John F. Slack

Boron isotopic composition of tourmaline from massive sulfide deposits and tourmalinites

Boron isotope ratios (11B/10B) have been measured on 60 tourmaline separates from over 40 massive sulfide deposits and tourmalinites from a variety of geologic and tectonic settings. The coverage of these localities is global (5 continents) and includes the giant ore bodies at Kidd Creek and Sullivan (Canada), Broken Hill (Australia), and Ducktown (USA). Overall, the tourmalines display a wide range inδ11B values from −22.8 to +18.3‰ Possible controls over the boron isotopic composition of the tourmalines include: 1) composition of the boron source, 2) regional metamorphism, 3) water/rock ratios, 4) seawater entrainment, 5) temperature of formation, and 6) secular variations in seawaterδ11B. The most significant control appears to be the composition of the boron source, particularly the nature of footwall lithologies; variations in water/ rock ratios and seawater entrainment are of secondary importance. The boron isotope values seem especially sensitive to the presence of evaporites (marine and non-marine) and carbonates in source rocks to the massive sulfide deposits and tourmalinites.

Evidence for early life in Earth’s oldest hydrothermal vent precipitates

Although it is not known when or where life on Earth began, some of the earliest habitable environments may have been submarine-hydrothermal vents. Here we describe putative fossilized microorganisms that are at least 3,770 million and possibly 4,280 million years old in ferruginous sedimentary rocks, interpreted as seafloor-hydrothermal vent-related precipitates, from the Nuvvuagittuq belt in Quebec, Canada. These structures occur as micrometre-scale haematite tubes and filaments with morphologies and mineral assemblages similar to those of filamentous microorganisms from modern hydrothermal vent precipitates and analogous microfossils in younger rocks. The Nuvvuagittuq rocks contain isotopically light carbon in carbonate and carbonaceous material, which occurs as graphitic inclusions in diagenetic carbonate rosettes, apatite blades intergrown among carbonate rosettes and magnetite–haematite granules, and is associated with carbonate in direct contact with the putative microfossils. Collectively, these observations are consistent with an oxidized biomass and provide evidence for biological activity in submarine-hydrothermal environments more than 3,770 million years ago.

Stratiform tourmalinites in metamorphic terranes and their geologic significance

Stratiform tourmalinites are significant minor rock types in many regional metamorphic terranes of the world. Tourmalinites are more widespread than previously recognized and are especially common in Proterozoic and early Paleozoic sequences dominated by clastic metasedimentary rocks. They consist of conformable layers made up primarily of quartz and abundant tourmaline, the latter typically exceeding 15% to 20% of rock volumes. A few tourmalinites display striking sedimentary structures such as graded bedding, cross-laminations, slump and flame structures, and rip-up clasts. These and other geologic features provide important constraints for assessing the origin of these rocks, and they suggest that tourmalinites form by the early diagenetic modification of a primary boron-rich chemical precipitate. Tourmalinites are significant in preserving a valuable record of unusual chemical and paleogeographic conditions in clastic sedimentary basins, and in their close association with a variety of stratabound mineral deposits.

Seafloor-hydrothermal Si-Fe-Mn exhalites in the Pecos greenstone belt, New Mexico, and the redox state of ca. 1720 Ma deep seawater

Mineralogical and geochemical data for ca. 1720 Ma Si-Fe-Mn seafloor-hydrothermal sedimentary rocks (exhalites) near the Jones Hill Zn-Cu-Pb-Ag-Au volcanogenic massive sulfide (VMS) deposit, northern New Mexico, provide valuable insights into the redox state of late Paleoproterozoic deep sea-water. Distal exhalites ~1200 m south of the deposit form beds 0.5–2 m thick composed of interlayered iron formation and metachert. The iron formation consists mostly of quartz and magnetite, and includes 0.3–3-cm-thick laminae of fine-grained garnet-quartz rock, which in places contains as much as 9.4 wt% MnO that resides chiefly in spessartine-rich garnet (coticule). Shale-normalized rare earth element data for an unaltered, low-Al quartz-magnetite iron formation show no Ce anomaly, which rules out fully oxic deep waters during exhalative mineralization. The garnet-quartz rocks and coticules mostly have small positive Ce anomalies, which are larger for calculated detrital-free compositions, thus precluding deposition in anoxic waters. Significant amounts of ferric iron are inferred for protoliths of the iron formation, based on the presence of abundant magnetite laminae, and of magnetite inclusions in cores of the spessartine garnets. Protoliths of the garnet-quartz rocks and coticules probably consisted largely of clays and Fe-Mn oxyhydroxides. Together these mineralogical and geochemical data suggest that the Jones Hill exhalites were deposited from deep sea-water having low concentrations of dissolved O2 corresponding to suboxic conditions, and not the sulfidic conditions proposed for late Paleoproterozoic deep seawater by other workers. Exhalites associated with Cu-rich VMS deposits, when effects of alteration and detrital components are considered, can be important proxies for evaluating the evolving redox state of ancient deep oceans.

Clastic metasediments of the Early Proterozoic Broken Hill Group, New South Wales, Australia: Geochemistry, provenance, and metallogenic significance

Whole-rock analyses of samples of pelite, psammite, and psammopelite from the Early Proterozoic Broken Hill Group (Willyama Supergroup) in the Broken Hill Block, New South Wales, Australia, reveal distinctive geochemical signatures. Major-element data show high Al2O3 and K2O, low MgO and Na2O, and relatively high Fe2O3TMgO ratios, compared to average Early Proterozoic clastic metasediments. High field strength elements (HFSE) are especially abundant, including Nb (most 15–27 ppm), Ta (most 1.0–2.2 ppm), Th (17–36 ppm), Hf (4–15 ppm), and Zr (most 170–400 ppm); Y (33–74 ppm) is also high. Concentrations of ferromagnesian elements are generally low (Sc = < 20 ppm, Ni = ≤ 62 ppm, Co = <26 ppm; Cr = most < 100 ppm). Data for rare earth elements (REEs) show high abundances of light REEs (LaCN = 116–250 × chondrite; LaCN = 437 in one sample), high LaCNYbCN ratios (5.6–13.9), and large negative Eu anomalies (EuEu∗ = 0.32–0.57). The geochemical data indicate derivation of the metasedimentary rocks of the Broken Hill Group by the erosion mainly of felsic igneous (or meta-igneous) rocks. High concentrations of HFSE, Y, and REEs in the metasediments suggest a provenance dominanted by anorogenic granites and(or) rhyolites, including those with A-type chemistry. Likely sources of the metasediments were the rhyolitic to rhyodacitic protoliths of local quartz + feldspar ± biotite ± garnet gneisses (e.g., Potosi-type gneiss) that occur within the lower part of the Willyama Supergroup, or chemically similar basement rocks in the region; alternative sources may have included Early Proterozoic anorogenic granites and(or) rhyolites in the Mount Isa and(or) Pine Creek Blocks of northern Australia, or in the Gawler craton of South Australia. Metallogenic considerations suggest that the metasediments of the Broken Hill Block formed enriched source rocks during the generation of pegmatite-hosted deposits and concentrations of La, Ce, Nb, Ta, Th, and Sn in the region. Li, Be, B, W, and U in pegmatite minerals of the district may have been acquired during granulite-facies metamorphism of the local metasediments.

Paleozoic and Mesozoic silica-rich seawater: Evidence from hematitic chert (jasper) deposits

Laterally extensive beds of highly siliceous, hematitic chert (jasper) are associated with many volcanogenic massive sulfide (VMS) deposits of Late Cambrian to Early Cretaceous age, yet are unknown in analogous younger (including modern) settings. Textural studies suggest that VMS-related jaspers in the Ordovician Lokken ophiolite of Norway were originally deposited as Si- and Fe-rich gels that precipitated from hydrothermal plumes as colloidal silica and iron-oxyhydroxide particles. Rare earth element patterns and Ge/Si ratios of the jaspers reflect precipitation from plumes having seawater dilution factors of 10 3 to 10 4 , similar to modern examples. We propose that silica in the ancient jaspers is not derived from submarine hydrothermal fluids-as suggested by previous workers-but instead was deposited from silica-rich seawater. Flocculation and precipitation of the silica were triggered inorganically by the bridging effect of positively charged iron oxyhydroxides in the hydrothermal plume. A model involving amorphous silica (opal-A) precursors to the jaspers suggests that silica contents of Cambrian-Early Cretaceous oceans were at least 110 mg/L SiO 2 , compared to values of 40-60 mg/L SiO 2 estimated in other studies. The evolution of ancient silica-rich to modern Fe-rich precipitates in submarine-hydrothermal plumes reflects a changeover from silica-saturated to silica-depleted seawater through Phanerozoic time, due mainly to ocean-wide emergence of diatoms in the Cretaceous.

Tourmaline in meta-evaporites and highly magnesian rocks: perspectives from Namibian tourmalinites

Tourmaline from meta-evaporitic tourmalinites of the Duruchaus Formation of central Namibia reveal a common compositional trend that occurs in tourmaline from other meta-evaporite localities. The meta-evaporitic tourmalines are generally sodic, magnesian, moderately-to-highly depleted in Al, and enriched in Fe 3+ and W O 2− (calculated). They typically follow this trend along a join between “oxy-dravite” [Na(Mg 2 Al)(Al 6 )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 (O)] and povondraite [Na(Fe 3 3+ ) (Fe 4 3+ Mg 2 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 (O)]. Similar trends occur in the meta-evaporites at Alto Chapare (Bolivia), Challenger Dome (Gulf of Mexico), and Liaoning (China). This chemical feature is attributed to the influence of oxidizing, highly saline, boron-bearing fluids that are associated with these lithologies. In the Namibian tourmalines there are some deviations from this trend, which are considered to be a consequence of later overprints related to sulfate–silicate interactions and/or influx of reactive fluid. Tourmalines occurring in the highly magnesian high-pressure rocks (whiteschists and pyrope–coesite rocks) are distinctly more magnesian and fall close to the dravite and “oxy-dravite” compositions. These latter tourmaline compositions likely reflect the metasomatic processes that produced these unusual bulk compositions and/or the influx of a reactive fluid that eliminated any earlier chemical signatures of meta-evaporitic fluids or protoliths.

Boron isotope systematics of tourmaline formation in the Sullivan Pb-Zn-Ag deposit, British Columbia, Canada

Abstract We report here the results of 54 boron isotope analyses of tourmaline associated with the giant Sullivan Pb–Zn–Ag deposit in southeastern British Columbia, Canada. The δ 11 B values range from −11.1 to −2.9‰, which is almost as great as the range found worldwide in tourmalines from 33 massive sulfide deposits and tourmalinites in dominantly clastic metasedimentary terranes. The major control on the overall δ 11 B values of the Sullivan tourmalinites is the boron source. Potential controls over the large range of the data also include: (1) differences in formation temperatures of the tourmalinites, (2) different stages of tourmaline formation, (3) variations in the proportions of dissolved boron incorporated into the tourmaline (Rayleigh fractionation), (4) seawater entrainment, and (5) post-depositional metamorphism. The boron isotope data at Sullivan are consistent with boron derivation from leaching of footwall clastic sediments. However, the great abundance of tourmaline in the Sullivan deposit suggests that the local clastic sediments were not the sole source of boron, and we argue that non-marine evaporites, buried deep below the orebody, are the most viable source of this additional boron. It is likely that some of the variation in tourmaline δ 11 B values reflect mixing of boron from these two sources. Comparison of the potential effects of these controls with geologic and other geochemical evidence suggests that major causes for the wide range of δ 11 B values measured at Sullivan are seawater entrainment and Rayleigh fractionation, although in places, post-depositional alteration and thermal metamorphism were important in determining δ 11 B values of some of the recrystallized tourmalinites.

Extraterrestrial demise of banded iron formations 1.85 billion years ago

In the Lake Superior region of North America, deposition of most banded iron formations (BIFs) ended abruptly 1.85 Ga ago, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. We propose a new model in which this impact produced global mixing of shallow oxic and deep anoxic waters of the Paleoproterozoic ocean, creating a suboxic redox state for deep seawater. This suboxic state, characterized by only small concentrations of dissolved O 2 (~1 μ M ), prevented transport of hydrothermally derived Fe(II) from the deep ocean to continental-margin settings, ending an ~1.1 billion-year-long period of episodic BIF mineralization. The model is supported by the nature of Precambrian deep-water exhalative chemical sediments, which changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter.

Sm-Nd dating of the giant Sullivan Pb-Zn-Ag deposit, British Columbia

We report here Sm and Nd isotope data for hydrothermal tourmalinites and sulfide ores from the giant Sullivan Pb-Zn-Ag deposit, which occurs in the lower part of the Mesoproterozoic Purcell (Belt) Supergroup. Whole-rock samples of quartz-tourmaline tourmalinite from the footwall alteration pipe yield a Sm-Nd isochron age of 1470 ± 59 Ma, recording synsedimentary B metasomatism of clastic sediments during early evolution of the Sullivan hydrothermal system. Data for variably altered (chloritized and/or albitized) tourmalinites from the hanging wall of the deposit, which are believed to have formed originally ca. 1470 Ma, define a younger 1076 ± 77 Ma isochron because of resetting of Sm and Nd isotopes during Grenvillian metamorphism. HCl leachates of bedded Pb-Zn ore yield a Sm-Nd isochron age of 1451 ± 46 Ma, which is consistent with syngenetic-exhalative mineralization ca. 1470 Ma; this age could also reflect a slightly younger, epigenetic hydrothermal event. Results obtained for the Sullivan deposit indicate that the Sm-Nd geochronometer has the potential to directly date mineralization and alteration in stratabound sulfide deposits that are not amenable to dating by other isotope methods.