Geology of the Bir Nifazi Quadrangle, Kingdom of Saudi Arabia

Abstract

The Bi'r Nifazi quadrangle is located in the north-central part of the Precambrian shield of Saudi Arabia between lat 24&deg; 45' and 25&deg; 00' N. and long 41&deg; 15' and 41&deg; 30' E. Approximately 75 percent of the area is underlain by large plutons that range in composition from gabbro to alkali granite. A nearly complete ophiolitic section, the Darb Zubaydah ophiolite, is preserved in septa between and in roof pendants within these plutons. The ophiolite appears to comprise the oldest rocks in the area. The age of the ophiolite (800-840 Ma) is based on a tentative correlation with dated volcanic rocks in the Nuqrah area, approximately 100 km north of the Bi'r Nifazi area. Emplacement of younger plutons has rotated the ophiolitic section into an eastward-facing subvertical homocline. The lowest exposed unit in the ophiolite within the quadrangle is composed of serpentinized peridotite that is intruded by gabbro and diabase dikes. The serpentinite is overlain by gabbro and microgabbro, massive diabase, a lower basalt sequence, a thick sandstone sequence named the Kaffan sandstone, lahar deposits, an upper basalt sequence, and interbedded tuff, basalt, and sedimentary rocks. Tectonized peridotite is not present at the base of the ophiolite, but roof pendants of tectonized peridotite are present in the granodiorite batholith west of the Bi'r Nifazi quadrangle. The composition of volcanic rocks and the abundance of pillow basalt, volcanic wacke, and coarse-grained andesitic to rhyolitic tuff and volcaniclastic rocks suggest that the ophiolite formed in the vicinity of an island arc. Strands of the left-lateral Najd fault system cut all of the ophiolitic and plutonic rocks and bound small basins in which sedimentary rocks of the Jibalah group were deposited. A north-trending, 10-km-long belt of gossans crops out within the ophiolite beneath the upper-basalt sequence at Jabal Mardah. Reconnaissance drilling indicates that one of the larger gossans is underlain by a steeply dipping, 15-m-thick, sulfide-rich volcanic wacke that averages 1 percent nickel locally. The ore is composed of pyrite, millerite, polydymite, and minor sphalerite that fill interstices between clasts of the wacke and are intimately intergrown with quartz and nickel-rich epidote and chlorite. These textures and assemblages suggest that the sulfides crystallized in situ from infiltrating hydrothermal fluids. Tuffs and basalt flows appear to have acted as impermeable barriers that channeled the hydrothermal fluids through the more permeable wacke where sulfides were deposited. Carbonate-replaced serpentinized peridotite at the base of the ophiolite is considered a potential source for the nickel. In contrast to most nickel deposits, the mineralized rocks at Jabal Mardah have extremely high Ni/Cu (130 to 260) and negligible concentrations (< 5 ppb) of platinum-group elements.