Geology and regional setting of the Al Masane ancient mine area, southeastern Arabian Shield, Kingdom of Saudi Arabia

Abstract

Stratiform zinc-copper massive-sulfide deposits at Al Masane occur in thin dolomitic interbeds within Proterozoic felsic crystal tuff and mafic flows and volcaniclastics. These strata dip steeply westward and are underlain by shale and shaly graywacke to the east and overlain by lapilli crystal tuff to the west. This section is part of the Habawnah fold or mineral belt that extends from the Wadi Wassat area southward into Yemen. Western parts of the Habawnah fold belt, including the Al Masane area, are characterized by a bimodal assemblage of of phenocryst-poor basalts and sodic rhyolite crystal tuff, and by zinc-copper mineral deposits. Strata in the eastern part of the belt, mostly east of the Ashara fault zone, contain abundant phenocryst-rich mafic volcanic rocks, little felsic crystal tuff, and barren or locally nickeliferous massive pyrite deposits. Stratified rocks and gabbro sills of the Al Masane area were isoclinally folded and metamorphosed to the greenschist facies. Structural analysis indicates that foliation, lineation, joints, major and minor folds, and one of three sets of faults formed during a single east-west compressional deformational episode. Axial-plane foliation dips steeply westward and is usually coincident with bedding. Lineation plunges steeply to the northwest throughout the map area and is parallel to the intersection of joints of two major sets and to the intersection of these joints with foliation. Major folds have north-trending, gently plunging axes to which the axis of a minor fold set is essentially parallel. A second minor fold-set has a steeply plunging axis parallel to lineation. The Saadah massive-sulfide body is elongated by a fold of this system. Unmetamorphosed Proterozoic felsic sills, quartz monzonite and gabbro plutons, porphyritic diorite dikes, mafic dikes, and basalt dikes, in that apparent order, have intruded the stratified section. Basalt dikes are offset by southwestward-dipping reverse faults and by faults parallel to bedding. Malachite and metal-oxide-bearing gossans in the Al Masane area were mined in ancient times. Radiocarbon dating of charcoal in slag indicates that the ores were smelted about 1,200 years ago. Gossans are depleted in copper, silver, and zinc, relative to underlying sulfide, by factors of about 0.33, 0.2, and 0.08, respectively. Evaluation of numerous analyses of gossans, carbonates, and silicate rocks indicates that exploration for zinc-copper massive-sulfide deposits in the region should center on detailed geochemical studies of dolomitic beds. Dolomitization of host siliceous shales and tuffs and the formation of the massive-sulfide bodies at Al Masane probably caused by submarine fumarolic hydrothermal activity during periods of relative volcanic quiescence. Further work in the Al Masane region should include a study of the dolomitized areas and a search for other potential alteration "pipe" products (e.g. chlorite) as a guide to location of proximal deposits. Zinc-copper deposits at Al Masane and in the Dhahar-Al Hajrah and Kutam-Farah Garan areas have characteristics in common and are associated with felsic volcanic centers in the western bimodal part of the Habawnah mineral belt. They probably have parallel origins related to sea-floor volcanism. Deposits at Al Masane and Dhahar-Al Hajrah are possibly in the same general stratigraphic interval. Characteristics and distribution of altered rocks at Kutam suggest that the observed structural control of mineralization may represent remobilization. Chemical characteristics of volcanic rocks at Al Masane and elsewhere, along with features such as zinc-copper-iron sulfide mineralization, rhyolite-basalt bimodality, and the quartz phenocryst-rich nature of the felsic rocks, are compatible with an unusually primitive tholeiitic island-arc origin for the strata and mineral deposits of the Habawnah mineral belt.