Mineral potential of felsic plutonic rocks in the north-central Arabian Shield, Kingdom of Saudi Arabia

Abstract

Fourteen plutons of Late Proterozoie age potentially favorable for rare-element mineralization have been identified in a geochemical and petrographic assessment of felsie plutonie rocks in the north-central Arabian Shield (lat 23 000'-25 0 00' N., long 40°00'-45°00' E.). The plutons are highly fractionated, leucocratie granitoids assigned to a major magmatie pulse that spanned the later stages of the Hijaz orogeny (about 610 Ma to about 550 Ma). Most of the targeted plutons are small or not deeply eroded. Two rock types are dominant: subsolvus, muscovite-bearing monzogranite or syenogranite; and hypersolvus, mieroeline perthite granite commonly containing sodie pyriboles. Enrichment in varied suites of the granitophile elements (Sn-W-NbTa-Zr-Y-Th-U) is characteristic. The plutons occupy the central part of a broad arcuate belt of geochemieally specialized plutons that conforms generally to the eastern limit of exposed Proterozoie basement. The muscovite-bearing monzogranites containing anomalous tin and tungsten in rock and (or) wadi sediment samples occur generally east of long 42°30' E. Two of these, Jabal Minya and Jabal Khinzir, are recommended for immediate follow-up studies. Of the alkali granites, most of which occur in the area west of long 42°30' E. , the composite plutons of Jabal Hadb ad Dayahin and Jabal Tuqfah have the highest potential for rare-element mineralization and warrant prompt systematic investigation. Evaluation of isolated oneor two-element anomalies should be coordinated with current high-density geochemical prospecting programs of the Riofinex Geological Mission. I/ U.S. Geological Survey, Reston, Virginia 22092 ACKNOWLEDGEMENTS Semiquantitative speetrographie analyses were performed by All Bone in the DGMR-USGS Chemical Laboratory under the direction of K. J. Curry. Mohsin Mohamed Ahmed provided able assistance in sampling and panning. Ali Jabarti determined most of the stained-slab modes; the remainder were determined by Ahmed al Bazli, who also assisted in preliminary compilation of the geochemieal data. E. A. du Bray introduced the author to Kevex analytical procedures and offered patient advice on the statistical determination of elemental threshold concentrations. C. Pellaton and J. C. Quinquis, Bureau de Recherches Geologiques et Minieres (BRGM), were instrumental in providing draft copies of 1:100,000-seale geologic maps in the north-central part of the study area. Organization and presentation of the results were substantially improved by the thoughtful review of C. R. Ramsay, Directorate General of Mineral Resources (DGMR). The work on which this report is based was performed in accordance with a work agreement between the Saudi Arabian Ministry of Petroleum and Mineral Resources and the U.S. Geological Survey (USGS). INTRODUCTION This report presents the initial results of a geoehemieal and petrogaphie assessment of Late Proterozoie felsie plutonie rocks in the north-central Arabian Shield to determine their potential for economic concentrations of tin, tungsten, and rare-element mineralization. The study area (fig. 1) extends from lat 23°00' to 25°00' N. and long 40°00' to 45°00 f E. and lies in a part of the central Arabian Shield mapped in reconnaissance manner at a scale of 1:100,000 by geologists of BRGM. The area of approximately 100,000 km2 is bounded on the west by basalt fields (harrats) of Cenozoic age and extends eastward approximately 450 km, where the Proterozoie basement is unconformably overlain by sedimentary rocks of Phanerozoic age. It is a broad and gently east-dipping peneplained surface with a base level of approximately 1000 m above sea level. Exposures over large parts are obscured by a thin veneer of eolluvial and eolian deposits. This surface of subdued relief is interrupted by belts of metamorphosed layered rocks and felsie plutons having local relief of several hundred meters. The geologic fabric of the Proterozoie basement in the north-central Shield is dominated by northto northwesttrending sequences of metamorphosed volcano-sedimentary, and plutonie rocks. According to prevailing theories these sequences represent intraoeeanie magmatic ares that were