Large numbers of igneous intrusions are emplaced in the magmatic arcs above subduction zones, but only a small fraction of them are mineralized with Cu ± Au ± Mo. Despite significant advances in recognizing the importance of magma source, volatile content, and redox condition in controlling mineralization, the magmatic processes required to form a mineral deposit are still poorly understood. Here, we demonstrate that mafic recharge to the magma chamber is critical for Cu-Au mineralization, as exemplified by the Bozymchak Cu-Au skarn deposit in the Chatkal-Kurama arc of Kyrgyzstan, West Tianshan. The Bozymchak intrusion associated with the mineralization comprises monzonite porphyry and granodiorite porphyry. A sensitive high-resolution ion microprobe zircon U-Pb age of 305 ± 2 Ma was obtained for the monzonite porphyry, which is consistent with the U-Pb age of zircon from the granodiorite porphyry (304 Ma) and an Re-Os age for the molybdenite (305 Ma). These ages indicate that both the monzonite porphyry and granodiorite porphyry were coeval with mineralization. The monzonite porphyry exhibits high-K calc-alkaline (K2O = 3.36−3.50 wt%), metaluminous and magnesian-rich characteristics, enrichment in large ion lithophile elements (LILEs; e.g., Cs, Th, Ba, K, and Pb), and negative anomalies in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). These geochemical signatures, combined with the Sr-Nd-Hf isotopic signatures [ISr(avg) = 0.7059, εNd(t) = −3.5 to −2.8, εHf(t) = −3.8 to +1.4], suggest that the magma was the product of partial melting of a metasomatized mantle wedge and was subjected to subsequent assimilation and fractional crystallization (AFC). The development of disequilibrium textures in both the monzonite porphyry and the granodiorite porphyry, such as acicular apatite, reverse zoning of plagioclase, and coexisting high-Al and low-Al amphiboles, indicates that the magma evolved in an open crustal magma chamber with mafic magma recharge. The magmas were moderately oxidized (∼ΔFMQ +1.0; FMQ is the fayalite-magnetite-quartz oxygen buffer) and water-rich, with 5.9 wt% and 6.5 wt% H2O in the granodiorite porphyry and monzonite porphyry, respectively. The repeated influx of hot mafic magma batches prolonged the existence of the magma chamber and promoted the maturation of the arc magma. In addition to introducing more Cu and Au, the replenishment of mafic magma in the chamber also introduced additional H2O and Cl, as shown by the presence of high-Al amphiboles and the elevated oxygen fugacity, which further promoted Cu-Au mineralization.
Carbon dioxide (CO2) is a major component of volcanic gases and ore-forming hydrothermal fluids. However, CO2 has contrasting effects on the speciation of different metal complexes and ore mineral solubility, but a molecular understanding of its effects is lacking. To address this deficiency, we conducted ab initio molecular dynamics (MD) simulations of the behavior of AuCl(aq) in the CO2–H2O system at 340 °C and 118–152 bar and 800 °C and 265–291 bar for CO2 mole fractions (XCO2) of 0.1–0.9. The MD simulations indicate that the linear [H2O–Au–Cl]0 structure of gold chloride is not affected by CO2 at XCO2 up to 0.8 at 340 °C and XCO2 up to 0.5 at 800 °C, whereas the "dry" [AuCl]0 species predominates at XCO2 > 0.8 at 340 °C and XCO2 > 0.5 at 800 °C. The number of water molecules hydrating the AuCl(aq) complex decreases systematically with an increasing CO2 mole fraction and increasing temperature. Results of Au solubility experiments at 340 °C in CO2–H2O solutions show that the addition of CO2 does not enhance Au solubility. We conclude that hydrated chloride species with linear geometry are the main means for transporting gold in CO2–H2O–HCl fluids and that Au solubility decreases in CO2-bearing hydrothermal fluids as a result of the decrease in hydration of the Au complexes. This contrasts with the behavior of divalent transition metals (e.g., Fe, Co, Ni, and Zn). We propose that the different solubility behaviors of Au and base metals are due to the changes in translational entropy as a result of the changes in coordination geometry (and associated hydration) of the complexes with increasing XCO2. The first-shell coordination of Au(I) complexes remains constant over wide ranges of XCO2, whereas first-row divalent transition metal complexes undergo entropy-driven geometric changes with a decreasing water activity.
Abstract Thorium is the most abundant actinide in the Earth’s crust and has universally been considered one of the most immobile elements in natural aqueous systems. This view, however, is based almost exclusively on solubility data obtained at low temperature and their theoretical extrapolation to elevated temperature. The occurrence of hydrothermal deposits with high concentrations of Th challenges the Th immobility paradigm and strongly suggests that Th may be mobilized by some aqueous fluids. Here, we demonstrate experimentally that Th, indeed, is highly mobile at temperatures between 175 and 250 °C in sulfate-bearing aqueous fluids due to the formation of the highly stable Th(SO 4 ) 2 aqueous complex. The results of this study indicate that current models grossly underestimate the mobility of Th in hydrothermal fluids, and thus the behavior of Th in ore-forming systems and the nuclear fuel cycle needs to be re-evaluated.
Abstract Niobium is a critical metal in high demand because of technological advances and the supply risk created by the fact that over 90% of its production is by a single country (Brazil). In this paper, we review the geology of the deposits that are currently being mined and other potentially economic deposits as well as develop models for their genesis. With the exception of the Lovozero deposit (Russia), which is hosted by a layered silica-undersaturated alkaline igneous complex, all the deposits that are currently being mined for niobium are hosted by carbonatites, and most of the deposits with economic potential are also hosted by these rocks. Niobium owes its concentration in carbonatites and alkaline silicate rocks to its highly incompatible nature and the small degree of partial melting of the mantle required to generate the corresponding magmas. The primary control on the concentration of niobium to economic levels in alkaline silicate magmas is fractional crystallization, partly prior to but mainly after emplacement. In the case of silica-undersaturated magmas, the final residue saturates in minerals like eudialyte and loparite to form niobium-rich horizons in the layered complexes that crystallize from these magmas. The final residue, in the case of silica-saturated magmas, crystallizes the pegmatites that are the hosts to the economic niobium mineralization, which commonly takes the form of pyrochlore. In contrast, carbonatitic magmas undergo little to no fractional crystallization prior to emplacement. Moreover, fractional crystallization on emplacement has minimal impact on the concentration of niobium to economic levels. Instead, we propose that the metasomatic interaction of the carbonatitic magmas with their hosts to form rocks like phlogopitite (glimmerite) consumes much of the magma, leaving behind a phoscoritic residue from which pyrochlore crystallizes in amounts sufficient to form economic deposits. Although many niobium deposits display evidence of intense hydrothermal alteration, during which there can be major changes in the niobium mineralogy, the extremely low solubility of niobium in aqueous fluids at elevated temperature precludes significant mobilization and, thus, enrichment of the metal by hydrothermal fluids. However, weathering of carbonatite-hosted niobium deposits leads to supergene enrichment (due largely to the dissolution of the carbonate minerals) that can double the niobium grade and make subeconomic deposits economic. Pyrochlore is the principal niobium mineral in these laterite-hosted deposits, although its composition differs considerably from that in the primary mineralization. This paper evaluates the processes that appear to be responsible for the genesis of niobium ores and provides a framework that we hope will guide future in-depth studies of niobium deposits and lead to more effective strategies for their successful exploration and exploitation.
Abstract From integrated textural and compositional studies of auriferous and barren pyrite/marcasite in the epithermal Axi gold deposit, China, we have identified a relationship between multiple gold mineralizing events, mafic magma recharge, and fluid-rock reactions. Three generations of pyrite (Py1–3) and four generations of marcasite (Mar1–4) record episodic gold mineralizing events, followed by silver-copper-lead-zinc-cadmium enrichment. The gold mineralizing events are recorded by high concentrations of subnanometer-sized gold in Py1, Py3, and Mar3 (max. = 147, 129, and 34 ppm, med. = 39, 34, and 12 ppm). Based on previous Re-Os age determinations of pyrite and U-Pb zircon ages of the andesitic wallrock, these gold events slightly postdate pulsed mafic magma recharge and represent the incursion of Au-As-S-rich magmatic volatiles into circulating meteoric water. Silver-Cu-Pb-Zn-Cd enrichment in Py2, Mar2, and Mar4 are consistent with quiescent degassing and gradual Ag-Cu-Pb-Zn-Cd enrichment in an evolved felsic magma. Barren Mar1 records the dominance of meteoric water and a limited magmatic fluid contribution. High-Co-Ni-V-Cr-Ti contents in porous cores of Py1 and Mar2 are attributed to wall rock alteration and dissolution-reprecipitation. The results provide convincing evidence that the metal budget (especially for Au, Ag, Cu, Pb, Zn, Sb) of the hydrothermal fluids and sulfides in epithermal systems are controlled by the influx of magmatic fluids and associated magma, whereas the enrichment of certain fluid-immobile elements, such as Co, Ni, V, Cr, and Ti, is caused in part by fluid-rock interaction.
The Calabona porphyry copper system is developed in a small, relatively deep seated ( nearly equal 5 km) dacitic intrusion located in northwestern Sardinia (Italy). Early hydrothermal alteration produced a potassic assemblage in the deep and central parts of the complex, and a peripheral propylitic halo. Sodic alteration was subsequently superimposed on the potassic zone and volumetrically dominant phyllic alteration overprinted the apical parts of the intrusion. Hypogene copper mineralization (chalcopyrite and minor bornite) was associated with potassic alteration. The earliest fluid that circulated in the Calabona porphyry complex had high salinity (40-60 wt % NaCl equiv), and is interpreted to have been exsolved directly from the crystallizing magma. However, major entrapment of this fluid only occurred after it had cooled to temperatures of about 400 degrees C. This fluid was responsible for potassic alteration and for precipitation of chalcopyrite and bornite in thin, discontinuous group 1 veins and irregular and widely spaced group 2 veins. The average Cu grade in the potassic zone is between 0.05 and 0.1 percent. The circulation of a lower temperature (270 degrees -330 degrees C), Ca-enriched fluid of meteoric origin in the peripheral parts of the system caused propylitic alteration. At an intermediate stage of hydrothermal evolution, waters of external origin entered the central parts of the system along a network of late fractures (group 3 veins) or reopened group 2 veins. Partial mixing of this meteoric-formational water with the high-salinity fluid already circulating in the system, created a fluid characterized by salinities ranging from 2 to 23 wt percent NaCl equiv. Circulation of this mixed fluid at relatively low fluid/rock ratios along a prograde thermal path caused sodic alteration. The continuous inflow of meteoric water, and the general temperature decrease in the system, produced progressively more oxidized and acidic fluids, which caused phyllic alteration and intense copper leaching. Late boiling in the apical parts of the phyllic alteration zone favored deposition of chalcopyrite and bornite but did not add significant copper (Cu grade <0.03%). Supergene enrichment led to copper grades higher than those in the phyllic zone (0.07%), but not enough to permit mining. The very low concentration of Cu in the Calabona deposits is surprising, in view of wall-rock alteration, style of mineralization, and a fluid evolution typical of those of many productive porphyry copper systems. We propose that this is mainly a consequence of the raltive deep level of emplacement of the intrusion and the dacitic composition of the magma. These factors are interpreted to have combined to retard melt saturation with alkali chlorife-enriched fluids until late stages of crystallization, which restricted the amount of exsolved fluid and Cu extracted form the melt. As a result, overpressuring in the apical parts of the system was limited, the related fracture density was low, and the system therefore failed to provide the focus for the mineralizing fluids needed to permit bulk concentration of copper to economic levels. Further reasons for the uneconomic nature of the Calabona porphyry were the lack of multiple intrusive events and the sulfide-destructive, Cu-leaching effects of phyllic alteration.
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