Distribution and Behaviour of Rare Earth Elements in the Auriferous Volcano-Genic Massive Sulphide Deposits and Host Rocks of the Ariab District, Red Sea Hills, Sudan

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Ibrahim, Samia Abdel Rahman
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The Ariab district, Red Sea Hills region of NE Sudan comprises numerous Late Proterozoic polymetallic sulphide bodies of exhalative origin. Major minerals making the massive sulphide ore bulks show diversity in textures and accordingly, the sulphide minerals are divided in general into facies that reflect environments of deposition that differ in space and time. The sequence of ore textures suggests deposition growth in a hydrothermal system with increasing thermal intensity that continued outward displacing faices 1 and 2 sulphides. Chlorite-sulphide-oxides equilibria, the mineral paragenesis, and fabric of massive sulphide phases all show the sulphide ores to have been affected by greenschist regional metamorphism at temperature between 315a02;, and 350a02;C, and pressures 2.6 to 3.2 Kb range. The hydrothermal alteration has converted the wall rocks into quartz-sericite-chlorite assemblage (quartz-sericite zone) and chlorite ± quartz (chlorite zone) occurring as a simple assemblage in both the footwall and the flanks of the orebodies. The alteration temperatures estimated with the chlorite geothermometer are from 239.81° to 285.81° and from 300.117a02; to 394.110°C for quartz sericite and chlorite zones, respectively. REE study of Ariab massive sulphides has illustrated a positive Eu anomaly preserved as the massive sulphides subjected to greenschist grade of metamorphism and subsequent supergene weathering. Gossans with positive Eu anomalies must be the products of supergene weathering of massive sulphides exhibiting a similar positive Eu anomaly. The positive Eu anomalies in hydrothermally altered schists and gossans can be used as a guide to massive sulphide mineralization discoveries. The process of gossanization can lead to an almost complete loss of ore metals (especially Cu and Zn) from the gossan, so the preservation of the positive Eu anomaly is a strong indicator of massive sulphide gossan or a signature for the influence of an intense hydrothermal metals-bearing sulphide system in the case of hydrothermally altered schists. A clear correlation between Eud and Pb has been established in massive sulphide ores. The observed REE patterns of Ariab mineral district rocks are best explained by an input of hydrothermal solutions to the site of chemical sedimentation. Because of the characteristic REE pattern and consistent stratigraphic positions of the chemical sediment zones the following sequence of events took place within the hydrothermal system: 1) Alteration of volcanic glass and or ferromagnesian minerals released Cu and Fe to the solution (Cu- rich sulphide); 2) Preferential alteration of feldspar by very concentrated brine enriched the solution in Pb, Zn, Ba, and heavy REE (silica barite rich); 3) Continued removal of Fe from the volcanic rocks without alteration of a large amount of the solid phases (iron-formation). From the geochemical characteristics of the Ariab VMS deposits, a simple model can be constructed: 1) low temperature hydrothermal processes produced the base metal massive pyrite with flat Eu anomaly; 2) high temperature hydrothermal processes produced massive pyrite with enrichment of Ba, Cu, Zn, Pb and Fe and the characteristic positive Eu anomaly; 3) subsequent uplift and erosion exposed the massive sulphide deposits and host rocks to oxygenated meteoric and groundwater; 4) intense supergene weathering altered the massive sulphide mineralogy, nevertheless, the REE profiles and positive Eu anomalies developed during hydrothermal deposition are preserved
246 Pages
Distribution, Behaviour, Rare Earth Elements,Auriferous Volcano-Genic,Massive Sulphide Deposits , Host Rocks,Ariab District,Sudan