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 ẟ¹⁵N of mantle is -8.6‰ to -1.7‰, and of granitoids is 5‰ to 10‰, both with generally low N contents of less 1-2 ppm and average 21-27 ppm respectively; meteoric water is 4.4 ± 2.0‰ with low N contents of 2 μmole; kerogen and Phanerozoic sedimentary rocks are 3 to 5‰, with high N contents of 100's to 1000's ppm; and metamorphic rocks are +1.0‰ to more than 17‰ for ẟ¹⁵N, 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 ẟD in a subset of robust hydrothermal micas. Also, given sparse data on ẟ¹⁵N 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 ẟ¹⁵N values of 10 to 24‰, and N contents of 20 to 200 ppm. Sedimentary rocks also have ẟ¹⁵N of 12 to 17‰, and 15 to 50 ppm N contents. In contrast, granites have ẟ¹⁵N values of -5 to 5‰, and N contents of 20 to 50 ppm. (2) Proterozoic micas in orogenic gold deposits have intermediate ẟ¹⁵N values of 8.0 to 16.1‰, and N contents of 30 to 240 ppm. Sedimentary rocks have ẟ¹⁵N of 9.1 to 11.6‰, and N contents of 150 to 450 ppm. (3) Phanerozoic micas in orogenic gold deposits have relatively low ẟ¹⁵N values of 1.6 to 6.1‰, and high N contents of 130 to 3500 ppm. Sedimentary rocks are also low ẟ¹⁵N of 3.5 ± 1.0‰, 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 ẟD values of micas from gold vein locations in the western North American Cordillera show any covariation over 30° latitude; the calculated ẟD and ẟ¹⁸O values of ore fluid range from -10 to -65‰, and 8 to 16‰, 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 of hydrothermal pyrite indicate crustal not mantle sources. A metamorphic fluid origin is further endorsed by 0¹³C 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 of deposit. Given that these gold-bearing quartz vein systems formed by metamorphic dehydration they may proxy for bulk crust δ¹⁵N. The new data of N contents and δ¹⁵N values from crustal hydrothermal systems and limited sedimentary rocks both show systematic trends over 2.7 billion years from the Archean (δ¹⁵N = 16.5 ± 3.30/00; 15.4 ± 1.9‰); through the Paleoproterozoic (δ¹⁵N = 9.5 ± 2.40/00; 9.7 ± 1.0‰); to the Phanerozoic (δ¹⁵N = 3.0 ± 1.2%; 3.5 ± 1.0‰). Crustal N content has increased in parallel from 84 ± 67 ppm, through 103 ± 91 ppm, to 810 ± 1106 ppm. If the initial mantle acquired a δ¹⁵N of 25‰ 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‰, the results may provide a specific mechanism for shifting δ¹⁵N in these reservoirs to their present-day values of -5‰ in the upper mantle and 0‰ for the atmosphere by early growth of the continents, sequestering of atmospheric N₂ into sediments, and recycling into the mantle.