500 Million years of rift- and unconformity-related Mn mineralization in the Middle East: A geodynamic and sequence stratigraphical approach to the recycling of Mn

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• 作者：H.G. Dill^a ; ^dill@bgr.de ; ; H. Pö ; llmann^b ; A. Techmer^c
• 关键词：Manganese ; Rifting ; Unconformity ; Sequence stratigraphy ; Plate tectonics ; Middle East
• 刊名：Ore Geology Reviews
• 出版年：2013
• 出版时间：September, 2013
• 年：2013
• 卷：53
• 期：Complete
• 页码：112-133
• 全文大小：4939 K

文摘

The crust along the northern boundary of Gondwana, what is called today Jordan, Israel, Turkey and Egypt, is enriched in Mn. During the geodynamic evolution of this crustal part, lasting for 500 million years, the opening of the Paleo- and the Neotethys Oceans was accompanied by the emplacement of several medium- to small-sized Mn deposits in the Middle East region.

Manganese concentration was linked in time and space closely to the evolution of triple junctions which mark the break-up of the crust in northern Gondwana and whose linear fault zones deeply penetrated into the upper mantle so that alkaline magmas in the Middle East and East Africa could use these plate boundaries as conduits for their ascent. The first triple junction on the Sinai Peninsula, active around 500 Ma ago, marked the beginning of rifting and Mn recycling at the northern edge of Gondwana, the second one in the Afar Region, Ethiopia, denotes factually the end of the first tectono-metallogenetic phase and the onset of another Mn cycle still being in its embryonic state.

In the upper crust Mn was recycled by episodic rifting to form Mn deposits during the Cambrian, the late Paleozoic, the Paleogene and the Quaternary in the Middle East. Manganese mineralization is bound to siliciclastic rocks containing Mn oxides and hydrates, e.g., birnessite, pyrolusite, and manganite. Ba, K, Pb, Co and Ni act as chemical qualifiers in these oxidic minerals and allow for a more subtle subdivision of different stages of Mn mineralization (romanechite, cryptomelane, coronadite, asbolane). Concentration of Mn involved deposition in near-shore marine basins under diagenetic, epigenetic hydrothermal, roll-type and supergene conditions. It was mediated by Cu mineralization and microbial processes and, in places, accompanied by baryte concentration.

The branches of the Middle East triple junctions were not equally operative through time:

(1)

The Najd strike-slip zone was active during the Precambrian and Cambrian and was only re-activated after a long period of tectonic quiescence within the Red-Sea Graben

(2)

The N-trending Dead-Sea Transform Fault (DSTF) was active all the way till the Recent.

(3)

The third branch of the triple junction, the SW Egypt Rift was episodically active, e.g., during the mid-Permian-Jurassic period, leading to the late Permian Mn concentration. Its NE prolongation, the Palmyra Trough, gave rise to an embryonic Neo-Tethys Ocean and paved the way for Turkey to drift away from the Afro-Arabian Plate leading to a separate Mn province during the Cenozoic.

By its drifting away, part of the Mn preconcentrated in the crustal section of Gondwana was also taken away and recycled later north of the Neo-Tethys in what is called today Turkey. During the late Mesozoic, advanced spreading in the Neo-Tethys Ocean generated oceanic crust and added another batch of Mn from the mantle to the crustal Mn cycle in form of Mn-bearing mud on top of the ophiolite sequences in the Troodos and Semail Ophiolites Cenozoic. Mn mineralization resulted from redeposition of an intracrustal Mn repository along the NW prolongation of the Red Sea Rift and along the N prolongation of the DSTF during the Eocene in Turkey and Egypt.

Unlike many of the so-called giant Mn deposits located on stable cratons that owe their mineral wealth to their geodynamic persistence and in-situ re-working of Mn, Middle East rift-related Mn concentration was later subjected to considerable dilution by plate motion, leading to a number of Mn occurrences displaced around the former triple junction instead of one unique ¡°giant deposit¡± on top of it. Mn concentration in the Middle East is confined to megasequence boundaries (AP1/AP2, AP5/AP6, AP10/AP 11), using sequence stratigraphic principles for a temporal subdivision. The crustal sections most productive for Mn concentration are those megasequences typical of maximum regression surfaces or, in other words, where conformities were correlative with unconformities. An important chemical factor of Mn enrichment related to these sequence stratigraphical changes can be microbial activity both in oxic Mn enrichments and anoxic, sulfidic environments.

These geodynamic issues, described in terms of sequence stratigraphy and plate tectonics, directly translate into the physico-chemical regime, when the periods of a maximum contrast among the redox conditions fostered the complete sequence of tetra- to bivalent oxidic Mn minerals to evolve. In conclusion, the most fertile areas on the globe to concentrate Mn are crustal sections characterized by hot-spots at a triple-junctions, truncated by surfaces of maximum regression that provoked the strongest contrast of redox conditions and thereby gave rise to the precipitation of a wide range of oxidic Mn minerals. During subduction Mn previously preconcentrated in the oceanic crust will be consumed and, hence, active margins rank lower than passive ones when it comes to Mn accumulation.