Se/S ratios in pyrite from these deposits using high precision Hexapole ICP-MS. The aim is to better constrain the ore-forming fluid and rock source reservoirs for this class of deposit. Orogenic gold quartz vein systems constitute a class of epigenetic precious metal deposit; they formed syntectonically and near peak metamorphism ofsubduction-accretion complex's ofArchean to Cenozoic age, spanning over 3 billion years of Earth's history. This class of gold deposits is characteristically associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustal structures. However, after many decades ofresearch their origin remains controversial: several contrasting genetic models have been proposed including mantle, granitoid, meteoric water, and metamorphic­ derived ore-forming fluids. Nitrogen, as structurally bound NH4+, substitutes for K in potassium-bearing silicates such as mica, K-feldspar, or its N-endmember buddingtonite; it also occurs as N2 in fluid inclusions. The content and isotope composition of nitrogen has large variations in geological reservoirs, which makes this isotope system a potentially important tracer for the origin of terrestrial silicates and fluids: The 015N of mantle is -8.6%0 to -1.70/00, and of granitoids is 50/00 to 10%0, both with generally low N contents of less 1-2 ppm and average 21-27 ppm respectively; meteoric water is 4.4 ± 2.0%0 with low N contents of 2!J.mole; kerogen and Phanerozoic sedimentary rocks are 3 to 50/00, with high N contents of 100's to 1000's ppm; and metamorphic rocks are +1.0%0 to more than 17%0 for 015N, and tens to >1000 ppm N content. Accordingly, nitrogen isotope systematics of hydrothermal micas from lode gold deposits might provide a less ambiguous signature of the ore-forming fluid source reservoir(s) than other isotope systems. The studied orogenic gold deposits were selected from a number of geological environments: ultramafic, turbidite, granitoid hosted in the late Archean gold provinces in the Superior Province of Canada and the Norseman terrane in Western Australia; Paleozoic turbidite-hosted gold province in the central Victoria, Australia; and the Mesozoic-Cenozoic western North American Cordillera from the Mother Lode gold in southern California, through counterparts in British Columbia and the Yukon, to the Juneau district in Alaska, providing ca 40° latitude. The latter sampling design is to test for latitudinal variations of oD in a subset of robust hydrothermal micas. Also, given sparse data on 015N in Archean silicate reservoirs, new N-isotope data on granites, sedimentary rocks, and organic-rich material were obtained. New data on N-isotopic compositions ofrobust hydrothermal K-silicates in orogenic gold deposits and other rock types show: (1) Archean micas in orogenic gold deposits have enriched 015N values of 10 to 24%0, and N contents of 20 to 200 ppm. Sedimentary rocks also have 015N of 12 to 17%0, and 15 to 50 ppm N contents. In contrast, granites have 015N values of-5 to 5%0, and N contents of 20 to 50 ppm. (2) Proterozoic micas in orogenic gold deposits have intermediate 015N values of 8.0 to 16.1 %0, and N contents of ii 30 to 240 ppm. Sedimentary rocks have 015N of 9.1 to 11.6%0, and N contents of 150 to 450 ppm. (3) Phanerozoic micas in orogenic gold deposits have relatively low 015N values of 1.6 to 6.1 %0, and high N contents of 130 to 3500 ppm. Sedimentary rocks are also low 015N of 3.5 ± 1.0%0, and high N contents of hundreds to thousands ppm. On the basis of these results, N in the micas from orogenic gold deposits rules out magmatic, mantle, or meteoric water ore fluids. Nor do oD values of micas from gold vein locations in the western North American Cordillera show any covariation over 30 0 latitude; the calculated oD and 0180 values of ore fluid range from -10 to -650/00, and 8 to 160/00, respectively, ruling out the meteoric water model based on fluid inclusions (secondary). Selenium contents and Se/S ratios in sulfide minerals have been used to constrain the genesis of sulfide mineralization given large differences in S/Se x 106 ratios of most mantle-derived magmas (230 to 350) and crustal rocks of <10's. Hydrothermal pyrites have Se abundances from 0.9 to 15.2 ppm (n = 28), corresponding to Se/S x 106 ratios of 2 to 34, hence Se/S systematics ofhydrothermal pyrite indicate crustal not mantle sources. A metamorphic fluid origin is further endorsed by 0l3C values of hydrothermal carbonates, coexisting with micas, decreasing with increasing metamorphic grade, consistent with progressive loss of 12C-enriched fluids during decarbonation reactions. Independent lines of geological and geochemical evidence such as restriction of these deposits to metamorphic terranes; lithostatic vein-forming fluid pressures; and consistently low aqueous salinity; demonstrate that metamorphic ore fluids were involved in this class ofdeposit. Given that these gold-bearing quartz vein systems formed by metamorphic dehydration they may proxy for bulk crust 815N. The new data of N contents and 815N values from crustal hydrothermal systems and limited sedimentary rocks both show systematic trends over 2.7 billion years from the Archean (8 15N = 16.5 ± 3.30/00; 15.4 ± 1.9%0); through the Paleoproterozoic (8 15N = 9.5 ± 2.40/00; 9.7 ± 1.00/00); to the Phanerozoic (8 15N = 3.0 ± 1.2%0; 3.5 ± 1.0%0). Crustal N content has increased in parallel from 84 ± 67 ppm, through 103 ± 91 ppm, to 810 ± 1106 ppm. Ifthe initial mantle acquired a 815N of­ 25%0 corresponding to enstatite chondrites as found in some diamonds, whereas the final atmosphere from late accretion of volatile-rich C1 carbonaceous chondrites was +30 to +42%0, the results may provide a specific mechanism for shifting 815N in these reservoirs to their present-day values of -50/00 in the upper mantle and 00/00 for the atmosphere by early growth of the continents, sequestering of atmospheric N2 into sediments, and recycling into the mantle. iii ACKNOWLEDGMENTS First and foremost, lowe my deepest thanks to my supervisor Rob Kerrich for his guidance, friendship and never-ending interest. He continually challenged and expanded my scientific boundaries, and allowed my Ph.D. experience to be an intellectual cornerstone for my future career. I also acknowledge that, during the course of my studies, I received the University of Saskatchewan Graduate Scholarships and the Hugh E. McKinstry Grant from the Society ofEconomic Geologists Foundation Inc. USA. This study would not have been possible without the support of abundant technical assistance in acquiring geochemical data for this thesis, for which the following people are duly acknowledged: N-isotopes - G. Parry and M. Stocki; C and a-isotopes - D. Pezderic and T. Prokopiuk; H-isotopes - E. Hoffman; ICP-MS - Q. Xie and G. Yip; Microprobe - T. Bonli; and Thin Sections - B. Novakovski. The thesis has been benefited greatly from the critical helpful discussions and comments of Drs. K.M. Ansdell, Y. Pan, B. Pratt, G.R. Davis, J. F. Basinger (Univ. of Saskatchewan); RJ. Goldfarb (U.S.G.S); and A. Delsemme (Univ. ofToledo, Ohio). Special thanks to L. Caron, L. Hunt, and C. Hart for field assistance in southern British Columbia, northern British Columbia, and Yukon respectively. I am also grateful to Drs. R.J. Goldfarb and C. Ash for supplying samples from the Fern and Christina in Alaska and the Bralorne, Snowbird in British Columbia. I wish to thank all of my fellow graduate students, namely A. Abeleira, P. Dong, S. Ivanov, J. King, J. Muise, T. Birkham, and K. Fanton, for their provocative discussions. Finally, I whole-heartedly thank my wife Xia, and my daughters Lingsa and Lilyan, for their support and encouragement throughout the years ofmy study. iv TABLE OF CONTENTS PERMISSION
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