ALBERT

Description: 〈div data-abstract-type="normal"〉〈p〉During a study of the ore minerals belonging to the recently discovered Shuangjianzishan Ag–Pb–Zn deposit in NE China, we have discovered exceptional selenium enrichment in canfieldite (up to 11.6 wt.% of Se). Incorporation of Se into canfieldite has been investigated by an integrated approach using field emission scanning electron microscopy, electron microprobe and single-crystal X-ray diffraction. Canfieldite has been identified as one of the dominant Ag-bearing ore minerals in the studied deposit, which occurs mostly in slate-hosted vein type Ag–Pb–Zn ore bodies. Selenium is either homogeneously or, remarkably, heterogeneously distributed in the different canfieldite fragments studied. Chemical variations of Se are mostly attributable to a series of retrograde reactions resulting in diverse decomposition and exsolution of primary phases during cooling, or alternatively, related to influxes of Se-rich fluids during the formation of canfieldite. To evaluate the effects of the Se-for-S substitution in the structure, a crystal of Se-rich canfieldite [Ag〈span〉7.98〈/span〉Sn〈span〉1.02〈/span〉(S〈span〉4.19〈/span〉Se〈span〉1.81〈/span〉)〈span〉Σ6.00〈/span〉] was investigated. The unit-cell parameters are: 〈span〉a〈/span〉 = 10.8145(8) Å and 〈span〉V〈/span〉 = 1264.8(3) Å〈span〉3〈/span〉. The structure was refined in the space group 〈span〉F〈/span〉〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190709073523038-0660:S0026461X18001585:S0026461X18001585_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉3〈span〉m〈/span〉 to 〈span〉R〈/span〉〈span〉1〈/span〉 = 0.0315 for 194 independent reflections, with 20 parameters. The crystal structure of Se-rich canfieldite was found to be topologically identical to that of pure canfieldite. If the short Ag–Ag contacts are ignored (due to the disorder), the two Ag atoms in the structure can be considered as three-fold (Ag1) and four-fold (Ag2) coordinated. Tin adopts a regular tetrahedral coordination. As in the case of Te-rich canfieldite, the refinement of the site-occupancy factor indicates that Se is disordered over the three anion positions.〈/p〉〈/div〉

Description: Strata-bound carbonate-replacement Pb-Zn-Ag deposits in the Lavrion district, Greece, are spatially related to a late Miocene granodiorite intrusion (7-10 Ma) and various sills and dikes of mafic to felsic composition. The Plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a Ca-Fe skarn. Carbonate-replacement deposits occur predominantly in marbles (Upper and Lower Marble of the Basal unit), Kaesariani schists, and along a major detachment fault that separates the Basal unit from the Upper unit. Orebodies are mainly strata bound carbonate-replacement, although sulfides also occur in veins. The mineralogy of carbonate-replacement deposits is dominated by base metal sulfides and sulfosalts of Ag, Bi, Sn, Sb, As, and Pb, particularly at Plaka and Kamariza. Carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite. Fluid inclusion studies of sphalerite, fluorite, calcite, and quartz in carbonate-replacement deposits suggest that they were deposited from 132{degrees} to 365{degrees}C from CO2-poor, low- to high-salinity fluids (1-20 wt % NaCl equiv). Carbon and oxygen isotope compositions of calcite ({delta}13C = -15.6 to -1.5{per thousand} and{delta} 18O = -9.2 to +17.3{per thousand}) intergrown with sulfides reflect variable exchange of the ore-bearing fluid with the Upper and Lower Marbles and proximity to the Plaka granodiorite. Post-Archean Australian Shale (PAAS)-normalized rare earth and yttrium patterns of the Upper and Lower Marbles, and calcite intergrown with sulfides show positive Eu and negative Ce anomalies as well as Y/Ho ratios between 40 and 80. Normalized rare earth and yttrium patterns of fluorite also have positive Eu and negative Ce anomalies. Such anomalies for both the carbonates and fluorite reflect the high pH or high fO2 conditions of the late-stage hydrothermal fluids and the likely derivation of calcium from marine carbonates (precursors of the Upper and Lower Marbles). The range of sulfur isotope compositions for sulfides ({delta}34S = -4.9 to +5.3{per thousand}, with one outlier of 9.4{per thousand}) in carbonate-replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate. Sulfur isotope compositions of barite from carbonate-replacement range from {delta}34S = 17.2 to 23.7 per mil and reflect Miocene seawater sulfate values. If a magmatic source of sulfur is assumed along with an average temperature of 250{degrees}C for the ore-forming fluids, as based on fluid inclusion studies, sulfides in carbonate-replacement deposits were deposited at values of log fO2 = -41 to -36 and a pH = 5.8 to 9.1. However, the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source. The carbonate-replacement deposits resemble manto-type sulfide deposits in Mexico, central Colorado, South Korea, Nevada, and northern Greece.

Description: The newly discovered Shuangjianzishan Ag-Pb-Zn deposit, with 145 Mt of ore grading 128.5 g/t Ag (locally up to 32,000 g/t) and 2.2 wt % Pb + Zn, is located in the Great Hinggan Range metallogenic belt, northeastern China, and is currently the largest Ag deposit in Asia. The Ag-Pb-Zn orebodies occur as veins and are hosted primarily by a Permian slate. Recent drilling and core logging have identified a partially Mo mineralized granite porphyry intrusion adjacent to the Ag-Pb-Zn mineralized veins. This well-preserved magmatic-hydrothermal system therefore offers an excellent opportunity to evaluate the possible temporal and genetic relationship between Mo-mineralized porphyry intrusions and Ag-Pb-Zn veins. Three primary paragenetic stages of veining have been recognized: (I) early pyrite + quartz ± K-feldspar, (II) main ore sulfide + sulfosalt + quartz + calcite + sericite + chlorite ± epidote, and (III) post-ore quartz. The silver mineralization occurs mainly in the late paragenetic part of Stage II, in which canfieldite (Ag8SnS6), argentite (Ag2S) and freibergite [(Ag, Cu)12Sb4S13] are the dominant Ag-bearing ore minerals. A combination of ore mineral chemical and sulfur isotope geothermometers and physicochemical calculations suggest that the Ag-Pb-Zn mineralization took place at a temperature of 250° to 200°C, a pH of 6.7 to 5.6, and a Δlogfo2 (HM) of –2.4 to –8.7. A conspicuous enrichment of Sn and Se in the ore, which is represented by minerals containing the metal suite Ag-Pb-Zn-(Cu-Sn-Se-Sb), likely reflects a close genetic association between the base metal mineralization and a magma. In situ analyses show that the δ34S values of the sulfides and Ag-bearing sulfosalts from the Ag-Pb-Zn mineralized veins vary from –4.67 to +2.44‰; the mean value is –2.11 ± 1.49‰ (n = 77). The calculated mean δ34SH2S value of the ore-forming fluid is –1.65 ± 0.83‰, which is indicative of a magmatic sulfur source. In situ Pb isotope analyses of the ore minerals yielded a narrow range of values (206Pb/204Pb of 18.243–18.310, 207Pb/204Pb of 15.503–15.563 and 208Pb/204Pb of 38.053–38.203, n = 59). Comparisons to corresponding isotopic data for the various rock units in the area and sulfides from nearby ore deposits indicate that there were substantial contributions of Pb and other metals (e.g., Ag and Zn) to the Shuangjianzishan deposit from a Mesozoic granitic source. Diorite-granodiorite dikes and dacite are crosscut by the Ag-Pb-Zn veins, and therefore, predate ore formation. These rock units have zircon U-Pb ages of 250.2 ± 2.0 and 133.9 ± 1.4 Ma, respectively. A concealed, weakly Mo mineralized granite porphyry intrusion proximal to the Ag-Pb-Zn mineralized vein system yielded zircon U-Pb ages of 134.4 ± 1.0 (MSWD = 0.1) and 134.4 ± 1.0 Ma (MSWD = 0.2), for coarse- and fine-grained facies, respectively. These ages are indistinguishable within the uncertainty from the zircon ages for the dacite and a granite intrusion ~2 km north of the mineralized veins, which has a weighted mean zircon U-Pb age of 135.2 ± 1.4 Ma (MSWD = 0.78). Molybdenite from three quartz vein/veinlet samples hosted by slate immediately above the porphyry intrusion yielded Re-Os model ages from 136.3 ± 0.9 to 133.7 ± 1.2 Ma and a weighted mean Re-Os age of 134.9 ± 3.4 Ma. Finally, three pyrite samples separated from the Ag-Pb-Zn mineralized veins have a weighted mean Re-Os model age of 135.0 ± 0.6 Ma. The very similar zircon U-Pb ages for the Mo-mineralized granite porphyry and dacite, and Re-Os ages for molybdenite and pyrite in the Shuangjianzishan ore district indicate that the Mesozoic magmatic-hydrothermal activity was restricted to a relatively short time interval (~136–133 Ma). They also suggest that the weakly Mo mineralized granite porphyry was likely the source of the fluids and metals that produced the Ag-Pb-Zn mineralization. Based on our geological observations and an extensive analytical database, a model is proposed for the genesis of the giant Shuangjianzishan Ag-Pb-Zn deposit in which the ore-forming fluid and its metals (i.e., Ag, Pb, and Zn) were exsolved during crystallization of the final phase of a composite granite porphyry intrusion. This fluid transported metals to the distal parts of the system, where they were deposited in preexisting faults or fractures created by the withdrawal of magma during the waning stages of the magmatic-hydrothermal event. The present study of the Shuangjianzishan Ag-Pb-Zn deposit and those of other magmatic-hydrothermal ore deposits in the region provide compelling evidence that the widespread Mesozoic felsic magmatism and Ag-Pb-Zn mineralization in the southern Great Hinggan Range took place in an intracontinental extensional tectonic setting, which was synchronous with, and spatially associated to, Paleo-Pacific slab rollback and lithospheric delamination and thinning.

Description: This paper investigates the interrelationships between the engineering properties of igneous aggregate rocks from Greece with the aid of the R-mode factor analysis. The collected samples represent mafic and ultramafic rocks from the ophiolite complexes of Gerania, Guevgueli, Veria-Naousa, and Edessa as well as intermediate-acidic rocks from the surrounding areas of the complexes. Factor analysis verifies the important interdependences among the engineering parameters like physical, mechanical, geometrical, and physicochemical properties by giving statistical significance. Variations of the petrographic characteristics of the investigated rocks influence their engineering properties as well as the interdependence among them. Factor 1, which is the most representative one (~36% of the total variance), shows interdependences between certain physical, mechanical, physicochemical properties such as total porosity (nt) with moisture content (w), nt with the Los Abrasion value (LA), and the uniaxial compressive strength (UCS) with point load index Is(50). Additionally, the second factor (~27% of the total data variability) correlates physical properties such as w, nt, physicochemical properties such as the methylene blue test (MBF), mechanical properties such as UCS, Is(50), and loss on ignition (LOI), which highlights the effect of mineralogy on these properties. Lastly, Factor 3 (~14% of the total data variability) expresses the interdependence of the flakiness index (IF), which is an elongation index (IE) relative to their alteration (LOI).

Description: During a study of the ore minerals belonging to the recently discovered Shuangjianzishan Ag–Pb–Zn deposit in NE China, we have discovered exceptional selenium enrichment in canfieldite (up to 11.6 wt.% of Se). Incorporation of Se into canfieldite has been investigated by an integrated approach using field emission scanning electron microscopy, electron microprobe and single-crystal X-ray diffraction. Canfieldite has been identified as one of the dominant Ag-bearing ore minerals in the studied deposit, which occurs mostly in slate-hosted vein type Ag–Pb–Zn ore bodies. Selenium is either homogeneously or, remarkably, heterogeneously distributed in the different canfieldite fragments studied. Chemical variations of Se are mostly attributable to a series of retrograde reactions resulting in diverse decomposition and exsolution of primary phases during cooling, or alternatively, related to influxes of Se-rich fluids during the formation of canfieldite. To evaluate the effects of the Se-for-S substitution in the structure, a crystal of Se-rich canfieldite [Ag 7.98 Sn 1.02 (S 4.19 Se 1.81 ) Σ6.00 ] was investigated. The unit-cell parameters are: a = 10.8145(8) Å and V = 1264.8(3) Å 3 . The structure was refined in the space group F 4¯ 3 m to R 1 = 0.0315 for 194 independent reflections, with 20 parameters. The crystal structure of Se-rich canfieldite was found to be topologically identical to that of pure canfieldite. If the short Ag–Ag contacts are ignored (due to the disorder), the two Ag atoms in the structure can be considered as three-fold (Ag1) and four-fold (Ag2) coordinated. Tin adopts a regular tetrahedral coordination. As in the case of Te-rich canfieldite, the refinement of the site-occupancy factor indicates that Se is disordered over the three anion positions.

Description: The Sandaowanzi gold deposit in the Great Hinggan Range metallogenic belt, northeast China, is unusual because the mineralization takes the form of Au- and Ag-bearing tellurides. This mineralization, which constitutes a resource of 28 t Au, is hosted in 20 NW-trending, syntaxial, layered quartz veins and tension gashes within almost coeval trachyandesites and andesitic breccias that overlie the Sandaowanzi monzogranite. Alteration halos are developed around the quartz veins and tension gashes. They consist of an inner silicic-pyritic zone, two intermediate zones containing quartz-illite-sericite and quartz-adularia, and an outer zone containing a quartz-calcite-kaolinite-chlorite assemblage. Fluid inclusion microthermometric data reveal that the Sandaowanzi ores precipitated from moderate-temperature (200°–280°C), low-pressure (70–130 bar), and low- to moderate-salinity (mostly 〈6.0 wt % NaCl equiv) hydrothermal fluids. Temperature and pressure are interpreted to have decreased with the evolution of the system. The 〈δ34SH2S (–2.2 to –0.2‰) values indicate that the sulfur is of magmatic origin and suggest that it was leached by the ore fluid from the host volcanic rocks. A magmatic origin is also interpreted for the metals, based on Pb isotope data (206Pb/204Pb = 18.2366–18.3146, 207Pb/204Pb = 15.5404–15.5624, 208Pb/204Pb = 38.0901–38.2293). In contrast, the δ18OH2O (–13.6 to –7.6‰) and δDH2O (–127 to –96‰) values indicate that the hydrothermal fluids were dominantly meteoric. Physicochemical modeling shows that sulfidation of the host rocks (decrease of αHS–(aq)) and condensation of H2Te-bearing vapors (increase of αHTe–(aq)) were the dominant controls on precipitation of the Au and Ag telluride ores. This study emphasizes the importance of fluid-rock interaction (sulfidation) and the mixing of low-density H2Te-bearing magmatic fluids with meteoric waters in producing large Au and Ag telluride deposits.