Alteration Patterns Related to Hydrothermal Gold Mineralizaition in Meta‐andesites at Dungash Area, Eastern Desert, Egypt

Abstract

: The hydrothermal alteration patterns associating with the gold prospect hosted by metavolcanics in the Dungash area, Eastern Desert of Egypt, were investigated in order to assign their relationship to mineralization. The metavolcanics of andesitic composition are generated by regional metamorphism of greenschist facies superimposed by hydrothermal activity. Epidote and chlorite are metamorphic minerals, whereas sericite, carbonates, and chlorite are hydrothermal alteration minerals. The auriferous quartz vein is of NEE‐SWW trend and cuts mainly the andesitic metavolcanics, but sometimes extends to the neighbouring metapyroclastics and metasediments. Quartz‐sericite, sericite, carbonate‐sericite, and chlorite‐sericite constitute four distinctive alteration zones which extend outwards from the mineralized quartz vein. The quartz‐sericite and sericite zones are characterized by high contents of SiO2, K2O, Rb, and As, the carbonate‐sericite zone is by high contents of CaO, Au, Cu, Cr, Ni, and Y, and the chlorite‐sericite zone is by high contents of MgO, Na2O, Zn, Ba, and Co. Gold and sulphide minerals are relatively more abundant in the carbonate‐sericite zone followed by the sericite one. The geochemistry of the alteration system was investigated using volume‐composition and mass balance calculations. The volume factors obtained for the different alteration zones, mentioned above (being 1.64, 1.19, 1.17, and 1.07, respectively), indicate that replacement had taken place with a volume gain. The mass balance calculations revealed addition of SiO2, K2O, As, Cu, Rb, Ba, Ni, and Y to the system as a whole and subtraction of Fe2O3 from the system. Initial high aK+ and aH+ for the invading fluids is suggested. As the fluids migrated into wallrocks, they became more concentrated in Mg, Ca, and Na with increasing activities of CO2 and S. The calculated loss‐gain data are in agreement with the microscopic observations. Breakdown of ferromagnesian minerals and feldspars in the quartz‐sericite, sericite, and chlorite‐sericite zones accompanied by loss in Mg, Fe, Ca, and Na under acidic conditions and low CO2/H2O ratio may obstruct the formation of carbonates and sulphides, and the precipitation of gold in these zones. The role of metamorphic fluids in the area is expected to be restricted to the liberation of Au and some associated elements from their hosts.

The use of ASTER data and geochemical analyses for the exploration of gold at Samut area, South Eastern Desert of Egypt

“…Furthermore, Almond et al (1984) and Hassaan and El Mezayen (1995) suggested the derivation of the ore fluids by metamorphism during ophiolite and/or island arc tectonic stages. This is consistent with Boyle (1979) who proposed a greenschist metamorphogenic model for the origin of gold (Helba et al 2001). Botros (2004) envisaged a combined metamorphic-magmatic origin for these fluids.…”

Mineral chemistry, bulk rock geochemistry, and S‐isotope signature of lode‐gold mineralization in the Bétaré Oya gold district, south‐east Cameroon

“…The veins depict alteration processes such as silicification, sulphidation, haematitization, carbonatization, sericitization, and K‐feldspar alteration (Figures ) that vary from the core zone to the selvages. According to Helba, Khalil, and Abdou (), gold and sulphide mineralization is particularly abundant in carbonate–sericite‐rich zones for intrusion‐related gold deposits. Both ore and gangue minerals define the margins of the veins.…”

“…Using ternary discrimination plots (Au–Ag–Cu and Au–Ag–Hg), the composition of the gold grains from Bétaré Oya was investigated (Figure ). Compositional fields (Chapman & Mortensen, ; Helba et al, ; Townley et al, ) were included in order to constrain the gold formation process and mineral system. A match to magmatic–hydrothermal vein‐type gold is suggested here for the Bétaré Oya gold grains (Figure ).…”

Behavior of Major and Trace Elements during Ore Deposition: Example from the Low‐Sulfidation Pantingan Gold System, Mount Mariveles, Bataan, Philippines

“…Signifi cant changes in SiO 2 in the samples can be attributed to silica inundation or silica leaching. Gains in Fe 2 O 3 + MgO accompanied by loss in CaO + Na 2 O are being attributed to the breakdown of plagioclase ( Helba et al , 2001;Querubin & Yumul, 2005 ). In contrast, the increase in CaO + Na 2 O suggests the possible occurrence of calcite.…”