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Abstract Exploration activities outlined a shear zone-hosted gold and base metal occurrence in low-grade metasedimentary rocks near the town of Workamba, Tigray State, northern Ethiopia. Gold resources are estimated at ~2 t Au with concentrations reaching up to 8 g/t Au in the ore. The geology of the study area is dominated by foliated or sheared, lower greenschist facies metavolcanic and metasedimentary rocks, which are correlated to the lower island-arc metavolcanic rocks of the ~860 to 750 Ma Tsaliet Group and upper carbonate and metasedimentary rocks of the ~740 Ma Tambien Group, respectively. The Tsaliet and Tambien groups represent the southern parts of the Arabian-Nubian Shield. The Tsaliet Group, which is exposed in the north-western parts of the study area, consists mainly of metavolcaniclastic rocks. The overlying slate, phyllite, sericite-chlorite schists and marbles of the Tambien Group occupy the south-eastern part of the area. A steeply dipping NE trending ductile shear zone occurs in the metasediments along the contact with the Tsaliet Group. It is intruded by monzogranite and lamprophyre dikes or sills. The shear zone is correlated with an early regional deformation event (D1), which also caused NE foliations and ENE trending folds. A second regional deformation phase (D2) resulted in NNE oriented weakly developed dextral shear bands and N-S trending parasitic folds. NW striking brittle faults postdate D1 and D2 ductile structures. Trace element patterns of the metasedimentary rocks support findings of previously published works suggesting that they are derived from the rocks of the Tsaliet Group. Major, trace, and rare earth element characteristics of the monzogranite dikes/sills permit the conclusion that they are related to the ~620 to 520 Ma post-tectonic magmatism in the Tigray region. The post-tectonic granitoids are resulted from mantle-derived magmas, which are modified by fluid components of the subducted slab or involvement of continental crust. Petrographic and geochemical results of the lamprophyres show that they are calc-alkaline in nature. The monzogranite dikes/sills have 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values between 37.176 to 37.310, 15.508 to 15.525, and 17.799 to 18.310, respectively. These lead isotope ratios are compatible with a significant mantle component in these rocks. Lead isotope ratios of the metasedimentary and metavolcanic rocks at Workamba are heterogeneous, suggesting that not all samples did retain their original Pb isotope signatures. However, the original Pb isotope composition of these rocks tends to be less radiogenic as those of the post-tectonic dikes/sills. Field and petrographic observations show that the metavolcanic rocks are locally affected by propylitic alteration and sericitization. The metasedimentary rocks suffered from pervasive silicification, sericitization, and carbonatization. The monzogranite and lamprophyre dikes/sills locally experienced pervasive sericitization, carbonatization, epidotization, and chloritization. Two generations of quartz veins occur cutting and/or parallel to D1 foliation on the Tsaliet and Tambien groups. The first generation quartz veins are folded and less abundant as compared to the unfolded second generations. The metasedimentary rocks and the dikes/sills are also cut by calcite ±quartz veinlets in the mineralized zone. Mass balance calculations of the metasedimentary rocks suggest that Na2O, and Sr are removed by hydrothermal fluids from the original composition of the rocks. K2O, MnO, CaO, Ba, and Pb are variably added or removed. The dikes/sills retained near original geochemical composition. The shear zone hosted metallic mineralization occurs within the metasedimentary rocks, in close spatial proximity to the dikes/sills. It took place under brittle conditions. Pyrite, sphalerite, galena, and chalcopyrite are the main sulfide phases, whereas pyrrhotite, arsenopyrite, and chalcocite are minor components. Gold is invisible and was only detected in geochemical analysis. Gangues include calcite, sericite, and quartz. The mineralized rocks are enriched in Au, As, Sb, and Ag. The Au/Ag ratio of all analyzed samples is less than 10. The paragenetic relationships show that pervasive sericitization and chlorite-epidote (propylitic) alteration occurred before an early quartz and/or calcite ±quartz veining event. The mineralization occurred roughly concomitant with carbonatization and calcite ±quartz veining, after the intrusion of the monzogranite and lamprophyre dikes/sills. Sulfur isotope signature of sulfides (δ34S(VCDT) = 1.8 to 9.1 ‰) is compatible with the hypothesis that S is derived from the country rocks through leaching by hydrothermal fluids. The carbon isotope signature of hydrothermal calcite (13C(VPDB) = –5.6 to +1.8 ‰) and that of a fluid in equilibrium with this calcite (13C(VPDB) = -3.7 to +3.7 ‰) are similar to those of Tambien Group carbonate rocks (13C(VPDB) = -4.5 to 7 ‰). Therefore, the C is dominantly derived from the Tambien Group. The calculated oxygen isotope signature (18O(VSMOW) values between 5.1 to 8.7 ‰) of a hydrothermal fluid in equilibrium with calcite is compatible with a fluid of magmatic or metamorphic origin. The 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values of sulfides, which range between 36.94 to 36.99, 15.47 to 15.49, and 17.36 to 17.38 respectively, differ markedly from those of monzogranite dikes/sills. The data implies that the mineralization is unrelated to post-orogenic magmatism and that the Pb isotope signature of the sulfides represents the original Pb isotope signature of the metasedimentary and metavolcanic rocks. Overall evaluation of S, C, and Pb isotopic data permits to conclude that metals, sulfur, and other ore components are derived from the country rocks through leaching by metamorphic fluids. Petrographic studies of fluid inclusions show the presence of coexisting liquid-only, liquid-rich and vapour-rich two phase fluids. The fluid inclusions are predominantly secondary or pseudo-secondary in nature. Microthermometric measurments show that all fluid inclusions are of low-salinity (~1 to 8 wt. % NaClequiv.) aqueous types. Average homogenization temperatures of fluid inclusions in calcite are around 300ºC, i.e. close to temperatures found in orogenic gold systems. The mineralization at Workamba was likely generated by devolatilization and dehydration processes during metamorphism of the Tsaliet and Tambien Group rocks. The major collision orogeny which waned in Tigray at ~630 Ma might have caused eviction of the mineralizing fluids from deep regions. These mineralizing fluids were then channeled in the shear zone. Gold was presumably transported as Au(HS)2- complexes at intermediate oxidation states by near-neutral fluids, and sulfidation of host rocks is proposed as the likely gold deposition mechanism. The sub- to greenschist facies metamorphism of the Tambien and Tsaliet Group rocks, the enrichment in Sb, and As, the brittle-ductile shearing experienced by the rocks, and the mineralization under brittle conditions suggest a depth of formation between 4 to 6 km. Generally, the findings of this study suggest that the genesis of metallic mineralization at Workamba is of orogenic gold type and unrelated to post-orogenic magmatism in the area, although a close spatial proximity to post-orogenic dikes/sills exists. The age of the mineralization is estimated to be late Neoproterozoic.

The physical properties, viscosity, density, heat capacity and thermal expansivity, of relaxed supercooled liquids in the temperature range just above the glass transition have been determined for ten compositions along the compositional binary Na2O---SiO2, in the range of 2–45 mole% Na2O, by a combination of scanning calorimetry, dilatometry and micropenetration viscometry. The viscosity, density, heat capacity and thermal expansivity in the glassy state have also been determined. The heat capacities illustrate a linear composition dependence for the glassy state and a smooth but strongly non-linear composition dependence for the supercooled liquid state. The thermal expansivities were determined by dilatometry up to the glass transition and, by a normalized comparison of relaxation behavior in the glass transition interval, to temperatures 50°C above the glass transition. The expansivity is a linear function of the molar composition in the glass but a strongly non-linear function of molar composition in the supercooled liquid. The viscosity data just above the glass transition temperature, combined with data from high temperature using the concentric cylinder method, illustrate that the composition dependence of viscosity is strongly non-linear and exhibits an inflection as a function of composition. The glass transition temperature, taken as the peak temperature of the calorimetric measurements, is not in general an isokom in this system. The data for these property determinations in the Na2O---SiO2 system provide much improved constraints on the partial molar properties of SiO2 liquid and partial molar properties of the SiO2 component in silicate melts. The complex behavior of the transport properties, i.e. the glass transition temperature and the viscosity, point to complexities in viscous flow beyond that of simple binary mixing of the Na2O and SiO2 components.

The densitiesa nd thermal expansivitieso f boron-bearingh aplogranitic glassesa nd Iiquids have been determined using a combination of scanning .florimetry and dilatomelry. B2O3 reduces the density of haplogranitic liquids (at 750'C) from 2.295 t 0.006 g cm-r to 2.237 + 0.005 g cm-3 wirh the addition of 8.92 wt. Vo 82o,. These densities have been converted into molar volumes in the binary system haplogranite - BrO3. The partial molar volume of 8203, calculated from a linear fit to the data at 750oC, is ,10.30 + 0.77 cmr mole-r in these melts. This value compares with a molar volume of pure B2O3 at this temperature of M.36 x. 0.22 cm3 mole-l (Napolitano et ol. 1965), indicating a negative excess volume of mixing along the haplogranite - B2O3 join. In comparison, at l3moc, the addition ot Na2O to B2O3 reduces the panial molar volume of B2O3 from 46.6 to 32.3 cm3 mole-r ar 45 molego Na2O (Riebling 1966).T he densityr esultsr eported here, along with the viscosity-reducinge ffect of B2O3o n granitic melts (Dingwell et al, 1992),s hould both significantlya cceleratep rocesseso f crystal-melt fractionation and facilitate the evolution of extremely fractionated igneous systems.

Na2O---B2O3---SiO2 and Na2O-Ga2O3 viscosities decrease in the order aluminosilicate > ferrosilicate, (galliosilicate ?) > borosilicate. Thp electronegativities of the trivalent cations are inversely correlated with the relative viscosities of melts in these systems. Similarly, the electronegativities of network-stabilizing cations are inversely correlated with melt viscosity for alkali and alkaline-earth aluminosilicate melt systems. The variation in the viscosity of tectosilicate melts is correlated with estimated average T-O-T bond angles, and exothermic heats of solution of quench glasses. Structural controls of viscosity discussed are tetrahedral ordering and relative bond strengths. The acmite component in natural, peralkaline, silicic volcanics will not contribute directly to high melt viscosities for these lavas.

The densities of 12 melts in the Na2O-FeO-Fe2O3-SiO2 system have been determined in equilibrium with air, in the temperature range of 1000–1500°C, using the double bob, Archimedean technique. Ferrous iron determinations of 100–200 mg samples, “dip” quenched from high temperature, indicate that all the melts investigated were highly oxidized under these experimental conditions. 57Fe Mössbauer spectra of glasses obtained by drop quenching 80 mg melt samples from loop equilibration runs yield Fe3+/Fe2+ data equivalent to that for the densitometry (dip) samples for all but the most viscous melt, and confirm that all but one melt equilibrated with air during the densitometry measurements. Melt densities range from 2.17 to 2.88 g/cm3 with a mean standard deviation (from replicate experiments) of 0.36%. Least squares regression of the density data at 1300, 1400 and 1500°C, was calculated, both excluding and including excess volume terms (herein named linear and nonlinear fits, respectively) and the root mean squared deviation (RMSD) of each regression was compared with the total experimental error. The partial molar volumes computed for linear fits for Na2O and SiO2 are similar to those previously reported for melts in the Na2O-Al2O3-SiO2 system (Steinet al., 1986). The partial molar volumes of Fe2O3 obtained in these linear fits are equal to those obtained by Shiraishi et al. (1978) in the FeO-Fe2O3-SiO2 system but 5 to 10% lower than reported by Mo et al. (1982) in multicomponent melts. The partial molar volume exhibited by Fe3+ in this system is representative of the partial molar volume of tetrahedrally coordinated Fe3+ in silicate melts.

The viscosities of two melts in the system Na2O-FeO-Fe2O3-SiO2 have been measured as a function of oxidation state. The experiments were conducted by concentric-cylinder viscometry, on melts equilibrated with gas mixtures in a vertical tube, gas-mixing furnace. Viscosity determinations were made during stepwise reduction and oxidation of the melts. 57Fe Mössbauer spectra were obtained on quenched melt samples recovered during the viscometry experiments. In addition, a series of loop fusion experiments were performed at calibrated ƒO2 values in order to relate viscosity determinations directly to ƒO2. The viscosities of acmite and NS4F40 (Na-rich and Si-poor relative to acmite) melts decrease with reduction of Fe in the melts, as nonlinear functions of , yielding a region of viscosity invariance at moderate to low values of (

The effect of fluorine on melt viscosities of five compositions in the system Na2O-Al2O3- SiO2h as been investigateda t one atmospherea nd 1000-1600'Cb y concentric-cylinder viscometry. The compositions chosen were albite, jadeite and nepheline on the join NaAlOlSiO2 and two others of the join at 75 mole percent SiO2, one peralkaline and one peraluminous. All melt viscosities were independent of shear rate over two orders of magnitude, indicating Newtonian behavior. All viscosity-temperature relationships were Arrhenian within error. Fluorine reduces the viscosities and activation energies of all melts investigated. The viscosity-reducing power of fluorine increases with the SiO2 content of melts on the join NaAlO2-SiO2 and is a maximum at Na/Al (molar) = I for melts containing 75 mole percent SiO2. Fluorine and water have similar effects on aluminosilicate melt viscosities, probably due to depolymerization of these melts by replacement of Si-O-(Si, Al) bridges with Si-OH and Si-F bonds, respectively. Evidence from slag systems shows that fluorine also reduces the viscosity of depolymerized silicate melts. The viscous flow of phonolites, trachytes and rhyolites will be strongly afected by fluorine. It appears that fluorine contents of igneous rocks may be combined with water in calculation schemes for determining the viscosity of natural melts.

Sun, 1 Jan 1984 12:00:00 +0100 http://epub.ub.uni-muenchen.de/5974/ http://epub.ub.uni-muenchen.de/5974/1/5974.pdf Dingwell, Donald B.; Harris, D.; Scarfe, Christopher M. Dingwell, Donald B.; Harris, D. und Scarfe, Christopher M. (1984): The solubility of water in melts in the system SiO2-Al2O3-Na2O-K2O at 1 to 2 kbars. In: Journal of Geology, Vol. 92: pp. 387-395. Geowis