Jabal Dhaylan Zinc-Lead Deposits, Geologic Setting, Genesis, and 1996-2000 Exploration Programs, with Geophysical Studies
Timothy S. Hayes, Stephen J. Sutley, Khalid A. Kadi, Mazen B. Balkhiyour, Alim A. Siddiqui, Zacharia Beshir, and Hisham I. Hashem with Geophysical Studies by Abdullah Showail, Mabrouk Basahel, Ahmed Zamzami, Hani Zahran, and Yahya Tarabulsi
Eleven outcropping occurrences of Zn-Pb-(Cu-Ag) minerals and barite were discovered along Jabal Dhaylan by Bureau de Recherches Géologiques et Minières (BRGM) geologists working on behalf of the Saudi Arabian Deputy Ministry for Mineral Resources, in 1968. Five occurrences are in Tertiary arkoses and are patchy and small. Four occurrences in overlying carbonates are larger and richer; these four have potential as mineable Zn-Pb deposits. At Calamine Hill prospect, 21 BRGM core holes demonstrated a weathered-zone resource recently calculated at 1.18 million tonnes containing 5.6 percent Zn and 1.4 percent Pb (Carlson, 1996). Recent studies in the district, reported here, include selective geological re-mapping, supplemental outcrop sampling, surveying a district-wide geophysics grid, extensive ground geophysical surveys, 13 new core holes totaling 2252 m, and supporting laboratory studies. Though this work did not find a new orebody, results were encouraging, and further drilling and geophysical surveying are warranted. Favored Jabal Dhaylan host rocks are the Early-to-Middle Miocene Maqna group carbonates that unconformably overlie Early Miocene Al Wajh formation arkoses (Saudi Aramco stratigraphic nomenclature). At places along the tip of the half-graben east of the Calamine Hill fault, the Maqna carbonates directly overlie Precambrian basement rocks that stand as a paleo-ridge up through the Al Wajh. Maqna group deposition was synchronous with and controlled by faulting on the Calamine Hill and other synsedimentary, down-to-the-Red Sea normal faults. The half-graben tip was apparently an island and a depositional paleohigh in early Maqna time. Coralline boundstones with interbedded skeletal grainstones were deposited against the shoreline, like the modern Red Sea coastal reefs. Next offshore, reef–clast rudstones were deposited. Farther offshore, the rudstone beds graded to lime-floatstone. And, on the offshore shelf, mostly below storm wave base, claystone, laminated anhydrite, fine-grained sandstone, and rare, thin beds of clay-rich lime floatstone were deposited. Zn-bearing minerals at the outcropping occurrences, in order of abundance, include zinc-rich cryptomelane, hemimorphite, hydrozincite, and additional zinc-bearing black manganese oxides. Cerrusite and hydrocerrusite are the most common weathered-zone Pb minerals, and plattnerite and pyromorphite occur locally, as do galena relicts. Weathered-zone Zn grades have been residually enriched to more than 30 percent Zn at places, by sulfide-localized, karstic weathering. No sulfide intercepts have yet been drilled that have comparable grades. Nonetheless, comparison with sulfide zones of Egyptian deposits and pseudomorphism in Jabal Dhaylan rocks suggest that hypogene Zn grades may be relatively high. Pseudomorphs of zinc-rich cryptomelane after sphalerite suggest hypogene grades of over 10 percent Zn, and potential tonnages are greater than 1 million tonnes. Sphalerite and galena are the only known zinc and lead minerals below the weathered-zone. Most Jabal Dhaylan sphalerite is uniquely cream-colored to white. Sphalerite in microcolloform intergrowths with marcasite replaces the dolostone adjacent to fractures, and galena, marcasite, and celestian barite, in that paragenetic sequence, later occlude both fractures and host rock primary pores. Zinc-lead(-barite) are thus zoned relative to fractures.
Hayes, T.S., Sutley, S.J., Kadi, K.A., Balkhiyour, M.B., Siddiqui, A.A., Beshir, Z., and Hashem, H.I., 2002, Jabal Dhaylan zinc-lead deposits, Geologic setting, genesis, and 1996-2000 exploration programs, with geophysical studies by Showail, A., Basahel M., Zamzami, A., Zahran, H., Tarabulsi, Y.: Kingdom of Saudi Arabia, Saudi Geological Survey Open-File Report SGS-OF-2001-5, 71 p., 33 figs., 2 tables, 5 apps., 2 pls. In two volumes.