Description: Opinion is divided over whether the fluid responsible for the formation of high sulfidation epithermal deposits is a vapor or a liquid, and whether it is entirely volcanic or of mixed volcanic-meteoric origin. Observations made at Kawah Ijen, an active stratovolcano (mainly andesitic in composition) located in the Ijen Caldera Complex in Java, Indonesia, are used to address these issues. The Kawah Ijen crater is approximately 1 km in diameter, and hosts one of the world’s largest hyperacidic lakes (pH ~0). On the lake edge is a small and actively degassing solfatara field, which is surrounded by a much larger area of acid-sulfate alteration. This area was exposed during a phreatomagmatic eruption in 1817, which excavated the crater to a depth of 250 m, and comprises zones of residual silica, alunite-pyrite, and dickite/kaolinite. Based on the fractionation of 34 S and 32 S between alunite and pyrite, the acid-sulfate alteration occurred at a temperature between 200° and 300°C. High sulfidation epithermal mineralization accompanied the alteration in the form of massive and vein-hosted pyrite that contains up to 192 ppb Au, 9.2 ppm Ag, 6,800 ppm Cu, and 3,430 ppm As; these elements are invisible at the highest resolution of scanning electron microscopy, and thus either occur in the form of nanoparticles or are in solid solution in the pyrite. Condensed fumarolic gases released from the solfatara field and sampled at temperatures between 330° and 495°C contain up to 3 ppm Cu and 3.8 ppm As; the concentrations of Au and Ag are below detection. The pH of the condensed gas (water vapor) is ~–0.5. The above observations support a model in which highly acidic gases condensed ~250 m beneath the floor of the crater. Depending on the fluid/rock ratio, the condensed liquids altered the andesitic host rocks by leaching them to leave behind a residue of "vuggy silica" (high fluid/rock ratio), by replacing the primary minerals with alunite and pyrite (intermediate fluid/rock ratio), or by converting them to dickite/kaolinite (lower fluid/rock ratio). Gold-, silver-, and copper-bearing phases were undersaturated in the condensed liquids. However, they were able to concentrate by adsorbing on the surfaces of the growing pyrite crystals, which developed p-type conductive properties as a result of the uptake of arsenic. The metals were incorporated in the pyrite either by their electrochemical reduction to form native metal nanoparticles or through coupled substitutions with arsenic for iron and sulfur. The results of this study provide compelling evidence that high sulfidation epithermal precious metal mineralization can form directly from condensed magmatic gases.

Description: The behavior of niobium and tantalum is poorly understood in rocks that have undergone significant hydrothermal alteration, and niobium-tantalum minerals of hydrothermal origin are rarely mentioned in the literature. Consequently, the mobility of these critical metals, although widely considered to be negligible, has not been evaluated. In this paper, we present the results of a study of the genesis of niobium and tantalum mineralization in the Nechalacho rare metal deposit, Northwest Territories, Canada, which contains one of the largest known resources of these metals in rocks that have undergone intense hydrothermal alteration. Analyses and examination of samples using the electron microprobe has led to the identification of a variety of niobium- and tantalum-bearing minerals in the Nechalacho deposit. Niobium-bearing zircon, columbite-(Fe), fergusonite-(Y), and samarskite-(Y) were identified in the ore zones of the deposit, uranopyrochlore, and columbite-(Fe) were found outside the ore zones, and magmatic fluornatropyrochlore was shown to be the sole niobium-tantalum mineral in relatively unaltered syenites below the Basal ore zone. Based on the paragenetic relationships among the above minerals, variations in the composition of the columbite group minerals as a function of location in the Nechalacho Layered Suite and the distribution of niobium, tantalum, zirconium, and uranium in the bulk rocks, we have developed a model to explain the occurrence of niobium and tantalum in the Nechalacho deposit. The first step in the concentration of these elements was the crystallization of niobium- and tantalum-bearing zircon and eudialyte in the subhorizontal Upper and Basal ore zones, respectively. This was accompanied by the crystallization of magmatic columbite-(Fe) in the Upper ore zone. Fergusonite-(Y) crystallized in the Basal ore zone and also formed due to the breakdown of eudialyte. Outside the ore zones, there was crystallization of pyrochlore and to a lesser extent magmatic columbite-(Fe). This step led to the development of strong spatial associations among niobium, zirconium, and uranium that are evident as strong positive correlations in the bulk-rock concentrations of these elements at the meter scale. During the ensuing intense and widespread hydrothermal alteration, niobium was locally remobilized. Hydrothermal columbite-(Fe) and fergusonite-(Y) formed at the cores of altered zircon grains. Wholesale replacement of magmatic columbite-(Fe) and fergusonite-(Y) by hydrothermal anhedral crystals occurred in the two ore zones. The estimated relative proportions of the sources of these minerals in the ore zones, although varying to some extent because of a dependence on the amount of niobium mobilized from zircon, is ~40/60. Outside the ore zones, columbite-(Fe) and uranopyrochlore are the present manifestations of the former pyrochlore. With the exception of magmatic fluornatropyrochlore in the fresher syenites below the Basal ore zone and a single example of magmatic columbite-(Fe) in an Upper ore zone sample, all niobium and tantalum minerals have a hydrothermal origin as a result of this pervasive alteration.

Description: Extreme enrichment and postmagmatic hydrothermal mobilization of the rare earth elements (REE), Zr, and Nb have been reported for a number of anorogenic peralkaline intrusions, including the world-class REE-Zr-Nb deposit at Strange Lake, Quebec, Canada. Establishing lithogeochemical vectors for these types of deposits is a challenging task because the effects of hydrothermal processes on element distribution are poorly known and the relationships of alteration types to mineralization stages have not been well documented. Here, we present results of a detailed mineralogical and geochemical investigation involving a dataset of over 500 mineral and bulk-rock analyses of a northeast-southwest section through the potential ore zone at Strange Lake. Based on these data, we develop a model that explains the role of hydrothermal processes in concentrating metals in peralkaline granitic systems and identify lithogeochemical vectors for their exploration. The B zone, located along the northwestern margin of the Strange Lake pluton, contains a lens-shaped, pegmatite-rich domain comprising subhorizontal sheets of pegmatites hosted by granites with a total indicated resource of 278 million tonnes (Mt) grading 0.93 wt % total rare earth oxides (TREO), of which 39% are heavy rare earth elements (HREE). Within this resource, there is an enriched zone containing 20 Mt of ore grading 1.44 wt % TREO, of which 50% are HREE. The pegmatites are characterized by a core enriched in quartz, fluorite, and light rare earth elements (LREE) fluorocarbonates, and a granitic border enriched in zirconosilicates and granitic minerals. The pegmatite sheets and surrounding granites evolved in three essential stages: a magmatic stage (I), a near-neutral hydrothermal stage involving their interaction with NaCl-bearing orthomagmatic fluids (II), and an acidic hydrothermal stage (III, comprising high-[IIIa] and low-temperature [IIIb] substages) that resulted from their interaction with pegmatite-sourced HCl-HF–bearing fluids. Stage IIIa led to pseudomorphic mineral replacement reactions (e.g., Na-Ca exchange during replacement of zirconosilicates) and formation of an aegirinization/hematization halo around the pegmatites. In contrast, stage IIIb, which was responsible for the hydrothermal mobilization of Zr and REE, is manifested by fluorite and quartz veins, zircon spherules, gadolinite-group minerals, gittinsite, ferriallanite-(Ce), and a pervasive replacement of the granite by these minerals. The distribution of REE, Zr, Nb, and Ti was controlled by the competition between hydrothermal fluids and the stability of primary REE-F-(CO 2 ) minerals (e.g., bastnäsite-(Ce) host to LREE), zirconosilicates (i.e., Na zirconosilicates and zircon host to HREE and Zr), and Nb-Ti minerals (i.e., pyrochlore host to Nb and narsarsukite host to Ti), and the stability of secondary LREE silicates (i.e., ferriallanite-(Ce)), HREE silicates (i.e., gadolinite-(Y)), zirconosilicates (i.e., gittinsite and zircon), and Nb-Ti minerals (i.e., titanite and pyrochlore). Lithogeochemical vectors were identified to distinguish between the high-temperature acidic alteration (IIIa), using CaO/Na 2 O (indicator of Ca metasomatism) and Fe 2 O 3 /Na 2 O ratios (indicator of aegirinization/hematization), and the low-temperature acidic alteration (IIIb), using the CaO/Al 2 O 3 ratio (indicator of Ca-F metasomatism). Bulk-rock compositional data show that alteration was accompanied by an enrichment in heavy rare earth oxides (HREO) and ZrO 2 at the deposit scale, whereas there was no selective enrichment in the light rare earth oxides (LREO). A 2-D geochemical model of the deposit indicates that the LREO are more dispersed, whereas HREO and ZrO 2 are selectively distributed. These variations in LREE/HREE are also reflected in the mineral chemistry, especially in hydrothermal zircon crystals showing an unusual LREE enrichment and HREE depletion, contrasting with pseudomorphs, which are enriched in HREE. Hydrothermal ferriallanite-(Ce) and gadolinite-group minerals also show a clear trend of REE depletion with Ca enrichment. Controlling factors for the hydrothermal mobilization of LREE, HREE, and Zr were temperature, pH, and the availability of fluoride ions (F – ) in the fluid for the dissolution of zircon, and chloride ions (Cl – ) for the complexation of the REE. The study of rare hydrothermal minerals in conjunction with field observations and the evaluation of variations in bulk-rock composition allowed us to develop a new model for the hydrothermal evolution stage of Strange Lake.

Description: With a current resource of 13.4 Moz Au, plus past production of 5.1 Moz Au, the Canadian Malartic deposit represents the first bulk tonnage (measured and indicated resources of 372.9 Mt at 1.02 g/t Au) mine in the Superior province. Canadian Malartic is thus an important example of a large-tonnage, low-grade Archean gold deposit in which the mineralization is disseminated (or in fine veinlets) and hosted partly by felsic to intermediate intrusions. The deposit is located in the Abitibi greenstone belt, Quebec, within and immediately south of the Cadillac-Larder Lake tectonic zone, and occurs in porphyritic monzodiorite intrusions as well as clastic metasedimentary rocks of the Pontiac Group and mafic-ultramafic volcanics of the Piché Group. These rocks have undergone pervasive potassic alteration, carbonatization, pyritization, and local silicification. The main ore minerals are native gold and subordinate gold tellurides, accompanied by pyrite and minor chalcopyrite, galena, sphalerite, hematite, molybdenite, and Ag-Pb-Bi-bearing tellurides. Gold is concentrated in two generations of thin, discontinuous veins, and as finely disseminated grains in alteration envelopes around the main ore-stage veinlets. The main-stage veinlets consist of quartz-carbonate ± biotite ± albite surrounded by alteration haloes of K-feldspar-biotite ± pyrite ± calcite, whereas later veins are dominated by quartz-pyrite-calcite ± muscovite ± biotite ± chlorite. Alteration and gold mineralization were accompanied by large mass gains in K and S, extremely large mass gains in Ag, Te, and Au, and significant mass gains in Sb, W, Bi, and Pb; Cu underwent significant mass loss. The oxygen and hydrogen isotope composition of the mineralizing fluid determined from the corresponding compositions of quartz, biotite, and hematite ( 18 O fluid of 5.2 to 9.8, D fluid of –52.0 to –45.0) is consistent with a predominantly magmatic source. A magmatic fluid source is supported by the composition of fluid inclusion leachates from quartz. Based on the isotopic composition of pyrite, sulfur was dominantly of magmatic origin but included a small contribution from a sedimentary source ( $${\delta }^{34}{\mathrm{S}}_{{\mathrm{H}}_{\hbox{ 2 }}\mathrm{S}}$$ from –4.5 to +3.3, with small, positive 33 S values). The deposit is interpreted to have formed at a temperature of ~475°C, based on oxygen and sulfur isotope geothermometry. Using this temperature in conjunction with the titanium-in-quartz geothermobarometer, the deposit is interpreted to have been emplaced at a pressure of ~3 kbar or a depth of ~10 km. Consistent with the constraints noted above, we propose a genetic model for the Canadian Malartic deposit in which felsic to intermediate, borderline alkaline to subalkaline magmas, emplaced at midcrustal levels, exsolved relatively oxidized $$(\mathrm{log}\phantom{\rule{.1em}{0ex}}{f}_{\hbox{ O }}{}_{{}_{2}}~-19)$$ , CO 2 - and sulfur-rich ( a S ≥ 0.1) auriferous fluids. These fluids rose to higher levels (~10 km; ~3 kbars), where they interacted with associated porphyritic monzodiorite intrusions, clastic Pontiac metasedimentary and Piché Group mafic to ultramafic rocks. The porphyries and metasedimentary rocks buffered the fluids to near-neutral pH, whereas the mafic-ultramafic rocks buffered the fluids to higher pH and lower $${f}_{\hbox{ O }}{}_{{}_{2}}$$ . Ore deposition resulted from pyritization of the host rocks and oxidation of the mineralizing fluid, which reduced $${a}_{{\mathrm{H}}_{2}\mathrm{S}}$$ and caused destabilization of gold bisulfide species, leading to precipitation of native gold.

Description: The Misery syenitic intrusion in northern Quebec is host to a potentially important, recently discovered rare earth element (REE)-Zr-Nb prospect, containing significant concentrations of both light and heavy REEs, and is conspicuous on aeromagnetic maps as a well-defined, ring-shaped anomaly. Felsic syenite composed of idiomorphic perthite with interstitial mafic minerals, comprising fayalite, hedenbergite, ferropargasite and annite dominates the exposed outer part of the intrusion (the core is covered by Misery Lake). This unit is accompanied by ferrosyenite containing up to 50 vol % mafic minerals, including cumulate fayalite. The ferrosyenite, which occurs as amoeboid-like inclusions in the felsic syenite, is interpreted to have formed by fractional crystallization of ferromagnesian minerals, leaving behind a residual magma which produced the felsic syenites. This latter magma was progressively enriched in alkalis and silica, and only became saturated in ferromagnesian minerals at a very late stage of crystallization. The bulk of the REE mineralization was initially concentrated in pods and layers of fluorapatite that accumulated as a result of gravitational settling. The fluorapatite crystals were partly or completely replaced by britholite-(Ce), which further enriched the rocks in LREEs. Locally, thin quartz-fayalite dikes cut the felsic syenite. These dikes contain up to 10 vol % fergusonite-(Y) and 8 vol % zircon, and are interpreted to be products of an immiscible FeO-SiO 2 -rich magma into which Zr, Nb, and HREEs were preferentially fractionated from the conjugate felsic syenite magma.

Description: Although gold in high-sulfidation epithermal deposits generally occurs as the native metal or electrum, in some deposits, a significant proportion of the gold is hosted in pyrite. Here we use a combination of petrography, whole-rock geochemistry, pyrite chemistry, crystallography, and phase stability relationships to determine how gold was transported and incorporated into pyrite in two relatively young high-sulfidation epithermal deposits, where the gold occurs almost exclusively in solid solution or as nanoparticles in pyrite. The genetically related Bawone and Binebase Au (Cu-Ag) deposits, located 1 km apart on the volcanic island of Sangihe, northeastern Indonesia, are hosted by andesitic volcaniclastic rocks that were altered to a proximal advanced argillic association of quartz + pyrite (py I) + pyrophyllite + natroalunite + alunite + dickite + kaolinite and a more distal intermediate argillic association of quartz + pyrite (py I) + kaolinite + dickite + illite. The economic mineralization takes the form of multiple generations of auriferous pyrite, the first of which, pyrite I (py I), developed during advanced argillic alteration. Mass balance calculations show that all elements were mobile with the exception of Nb, Ti, some rare earth elements, and possibly Al. The highest gold concentration is in pyrite II (py II), which occurs in veins that cut pyrite I. This drusy variety of pyrite is characterized by complex growth and sector zoning, and contains as much as 6.0 wt % Cu. The elevated Cu concentrations correlate positively with Au and As concentrations, whereas the Ag concentration correlates strongly with Au but not Cu. Later barite-enargite mineralization exploited py II veins and vugs, and significant concentrations of Ag and Au are hosted by enargite, although the Au concentration in enargite is lower than in py II or py I. A model is presented in which the fluid responsible for advanced argillic and intermediate argillic alteration and associated stage 1 gold mineralization was a condensed magmatic vapor derived from an oxidized magma. The gold and other metals were transported as hydrated species that ascended through the volcanic pile via fractures and zones of enhanced permeability to a depth between 900 and 1300 m, where the vapor condensed at a temperature between 250 and 340°C to form an acidic liquid with a pH of ~2.5; f o 2 ranged up to four log units above the hematite-magnetite buffer. Interaction of this liquid with the host andesites caused advanced argillic and intermediate argillic alteration, including sulfidation of mafic minerals to form py I. During crystallization of py I, Au, Cu and Ag were adsorbed onto the surface of the pyrite and deposited as nanoparticles, or were incorporated in the pyrite structure. Adsorption of Au, Cu, and Ag from the condensed vapor reached a peak during the crystallization of vein-hosted py II, and the uptake of Ag and minor Au continued during later crystallization of enargite. From the distribution of metals among growth and sector zones in py II, incorporation of gold and other metals appears to have been maximized when physicochemical conditions were relatively stable. This is in contrast to the requirement for native gold precipitation, namely that physicochemical gradients be steep to ensure supersaturation of gold in the ore fluid.

Description: Despite the numerous industrial and scientific applications of gallium, its behavior in nature and the processes that concentrate it to potentially economic levels are poorly understood. Although the main supply of this metal is as a by-product of the mining of bauxite, it is also concentrated by magmatic-hydrothermal processes in peralkaline igneous systems. Here we report the results of a study of the distribution of gallium and the controls on this distribution in the Nechalacho rare metal deposit, Northwest Territories, Canada, which has been shown to contain significant reserves of this critical metal. Electron microprobe analyses and X-ray element maps of gallium-bearing minerals were used to determine the mineralogical distribution of gallium in the Nechalacho intrusive suite. Elevated gallium concentrations were identified in albite, biotite, orthoclase, chlorite, and allanite. Of these aluminum-bearing minerals, the most important hosts of gallium are albite, biotite, and orthoclase. Ferric iron-bearing minerals, including magnetite and aegirine, which were considered potential candidates for gallium sequestration, contain relatively low concentrations of the metal. This behavior of gallium, at least from a magmatic perspective, is consistent with its predicted partitioning between phenocrysts and melt. However, there is also evidence that gallium was redistributed by hydrothermal fluids. Chloritization of biotite resulted in the enrichment of gallium in the secondary mineral (chlorite), and the development of secondary albite (albitization) led to a depletion of gallium in primary albite. On the basis of these results, we argue that the overall distribution of gallium within the Nechalacho deposit was controlled by magmatic crystal fractionation, whereas hydrothermal processes led to local remobilization of the metal. During fractional crystallization, gallium was moderately compatible in minerals such as albite and biotite, whereas it bordered between compatibility and incompatibility in minerals such as magnetite and orthoclase, and was incompatible in aegirine. This resulted in a relatively constant bulk gallium concentration in the Nechalacho deposit, although locally, gallium was remobilized hydrothermally, particularly within the most altered parts of the intrusion, notably, the albitite.

Description: The Cenozoic Ambohimirahavavy alkaline complex in Madagascar consists of several syenitic to granitic intrusions (24.2 ± 0.6 Ma) the largest of which, the Ampasibitika intrusion, is characterized by the presence in its outer flanks of late peralkaline granitic dikes intruding mudstone and limestone of the Isalo Group. A network of dikelets and veinlets propagates from these dikes, intruding along bedding or obliquely to bedding. At the contact between the dikes and dikelets and a limestone, a reaction zone enriched in rare metals, dominated by calc-silicate minerals such diopside and andradite-grossular, forms a rare example of skarn resulting from peralkaline igneous activity. Much of the rare-metal mineralization (REE, Zr, Nb, Th, Sn, and Ti) occurs as secondary phases in the dikelets and skarn. In the dikelets and endoskarn, high field strength element (HFSE)-rich phases consist mainly of zircon, bastnäsite-(Ce), and Ca-REE-, and Ca-HFSE-rich phases in pseudomorphs after aegirine-augite. In the exoskarn, the main HFSE-rich phases are bastnäsite-(Ce), zircon, pyrochlore, Nb-rich titanite, and an unidentified F-rich Ca-zirconosilicate finely disseminated in a matrix composed of calcite, diopside, andradite, phlogopite, quartz, fluorite, and fluorapatite. The secondary zircon is characterized by a low Zr content and by the presence of REE, Ca, Al, and Fe. Three types of primary fluid inclusions were observed in dikelets and skarn in quartz, calcite, and diopside: liquid-rich inclusions (L-V) (20 to 40 vol % vapor) occur in all three minerals and homogenize to liquid; vapor-rich inclusions (V) (〉90 vol % vapor) occur in diopside and quartz, and homogenize to vapor; and halite-bearing L-V inclusions (L-V-H) occur in diopside and quartz, and homogenize either by disappearance of the vapor bubble or by halite dissolution. The L-V inclusions have low to intermediate salinity and homogenize at temperatures ranging from 200° to 380°C. The V inclusions have low salinity and homogenize at higher temperature (350°–395°C). The L-V-H inclusions mainly contain NaCl (35–45 wt % NaCl equiv), and homogenize by three modes, namely, bubble disappearance (mode A), halite dissolution (mode B), and simultaneous bubble and halite disappearance (mode C); the homogenization temperatures range from 260° to 380°C. We propose a model in which rare metals were transported by a Cl – , F – and HFSE-rich orthomagmatic fluid exsolved at 400° to 450°C and about 20 MPa. At these conditions, the fluid was in the two-phase region and vapor dominated. The rare metals were deposited as a result of the interaction of this fluid with limestone and mixing with an external fluid. This interaction/mixing buffered the orthomagmatic fluid to higher pH and lower temperature, resulting in the destabilization of REE-chloride complexes and deposition of fluorocarbonate minerals in the limestone; Zr and Nb, which were likely transported as hydroxyl-fluoride complexes, precipitated as zircon and pyrochlore due to deposition of fluorite and a consequent decrease in fluoride activity.

Description: The Wicheeda carbonatite is a deformed plug or sill that hosts relatively high grade light rare earth elements (LREE) mineralization in the British Columbia alkaline province. It was emplaced within metasedimentary rocks belonging to the Kechika Group, which have been altered to potassic fenite near the intrusion and sodic fenite at greater distances from it. The intrusion comprises a ferroan dolomite carbonatite core, which passes gradationally outward into calcite carbonatite. The potentially economic REE mineralization is hosted by the dolomite carbonatite. Three types of dolomite have been recognized. Dolomite 1 constitutes the bulk of the dolomite carbonatite, dolomite 2 replaced dolomite 1 near veins and vugs, and dolomite 3 occurs in veins and vugs together with the REE mineralization. Carbon and oxygen isotope ratios indicate that the calcite carbonatite crystallized from a magma of mantle origin, that dolomite 1 is of primary igneous origin, that dolomite 2 has a largely igneous signature with a small hydrothermal component, and that dolomite 3 is of hydrothermal origin. The REE minerals comprise REE fluorocarbonates, ancylite-(Ce), and monazite-(Ce). In addition to dolomite 3, they occur with barite, molybdenite, pyrite, and thorite. Minor concentrations of niobium are present as magmatic pyrochlore in the calcite carbonatite. A model is proposed in which crystallization of calcite carbonatite preceded that of dolomite carbonatite. During crystallization of the latter, an aqueous-carbonic fluid was exsolved, which mobilized the REE as chloride complexes into vugs and fractures in the dolomite carbonatite, where they precipitated mainly in response to the increase in pH that accompanied fluid-rock interaction and, in the case of the REE fluorocarbonates, decreasing temperature. These fluids altered the host metasedimentary rock to potassic fenite adjacent to the carbonatite and, distal to it, they mixed with formational waters to produce sodic fenite.