A geological and geochemical reconnaissance of the Tathlith one-degree quadrangle, sheet 19/43, Kingdom of Saudi Arabia

Abstract

The Tathlith one-degree quadrangle occupies an area of 11,620 sq km in the northeastern Asir region of the Kingdom of Saudi Arabia, in the southeastern part of the Precambrian shield. In the eastern part of the quadrangle the Precambrian rocks are covered by exposures of easterly-dipping sandstone of Cambrian or Ordovician age. A welldeveloped and highly integrated drainage system trending northward is worn into the Precambrian rocks, but for most of the year the wadis are dry. The Precambrian rocks of the quadrangle consist of an old, non-metamorphosed to variably metamorphosed sequence of volcanic and sedimentary rocks intruded by three main successions of plutonic and hypabyssal igneous rocks. The interlayered volcanic and sedimentary rocks occupy arcuate, north-trending fold belts in which old, rather tight north-trending folds have been refolded at least once by open folds with nearly east-trending axes. Old, northtrending left-lateral faults are associated with the fold belts and are themselves intersected by younger, northwest-trending faults. Motion on both sets of faults has been reactivated several times. An igneous differentiation suite ranging in composition from gabbro to quartz porphyry forms prominent homogeneous to zoned subcircular granitic plutons and stocks of gabbro and diorite in the older rocks. Pyroxene is present in some of the granitic rocks, and the suite itself is called the peralkalic magma series. Geochemically, the granites of the series are characterized by greater amounts of beryllium, gallium, lanthanum, niobium, lead, yttrium, and zirconium than the older granites, and magnetite from the series is typically enriched in zinc. These Precambrian rocks are overlain in the east by the Cambrian-Ordovician ferruginous Wajid Sandstone. Patches of uneroded saprolite are locally present on the surface of the Precambrian rocks under the sandstone. Earlier and more extensive weathering was a factor in supplying silt to certain valleys in the southern part of the area. Evidence for cyclic climate change is preserved in these silts. A series of interrelated research projects is needed to study the silts and other materials, including sabkhahs, Quaternary deposits, and associated anthropological, archaeological, and historic evidence, to evaluate the factors and processes of regional climate change in Arabia. Research of this type might also allow estimates to be made of the possibility for the existence of factors that could reverse the trend to regional aridity. The major ore deposit in the area is the Ash Sha'ib ancient mine for which G. H. Allcott (1970) estimated 2,170,000 tons of sulfide ore to a depth of 170 m worth $40 million for the contained zinc, copper, and silver. However, only $20 million of the ore was estimated to be worth $25 per ton or more; hence, the deposit was regarded as too small to exploit. Detailed geologic work is needed to determine if there are other similar deposits nearby. Chrysotile asbestos of good quality is present a few kilometers south of the quadrangle, an...

Composition of coarse-grained magnetite from pegmatite dikes related to plutons of quartz monzonite in the Jabal Lababa area, Kingdom of Saudi Arabia

“…The results of these analyses do show that niobium, tin, and yttrium are enriched in magnetite from the only part of the Jabal Lababa area where positive anomalies for these elements are present in other media. These results also show a remarkable continuity in values for copper, molybdenum, and zinc between this new work and the results of early investigations of the trace-element composition of detrital magnetite in Arabia (Overstreet and Rossman, 1970;Kahr and others, 1972;Overstreet, 1978).…”

Hamdah ancient gold mines, Kingdom of Saudi Arabia

“…USGS geologists returned to the Hamdah area in 1976 in connection with a comprehensive exploration of the Jabal Ishmas-Wadi Tathlith gold belt (Worl, 1979). Overstreet (1978) incorporated field data first acquired during reconnaissance exploration by the USGS in 1965 in a geologic map of the area north of 19° N. (Tathlith one-degree quadrangle) ( fig. 2).…”

“…2). The map by Overstreet (1978) includes the northern part of the Hamdah region and is an illustration in an account of asbestos and gold mineralization in the region. In 1976, the Hamdah group of mines was mapped at 1:10,000 scale and the Hamdah prospect was mapped at 1:2,000 scale (Helaby and Worl, 1980).…”

“…However, because such issues of regional extent have limited relevance to the immediate objectives of exploration for gold at the Hamdah prospect, they are not further discussed. (For more information on the tectonic setting of the region see Schmidt and others, 1978;Overstreet, 1978;Greenwood and others, 1982;Johnson and Vranas, 1984;Greenwood, 1985aGreenwood, , 1985bStoeser and Camp, 1985;Bookstrom and others, 1989).…”

“…A similar deposit is found in the constricted part of Wadi Tathlith upstream from Hamdah. According to Overstreet (1978), some silt was also deposited in historic times upstream from weirs or constrictions made e irrigated agricultural purposes. On the basis of a st and structure, G.O.…”