Mineral deposits of the northwestern Hijaz quadrangle, Kingdom of Saudi Arabia

A reconnaissance of portions of the Northwestern Hijaz quadrangle in the Kingdom of Saudi Arabia was made during 1964 and 1965 as part of a mineral survey of the Precambrian crystalline rocks of the country. The survey is being made under the terms of an agreement between the Saudi Arabia Ministry of Petroleum and Mineral Resources and the United States Geological Survey. Ancient gold and copper mines occur in the area, and deposits of iron minerals, chromite, copper minerals, asbestos, magnesite, gypsum, and glass sand have been discovered in recent years.

The land surface of the quadrangle consists of a narrow coastal plain, a mountainous belt, and a plateau. The mountainous bolt is made up of Precambrian rocks that include three major stratigraphic units separated by unconformities. The two older units are slightly metamorphosed and are intruded by plutonic rocks of many kinds. Granitic rocks predominate but intrusions of syenite, diorite, gabbro, and peridotite are known. Dikes are abundant throughout the mountainous area. The Precambrian rocks have been folded on north- to northwest-trending axes. A major northwest-striking wrench fault zone crosses the central portion of the quadrangle. North-, northeast-, and east-trending faults are locally prominent.

The plateau lies in the northeastern part of the quadrangle. It is made up of unmetamorphosed gently dipping sandstone of Paleozoic age overlain in part by flood basalts of Tertiary to Quaternary age. Sedimentary rocks of Miocene(?) age crop out in the coastal area.

Gold was mined in the area during the eighth and ninth centuries A. D. The ancient gold mines were reexamined in the 1930s but wore found to be too small and too low grade for mining. Exploration for minerals other than gold began in 1950.

The present work consisted of the examination of geological features that are potentially favorable for the presence of mineral deposits such as intrusive contacts, fault zones, quartz veins, and hydrothermally altered areas. Samples of wadi sediment were collected in areas that appeared most favorable. The samples were analyzed spectrographically for trace amounts of 27 elements to confirm the presence or absence of mineralization.

Massive magnetite bodies that range in size from a few hundred to a few thousand tons wore discovered in the course of the work as well as widely scattered traces of secondary copper minerals. The wadi samples disclosed several areas that contain from 5 to 10 times the average trace amounts of base metals and molybdenum; these areas should be prospected in more detail.