Ordovician reef-hosted Jiaodingshan Mn–Co deposit and Dawashan Mn deposit, Sichuan Province, China

Abstract

The Jiaodingshan Mn–Co and Dawashan Mn deposits are located in the approximately 2-m thick Daduhe unit of the Wufengian strata of Late Ordovician (Ashgill) age. Paleogeographic reconstruction places the deposits at the time of their formation in a gulf between Chengdu submarine rise and the Kangdian continent. The Jiaodingshan and Dawashan deposits occur in algal-reef facies, the former in an atoll-like structure and the latter in a pinnacle reef. Ores are mainly composed of rhodochrosite, kutnahorite, hausmannite, braunite, manganosite, and bementite. Dark red, yellowish-pink, brown, green-gray, and black ores are massive, banded, laminated, spheroidal, and cryptalgal (oncolite, stromatolite, algal filaments) boundstones. Blue, green, and red algal fossils show in situ growth positions. Samples of high-grade Jiaodingshan and Dawashan ores assay as much as 66.7% MnO. Jiaodingshan Mn carbonate ores have mean contents of Ba, Co, and Pb somewhat higher than in Dawashan ores. Cobalt is widely distributed and strongly enriched in all rock types as compared to its crustal mean content. Cobalt is correlated with Cu, Ni, and MgO in both deposits and additionally with Ba and Zn in the Dawashan deposit. The δ 13 C PDB values of Mn carbonate ores (−7.8 to −16.3‰) indicate contributions of carbon from both seawater bicarbonate and the bacterial degradation of organic matter, the latter being 33% to 68%, assuming about −24‰ for the δ 13 C PDB of the organic matter. Host limestones derived carbon predominantly from seawater bicarbonate ( δ 13 C PDB of +0.2 to −7‰). NW-trending fault zones controlled development of lithofacies, whereas NE-trending fault zones provided pathways for movement of fluids. The source of Co, Ni, and Cu was mainly from weathering of mafic and ultramafic rocks on the Kangdian continent, whereas contemporaneous volcanic eruptions were of secondary importance. The reefs were likely mineralized during early diagenesis under shallow burial. The reefs were highly porous and acted as the locus for metasomatic replacement by Mn that combined with CO 2 produced during oxidation of organic matter in the zone of sulfate reduction and seawater bicarbonate. That metasomatic replacement formed the rhodochrosite ores.