ALBERT

Description: Interface coupled dissolution-reprecipitation reactions (ICDR) are a common feature of fluid-rock interaction during crustal fluid flow. We tested the hypothesis that ICDR reactions can play a key role in scavenging minor elements by exploring the fate of U during the experimental sulfidation of hematite to chalcopyrite under hydrothermal conditions (220–300 °C). The experiments where U was added, either as solid UO 2+x (s) or as a soluble uranyl complex, differed from the U-free experiments in that pyrite precipitated initially, before the onset of chalcopyrite precipitation. In addition, in UO 2+x (s)-bearing experiments, enhanced hematite dissolution led to increased porosity and precipitation of pyrite+magnetite within the hematite core, whereas in uranyl nitrate-bearing experiments, abundant pyrite formed initially, before being replaced by chalcopyrite. Uranium scavenging was mainly associated with the early reaction stage (pyrite precipitation), resulting in a thin U-rich line marking the original hematite grain surface. This "line" consists of nanocrystals of UO 2+x (s), based on chemical mapping and XANES spectroscopy. This study shows that the presence of minor components can affect the pathway of ICDR reactions. Reactions between U- and Cu-bearing fluids and hematite can explain the Cu-U association prominent in some iron oxide-copper-gold (IOCG) deposits.

Description: A growing literature is demonstrating that platinum (Pt) is transformed under surface conditions; yet (bio)geochemical processes at the nugget-soil-solution interface are incompletely understood. The reactivity of Pt exposed to Earth-surface weathering conditions, highlighted by this study, may improve our ability to track its movement in natural systems, e.g., focusing on nanoparticles as a strategy for searching for new, undiscovered sources of this precious metal. To study dissolution/re-precipitation processes of Pt and associated elements, grains of Pt-Fe alloy were collected from a soil placer deposit at the Fifield Pt-field, Australia. Optical- and electron-microscopy revealed morphologies indicative of physical transport as well as chemical weathering. Dissolution "pits," cavities, striations, colloidal nano-particles, and aggregates of secondary Pt platelets as well as acicular, iron (Fe) hydroxide coatings were observed. FIB-SEM-(EBSD) combined with S-μ-XRF of a sectioned grain showed a fine layer of up to 5 μm thick composed of refined, aggregates of 0.2 to 2 μm sized crystalline secondary Pt overlying more coarsely crystalline Pt-Fe-alloy of primary magmatic origin. These results confirm that Pt is affected by geochemical transformations in supergene environments; structural and chemical signatures of grains surfaces, rims, and cores are linked to the grains’ primary and secondary (trans)formational histories; and Pt mobility can occur under Earth surface conditions. Intuitively, this nanophase-Pt can disperse much further from primary sources of ore than previously thought. This considerable mineral reactivity demonstrates that the formation and/or release of Pt nanoparticles needs to be measured and incorporated into exploration geochemistry programs.

Description: Chalcopyrite (CuFeS 2 ) and bornite (Cu 5 FeS 4 ) are the most abundant Cu-bearing minerals in hydrothermal Cu deposits, forming under a wide range of conditions from moderate-temperature sedimentary exhalative deposits to high-temperature porphyry Cu and skarn deposits. We report the hydrothermal synthesis of both chalcopyrite and bornite at 200–300 °C under hydrothermal conditions. Both minerals formed via the sulfidation of hematite in solutions containing Cu(I) (as a chloride complex) and hydrosulfide, at pH near the pK a of H 2 S(aq) over the whole temperature range. Polycrystalline chalcopyrite formed first, followed by bornite. Assuming that Fe behaves conservatively, the transformation of hematite to chalcopyrite involves a large increase in volume (~290%). The reaction proceeds both via direct replacement of the existing hematite and via overgrowth around the grain. Chemical exchanges between bulk solution and hematite are enabled by a network of micrometer-size pores. However, in some cases the chalcopyrite overgrowth develops large grain sizes with few apparent pores and in these cases fluid transport may have been via a network of fractures. Similarly to the replacement of hematite by chalcopyrite, bornite forms via the replacement of chalcopyrite. The reaction has a large positive volume (~230%), and proceeds both via chalcopyrite replacement and via overgrowth. This study shows that replacement reactions can proceed via coupled dissolution-reprecipitation even where there is a large volume increase between parent and product mineral. This study also provides further evidence about the controls of reaction pathways onto the final mineral assemblage. In this case, the host initial fluid was undersaturated with respect to Fe-bearing minerals. Upon slow release of Fe at the surface of hematite, a mineral assemblage of chalcocite, bornite, and finally chalcopyrite is expected. However, in practice chalcocite did not nucleate on the surface of hematite. Rather relatively slow nucleation of bornite enabled high concentrations of Fe to build up near the dissolving hematite, so that chalcopyrite (high-sulfidation experiments) or chalcopyrite+pyrite (low sulfidation) crystallized first.

Description: Porosity plays a key role in the formation and alteration of sulfide ore minerals, yet our knowledge of the nature and formation of the residual pores is very limited. Herein, we report the application of ultra-small-angle neutron scattering and small-angle neutron scattering (USANS/SANS) to assess the porosity in five natural sulfide minerals (violarite, marcasite, pyrite, chalcopyrite, and bornite) possibly formed by hydrothermal mineral replacement reactions and two synthetic sulfide minerals (violarite and marcasite) prepared experimentally by mimicking natural hydrothermal conditions. USANS/SANS data showed very different pore size distributions for these minerals. Natural violarite and marcasite tend to possess less pores in the small size range (〈100 nm) compared with their synthetic counterparts. This phenomenon is consistent with a higher degree of pore healing or diagenetic compaction experienced by the natural violarite and marcasite. Surprisingly, nanometer-sized (〈20 nm) pores were revealed for a natural pyrite cube from La Rioga, Spain, and the sample has a pore volume fraction of ~7.7%. Both chalcopyrite and bornite from the massive sulfide assemblage of the Olympic Dam deposit in Roxby Downs, South Australia, were found to be porous with a similar pore volume fraction (~15%), but chalcopyrite tends to have a higher proportion of nanometer-size pores centered at ~4 nm while bornite tends to have a broader pore size distribution. The specific surface area is generally low for these minerals ranging from 0.94 to 6.28 m 2 /g, and the surfaces are generally rough as surface fractal behavior was observed for all these minerals. This investigation has demonstrated that USANS/SANS is a very useful tool for analyzing porosity in ore minerals. We believe that with this quantified porosity information a deeper understanding of the complex fluid flow behavior within the porous minerals can be expected.

Description: We report the replacement of chalcopyrite by bornite under hydrothermal conditions in solutions containing Cu(I) and hydrosulfide over the temperature range 200–320 °C at autogenous pressures. Chalcopyrite was replaced by bornite under all studied conditions. The reaction proceeds via an interface coupled dissolution-reprecipitation (ICDR) mechanism and via additional overgrowth of bornite from the bulk solution. Initially, the reaction is fast and results in a bornite rim of homogeneous thickness. Reaction rates then slow down, probably reflecting healing of the porosity, and the reaction proceeds predominantly along twin boundaries of the chalcopyrite. The composition of the bornite product is generally Cu-rich, corresponding to the bornite-digenite (Cu 5 FeS 4 -Cu 9 S 5 ; Bn-Dg) solid solution ( bdss ). The Cu and Fe contents were controlled principally by temperature, with solution pH having only a small effect. The percentage of Cu in bdss decreased and the percentage of Fe increased with increasing reaction temperature: at 200 °C a composition of Bn 47 Dg 53 was obtained; at 300 °C the composition was Bn 90 Dg 10 and at 320 °C it was near-stoichiometric bornite. The influence of temperature rather than solution chemistry on the composition of bdss , as well as the homogeneity of the bornite product grown both via replacement of chalcopyrite and from the bulk solution as overgrowth, are interpreted to reflect buffering of the bornite activity in bdss via solids (e.g., reaction chalcopyrite + 2 chalcocite = bornite). Only the end-member compositions of the bdss are found in nature, indicating that the products obtained are metastable, and illustrating the importance of reaction mechanism for controlling the chemistry of the mineral product. The unique features of the chalcopyrite to bornite reaction investigated here are related to interaction between a solution controlled ICDR reaction with solid-state diffusion processes driving porosity healing.

Description: Calaverite, krennerite, and sylvanite are Au-Ag-tellurides with close compositions and related crystal structures. Previous experimental studies show that both calaverite and sylvanite transform to porous "mustard" gold under hydrothermal conditions; however the transformation of sylvanite follows a complex reaction path with several intermediary products, contrasting with the simple replacement of calaverite by gold. Here we report results of an experimental study of the transformation of krennerite, a phase with Ag contents intermediate between those of calaverite and sylvanite. Krennerite was replaced by Au-Ag alloy under all experimental conditions explored (160 to 220 °C; pH T ~ 3 and 9; varying availability of oxygen). No reaction was observed at the same temperature under dry conditions. The replacement was pseudomorphic and the resulting Au-Ag alloy was porous, consisting of worm-like aggregates with diameters ranging from 200 nm to 1 μm. The replacement of krennerite proceeds via an interface coupled dissolution-(re)precipitation (ICDR) reaction mechanism. Tellurium is lost to the bulk solution as Te(IV) complexes, and may precipitate away from the dissolution site. In contrast, Au-Ag alloy precipitates locally near the krennerite dissolution site. Overall, the hydrothermal alteration of krennerite is very similar to that of calaverite, but differs from the alteration of sylvanite, for which multi-step reaction paths led to complex products and textures under similar conditions. These striking differences are driven by the competition between solid-state reactions and ICDR reaction in sylvanite. This reflects the fact that a metastable, Ag-rich calaverite nucleates on sylvanite during the early steps of its dissolution, as a result of the close relationship between the structures of these two minerals and the enrichment in Au, Ag, and Te in solution at the reaction front. In contrast, for calaverite and krennerite, no such phase precipitates, and both minerals are transformed in a pseudomorphic manner into Au-Ag alloy.

Description: This paper describes the new mineral nataliyamalikite, the orthorhombic form of thallium iodide (TlI), from high-temperature fumaroles from the Avacha volcano, Kamchatka Peninsula, Russia. We also present some chemical analyses showing extreme enrichment of Tl in the volcanic gases at the Avacha volcano, and a review of thallium geochemistry that highlights the fascinating processes that led to the formation of nataliyamalikite. Nataliyamalikite occurs as pseudo-cubic nanocrystals (≤0.5 μm) within vacuoles in an As-(Te)-rich amorphous sulfur matrix and rarely as irregularly shaped aggregates up to ~50 μm in diameter within the amorphous sulfur matrix. Associated minerals include an unidentified Tl-As-S mineral, barite, and rare inclusions of a Re-Cu-bearing phase. The mean empirical composition based on four EDS analyses is Tl 1.00 (I 0.95 Br 0.03 Cl 0.02 ), corresponding to the ideal formula TlI. Nataliyamalikite crystallizes in the orthorhombic system, space group Cmcm , which is consistent with the low-temperature (〈175 °C) synthetic TlI polymorph. EBSD data reveal that some grains retain the cubic symmetry ( Pm m ) of the high-temperature polymorph, although most analyzed grains display the orthorhombic symmetry. Single-crystal X-ray studies of material extracted by the focused ion beam-scanning electron microscopy (FIB-SEM) technique, and carried out on the MX2 macromolecular beamline of the Australian Synchrotron, determined the following cell dimensions: a = 4.5670(9), b = 12.803(3), c = 5.202(1) Å, V = 304.2(1) Å 3 , and Z = 4. The six strongest calculated X-ray reflections and their relative intensities are: 3.31 (100), 2.674 (73), 3.20 (43), 2.601 (28), 2.019 (21), and 2.284 Å (19). The combination of EBSD analysis (providing an efficient test of the crystallinity and crystal symmetry of a population of micrometer-sized grains) and synchrotron single-crystal X-ray micro-diffraction (beam size ~7.5 μm) on micro-aggregates extracted using FIB-SEM opens the way to the characterization of challenging specimen—in this case, the sulfur matrix is highly beam sensitive, and the nataliyamalikite grains could not be isolated using optical microscopy. The high-temperature (〉600 °C) sulfidic (~1.2 wt% S) vapors at Avacha are extremely enriched in thallium; with 34 ppm, they contain an order of magnitude more Tl than the richest volcanic gases analyzed to date and ~100 x more Tl than most metal-rich fumarolitic fluids associated with volcanic arcs. The formation of nataliyamalikite illustrates the complex processes that control thallium geochemistry in magmatic arc systems. Thallium minerals have now been reported in andesitic (Avacha), basaltic (Tolbachik, Kamchatka), as well as rhyolitic (Vulcano, Eolian Islands, Italy) volcanoes. Ultimately, these thallium minerals result from the transfer of thallium from subducted sediments to volcanic gases in arc volcanoes. We suggest that the extremely thallium-enriched vapors from which nataliyamalikite formed result from complex and transient interactions between Tl-rich sulfosalt melts and magmatic vapors, a process that may be important in controlling metal distribution in boiling epithermal systems.

Description: The new mineral graţianite, MnBi 2 S 4 , is described from the Bǎia Bihor skarn deposit, Bihor County, Romania. Graţianite occurs as thin lamellae, intimately intergrown with cosalite and bismuthinite, or as flower-shaped blebs within chalcopyrite, where it is associated with cosalite and tetradymite. Graţianite displays weak to modest bireflectance in air and oil, respectively, and strong anisotropy. The mean empirical composition based on 20 electron probe microanalyses is: (Mn 0.541 Fe 0.319 Pb 0.070 Cu 0.040 Cd 0.009 Ag 0.001 ) S0.980 (Bi 1.975 Sb 0.018 ) S1.993 (S 4.008 Se 0.012 Te 0.007 ) S4.027 , corresponding to the ideal formula MnBi 2 S 4 . Graţianite crystallizes in the monoclinic system (space group C 2/ m ). Single-crystal X-ray studies of material extracted by the focused ion beam-scanning electron microscopy (FIB-SEM) technique, and carried out on the MX2 macromolecular beamline of the Australian Synchrotron determined the following cell dimensions: a = 12.6774(25) Å, b = 3.9140(8) Å, c = 14.7581(30) Å, b = 115.31(3)°, V = 662.0(2) Å 3 , and Z = 4. The six strongest X-ray reflections and their relative intensities are: 3.448 Å (100), 2.731 Å (77), 2.855 Å (64), 3.637 Å (55), 3.644 Å (54), and 3.062 Å (51). Graţianite is the monoclinic analog of berthierite (FeSb 2 S 4 ), garavellite [Fe(Bi,Sb) 2 S 4 ] and clerite [Mn(Sb,As) 2 S 4 ] (Nickel-Strunz class 02.HA.20). It is isostructural with synthetic sulfides and selenides in the MnBi 2 S 4 –MnSb 2 S 4 and MnBi 2 Se 4 –MnSb 2 Se 4 series, and with grumiplucite (HgBi 2 S 4 ) and kudriavite, [(Cd,Pb)Bi 2 S 4 ], 3 P members of the pavonite homologous series. The mineral is named for Graţian Cioflica (1927–2002), formerly Professor in Mineralogy and Ore Deposits at the University of Bucharest, Romania. The Băia Bihor skarn, like others within the same belt, is geochemically complex. The availability of Cu, Zn, and Pb, but also Ag, Bi, Mo, and B, as well as a wide range of minor elements, has created an environment allowing for crystallization of an unusually diverse range of discrete minerals. Graţianite is part of the peculiar associations of Bi–Pb-sulfosalts and Bi-chalcogenides that are genetically related to Au-enrichment. This study demonstrates the versatility of FIB-SEM techniques for in situ extraction of small volumes of well-characterized material, coupled with single-crystal X-ray analysis using synchrotron radiation, for the characterization of new minerals.

Description: Recent experiments have shown that microporous gold can be obtained via the oxidative dealloying of Au(Ag)-tellurides such as calaverite (AuTe 2 ), krennerite (Au 3 AgTe 8 ), and sylvanite [(Au,Ag) 2 Te 4 ] under mild hydrothermal conditions. The same Au textures have been found in natural gold-telluride ores from the Late Miocene epithermal Aginskoe Au-Ag-Te deposit in Kamchatka, Russia. This confirms that natural microporous gold can form via the replacement of telluride minerals. This replacement may take place under hydrothermal conditions, e.g., during the late stage of the ore-depositing event, explaining the wide distribution of "mustard gold" in some deposits. At Aginskoe, the oxidation of Au-tellurides appears to have resulted only in local redistribution of Au and Te, because the associated oxidation of chalcopyrite scavenged the excess Te, inhibiting the crystallization of secondary Te minerals more than a few micrometers in size. Such cryptic mobility may explain the lack of reported secondary Te minerals in many Te-bearing deposits.

Description: Although the crustal abundance of tellurium (Te) is about half of that of gold (Au), several classes of Au deposits are highly enriched in Te. Our understanding of the nature of this Au-Te association is hampered by the lack of experimental studies of Te geochemistry at elevated temperature. We characterized the structure of polytelluride solutions from room temperature to 599 °C at 800 bar using in situ X-ray absorption spectroscopy. Both ab-initio XANES and EXAFS fits show that polytellurides are stable up to the highest temperature, with planar structures (four- or threefold coordination of Te) giving way to linear chains (e.g., Te 2 2– ion) at temperatures above ~200 °C. This is the first experimental confirmation of the thermal stability of polytelluride species. The data show that polytellurides play an important role in Te transport in reduced S-rich or CO 2 -rich solutions and vapors.