Tourmaline from thirteen volcanogenic massive sulfide (VMS) deposits, three clastic-dominated (CD) Zn-Pb deposits, and four barren sediment-hosted tourmalinites were analyzed for major, minor, and trace elements to identify chemical criteria that distinguish tourmaline in VMS and CD deposits from other barren and mineralized environments. Tourmaline from VMS is mainly dravitic, with some deposits showing schorlitic and uvitic compositions, whereas CD Zn-Pb and barren tourmalinite tourmaline spans the schorl-dravite transition and dravitic compositions, with few exceptions showing uvitic and foititic compositions. Tourmaline major element composition from VMS and CD is governed by heterovalent coupled substitutions, reflecting local processes specific to each deposit. In barren tourmalinites, the compositions are mainly controlled by the chemical composition of the host metasedimentary units. The VMS tourmaline shows higher Al, Mg, and V, whereas CD has higher Ca, Co, K, Mn, Li, La, Ce, Eu, Zn, and Pb; tourmalinites show higher Fe, Na, Ti, Sr, Cr, Sc, Sn, Ga, and Cu. A partial least-squares discriminant analysis (PLS-DA) model is used to classify VMS deposits, CD Zn-Pb deposits, and barren tourmalinites. The results show that VMS tourmaline correlates with Mg, Sn, Ga, and V; CD Zn-Pb tourmaline correlates with Pb, Zn, K, Mn, and Eu; and tourmaline from barren tourmalinites correlates with Fe, Ti, and Sr. These elements were used to build bivariate classification plots using element ratios that enhance the geochemical distinction of VMS and CD Zn-Pb tourmaline from tourmaline in granite-related Sn-W, orogenic gold, porphyry Cu-Mo-Au, iron oxide-copper-gold, and barren geological environments.

This special issue of Mineralium Deposita brings together researchers from multiple disciplines to showcase recent advances in understanding the origin and exploration of selected Li-Cs-Ta (LCT) and Sn pegmatite deposits worldwide. The issue comprises ten contributions on topics ranging from mechanisms responsible for the exceptional enrichment of critical metals, through mineralization processes, to mineral exploration.

Metasedimentary rocks of the Mesoproterozoic Belt Supergroup in the United States and correlative Purcell Supergroup in Canada contain diverse types of mineral deposits with a variety of commodities including base metals, precious metals, and critical metals. The economically most important deposits (≥ 10 Mt past production + reserves + resources) are stratabound Pb–Zn–Ag ores at the Sullivan mine in southeastern British Columbia, Ag–Pb–Zn–(Cu) veins of the Coeur d’Alene district in northern Idaho, stratabound sediment-hosted Cu–Ag deposits of the Spar Lake, Montanore, and Rock Creek deposits in western Montana, stratabound Co–Cu–Au–(Bi–Y–REE) ores of the Idaho cobalt belt, and stratiform Cu–Co–Ag at the Black Butte deposit in western Montana. Minor deposits in the region, also hosted in strata of the Belt and Purcell supergroups, are stratabound Pb–Zn–Ag deposits in southeastern British Columbia, and Th–REE–(Cu) veins of the Lemhi Pass district in northern Idaho and western Montana. Small prospects and occurrences include gold-quartz veins, Ag–Pb–Zn veins, Fe–(Cu–Au) deposits, and a variety of rare metals (Th, REE, Nb, Be, Sn) in pegmatites and carbonatites. High-precision geochronology of ore and gangue minerals, like in the Sullivan deposit, has yielded only Mesoproterozoic ages, but other deposits (Blackbird, Rock Creek, Coeur d’Alene, Lemhi Pass) have U–Pb and/or Re–Os ages that suggest Neoproterozoic, Paleozoic, and/or Mesozoic to Tertiary overprinting of older Mesoproterozoic mineralization, possibly linked in some cases to fluid flow during reactivation of deep basement structures. Some of the Paleozoic and younger ages are also attributed here to dissolution-reprecipitation of dated minerals and not to new metal introduction. An integrated assessment of the geochronological data reveals growing evidence for previously underappreciated roles of major mineralizing events occurring during the East Kootenay orogeny (1379 − 1325 Ma) and the Grenvillian orogeny (1150 to 980 Ma), with important implications for mineral exploration in many parts of the Belt-Purcell Basin.

Mid-Paleozoic base metal mineralization in the Northern Cordillera occurs in Laurentian margin/peri-Laurentian terranes and the Arctic Alaska terrane. In the Laurentian margin and peri-Laurentian terranes, Late Devonian carbonate-hosted Zn-Pb, clastic-dominated (CD) Pb-Zn, and polymetallic Zn-Cu-Pb-Ag-Au volcanogenic massive sulfide (VMS) deposits formed between 384 and 361 Ma. These formed in response to tectonic, magmatic, and basin conditions related to arc magmatism and rifting in the Yukon-Tanana terrane, initial opening of the Slide Mountain back-arc basin, and extension (+/- coincident magmatism) within the Laurentian margin. In the Arctic Alaska terrane, base metal mineralization consists of Late Devonian and Mississippian (similar to 384-377 Ma) carbonate-hosted Cu-Co (Bornite) and polymetallic Zn-Cu-Pb-Ag VMS mineralization in the Ambler district. This mineralization likely formed in response to arc rifting along an older Arctic-Alaska basement crustal block with deposits forming proximal (e.g., VMS) and distal (e.g., carbonate replacement?) to magmatic centers. The Arctic-Alaska terrane also hosts world-class Mississippian (338- 334 Ma) Zn-Pb-Ag deposits in the Red Dog district that likely formed in a west-facing continental margin setting. The deposits in the Northern Cordillera formed in response to the complex interplay of regional tectonics, magmatism, heat flow, and basin architecture at the plate- to depositscale.

The most important mineral deposits in the Mesoproterozoic Belt-Purcell Basin, based on past production and resources, are the stratabound Pb-Zn-Ag ores at Sullivan, British Columbia, and the Ag-Pb-Zn(-Cu) veins of the Coeur d'Alene district in Idaho. Other important types include the stratiform Cu-Co-Ag deposit at Black Butte in Montana, the stratabound Co-Cu-Au(-Bi-Y-REE) Blackbird and related deposits of the Idaho cobalt belt, the stratabound Spar Lake and related Cu-Ag deposits in Montana, and the Th-REE(-Cu) vein deposits of the Lemhi Pass area in Idaho. Additional deposit types known in the region include gold-quartz veins, Ag-Pb-Zn veins (exclusive of the Coeur d'Alene district), Fe(-Cu-Au) deposits of possible IOCG affinity, and a variety of rare metals (Th, REE, Nb, Be, Sn) in pegmatites, veins, and carbonatites. High-precision geochronology on some of the deposits like Sullivan has yielded only Meso-proterozoic ages, but for other deposits (Blackbird, Spar Lake, Coeur d'Alene, Lemhi Pass), U-Pb and Re-Os dates suggest Paleozoic and/or Mesozoic-Tertiary overprinting of older Mesoproterozoic mineralization.

Elizabeth volcanogenic massive sulfide (VMS) deposit in Vermont is atypical among such deposits in containing both stratiform Fe-Cu-Zn-Mn mineralization and stratabound Al-Na-K-B enrichments and Ca-Mg depletions in wall rocks. This mafic-siliciclastic (Besshi-type) deposit is contained in a thick sequence of Lower Devonian pelitic schist, minor quartzite, and sparse amphibolite. Wall rocks to the sulfide ores are predominantly amphibole-bearing rocks lacking quartz. The deposit has been metamorphosed to middle amphibolite facies and is complexly deformed by two folding events and syndeformational thrust faults. Whole-rock analyses of altered metabasaltic wall rocks reveal locally high concentrations of SiO2 (up to 85.0 wt %), Al2O3 (up to 32.5 wt %), K2O (up to 4.02 wt %), or Na2O (up to 6.02 wt %). Data for relatively immobile trace elements (Sc, Cr, Ti, Zr, Th) and rare earth elements (REEs) indicate protoliths of low-Ti tholeiitic basalt broadly of normal mid-ocean ridge basalt (N-MORB) affinity. The altered wall rocks are mineralogically distinctive in containing, in many samples, abundant muscovite, phlogopite, albite, dolomite, tourmaline, and/or tremolite-actinolite. One very aluminous unit of coarse garnet-mica schist, also with a tholeiitic basalt protolith, has local domains composed of abundant staurolite, minor margarite and sillimanite, and rare corundum. All of these altered wall rocks are stratabound but generally discontinuous along strike, in contrast to Mn-rich stratiform lenses (coticules), for which mineralogical and geochemical data suggest deposition as VMS-related chemical sediments. Al-normalized calculations of whole-rock geochemical data for mineralogically different types of altered basalts, relative to an inferred least altered precursor in the mine sequence, show average major additions (>50%) of Mn, Na, and K, and major losses (>50%) of Mg and Ca for most types. Changes in Si, Ti, and Fe are generally negligible (+/- 5%), except for siliceous, tourmaline-rich rocks that display on average a major addition of 254% Si. Whole-rock delta O-18 values of silica-poor samples with metabasaltic precursors range from 10.7 to 13.4 parts per thousand. These values are uncorrelated with SiO2 contents and are mostly higher than that of the least altered metabasalt in the Elizabeth mine area (delta O-18 = 11.1 parts per thousand), suggesting premetamorphic subseafloor alteration by relatively low-temperature (ca. 150 degrees-250 degrees C) VMS-related hydrothermal fluids. A lack of depletion of light REEs for most of the altered metabasalts, compared to the least altered precursor, is also consistent with a low-temperature alteration process. Sulfur isotope values for pyrrhotite, chalcopyrite, sphalerite, and pyrite in massive sulfide and disseminations in wall rocks range from 4.3 to 9.3 parts per thousand, a typical range for sediment-hosted sea-floor hydrothermal systems, reflecting mixed sulfur sources derived mainly from footwall basalts and coeval seawater sulfate. Shale-normalized REE data for coticules show small negative Ce and Eu anomalies that suggest deposition of precursor Mn-rich sediments in mildly oxic waters. In contrast, most samples of massive sulfides lack Ce anomalies but have small to moderate positive Eu anomalies, reflecting mineralization under anoxic and reducing conditions at or above 250 degrees C. The presence of negative Ce anomalies in the coticules and two samples of massive sulfide, but the lack of such anomalies in other massive sulfide samples, suggest deposition within a stratified water column in which the redoxcline fluctuated due to hydrothermal venting of reductants such as Fe2+, Mn2+, H2S, H-2, and CH4, which episodically produced anoxic bottom waters during VMS mineralization. The length (>= 3.4 km) and relatively narrow width (similar to 500 m) of the Elizabeth sulfide deposit is attributed to formation in an elongate sea-floor graben that served as a locus of tholeiitic basaltic volcanism and hydrothermal mineralization. Morphological and geochemical data for highly altered metabasaltic wall rocks of the deposit provide evidence for pervasive subseafloor alteration, during and after exhalative chemical precipitation of sulfides and Mn-Fe sediments. Possible modern analogs are the predominantly sediment-hosted VMS deposits in Middle Valley and Escanaba trough in the northeast Pacific Ocean and Guaymas basin in the Gulf of California. The metalliferous sulfide deposits of the Atlantis II Deep in the Red Sea may be the best modern analog, based on their sheetlike morphology, sulfur isotope values, and associated Fe and Mn oxyhydroxide sediments that are potential protoliths of the coticule rocks in the Elizabeth mine sequence.

The Sheep Creek prospect is a stratabound Zn–Pb–Ag–Sn massive sulfide occurrence in the Bonnifield mining district, northern Alaska Range. The prospect is within a quartz–sericite–graphite–chlorite schist unit associated with Devonian carbonaceous and siliceous metasedimentary rocks. Volcanogenic massive sulfide (VMS) deposits in the district are hosted in felsic metavolcanic rocks (362 ± 2 Ma) associated with siliciclastic and carbonaceous sedimentary rocks that overlie the stratigraphic sequence hosting the Sheep Creek prospect. Felsic metaigneous rocks in underlying units are 372 ± 4 to 366 ± 4 Ma. Sheep Creek is atypical of the other sulfide deposits in the district in (1) having Sn grades up to 1.2%; (2) being contained in fine-grained, quartz-rich rocks and quartz–pebble conglomerate that likely originated as chert and chert-clast sediment, respectively; and (3) showing minimal evidence of volcanic components in the host rocks. Comparison of immobile trace-element proportions for graphitic and siliceous rocks from the Sheep Creek area with those for argillite associated with the Bonnifield VMS deposits indicates a continental volcanic-arc provenance for the former and a within-plate and passive margin provenance for the latter. In contrast to previously published interpretations, our data analysis supports a clastic-dominated (CD) rather than a VMS affinity for the Sheep Creek prospect. In our model, Zn–Pb–Ag–Sn mineralization formed by syngenetic or early diagenetic processes on or beneath the seafloor, possibly in the shallow-water environment of an outer continental shelf setting. Potential analogues are the Paleozoic CD deposits in the Canadian Selwyn Basin outboard of the Laurentian continental margin.

Lead isotope values for volcanogenic massive sulfide (VMS) deposits provide important insights into metal sources and the nature of pre -accretionary tectonostratigraphic terranes and underlying basements. Deposits of this type in New England formed in diverse tectonic settings including volcanic arcs and backarcs, a supra-subduction zone arc, a rifted forearc foreland basin, and a rifted continental margin. Following VMS mineralization on or near the seafloor, components of the tectonostratigraphic assemblages--volcanic +/- sedimentary rocks, coeval intrusions, sulfide deposits, and underlying basements--were diachronously accreted to the Laurentian margin during the Paleozoic. Lead isotope data for galena show relatively large ranges for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb. Evaluation of potential lead sources, using for comparison Pb-isotope data from modern and ancient settings, suggests that principal sources include the mantle, volcanic +/- sedimentary rocks, and deeper basement rocks. Integration of the Pb-isotope values with published data such as Nd isotopes for the volcanic rocks and from deep seismic reflection profiles points to the involvement of several basements, including those of Grenvillian, Ganderian, Avalonian, and West African (and (or) Amazonian) affinity. Clustering of Pb-isotope data for VMS deposits within individual Cambrian and Ordovician volcanic and volcanosedimentary settings, delineated by differences in 206Pb/204Pb and mu (238U/204Pb) values, are consistent with lead derivation from at least four and possibly five different tectonostratigraphic assemblages with isotopically distinct basements. Collectively, our Pb-isotope data for New England VMS deposits provide a novel window into the nature of subarc basement rocks during pre -accretionary sulfide mineralization outboard of Laurentia during early Paleozoic time.

Reported here are geological, geophysical, mineralogical, and geochemical data on a previously unknown tra-chyte-hosted rare earth element (REE)-Nb-Zr occurrence at Pennington Mountain in northern Maine, USA. This occurrence was newly discovered by a regional multiparameter, airborne radiometric survey that revealed anomalously high equivalent Th (eTh) and U (eU), confirmed by a detailed ground radiometric survey and by portable X-Ray fluorescence (pXRF) and whole-rock analyses of representative rock samples. The mineralized area occurs within an elongate trachyte body (-1.2 km2) that intrudes Ordovician volcanic rocks. Geologic con-straints suggest that the trachyte is also Ordovician in age. The eastern lobe (-900 x-400 m) of the trachyte is pervasively brecciated with a matrix containing seams, lenses, and veinlets composed mainly of potassium feldspar, albite, and fine-grained zircon and monazite. Barite is locally abundant. Minor minerals within the matrix include columbite, bastnasite, euxenite, chlorite, pyrite, sphalerite, and magnetite. The pXRF analyses of 22 samples (App. Table A1) collected from the eastern lobe demonstrate that this entire part of the trachyte is highly mineralized. Whole-rock geochemical analyses for samples from the eastern lobe document high average contents of Zr (1.17 wt %), Nb (1,656 ppm), Ba (3,132 ppm), Y (1,140 ppm), Hf (324 ppm), Ta (122 ppm), Th (124 ppm), U (36.5 ppm), Zn (689 ppm), and Sn (106 ppm). Among light REE, the highest average concentrations are shown by La (763 ppm) and Ce (1,479 ppm). For heavy REE (HREE), Dy and Er are the most abundant on average (167 and 114 ppm, respectively). No HREE-rich minerals such as xenotime have been identified; the HREE may reside chiefly in monazite and bastnasite, and within the fine-grained zircon. Very strong positive correlations (R2) of 0.92 to 0.98 exist between Th and Zr, Nb, Y, Ce, Yb, and Sn, indicating that the radiometric data for eTh are valid proxies for concentrations of these metals in the mineralized rocks. Trachyte-hosted REE-Nb-Zr deposits like the occurrence at Pennington Mountain also are known in eastern Australia and in the south Qinling belt of Central China. Based on comparisons with these deposits, and the lack of detailed geologic mapping in the Pennington Mountain region, we suggest that other rare-metal occurrences contained in trachyte may exist elsewhere in northern Maine, and more widely in the Appalachian-Caledonian orogen where volcanic and subvolcanic trachytes have been recognized.

The oldest putative fossils occur as hematite filaments and tubes in jasper-carbonate banded iron formations from the 4280- to 3750-Ma Nuvvuagittuq Supracrustal Belt, Québec. If biological in origin, these filaments might have affinities with modern descendants; however, if abiotic, they could indicate complex prebiotic forms on early Earth. Here, we report images of centimeter-size, autochthonous hematite filaments that are pectinate-branching, parallel-aligned, undulated, and containing Fe2+-oxides. These microstructures are considered microfossils because of their mineral associations and resemblance to younger microfossils, modern Fe-bacteria from hydrothermal environments, and the experimental products of heated Fe-oxidizing bacteria. Additional clusters of irregular hematite ellipsoids could reflect abiotic processes of silicification, producing similar structures and thus yielding an uncertain origin. Millimeter-sized chalcopyrite grains within the jasper-carbonate rocks have 34S- and 33S-enrichments consistent with microbial S-disproportionation and an O2-poor atmosphere. Collectively, the observations suggest a diverse microbial ecosystem on the primordial Earth that may be common on other planetary bodies, including Mars.

Iron in the early anoxic oceans of Archean age (4000-2500 million years ago) is believed to have been oxidized to form banded iron formations (BIF). Previously, it has been proposed that iron was oxidized either by free oxygen, H2O2, microbial oxidation, or photo-oxidation. However, these mechanisms are difficult to reconcile with evidence for the oceans at that time having been largely devoid of dissolved oxygen and oxidants, together with the rarity of microbial remains in BIF and restrictively slow rates of photo-oxidation. Experiments reported here show that ferrous iron readily oxidizes in analogs of Archean anoxic seawater following the precipitation of ferrous hydroxide. Once precipitated, ferrous hydroxide undergoes decomposition to elemental iron that reacts with water at room temperature to form ferric iron and release hydrogen gas. The ferric iron may then be incorporated into green rust, a mixed ferrous ferric phase that ages into iron minerals commonly found in BIF. Our finding suggests that anoxic iron oxidation may have contributed to the formation of oxide-facies BIF, especially Algoma-type BIF that likely formed in semi-restricted basins where ferrous hydroxide saturation was more easily achieved. Additionally, ferrous hydroxide decomposition would have contributed to early Earth's oxidation, as a result of hydrogen escape to space, thus providing new insights into environmental and biological conditions on early Earth. (c) 2022 Elsevier B.V. All rights reserved.

An analysis of the potential for deposits of critical minerals and elements in Maine presented here includes data and discussions for antimony, beryllium, cesium, chromium, cobalt, graphite, lithium, manganese, niobium, platinum group elements, rhenium, rare earth elements, tin, tantalum, tellurium, titanium, uranium, vanadium, tungsten, and zirconium. Deposits are divided into two groups based on geological settings and common ore-deposit terminology. One group consists of known deposits (sediment-hosted manganese, volcanogenic massive sulphide, porphyry copper-molybdenum, mafic- and ultramafic-hosted nickel-copper [-cobalt-platinum group elements], pegmatitic lithium-cesium-tantalum) that are in most cases relatively large, well-documented, and have been explored extensively in the past. The second, and much larger group of different minerals and elements, comprises small deposits, prospects, and occurrences that are minimally explored or unexplored. The qualitative assessment used in this study relies on three key criteria: (1) the presence of known deposits, prospects, or mineral occurrences; (2) favourable geologic settings for having certain deposit types based on current ore deposit models; and (3) geochemical anomalies in rocks or stream sediments, including panned concentrates. Among 20 different deposit types considered herein, a high resource potential is assigned only to three: (1) sediment-hosted manganese, (2) mafic- and ultramafic-hosted nickel-copper(-cobalt-platinum group elements), and (3) pegmatitic lithium-cesium-tantalum. Moderate potential is assigned to 11 other deposit types, including: (1) porphyry copper-molybdenum (-rhenium, selenium, tellurium, bismuth, platinum group elements); (2) chromium in ophiolites; (3) platinum group elements in ophiolitic ultramafic rocks; (4) granite-hosted uranium-thorium; (5) tin in granitic plutons and veins; (6) niobium, tantalum, and rare earth elements in alkaline intrusions; (7) tungsten and bismuth in polymetallic veins; (8) vanadium in black shales; (9) antimony in orogenic veins and replacements; (10) tellurium in epithermal deposits; and (11) uranium in peat.

Detailed mineralogical and geochemical analysis of drill core samples from three previously unstudied localities (Sheps Lake, Lac Ritchie, Hayot Lake) of the ca. 1.88 Ga Sokoman continental margin-type iron formation (IF) was undertaken to better understand tectonically stable, shallow-marine environments and surface redox conditions during the late Paleoproterozoic. Suboxic (Fe-oxide-rich including paragenetically early hematite) and anoxic (Fe-silicate/carbonate-rich) mineral paragenetic pathways operated during IF deposition. Post-depositional alteration beyond late diagenesis/metamorphism was negligible, based on petrographic examination and analysis of bulk Fe(III)/Fe(II) ratios. High-precision trace element (TE) data of the Sokoman IF, in the context of new analyses of IF/iron ore reference materials (IOC-1, FeR-3, FeR-4), reveal similarities to contemporaneous continental margin-type IF. However, both the analytical approach and integration of chemostratigraphic variations in detrital element, rare earth element and yttrium (REE + Y), and other TE (Cr, V, U, Ni, Co, Zn) parameters with a previously published sequence-stratigraphic framework provides refined insight into the ca. 1.88 Ga marine surface environment. Specifically, this study dissects new details on the effects of base-level fluctuations, terrigenous input, basin redox stratification, and microbial activity that are collectively captured within the mineralogically and texturally complex units of the Sokoman IF. The REE + Y signature of the Sokoman IF is confirmed to have developed during deposition/early diagenesis through a comparison of geochemical signatures of chert (jasper) intraclasts and surrounding bulk IF. Furthermore, the Sokoman IF REE + Y data show patterns reminiscent of modern seawater (LREE depletion, small negative Ce anomalies, small positive La, Gd, and Y anomalies), but in some cases also strong positive Ce anomalies. Modelling of hyperbolic trends in Ce/Ce*-Pr/Pr* plots, preserved despite varying detrital admixtures, provides supporting evidence for interaction of dissolved REE + Y with marine Fe- and Mn-(oxyhydr)oxides, and quantitatively constrains the amount of detritus required to overprint Ce anomalies. The co-existence of positive and negative bulk-rock Ce anomalies, similar to those of other ca. 1.88 Ga IF, implies the presence of a shallow marine redoxcline at that time. However, the absence of any strong covariations between these Ce anomalies and (1) Mn- or Fe-enrichments, (2) Y anomalies, (3) LREE/HREE ratios, or (4) tetrad coefficients (tau) is best explained by the separation of a shallow Mn-redoxcline from a slightly deeper and more diffuse Fe-redoxcline inferred here to be controlled by cyanobacteria and photoferrotrophs, respectively. Combined plots of chemostratigraphic and TE/Sigma Fe vs. enrichment factors highlight variable input/scavenging of different TEs within the Sokoman IF; authigenic TE enrichment is more readily captured in deeper, suboxic to anoxic units relative to shallower, nearshore units where even low amounts of continental detritus can obscure low-magnitude, authigenic redox signatures. This approach confirms the low magnitude and limited range of authigenic enrichments in redox-sensitive and nutrient-type TEs in the Sokoman IF as being similar to those of other ca. 1.88 Ga IF localities, but reveals which depositional environments best capture specific authigenic signatures (e.g., Cr-V-U-P enrichments). Detritus-poor samples record highly fractionated Nb/Ta and Zr/Hf ratios (m/m; Nb/Ta: median 56.4, range 15.5-680; Zr/Hf: median 97.8, range 42.9-409) that exceed those observed in the modern hydrosphere, and are interpreted to reflect greater interaction of the "dissolved" load of these elements with abundant marine Fe/Mn colloids/fine-particulates. Detritus-rich samples have Nb/Ta and Zr/Hf ratios converging towards crustal values similar to those of shales within the Sokoman basin; both datasets support a model for a predominantly felsic (Archean plutonic/metamorphic rock) source. New inferences from our data on the Sokoman IF support a close link between atmosphere-ocean oxygenation and microbial ecosystems via continental weathering under an oxygen-poor atmosphere (aided locally by arid conditions). In this model, such conditions limited the terrestrial supply of redox-sensitive and nutrient-type elements (most notably P) into the ocean, largely restricting the spatial extent of primary productivity to the photic zone of coastal regions. These processes are consistent with collective evidence from other ca. 1.88 Ga IF deposits that suggest low-O-2 and nutrient-limited Earth surface conditions relative to preceding time intervals in the Paleoproterozoic.

Stratabound tourmalinites are metallogenically important rocks that locally show a close spatial association with diverse types of mineralization, especially volcanogenic massive sulfides (VMS) and clastic-dominated (CD) Zn-Pb deposits. These tourmalinite occurrences span the geologic record from Eoarchean to Jurassic. Host lithologies are dominated by clastic metasedimentary rocks but in some areas include metavolcanic rocks, marble, or metaevaporites. Stratabound and stratiform (conformable) tourmalinites commonly display sedimentary structures such as graded beds, cross-beds, and rip-up clasts. In most cases, field and microtextural relationships are consistent with a synsedimentary to early diagenetic introduction of boron as a precursor to tourmaline formation. Whole-rock geochemical data for major, trace, and rare earth elements (REE) provide valuable insights into tourmalinite origins. Al-normalized values relative to those for least-altered host metasedimentary rocks suggest that tourmalinites in proximal settings at or near hydrothermal vent sites characterized by high fluid/rock regimes (e.g., Sullivan Pb-Zn-Ag deposit, Canada) have very different signatures than those in low fluid/rock, distal settings (e.g., Broken Hill Pb-Zn-Ag deposit, Australia). The high fluid/rock regimes at Sullivan show large mass changes of +60 % for Mg and +180 % for Mn, as well as large variations in abundances of light and middle REE. In contrast, tourmalinite formation in low fluid/rock regimes yields minimal Al-normalized changes in major elements, trace elements, and REE. Boron isotope values of tourmalinite-hosted tourmaline vary widely from -26.1 to +27.5 parts per thousand, and are attributed mainly to boron sources (e.g., sediments, evaporites) with generally minor influence from processes such as formational temperature, fluid/rock ratio, and secular variation in seawater delta 11B values. Laterally extensive stratiform tourmalinites formed mainly by syngenetic or early diagenetic processes on or beneath the seafloor. The syngenetic process is attributed to the interaction of vented B-rich brines with aluminous minerals in sediments, whereas the diagenetic process involves the selective replacement of aluminous sediments by B-rich fluids. Modern examples of tourmalinites, as yet undiscovered, may exist in metalliferous sediments of the Red Sea and the eastern Pacific Ocean, in altered volcaniclastic sediments within active seafloor-hydrothermal systems of the South Pacific, and in hydrothermal mounds and vents associated with mafic sill complexes in extensional basins as in the North Sea and South China Sea. Stratabound tourmalinites that contain base-metal sulfides, high Mn concentra-tions (>1 wt. % MnO), or positive Eu anomalies can be valuable exploration guides for base-metal sulfide deposits in sedimentary and volcanic terranes.