Mineral Potential of Felsic Plutonic Rocks in the North-Central Arabian Shield, Kingdom of Saudi Arabia By
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DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Mineral potential of felsic plutonic rocks in the north-central Arabian Shield, Kingdom of Saudi Arabia by !/ William J. Moore Open-File Report 85- Prepared for the Ministry of Petroleum and Mineral Resources, Deputy Ministry for Mineral Resources, Jiddah, Kingdom of Saudi Arabia This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature, I/ U.S. Geological Survey, Reston VA 22092 1985 CONTENTS Page ABSTRACT................................................ 1 ACKNOWLEDGEMENTS........................................ 2 INTRODUCTION............................................ 2 PAST WORK............................................... 4 OBJECTIVES.............................................. 5 METHODS................................................. 6 FELSIC PLUTONIC ROCKS................................... 10 Lithology........................................... 10 Distribution........................................ 12 Geochronology....................................... 13 Geochemistry........................................ 19 Radiometric responses............................... 24 DISCUSSION.............................................. 26 RECOMMENDATIONS......................................... 29 DATA FILE............................................... 29 APPENDIX................................................ 34 REFERENCES CITED........................................ 39 ILLUSTRATIONS Figure 1. Index map showing location of study area...... 3 2. Map showing distribution of felsic plutonic rocks in the north-central Arabian Shield... 9 3. Ternary diagram showing modal proportions of quartz, alkali feldspar, and plagioclase in felsic plutonic rocks of the north-central Shield...................................... 11 4. Map showing radiometric ages of plutonic rocks in study area......................... 15 5. Map showing sample locations and concentra tions of selected elements in geochemically anomalous samples of felsic plutonic rocks of the north-central Shield.... 23a 6. Map showing sample locations and concentra tions of selected elements in geochemically anomalous samples of felsic plutonic rocks from Jabal Hadb ash Sharar and Jabal Hadb ad Dayahin.................................. 23b 7. Map showing sample locations and concentra tions of selected elements in geochemically anomalous panned concentrates of wadi sediment derived from felsic plutons of the north-central Shield........... 23c 8. Total-count scintillometer responses of felsic plutonic rocks of the north-^ central Shield................. ^ 25 9. Total count scintillometer responses of felsic plutonic rocks from Jabal Hadb ash Sharar and Jabal Hadb ad Dayahin........ 27 10. Map showing Shieldwide mineral occurrences and prospects associated with postorogenic granites, and geochemically anomalous plutons identified in this study............ 33 TABLES Table 1. Radiometric ages of plutonic rocks in the north-central Arabian Shield................ 16 2. Summary of mineralization types and potential prospecting targets in the felsic plutonic rocks of the Arabian Shield................. 20 3. Elemental geometric means of semiquantitative spectrographic data for rock and panned concentrate samples......................... 21 4. Threshold concentrations of semiquantitative spectrographic data for rock and sediment samples..................................... 22 5. Features of potentially mineralized felsic plutons in the north-central Arabian Shield...................................... 30 ii MINERAL POTENTIAL OF PELS 1C PLUTONIC ROCKS IN THE NORTH-CENTRAL ARABIAN SHIELD, KINGDOM OF SAUDI ARABIA by William J. Moorel/ ABSTRACT Fourteen plutons of Late Proterozoie age potentially favorable for rare-element mineralization have been identi fied 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 monzo- granite or syenogranite; and hypersolvus, mieroeline per- thite granite commonly containing sodie pyriboles. Enrich ment in varied suites of the granitophile elements (Sn-W-Nb- Ta-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 war rant prompt systematic investigation. Evaluation of isolat ed one- or 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 determin ed by Ahmed al Bazli, who also assisted in preliminary com pilation 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 substan tially 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 geoehem- ieal and petrogaphie assessment of Late Proterozoie felsie plutonie rocks in the north-central Arabian Shield to deter mine 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. Expo sures 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 north- to northwest- trending sequences of metamorphosed volcano-sedimentary, and plutonie rocks. According to prevailing theories these se quences represent intraoeeanie magmatic ares that were 36 Ad Da Ho ah / 20°- EXPLANATION A. This report B. Jackson and others, 1982 C. duBray, 1983; duBray and others 16 1982 Figure 1. Index map showing location of study area in the north-central Arabian Shield. successively combined during at least two periods of plate convergence or accretion between approximately 760 Ma and 625 Ma (Sehmidt and Brown , ^ ^njw^) . The African eraton probably provided the buttress against which these are complexes were consolidated along east-dipping subduetion zones. Final eratonization followed a major continental collisional event about 660 Ma (Sehmidt and others, 1979). Accretion of primitive erust to the African eraton at about 760+40 Ma gave rise to the first major pulse of syntee- tonie plutonism. The plutonie rocks assigned to this event consist chiefly of foliated diorites, tonalites, and grano- diorites, the batholithie and subvoleanie intrusive equiva lents of eomagmatie andesitie volcano-sedimentary layered roek sequences. A younger period of syntectonie plutonism was initiated by an inferred continental collision from the east at about 640+20 Ma, according to Sehmidt and others (1979), and is represented by large gneissie granodiorite and monzogranite domes. It resulted in significant thicken ing of the primitive erust. Emplacement of the more potas- sie granites of the Arabian Shield, those of primary inter est here, began in the waning stages of eompressive defor mation that resulted from continental collision (Sehmidt and others, 1979). These "postorogenic" plutons were emplaeed passively by stoping or