Application of LA-ICP-MS to sedimentary phosphatic particles from Tunisian phosphorite deposits: Insights from trace elements and REE into paleo-depositional environments     

Hechmi Garnit  Salah Bouhlel  Donatella Barca  Chaker Chtara 《Chemie der Erde / Geochemistry》2012,72(2):127-139

Laser ablation ICP-MS was used to determine trace (TE) and rare earth element (REE) contents of sedimentary phosphatic pellets and coprolites collected from late Paleocene-early Eocene phosphorite deposits in Tunisia. TE and REE contents in the investigated deposits exhibit a close relationship with grain types and paleogeography. Phosphatic pellets seem to concentrate more TE and REE than co-existing coprolites. Both particle types display modern oxic-suboxic seawater patterns. In most coprolites, ∑REE, Y, Cr, Ni, Th, and V show an increase from core to rim, whereas Ba, Cd, Cu, and U abundances decrease from core to rim, reflecting adsorption processes during early diagenesis consistent with changing redox and productivity conditions.The northern basins (Sra Ouertane) within open marine and platform settings tended to be more oxic-suboxic with relative high organic matter flux, while the eastern basins (Jebel Jebs) and Gafsa-Metlaoui basin are characterized by semi-restricted situation associated with suboxic-oxic conditions. This result is consistent with previous reconstructions which suggest that phosphorites of northern basins had a more direct connection to the open ocean than those to the south and east of the emerged area.This study highlights the potential for using laser ablation ICP-MS as a tool to reconstruct phosphorite depositional paleoenvironments.  相似文献   

Phosphorite-hosted zinc and lead mineralization in the Sekarna deposit (Central Tunisia)     

Hechmi?GarnitEmail author  Salah?Bouhlel  Donatella?Barca  Craig?A.?Johnson  Chaker?Chtara 《Mineralium Deposita》2012,47(5):545-562

The Sekarna Zn–Pb deposit is located in Central Tunisia at the northeastern edge of the Cenozoic Rohia graben. Mineralization comprises two major ore types: (1) disseminated Zn–Pb sulfides that occur as lenses in sedimentary phosphorite layers and (2) cavity-filling zinc oxides (calamine-type ores) that crosscut Late Cretaceous and Early Eocene limestone. We studied Zn sulfide mineralization in the Saint Pierre ore body, which is hosted in a 5-m-thick sedimentary phosphorite unit of Early Eocene age. The sulfide mineralization occurs as replacements of carbonate cement in phosphorite. The ores comprise stratiform lenses rich in sphalerite with minor galena, Fe sulfides, and earlier diagenetic barite. Laser ablation–inductively coupled plasma mass spectrometry analyses of sphalerite and galena show a wide range of minor element contents with significant enrichment of cadmium in both sphalerite (6,000–20,000 ppm) and galena (12–189 ppm). The minor element enrichments likely reflect the influence of the immediate organic-rich host rocks. Fluid inclusions in sphalerite give homogenization temperatures of 80–130°C. The final ice melting temperatures range from −22°C to −11°C, which correspond to salinities of 15–24 wt.% NaCl eq. and suggest a basinal brine origin for the fluids. Sulfur isotope analyses show uniformly negative values for sphalerite (−11.2‰ to −9.3‰) and galena (−16‰ to −12.3‰). The δ³⁴S of barite, which averages 25.1‰, is 4‰ higher than the value for Eocene seawater sulfate. The sulfur isotopic compositions are inferred to reflect sulfur derivation through bacterial reduction of contemporaneous seawater sulfate, possibly in restricted basins where organic matter was abundant. The Pb isotopes suggest an upper crustal lead source.  相似文献