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Genetic type,formation age and tectonic setting of the Waitoushan banded iron formation, Benxi,Liaoning Province

2012; Science Press; Linguagem: Inglês

2095-8927

Dai Yan,

Geoscience and Mining Technology

Anshan-Benxi area in Liaoning Province, where plenty of large-superlarge banded iron formations are distributed, is located at the northeastern part of the North China craton. Petrology, mineralogy and geochronology of the Waitoushan iron deposit in Anshan-Benxi area are studied in this paper. Iron ores in the Waitoushan deposit are predominantly banded, with a scrap of massive. The Archean amphibolites in the Anshan Group were developed as main wall rocks and interlayer among the orebodies. Geochemical analyses show the iron ores are enriched in HREE [(La/Yb)PAAS=0.24～0.33], and characterized by positive La anomaly (La/La*=1.43～1.61), positive Eu anomaly (Eu/Eu*=2.40～4.54) and positive Y anomaly (Y/Y*=1.10～1.30) with Y/Ho=30.59, Sr/Ba=17.62, Ti/V=19.45, indicating the ore-forming materials were derived from the intermixture of high temperature hydrotherm and seawater. There is no distinct negative Ce anomaly (Ce/Ce*=0.92～1.06), imlying the seawater was under anoxic condition during the deposition of the Waitoushan BIF. Apart from Fe2OT3 and SiO2, the contents of other oxides are rather low, with depletion in continental origin elements (such as Tu, U, Zr), implying the continental detrital materials contributed little to the metallogenic process of the Waitoushan BIF. The chondrite-normalized REE pattern of amphibolites is flat [(La/Yb)N=0.80～1.10] with no distinct Ce anomalies (Ce/Ce*=0.95～0.99) and Eu anomalies (Eu/Eu*=0.88～1.16), while the N-MORB-normalized trace element pattern shows enrichment in LILE without pronounced depletion in HFSE. Geochemical analyses show the protolith of amphibolites was probably basic volcanics originating in back-arc basin setting. Morphology and trace element analyses of zircon grains indicate the zircons selected from amphibolites were all magmatic. SIMS zircon U-Pb dating shows the protolith of amphibolites was formed at 2533±11Ma, standing for the formation age of the Waitoushan iron deposit. During the eruption of basaltic magma, a group of zircons with the age of 2610±5Ma were captured. Electron microprobe analysis shows the composition of magnetites is pure and single (FeOT=92.04%～93.05%), and the typomorphic studies indicate the Waitoushan deposit is attached to metamorphosed sedimentary iron deposit. To sum up, we propose the Waitoushan deposit belongs to the Algoma-type BIF, showing an affinity with the magmation in back-arc basin setting and representing a Neoarehean metallogenic event of BIF widely developed in the North China craton.