Geology and Mineral Resources of the Ar Rayn Terrane, Eastern Arabian Shield, Kingdom of Saudi Arabia

SGS-TR-2005-2 Jeff L. Doebrich, Abdullah M. Al-Jehani, Alim A. Siddiqui, Timothy S. Hayes, Yasin Saleh, Joseph L. Wooden, Peter R. Johnson, Fayek H. Kattan, Bandar Shaikan, Mabrouk Basahal, Hani Zahran, and Abdullah Al-Shammari
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The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield.
The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar faul , and nonconformably overlain on the east by Phanerozoic sedimentary rocks.
The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions.
The layered rocks of the arc, the Al Amar group (> 689 Ma to ~ 625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates.
Al Amar group rocks are divided here into an eastern and western sequence.
Eastern sequence rocks possess an F1 metamorphic fabric not seen in western sequence rocks.
Western sequence dacite and rhyolite contain higher Na and lower Rb and Sr, relative to equivalent rocks in the eastern sequence.
Plutonic rocks of the terrane form three distinct lithogeochemical groups:
1- low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity, emplaced between 632 and 616 Ma in the western part of the terrane.
2- high-Al TTG/adakite of arc affinity, emplaced between 689 and 617 Ma in the central and eastern part of the terrane.
3- syn- to post-orogenic alkali granite emplaced at about 614-607 Ma.  A west-dipping subduction zone beneath the Ar Rayn terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG.
The Ar Rayn terrane contains significant resources in:
1- epithermal Au-Ag-Zn-Cu-barite.
2- enigmatic stratiform volcanic-hosted Zn-Cu-Fe oxide (Khnaiguiyah-type).
3- Fe-oxide Cu-Au, and
4- orogenic Au vein deposits, and the potential for significant resources in
5- porphyry Cu.
6- stockwork Mo-W, and
7- intrusion-related gold vein deposits.
Khnaiguiyah-type Zn-Cu deposits formed before or during early deformation of the Al Amar group eastern sequence.
Epithermal Au-Ag-Zn-Cu-barite deposits formed in Al Amar group western sequence rocks proximal to sodic rhyolite and dacite domes, which are also areas of porphyry Cu potential.
Fe-oxide Cu-Au deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane.
Orogenic gold veins are largely associated with north-striking faults, particularly in and near the Al Amar fault zone, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes.
Stockwork-related Mo-W-Cu systems are hosted by and possibly related to group-3 syn-to post-orogenic intrusions.
Geologic, structural and metallogenic characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile.
The collective presence of epithermal, porphyry copper, and Fe-oxide Cu-Au systems, and the general absence of classic volcanogenic massive sulfide deposits in the Ar Rayn terrane are very similar to the Chilean coastal belt.
The concentration of hydrothermal mineral systems, of central arc affiliation, along the Al Amar fault zone suggests that it is not an ophiolitic suture zone, as previously proposed, but originated as a major intra-arc fault that localized magmatism and mineralization.
Our preferred interpretation is that west-directed oblique subduction and ultimate collision with a land mass from the east resulted in major transcurrent displacement along the Al Amar fault, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the Al Amar fault against back-arc basin sediments now represented by the Abt schist of the Ad Dawadimi terrane.



Doebrich, J.L., Al-Jehani, A.M., Siddiqui, A.A., Hayes, T.S., Saleh, Y., Wooden, J.L., Johnson, P.R., Kattan, F.H., Shaikan, B., Basahal, M., Zahran, H., and Al-Shammari, A., 2005, Geology and mineral resources of the Ar Rayn terrane, Eastern Arabian shield, Kingdom of Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2005-2, 54 p., 6 tables, 32 figs, 9 apps.