Composition and geochemical Characteristics of Ferromanganese crusts from Southwesthern of the Central Oceanic Basin, East Vietnam Sea

Pham Tich Xuan1, Nguyen Van Pho1, Nguyen Nhu Trung2, Pham Thanh Dang1, Nguyen Xuan Qua1, Nguyen Dinh Dan3, Doan Thu Tra1, Nguyen Thi Lien
Received: 3 September 2020; Accepted: 26 November 2020
 

Abstract
This paper presents the study results of composition and geochemistry of Fe-Mn crust samples collected from 6 locations in the Southwest of the Cental Oceanic Basin, East Sea to clarify their origin and formation conditions and contribute to building a scientific basis for assessing the prospect of this mineral resources.The results of mineral composition analysis show that the crusts consist mainly of carbonate minerals such as carbonate-fluoroapatite (9 - 70%), or carbonate containing Mn as ankerite (2 - 40%), mangano-calcite (6 - 34%), calcite, and lesser todorokite (0 - 12%) and amorphous Fe oxides/hydroxides minerals such as hematite, goethite, etc. Other Mn-containing minerals such as rhodorite, kutnohorite, jacobsite, kanoite encountered with negligible amounts. Images under the mineralographic microscope show that Fe-Mn minerals are very poor crystalline minerals. Notable that the crusts contain a significant amount of terrigenous detritus, suggesting that coastal Fe-Mn deposits are strongly influenced by terrigenous sedimentation. The results of geochemical
analysis show that the Fe-Mn crusts/nodules have MnO content ranging from 9.2 to 21.7% and Fe2O3 (total) ranges from 6.31 to 22.83% respectively, similar to Fe-Mn crusts/nodules in the central part of Central Oceanic Basin. The composition of trace and rare earth elements shows the concentration of Co (100-1700 ppm), Cu (226.6 -639.6 ppm), Ni (300-4100 ppm), Mo (0.00 -100 ppm) and Zn (800-1200 ppm) lower than open ocean crusts, but the concentrations of REY (REE + Y) (164.77-1993.18 ppm), Ba (400-1400ppm), Pb (200-1700 ppm), Sr (1000-1400 ppm), Te (1.50-3.90 ppm) and Tl (4.11-60.8 ppm) are relatively high. The most economic important elements such as Co, Cu, Ni and Zn are all significantly enriched, in which Ni has a enrichment factor of 36.5, Co: 32.8, Cu: 14 and Zn: 5.5, total rare earth has enrichment factor ~ 4.5. Aplying different models for analysis data shows that the crusts in SW ESB were formed by chemical and sedimentation processes from the surrounding sea water, including participation at different degree of three processes: hydrogenetic, diagenetic and hydrothermal. However, the hydrogenetic process still plays a major role with the contribution of pore water in the surrounding sediments (diagenetic). The role of hydrothermal still is unclear and may be only minor and needs further clarification.
Keywords: Fe-Mn crust, East Vietnam Sea, hydrogenetic, diagenetic, hydrothermal.