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Gebremariam, Solomon (2009): Nature and characteristics of metasedimentary rock hosted gold and base metal mineralization in the Workamba area, central Tigray, northern Ethiopia. Dissertation, LMU München: Fakultät für Geowissenschaften



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.