The Anatomy of Gold-Bearing Juvenile Neoproterozoic Crust (~870–550 Ma) in the Egyptian Segment of the Nubian Shield

Abstract

There is a wide range of styles of gold mineralization in the juvenile crust of the Eastern Desert of Egypt that formed during the late Neoproterozoic time through the accretion of intra-oceanic arcs. These styles include exhalative, orogenic, intrusion-related and placer gold deposits. Gold mineralization of these different styles was formed during distinct epochs of the crustal evolution of the Egyptian segment of the Arabian-Nubian Shield (ANS), with size (where gold occurs as the primary commodity or as an accessory), type (i.e. the vein-type or disseminated), tectonic setting (i.e. compressional or tensional) and nature of mineralization (i.e. syngenetic or epigenetic) varied among these styles. At ~750 Ma, exhalative gold-bearing deposits attributed to volcanically derived hydrothermal fluids were formed. This style of mineralization is represented by auriferous Algoma-type Banded Iron Formation (BIF) localized in the central parts of the Eastern Desert with the exception of two occurrences at Semna and Abu Marawat further north and Canadian-type auriferous Volcanogenic Massive Sulphide Deposits (VMSDs) distributed in the southern parts of the Eastern Desert. Orogenic gold deposits were formed in two successive epochs. The first epoch (~700–650 Ma) was linked to the collision of island arcs and is concomitant with the emplacement of calc-alkaline magmatism that was a major fluid source for the mineralizing fluids. Deposits of this epoch are represented by listwaenite (i.e. silicified and carbonatized rock) associated with ophiolitic serpentinites in thrust contacts, nappe complexes marking suture zones between terranes, and the auriferous quartz veins hosted in a metavolcano-sedimentary assemblage and/or I-type calc-alkaline granites that surround them. Some of the gold deposits in the southern part of the Eastern Desert particularly in the Allaqi district and some deposits in the central Eastern Desert were formed in this epoch. Orogenic gold deposits of the second epoch are linked to the accretion of the composite arc terrane against the eastern flank of West Gondwanaland around ~650–630 Ma coeval with the beginning of Gondwana assembly. During this epoch, regional metamorphism at greenschist–amphibolite facies took place associated with a pronounced, but short magmatic pulse between ~635 and ~630 Ma that was a major fluid source for mineralization and a major cause for the regional metamorphism. Deposits formed within this epoch are represented by hydrothermal–vein–type crossing granites of this distinct magmatic pulse and/or the surrounding supracrustal sequence of the ophiolitic and island arc assemblage. The ~635–630 Ma magmatic pulse represents a transition from orogenic gold deposits to intrusion-related deposits. This is consistent with the studies that highlight the broad spatial and temporal relationship between orogenic gold deposits and intrusion-related deposits. The transition of magmatism from calc-alkaline to K-rich calc-alkaline and alkaline magmatism during ANS crustal tectonomagmatic evolution has been explained in terms of lithospheric delamination processes. Intrusion-related deposits evolved with time from intrusion-related gold deposits with minor rare metal mineralization formed at 610–590 Ma, contemporaneously with the deposition of the Hammamat molasse sediments and eruption of the late-orogenic Dokhan volcanics, to intrusion-related rare metal deposits with minor gold linked to alkaline to peralkaline magmatism emplaced in the maturation stage (~580–550 Ma) and later. Placer gold deposits were exploited by ancient Egyptians, with most of these deposits located close to auriferous quartz veins.