Geochemistry and origin of the Paleocene phosphorites from the Hazm Al-Jalamid area, northern Saudi Arabia

Highlights

• Phosphatic grains were reworked from a preexisting authigenic phosphatic mud.

• REE patterns indicated reducing marine formational conditions of phosphatic grains.

• Majority of REE occurs in the francolite crystal structure.

• Low REE is mainly due to the dilution of non-phosphatic constituents.

• Toxic element concentrations lie in the world phosphorite levels.

Abstract

The Paleocene phosphorites at the Hazm Al-Jalamid area, northern Saudi Arabia that belong to the Middle East-North Africa phosphate belt, were subjected to petrographic, mineralogical, and geochemical investigations to discuss their origin as well as the distribution and mode of occurrence of toxic elements.

Petrographic and mineralogical investigations revealed that the studied phosphorites are composed of phosphatic mudclasts and phosphatic bioclasts of francolite composition with traces of detrital quartz that are cemented by gypsum, chalcedony, calcite, halite, and dolomite. Phosphatic mudclasts are homogeneous and internally structureless but sometimes contain bone fragments, suggesting a reworked origin of these grains from pre-existing authigenic phosphatic mud. The presence of bone fragments within some of the phosphatic mudclasts, the occurrence of phosphatic mud filling bone cavities, and the similar appearance of the matrix of phosphatic mudclasts and the phosphatic mud in the bone cavities suggest that the bioclasts are also of reworked origin and washed out from the phosphatic mud. Scanning electron microscope observations of the mudclasts have revealed the capsule-like texture that is characteristic of bacterially mediated authigenic phosphorites.

The pronounced negative Ce and slight to moderate negative Eu anomalies of the studied phosphorites suggest their formation under reducing marine conditions. However, chondrite-normalized REE patterns show relative enrichments of LREE over the HREE and flat HREE patterns, which are obviously different from the seawater REE pattern suggesting post-depositional modifications on the REE distributions. The lower ∑ REE contents in the studied phosphorites compared to the seawater and other upper Cretaceous-lower Tertiary phosphorites are attributed to the dilution effect of non-phosphatic constituents and/or post-depositional modifications.

The concentrations of toxic elements (As, Cd, Hg, Pb, and Zn) in the Hazm Al-Jalamid phosphorites are either lower or in the acceptable levels for use of phosphorites in the fertilizers. Moreover, these elements did not show any correlations with the P₂O₅ contents indicating that the beneficiation and manufacturing processes of the studied deposits are not expected to enrich these elements in the intermediate and final products. Exception is only As that shows moderate positive correlation with the P₂O₅ contents. The analysis of As in the intermediate and final products is then recommended to follow up the level of As in these products.

Introduction

Upper Cretaceous–lower Tertiary deposits in the Middle East-North Africa phosphate belt represent one of the four major phosphogenic provenances in the geologic history (e.g. Klemme, 1958, Sheldon, 1981). The province holds the greatest accumulation of phosphorites in geological history, possibly in excess of 70 billion metric tons. It extends from Morocco in the west to Iraq in the east covering an age from Maastrichtian (e.g. deposits in Egypt) to Eocene (e.g., deposits in Iraq). Due to their economic and geologic significance, the phosphorite deposits in this province were subjected to geological, stratigraphic, mineralogical, and geochemical investigations in Morocco (e.g. Prévôt, 1988, Trappe, 1991), Tunisia (e.g. Belayouni et al., 1990, Ounis et al., 2008), Egypt (e.g. Awadalla, 2010, Baioumy, 2011, Baioumy and Tada, 2005, Baioumy et al., 2007, Glenn and Arthur, 1990), Jordan (e.g. Abed et al., 2007, Pufahl et al., 2003, Sadaqah et al., 2007), and Iraq (e.g. Al-Bassam and Al-Allak, 1985, Al-Bassam and Hagopian, 1983) to discuss their properties and origin. Many theories have been postulated to explain the origin of these deposits in different regions.

Paleocene–Eocene sedimentary phosphorites in the Northern region of Saudi Arabia, which were identified in 1965, represent part of the Upper Cretaceous–lower Tertiary deposits in Middle East–North Africa phosphate belt. To the best knowledge of the authors, nothing was published about the petrography, mineralogy, geochemistry, and origin of these deposits. The current study tries to discuss the origin of these deposits through detailed petrographic, mineralogical, and geochemical investigations. It focuses only on the Hazm Al-Jalamid area because of its huge reserves of phosphorite deposit. It is probably the most important phosphorite occurrence, so far discovered, in the Sirhan-Turayf Sub-basin and contains the majority of both the demonstrated (1070 million metric tons with 20.2 g/kg P₂O₅) and the identified resources (4120 million metric tons with 19.3 g/kg P₂O₅) in the region (Riddler et al., 1986). The deposits are currently produced from Hazm Al-Jalamid area to utilize local natural gas and sulfur resources to manufacture Diammonium Phosphate “DAP”. Due to the geological significance as part of the Middle East–North Africa phosphate belt of the Hazm Al-Jalamid phosphorites, this study integrated petrographic, mineralogical, and geochemical investigations on these deposits to examine their origin and compare their geochemical characteristic with other deposits in this belt. Besides, the utilization of these phosphorites as a raw material for the manufacture of Diammonium Phosphate (DAP) adds another important objective to the current study to discuss the environmental impact of the deposits through the concentration and distribution of environmentally important elements such as As, Cd, Hg, Pb, and Zn. The geological and economic significance of the rare earth elements in the deposits will be also addressed.