Nitrogen isotope characteristics of orogenic lode gold deposits and terrestrial reservoirs

Abstract

This doctoral research reports the first N-isotope characteristics of micas from orogenic gold deposits utilizing continuous flow isotope ratio mass spectrometer (CFIRMS), 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 of subduction-accretion complex's of Archean 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 of research 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 &#94815N of mantle is -8.6&#137 to -1.7&#137, and of granitoids is 5&#137 to 10&#137, both with generally low N contents of less 1-2 ppm and average 21-27 ppm respectively; meteoric water is 4.4 ± 2.0&#137 with low N contents of 2&#956mole; kerogen and Phanerozoic sedimentary rocks are 3 to 5&#137, with high N contents of 100's to 1000's ppm; and metamorphic rocks are +1.0&#137 to more than 17&#137 for &#94815N, 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 &#948D in a subset of robust hydrothermal micas. Also, given sparse data on &#94815N in Archean silicate reservoirs, new N-isotope data on granites, sedimentary rocks, and organic-rich material were obtained.

New data on N-isotopic compositions of robust hydrothermal K-silicates in orogenic gold deposits and other rock types show: (1) Archean micas in orogenic gold deposits have enriched &#94815N values of 10 to 24&#137, and N contents of 20 to 200 ppm. Sedimentary rocks also have &#94815N of 12 to 17&#137, and 15 to 50 ppm N contents. In contrast, granites have &#94815N values of -5 to 5&#137, and N contents of 20 to 50 ppm. (2) Proterozoic micas in orogenic gold deposits have intermediate &#94815N values of 8.0 to 16.1&#137, and N contents of 30 to 240 ppm. Sedimentary rocks have &#94815N of 9.1 to 11.6&#137, and N contents of 150 to 450 ppm. (3) Phanerozoic micas in orogenic gold deposits have relatively low &#94815N values of 1.6 to 6.1&#137, and high N contents of 130 to 3500 ppm. Sedimentary rocks are also low &#94815N of 3.5 ± 1.0&#137, 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 &#948D values of micas from gold vein locations in the western North American Cordillera show any covariation over 30&#176 latitude; the calculated &#948D and &#94818O values of ore fluid range from -10 to -65&#137, and 8 to 16&#137, 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