Geology of the Ar Rahail ancient gold mine, Kingdom of Saudi Arabia

Abstract

The Ar Rahail ancient gold mine is a collection of ancient workings in metasedimentary rocks of the Murdama Group adjacent to, and above the flanks of the Rahail stock. Injection of mafic dikes of regional origin was followed by emplacement of the Rahail stock, which consisted of early dikes of porphyritic biotite-bearing hornblende granodiorite, and a later cupola of porphyritic monzogranite. Crystallization of the cupola produced a molybdenum-bearing quartz vein stockwork. Intrusion-heated fluids developed chlorite and sericite alteration zones in the older granodiorite dikes. Pre-existing northwest-trending faults, possibly reopened by stock emplacement, were invaded by later fluids that precipitated barren quartz veins and, in the adjacent faulted wall rocks, anomalous gold and arsenic. Gold, however, is restricted to the narrow structures, and, although values as much as 4.2 g/t are present, the tonnages are inadequate for profitable mining. No further work is recommended, because the hoped for dissemination of gold between faults does not exist. Polished thin sections and fused rock glasses were prepared by the USGS mineralogy-petrology laboratory under the supervision of C. T. Thornber. Between 2000 and 2500 points were counted per section. R. Kamilli and C. Meissner provided stimulating manuscript comments. This study was performed in accordance with a work agreement between the U.S. Geological Survey and the Saudi Arabian Ministry of Petroleum and Mineral Resources as subprojects 3.01.61 and 3.11.05. METASEDIMENTARY ROCKS OF THE MURDAMA GROUP The metasedimentary country rock at the Rahail ancient mine site is mostly interlayered light-gray siltstone and gray-green claystone, much finer grained than rocks of the same age in adjacent areas (Cole, 1985) (pi. 1). Bedding is best This terminology is supported by the close comparison between the whole rock chemistry of Rahail rocks and that of the average granodiorite and adamellite of Nockolds (1964, p. 1014), respectively (table 1). The adamellite of Nockolds would most closely approximate the monzogranite of Streckeisen (1976, p. 12). Differences, such as the lower FeO, MgO, CaO, and higher Na2O in the cupola as opposed to the average adamellite, may be explained by the albitization of oligoclase and destruction of mafics in Rahail samples.