International Geologiical Congress - Oslo 2008

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MRD-01 General contributions to mineral deposits

 

Metasedimentary rock hosted Au-base metal mineralization in the Workamba area, central Tigray, northern Ethiopia

 

Solomon Gebresilassie, Ludwig-Maximilians University (Germany)
Robert Marschik, Ludwig-Maximilians University (Germany)
Stefan Hölzl, Bayerische Staatssammlung für Paläontologie und Geologie (Germany)
Kibret Sifeta, National Mining Corporation (Ethiopia)
 

 

Exploration has outlined a shear-zone controlled Au-base metal rich zone (estimated resources of about 2 t Au) within lower greenschist facies talc-chlorite and quartz-carbonate-rich chlorite schists at Workamba, Tigray province. The prospect forms part of the southern Arabian-Nubian Shield (ANS) and is composed of sheared metasedimentary and metavolcanic rocks, which are intruded by I-type, calc-alkaline granitoids. NE-trending shear zones and foliation, and NW-oriented brittle faults are the most prominent structural features. Two types of quartz veins are present in the area. An older generation defines parasitic folds in folded metasedimentary rocks. A younger generation occurs as NE-trending swarm parallel to the foliation, or along the NW-trending brittle faults cutting the folded rocks and foliation. Quartz-carbonate veins are present in the mineralized shear zone. They cut the foliation and may locally contain sulfide minerals. Most of the Au and base metals, however, occur in discontinuous veinlets or dispersed within the sheared talc-chlorite and quartz-carbonate-chlorite schists. The Au concentrations in drill core composite samples are up to around 8 ppm. Pyrite, sphalerite, and galena are the main sulfide phases. Chalcopyrite, pyrrhotite, neodigenite, and arsenopyrite are minor. The sulfides post-date shear deformation. Quartz and quartz-carbonate veins contain one-phase (liquid-only), and two-phase (liquid-rich or vapour-rich) fluid inclusions, which occur mainly as planar trails suggesting a secondary or pseudosecondary origin. Inclusions in mineralized samples yield ice melting temperatures from -0.5 to -3 °C and salinities between 1 and 5 wt.% NaClequiv.. Homogenization temperatures of the liquid-rich fluid inclusions average 330 °C, which we take as proxy for the mineralization temperature. Sulfides have δ34S(VCDT) between 1.8 and 9.1 ‰, suggesting sulfur derived from the host rock sequence with or without a magmatic sulfur contribution. These values are similar to that of orogenic gold deposits elsewhere. Lead isotope signatures of galena, sphalerite and pyrite indicate an oceanic crust or depleted mantel Pb source. The Pb isotope values plot in the same field as those of galenas from gold-quartz vein or VHMS deposits of the northeastern part of the ANS. Hydrothermal calcite yield δ18O(VPDB) values between -19.1 and -15.5 ‰ and δ13C(VPDB) values between -5.6 and +1.8 ‰. The calculated stable isotope composition of a fluid in equilibrium with the hydrothermal calcite at a temperature of 330 °C, as assumed for the mineralization, would have δ18O(VPDB) values between -23.9 and -20.3 ‰ and δ13C(VPDB) values between -3.3 and +4.1 ‰. This stable isotope signature is compatible with a metamorphic fluid source. Setting, host rocks, mineralogy, and isotope geochemical data are in favor of an orogenic gold deposit model to explain the nature and characteristics of the Au and base metal mineralization at Workamba.

 

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