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The iron oxide-apatite (IOA) deposits of the eastern Adirondack Highlands, New York, are historical high-grade magnetite mines that contain variable concentrations of rare earth element (REE)-bearing apatite crystals. The majority of the deposits are hosted within sodically altered Lyon Mountain granite gneiss, although some deposits occur within paragneiss, gabbro, anorthosite, or potassically altered Lyon Mountain granite gneiss. The IOA deposits and the waste and/or tailings piles associated with them have potential as an unconventional resource for REEs. Reprocessing of these piles would have the advantage of partial recycling of the waste material to produce a set of critical elements. Thirty-four ore, nine rock, 25 waste-pile, and four tailings-pile samples were collected and analyzed for major, minor, and trace elements. At the tailings- and waste-pile sites, composite samples were collected by combining 30 to >50 subsamples randomly distributed over each pile. The total REE content of the waste and tailings piles varied from approximately 10 to 22,000 ppm, whereas the ore sample concentrations ranged from approximately 15 to 48,000 ppm total REEs. A positive correlation exists between the total REE content of ore and its associated waste pile. Median light REE/heavy REE values were 2.14 for waste/tailings piles and 2.25 for ore, which is a substantial relative enrichment in the heavy REEs in comparison to many developed REE mines, such as the mined carbonatites of Bayan Obo, China, and Mountain Pass, California. Importantly, the ore and waste samples are significantly enriched in both Y and Nd compared to other REEs in the samples. Other minor components such as Th are also elevated. Airborne radiometric surveys show large positive eTh and eU anomalies corresponding to tailings piles. Although it is a limited data set, geochemical data of unaltered and altered host rocks suggest a speculative new model for IOA ore formation in the Adirondack Highlands that is consistent with the geology and previously published data. The ferroan ore-hosting Lyon Mountain granite gneiss underwent localized potassic alteration that enriched the altered rock in Fe, REEs, Th, and other metals. A later sodic alteration event affected the previously potassically altered Lyon Mountain granite gneiss, which increased rock porosity and remobilized Fe, REEs, and other elements from the host rock into the iron ore seams. The sodic fluids responsible for ore formation were enriched in F and Cl.

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High-grade hematitic iron ores (or HIF, containing 60–65 wt% Fe) have been mined in Mauritania since 1952 from Superior-type iron deposits of the F’derik-Zouerate district. Depletion of the high-grade ores in recent years has resulted in new exploration projects focused on lower-grade magnetite ores occurring in Algoma-type banded iron formation (or BIF, containing ca. 35 wt% Fe). Mauritania is the seventeenth largest iron producer in the world and currently has about 1.1 Gt of crude iron ore reserves. The main host for Algoma-type iron ore in the district is magnetite quartzite layers (formerly BIF) within Mesoarchean granulite-facies rocks of the Tiris Complex. Superior-type iron ores are restricted to the allochthonous Paleoproterozoic sequences of the Kediat Ijil and Guelb El Mhaoudat, which overlie the Tiris Complex. Paleoproterozoic BIF are present in the Sfariat belt that hosts at least three known iron occurrences, and at Guelb Zednes, all of which are interpreted as fragments of Superior-type BIF that were imbricated with, or allochthonously deposited on, the Mesoarchean-Paleoproterozoic suture zone during the Birimian orogeny. Prominent linear, high-amplitude magnetic anomalies associated with BIF are characteristic of the region. Paleoproterozoic rocks in the Kediat Ijil, a klippe of metasedimentary rocks including ferruginous chert capped by a distinctive conglomerate unit, and in the El Mhaoudat Range, produce very prominent, broad, high-amplitude magnetic anomalies that can be extended at least 150 km along strike projected beneath the Taoudeni Basin to depths of >2000 m. New petrographic, geochemical, and geochronologic data presented here elucidate several features of the iron deposits, their tectonic history, and possible processes of enrichment of BIF protore to HIF. The Zouerate district iron ores are remarkably pure, consisting almost wholly of hematite and quartz. Contents of all major elements other than iron and silica are well below global averages for hematitic iron ores; minor and trace element contents are similar to those of other deposits worldwide. However, Zouerate HIF shows a general depletion of REE, a positive Eu anomaly, and a preferential enrichment of HREE compared to LREE. U–Pb ages of detrital zircons in the Tazadit and Seyala Conglomerate Formations of the Kediat Ijil are consistent with their derivation from the Tiris Complex. This interpretation suggests a passive margin depositional environment on the northeastern edge of the Mesoarchean Rgueibat Shield, unaffected by clastic input from early Birimian tectonic elements. In contrast, ages of detrital zircons from rocks faulted against the Mhaoudat Formation show depositional peaks consistent with input from Birimian and Neoproterozoic sources. These age data imply that tectonic emplacement of Guelb El Mhaoudat could have occurred as a result of Pan African orogenic events; such a Pan African record represents a previously unrecognized tectonic element in this part of the Rgueibat Shield. Timing of enrichment of protore BIF to HIF is poorly constrained and may have resulted from multiple metamorphic-hydrothermal events from 2.83 to 1.6 Ga. Geochemical trends are consistent with interaction by a relatively high-temperature, oxidizing, and possibly alkaline fluid. A supergene weathering profile exists in the Zouerate district and began forming during uplift related to opening of the Atlantic Ocean at about 160 Ma, based on a new apatite fission track age. However, effects of this supergene enrichment are relatively minor, being superimposed on one or several metamorphic-hydrothermal events responsible for the transformation of BIF to HIF.

High-Fe and low-Fe sphalerite samples were reacted under controlled pH conditions to determine nonoxidative rates of release of Zn and trace metals from the solid-phase. The release (solubilization) of trace metals from dissolving sphalerite to the aqueous phase can be characterized by a kinetic distribution coefficient, ( D tr ), which is defined as [( R tr / X (tr) Sph )/( R Zn / X (Zn) Sph )], where R is the trace metal or Zn release rate, and X is the mole fraction of the trace metal or Zn in sphalerite. This coefficient describes the relationship of the sphalerite dissolution rate to the trace metal mole fraction in the solid and its aqueous concentration. The distribution was used to determine some controls on metal release during the dissolution of sphalerite. Departures from the ideal D tr of 1.0 suggest that some trace metals may be released via different pathways or that other processes (e.g., adsorption, solubility of trace minerals such as galena) affect the observed concentration of metals. Nonoxidative sphalerite dissolution (mediated by H + ) is characterized by a “fast” stage in the first 24–30 h, followed by a “slow” stage for the remainder of the reaction. Over the pH range 2.0–4.0, and for similar extent of reaction (reaction time), sphalerite composition, and surface area, the rates of release of Zn, Fe, Cd, Cu, Mn and Pb from sphalerite generally increase with lower pH. Zinc and Fe exhibit the fastest rates of release, Mn and Pb have intermediate rates of release, and Cd and Cu show the slowest rates of release. The largest variations in metal release rates occur at pH 2.0. At pH 3.0 and 4.0, release rates show less variation and appear less dependent on the metal abundance in the solid. For the same extent of reaction (100 h), rates of Zn release range from 1.53 × 10 −11 to 5.72 × 10 −10 mol/m 2 /s; for Fe, the range is from 4.59 × 10 −13 to 1.99 × 10 −10 mol/m 2 /s. Trace metal release rates are generally 1–5 orders of magnitude slower than the Zn or Fe rates. Results indicate that the distributions of Fe and Cd are directly related to the rate of sphalerite dissolution throughout the reaction at pH 3.0 and 4.0 because these two elements substitute readily into sphalerite. These two metals are likely to be more amenable to usage in predictive acid dissolution models because of this behavior. The Pb distribution shows no strong relation to sphalerite dissolution and appears to be controlled by pH-dependent solubility, most likely related to trace amounts of galena. The distribution of Cu is similar to that of Fe but is the most-dependent of all metals on its mole fraction ratio (Zn:Cu) in sphalerite. The Mn distributions suggest an increase in the rate of Mn release relative to sphalerite dissolution occurs in low Mn samples as pH increases. The Mn distribution in high Mn samples is nearly independent of pH and sphalerite dissolution at pH 2.0 but shows a dependence on these two parameters at higher pH (3.0–4.0).

A belt of unusual volcanogenic massive sulfide (VMS) occurrences is located along the eastern margin of the Alexander terrane throughout southeastern Alaska and northwestern British Columbia and exhibits a range of characteristics consistent with a variety of syngenetic to epigenetic deposit types. Deposits within this belt include Greens Creek and Windy Craggy, the economically most significant VMS deposit in Alaska and the largest in North America, respectively. The occurrences are hosted by a discontinuously exposed, 800-km-long belt of rocks that consist of a 200- to 800-m-thick sequence of conglomerate, limestone, marine clastic sedimentary rocks, and tuff intercalated with and overlain by a distinctive unit of mafic pyroclastic rocks and pillowed flows. Faunal data bracket the age of the host rocks between Anisian (Middle Triassic) and late Norian (late Late Triassic). This metallogenic belt is herein referred to as the Alexander Triassic metallogenic belt. The VMS occurrences show systematic differences in degree of structural control, chemistry, and stratigraphic setting along the Alexander Triassic metallogenic belt that suggest important spatial or temporal changes in the tectonic environment of formation. At the southern end of the belt, felsic volcanic rocks overlain by shallow-water limestones characterize the lower part of the sequence. In the southern and middle portion of the belt, a distinctive pebble conglomerate marks the base of the section and is indicative of high-energy deposition in a near slope or basin margin setting. At the northern end of the belt the conglomerates, limestones, and felsic volcanic rocks are absent and the belt is composed of deep-water sedimentary and mafic volcanic rocks. This northward change in depositional environment and lithofacies is accompanied by a northward transition from epithermal-like structurally controlled, discontinuous, vein- and pod-shaped, Pb-Zn-AgBa-(Cu) occurrences with relatively simple mineralogy, to sulfosalt-enriched VMS occurrences exhibiting characteristics of vein, diagenetic replacement, and exhalative styles of mineralization, and finally to Cu-Zn-(Co-Au) occurrences with larger and more clearly stratiform orebody morphologies. Occurrences in the middle of the belt are transitional in nature between structurally controlled types of mineralization that formed in a shallow-water, near-arc setting, to those having a more stratiform appearance, formed in a deeper water, rift-basin setting. The geologic setting in the south is consistent with shallow subaqueous emplacement on the flanks of the Alexander terrane. Northward, the setting changes to an increasingly deeper back- or intra-arc rift basin. Igneous activity in the Alexander Triassic metallogenic belt is characterized by a bimodal suite of volcanic rocks and a previously unrecognized association with mafic-ultramafic hypabyssal intrusions. Immobile trace and rare earth element (REE) geochemical data indicate that felsic rocks in the southern portion of the belt are typical calc-alkaline rhyolites, which give way in the middle of the belt to peralkaline rhyolites. Rhyolites are largely absent in the northern part of the belt. Throughout the belt, the capping basaltic rocks have transitional geochemical signatures. Radiogenic isotope data for these rocks are also transitional (basalts and gabbros: eNd = 4–9 and 87Sr/86Sr initial at 215 Ma = 0.7037–0.7074). Together these data are interpreted to reflect variable assimilation of mature island-arc crust by more primitive melts having the characteristics of either mid-ocean ridge (MORB) or intraplate (within-plate) basalts (WPB). The ore and host-rock geochemistry and the sulfosalt-rich mineralogy of the deposits are strikingly similar to recent descriptions of active sea-floor hydrothermal (white smoker) systems in back arcs of the southwest Pacific Ocean. These data, in concert with existing faunal ages, record the formation of a belt of VMS deposits and occurrences in a propagating intra- to back-arc rift tectonic setting during the Late Triassic. A modern analogue having similar tectonic and metallogenic features is the southward projection of the Lau basin, from the active sea-floor hydrothermal vents of the Valu Fa Ridge to the Taupo volcanic zone of the North Island, New Zealand.

Sphalerite [(Zn,Fe)S] from Mississippi Valley type (MVT), volcanogenic massive sulfide (VMS), and polymetallic vein (PMV) ore deposits show variable rates of dissolution and resulting aqueous metals concentrations under acid conditions. An important control on dissolution rates is solubilized iron that eventually produces Fe(III), a strong oxidant of sphalerite. VMS (high Fe and Cd) and PMV (high Fe and metals) sphalerite samples showed faster rates of sphalerite dissolution (10 -9 to 10 -10 mol/L/s Zn +2 ) and Fe solubilization (10 -9 to 10 -10 mol/L/s total Fe), and higher aqueous trace metals compared to MVT sphalerite that has low Fe and trace metals; these characteristics are manifested by slower rates of sphalerite dissolution (10 -10 to 10 -11 mol/L/s Zn +2 ) and Fe solubilization (10 -12 mol/L/s total Fe), and low aqueous metals compared to VMS and PMV samples. Aqueous concentrations of major (Fe, Zn) and trace (Cu, Cd, Pb, Mn) elements depended on the original composition and rate of dissolution of sphalerite. Fine-grained, high-Fe VMS sphalerite (6.7 wt.%) leached at pH 2-3 (25° C) produced dissolved Fe and Zn approaching 50 and 180 mg/L, respectively, within 1 week; Cu and Pb were 3 and 250 g/L, respectively. The same sphalerite leached at pH 4.0 or higher took two months to yield comparable concentrations. Coarse-grained VMS sphalerite leached for several months at a pH of 4.0 yielded only 100g/L Fe and 9.2 mg/L Zn. PMV sphalerite (4.2 wt.%) had a Zn solubilization rate and trace metal concentrations similar to the VMS sample, but a faster Fe solubilization rate (10 -8 mol/L/s Fetot). Low-iron sphalerite (0.2-0.3 wt.%) from MVT deposits produced Zn concentrations similar to the VMS sphalerite at pH 2.0 but lower concentrations at pH 4.0. At both pH values, aqueous Fe from the high-Fe VMS sample was 100 times that from the MVT sample. Trace metals showed a range of concentrations that generally depended on their original abundance in the solid and their solubilities in the acid solution. Because sphalerite from different ore deposits can be major sources of aqueous metals that may affect the composition of nearby surface and ground water, the potential metal contribution of non-pyritic minerals such as sphalerite should be a major consideration during all stages of economic development of an ore deposit. Additional