Origin of graphite in early precambrian banded iron formation in Anshan, China

Abstract

Graphite which occurs in the early Precambrian banded iron formation (BIF) (3.1x10⁹yr) at Gongchangling, Anshan, China, can be divided into two genetic types on the basis of its modes of occurrence: biogenic and inorganic; the former occurs in garnet-mica-quartz schist and the latter in rich magnetite ore.

The garnet-mica-quartz schist is located at the bottom of the formation. Its original rock is a volcanic tuff-bearing clayey siltstone. Graphite is fairly uniformly disseminated in the schist Chemical analysis of 20 samples of graphite yields an average content of 0.29±0.22%. The average δ¹³C value of 4 samples is -26.6 ±0.6‰ (PDB).

Rich magnetite ore bodies occur in the form of lenses and layers within the banded magnetite quartzite, and wallrock alteration is also noticed. Graphitebearing rich magnetite ore is composed of magnetite, maghemite and minor graphite. Late chlorite and siderite are recognized locally. Disseminated graphite is generally distributed in scaly aggregates interstitial to the grains of magnetite, occasionally found within the grains of magnetite. It is non-uniformly distributed in the horizon of rich ore, mainly in the core. No graphite is found in the outer part of the rich ore, poor ore in the same horizon, wallrock near the rich ore and altered rock, indicating that graphite has a great bearing on the rich ore. Chemical analysis of 15 samples gives an average graphite content of 0.89±0.51%. The average δ¹³C value of 18 samples is-4.7 ±2.1%.(PDB). This kind of graphite seems to have been formed by the following reaction: 6 FeCO₃=2Fe₃O₄ + 5CO₂+C in the primary sedimentary siderite under the condition of amphibole-facies regional metamorphism.