Sedimentary origin of the Mn-Fe ore of Um Bogma, southwest Sinai: Geochemical and paleomagnetic evidence

Paleomagnetic, mineralogical, and geochemical investigations have been carr ied out to estimate the origin and age of the Mn-Fe ore of Um Bogma Formati on of the early Carboniferous. For this purpose, oriented samples were coll ected from 26 sites in the Mn-Fe ore and host rocks of Um Bogma Formation a nd the overlying sandstone of Abu Thora Formation. The paleomagnetic pole, determined from the characteristic remanent magnetization of the Mn-Fe ore from 20 sites and the hosting rocks of Um Bogma Formation, is located at 22 degrees S, 97 degrees E (Dp = 3.9 degrees, Dm = 7.4 degrees). This paleoma gnetic pole position falls close to the Carboniferous poles of Africa, sugg esting that the Mn-Fe ore and associated rocks of Um Bogma Formation were d eposited during the Carboniferous. Furthermore, the paleolatitude determine d for these rocks, based on the mean paleomagnetic inclination, suggests th at the Mn-Fe ore of Um Bogma was deposited at a latitude 10 degrees south o f the equator, i.e., in tropical conditions. Mineralogical and geochemical investigations support the paleomagnetic results. The ore is enriched with Na, K, Ca, Mg, Sr, Co, and Ni, suggesting that the Mn-Fe ore of Um Bogma is of sedimentary marine origin. The enrichment of some ore samples with hydr othermal elements (As, Zn, Cu, Pb, V, W, and F) is restricted to faulted ar eas, indicating that a younger phase of hydrothermal activity affected the ore after its deposition. Samples from the overlying Abu Thora sandstone yield a mean paleomagnetic d irection of D = 326 degrees, I = 42 degrees, k = 19.3; alpha(95) = 17.9 deg rees, which provides a paleomagnetic pole position at latitude 59 degrees, longitude 302 degrees (Dp = 13.5 degrees, Dm = 22 degrees). This magnetizat ion is significantly different from that of the Mn-Fe ore and host rocks an d is assigned a Cretaceous age. The porous sandstone acquired its magnetiza tion long after deposition, most likely by diagenetic chemical processes as sociated with red pigmentation.