New δ 15 N analyses combined with a literature compilation reveal that shale kerogen, VMS-micas, and late-metamorphic vein micas show a secular trend from enriched values in the Archean, through intermediate values in Proterozoic terranes, to the Phanerozoic mode of 3‰–4‰. Kerogen in metashales from the 2.7 Ga Sandur Greenstone Belt, eastern Dharwar Craton, India, is characterized by δ 15 N 13.1‰ ± 1.3‰, and C/N 303 ± 93. A second population has δ 15 N 3.5‰ ± 0.9‰, and C/N 8 ± 0.4, close to the Redfield ratio of modern microorganisms, and is interpreted as precipitates of Proterozoic or Phanerozoic oilfield brines that penetrated the Archean basement. Kerogen from 1.7 Ga carbonaceous shales of the Cuddapah Basin average 5.0‰ ± 1.2‰, close to the mode at 3‰–4‰ for kerogen and bulk rock of Phanerozoic sediments. Biotites from late-metamorphic quartz-vein systems of the 2.6 Ga Kolar gold province, E. Dharwar Craton, that proxy for average crust, are also enriched at 14‰–21‰ for three samples, confirming that the N–budget of the hydrothermal fluids is dominated by sedimentary rocks. Muscovites from altered volcanic rocks in 2.7 Ga Abitibi belt VMS deposits have δ 15 N 12‰–20‰, in keeping with published data for shale kerogen from the same terrane, whereas equivalents in the 1.8 Ga Jerome VMS span 11.7‰–14.1‰. 15 N-enriched values in Precambrian rocks cannot be caused by N-isotopic shifts due to metamorphism or Rayleigh fractionation because (1) pre-, and post-metamorphic samples from the same terrane are both enriched in 15 N; (2) there is no covariation of δ 15 N with N, C/N ratios, or metamorphic grade; and (3) the magnitude of fractionations of 1‰ (greenschist) to 3‰ (amphibolite facies) during progressive metamorphism of sedimentary rocks, as constrained from empirical observations and experimental studies, is very small. Nor can 15 N-enriched values stem from long-term preferential diffusional loss of 14 N, as samples were selected from terranes where 40 Ar/ 39 Ar ages are within a few million years of concordant U-Pb ages; nitrogen is structurally bound in micas, whereas Ar is not. It is possible that the 15 N-enriched values stem from a different N-cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations between atmospheric N 2 and organic nitrogen observed exceeds any presently known, and chemoautotrophic communities tend to depleted values. Earlier results on Archean cherts show a range of δ 15 N from −6‰ to 30‰. Given the temporal association of chert–banded iron formation (BIF) with mantle plumes, the range is consistent with mixing between mantle N 2 of −5‰ and the 15 N-enriched marine reservoir identified in this study. The 15 N-enriched Archean atmosphere-hydrosphere reservoir does not robustly constrain Archean redox-state. We attribute the 15 N-enriched reservoir to a secondary atmosphere derived from CI-chondrite-like material and comets with δ 15 N of +30‰ to +42‰. Shifts of δ 15 N to its present atmospheric value of 0‰ can be accounted for by a combination of early growth of the continents with sequestration of atmospheric N 2 into crustal rocks, and degassing of mantle N ∼−5‰. If Earth's surface environment became oxygenated ca. 2 Ga, then there were no associated large N-isotope excursions.

Abstract Potassic alteration domains of greenstone belt lode gold deposits, typified by the Kerr Addison mine, Ontario, are characterized by systematic partitioning between different groups of incompatible elements such that Al, Ga, Th, U, Ti, and V are decoupled from K, Rb, Ba, Cs, Li, and Tl. The former group of elements have concentrations and interelement ratios in alteration domains which reflect host-rock control, implying isochemical behavior. In contrast, the lithophile elements K, Rb, and Ba are generally coenriched and linearly correlated over three orders of magnitude in abundance, where K/Rb = 230 to 380, and K/Ba = 35 to 100. K/Rb and K/Ba ratios trend toward higher values with respect to increasing concentrations of Rb and Ba, possibly as a result of mixing between host rock and hydrothermal reservoirs of the lithophile elements. K/Cs and K/Tl are weakly correlated, and lithium abundances and Rb/Sr ratios are erratic in altered rocks. These interelement trends, collectively, are present in deposits variously hosted by ultramaflc, mafic, or felsic volcanic rocks, and sediments or granitoids. Magmatic processes involving crystal fractionation of biotite, K feldspar, and plagioclase generate trends to systematically diminished K/Rb (≤50), K/Li, K/Cs, K/Tl, and Al/Ga but enhanced K/Ba (≥8 x 10 3 ) and Rb/Sr in most late-stage differentiates. Such late-stage trends are the rule in magmatophile deposits including the Archean Cadillac molybdenite deposit, Phanerozoic Cu, and Mo porphyry deposits, Sn-W greisens, and most pegmatites. Accordingly, magmatic processes of this type can be ruled out as the dominant source of volatiles for gold-forming systems. In most granulites, especially Archean examples, lithophile element depletion is a primary feature rather than being acquired during carbonic metamorphism. Consequently there is no complementarity between large ion lithophile element-depleted granulites and the K, Rb enrichment characteristic of gold deposits. Moreover, the high K/Rb, K/Cs, and Rb/Cs ratios but low K/Ba ratios of most depleted granulites are not reflected in the gold deposits, where K/Rb and K/Ba ratios approximate average crustal values. The compliance of K/Rb and K/Ba ratios in potassic alteration domains of Au deposits with values characteristic of main trend igneous rocks, or average crust, implies that K, Rb, and Ba were partitioned into the hydrothermal ore-forming fluids in approximately the same ratios as in the source rocks. Dehydration reactions in the source, or equilibration of fluids with source rocks under conditions of low water/rock ratio, rather than purely magmatic or granulitization processes, may satisfy the requirement for proportional K, Rb, and Ba coenrichment.

Abstract Archean snosnonitic lampropnyres are cotemporal and cospatial with gold mineralization in the Superior province, both being emplaced along major translithospheric structures that demark subprovince boundaries. They occur internal to greenstone belts such as the Abitibi. Lamprophyres constitute a volumetrically minor but widespread component of late kinematic magma-tism, which is most prominently developed in fault graben or dilational jogs where the dikes are associated with alkali gabbros, trachytes, and molasse sediments. By analogy with geochemically similar Phanerozoic counterparts, the dikes are a product of specific plate interactions rather than a deep asthenosphere plume-initiated event. Fresh shoshonitic dikes are characterized by normal background gold contents of 3.9 ±8.1 ppb (lσ) close to the value of 3.0 ppb for the bulk continental crust. Average abundances of As (1.4 ± 0.5 ppm), Sb (0.25 ± 0.25), Bi (0.09 ± 0.02), W (1.9 ± 2.2), Tl (0.11), B (6.2 ± 3.3), Cu (71 ± 38), Pb (7 ± 5), Zn (93 ± 18), and Mo (1.4 ± 1.5) are also close to values of 1.0, 0.2, 0.06,1.0, 0.36, 10, 75, 8, 80, and 1.0 ppm, respectively, in bulk continental crust. Fresh lamprophyres are not intrinsically enriched either in Au or elements affiliated with gold in mesothermal deposits, and accordingly, do not constitute a special source rock. Platinum-group element contents (Ir = 0.84 ± 0.58 ppb; Pt = 5.9 + 26.5, Pd = 5.5 ± 1.8) in conjunction with Cu, Au, and Ni abundances define approximately flat patterns on primitive mantle-normalized diagrams, consistent with derivation of the alkaline magmas from a depleted mantle source variably enriched by incompatible elements. Sporadically elevated Au abundances in lamprophyres proximal to gold deposits are interpreted as a secondary overprint in dispersion halos. Post-Archean ultramafic lamprophyres from the Superior province have suffered little crustal interaction and do not possess enhanced abundances either of Au (mean = 0.5 ppb) or most Au-associated elements. Where anomalously rich gold contents occur in alkaline rocks, Au invariably defines a peak relative to Cu and neighboring platinum-group elements on normalized diagrams such that the anomaly is likely the result of a secondary overprint rather than an intrinsic feature of the noble metal budget of alkaline magmas. Comparable abundances and ratios of Pd/Au, Os/Ir, and Ru/Ir in Archean lamprophyres, Archean komatiites, and Gorgona komatiites (Brugman et al., 1987) signify that the Archean and Phanerozoic upper mantle had similar noble metal contents such that the prolific greenstone belt Au-Ag vein deposits cannot be explained by secular variations in upper mantle Au abundance alone. The lack of covariation between Au and light rare earth elements in lamprophyres rules out mantle metasomatism as a process generating intrinsically Au-rich magmas. Emplacement of the lamprophyres was diachronous from north (2,705 Ma) to south (2,674 Ma) in the Superior province, as was the gold mineralization. Both were related to late transpressional tectonics during successive accretions of individual subprovinces. Alkaline magmatism and gold mineralization are temporally and spatially related because they share a common geodynamic setting, but they are otherwise the products of distinct processes; the alkaline magmas were derived from depths of 40 to 120 km in the continental mantle lithosphere and asthenosphere, whereas the gold-mineralizing systems were confined to the continental crust.