Occurrence of Anthraxolite (Bitumen) Spheroids in Xiangtan-Type Manganese Carbonate Deposits of South China

Xiangtan-type manganese carbonate deposits occur in early Sinian interglacial black shales, which are distributed widely and contain large manganese reserves in China. This type of manganese ore is black or greyish-black in colour and has massive, laminated, and banded structures. Black anthraxolite spheroids with white encircling haloes of quartz-Mn-calcite occur, mainly in the massive manganese ores. The geological setting, distribution, mineralogy, and chemical composition of the anthraxolite spheroids as well as the manganese carbonate ores are reported in detail. It is proposed that the anthraxolite spheroids were formed during the catagenetic stage, and underwent a rather low degree of thermal metamorphism (about 200°C). White quartz-Mn-calcite haloes and penetrating veinlets in the anthraxolite spheroids were formed later.