Gold in Volcanogenic Massive Sulfide Deposits of Egypt
摘要
Volcanogenic Massive Sulfide (VMS) deposits are stratiform accumulations rich in base metal sulfides such as copper, zinc, and lead, and they often contain significant amounts of gold as well, constituting a significant portion of the world's primary gold reserve (thirteen world-class > 30 t Au). Extracting gold from VMS deposits is crucial for the economic development of nations. Given gold’s high economic and social-political importance, ensuring sustainable production is imperative and expansion of the gold supply is necessary. Gold in VMS deposits ranges from invisible nano-scale particles to visible disseminated micro grains and large native forms. In the Arabian–Nubian Shield (ANS), prolonged subduction of converging plates from the Mozambique Ocean resulted in the formation of island arcs with episodic magmatic-hydrothermal systems. Over approximately 300 million years of ocean closure, several gold-rich VMS districts were formed within the juvenile Neoproterozoic crust, including notable examples such as Hamama in Egypt, Ariab in Sudan, Jabal Sayid in Saudi Arabia, Asmara in eastern Eritrea, and Bisha in western Eritrea. The Eastern Desert of Egypt (Northwest ANS) features two main VMS districts: (a) Wadi Hamama district, a recently discovered gold-rich VMS deposit with an estimated ore reserve of more than 12 Mt, and (b) Shadli metavolcanic district, which is recognized as Egypt's largest VMS deposit with significant Au and Ag anomalies and which has received minimal attention throughout the last quarter of the twentieth century. This chapter provides a concise summary of current information on Egyptian VMS deposits, delving into detailed discussions on geological characteristics, deposit-scale features, ore assemblages, alteration patterns, and distribution and forms of gold. The Egyptian VMS deposits are located in the Neoproterozoic arc assemblage sequence. These deposits, predominantly composed of volcanic-sedimentary units (bimodal-felsic type), have undergone variable alteration, metamorphism, and deformation. Ore bodies in these deposits primarily show layered structures and are characterized by an upper oxide-gossan zone, middle massive sulfide ore bodies, and lower mineralized stockwork-chloritic-sericitic alteration zone. The massive ore bodies are predominantly distributed along conformable horizons between felsic lava and pyroclastics. Microscopic observation and mineral chemistry data confirm gold occurrence as traces and nanometer- to micron-sized inclusions scattered in electrum, pyrite, galena, sphalerite grains, and Ag–Te assemblages. Additionally, gold is found as native grains, filling interstitial spaces in the host sulfide structure. The concentration of gold is notably high on the top of the iron-oxide-gossan zone and talc-carbonate layer, with varying amounts and grades being recorded in the massive ore bodies and stockwork zone. The Hamama and Shadli regions are renowned historical gold mines, where exploration ceased in the last century due to low reserves. However, a recent exploration program by the Aton Canadian company in the ancient Hamama mining area has revealed new reserves in the central part of the Eastern Desert. Comparatively, the ongoing exploration of the Hamama gold-rich VMS deposit, in conjunction with Um Samiuki, Hilgit, Maaqal, Derhib, Abu Gurdi, Um Selimat, and El Atshan polymetallic prospects, represent promising sites for prime exploration targets in the South Eastern Desert of Egypt. Both regions shear similar geological backgrounds, polymetallic nature, ore characteristics, and mineralization styles. Furthermore, the identification of silver-bearing minerals such as cervelleite, hessite, acanthite, freibergite, mckinstryite, benleonardite, stephanite, and native silver suggests that mineralization occurred under high ƒTe2 conditions, revealing the suitable condition for Au–Ag complexes. Consequently, the Shadli metavolcanic district of the South Eastern Desert still possesses economically significant blind ore bodies of Zn–Cu–Pb deposits. Exploration utilizing advanced techniques is recommended, accompanied by comprehensive geological and geophysical studies, to unlock the full potential of these VMS deposits.