International Geologiical Congress - Oslo 2008


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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts


Chahmir deposit: A sedimentary-exhalative Zn-Pb deposit in Bafq basin, Central Iran


Abodorrahman Rajabi, Tarbiat Modares University (Islamic Republic of Iran)
Ebrahim Rastad, Tarbiat Modares University (Islamic Republic of Iran)
Nematollah Rashidnejad Omran, Tarbiat Modares University (Islamic Republic of Iran)
Ramin Mohammadi Niaei, Iran Zinc Mienes Development Company (Islamic Republic of Iran)
Ramin Mohammadi Niaei, Iran Zinc Mienes Development Company (Islamic Republic of Iran)


The Chahmir zinc-lead deposit, located in the southeast of the Bafq basin, Central Iran, is one of several sediment hosted Zn-Pb SEDEX deposits (e.g., Koushk, Zarigan and Dareh Dehu deposits) hosted within a Lower Cambrian volcano-sedimentary sequence. The Bafq basin consists of a series of horsts and grabens along N-S trending faults produced by intercontinental rifting between 750 and 583 Ma.

The host sequence of the Chahmir deposit includes organic-rich tuffaceous shale and calcareous siltstone crosscut by mafic intrusions and underlain by calcareous felsic tuffs. Sulfide minerals are mainly pyrite, marcasite, sphalerite, galena and minor chalcopyrite.
The Chahmir deposit can be divided into two different parts, including a thicker part of generally massive higher grade ores (vent complex) and a thinner part of bedded and banded lower grade ores (bedded ore). Ore minerals are observed as laminated, disseminated and banded in the bedded ore, and mainly as massive, replacement, and veinlets in the vent complex. Pyrite, the dominant sulfide mineral in Chahmir, occurs as framboids in the bedded portion and as brecciated colloforms and massive in the vent complex. Main wall-rock alterations in Chahmir are silicification and carbonitization in the vent complex with carbonatization extending outward into adjacent bedded ore.

The Chahmir deposit with reserve in excess of one million tons of 5.5% Zn and 1.5% Pb, is strongly zoned, with Zn/Pb, Pb/Ag, and Zn/Fe ratios increasing, and Cu/(Zn+Pb) ratios decreasing, upward.

Sedimentological, textural, and mineralogical studies of the Chahmir deposit indicate that sulfide minerals were deposited by sedimentary-exhalative process(es) within an extensive submarine basin formed adjacent to a major synsedimentary regional fault. Accordingly, Chahmir is regarded as a Vent Proximal SEDEX deposit in the Bafq basin.
Key words: Chahmir, Bafq, SEDEX, Vent Proximal


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