鞍山前寒武纪条带状含铁建造中石墨的成因

ORIGIN OF GRAPHITES IN EARLY PRECAMBRIAN BANDED IRON FORMATION IN ANSHAN,CHINA

Graphites which occur in the Early Precambrian banded iron formation (BIF)(3.1×10^9 yr, Algoma type) at Gongchangling, Anshan, China can be divided into two genetie types on the basis of their 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 disseminates fairly uniformly in the schist. Chemical analysis of 20 samples of graphite yields an average content of 0.29±0.22%. The average δ^13C value of 4 samples is -26.6±0.6‰ (PDB). Rich magnetite ore bodies occur in the form of lens and layer within the banded magnetite quartzite, and wall-rock alteration is also noticed. Graphite-bearing 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 shoot, mainly in the core of the shoot. No graphite is found in the outer part of the shoot, poor ore in the same horizon, wallrock near the shoot and altered rock, indicating that graphite has a great bearing on the shoot. Chemical analysis of 15 samples gives an average graphite content of 0.89±0.51%. The average δ^23C 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=2 Fe3O4+5 CO2+C in the primary sedimentary siderite under condition of amphibole-facies regional metamorphism.

Graphites which occur in the Early Precambrian banded iron formation (BIF)(3.1×10~9 yr, Algoma type) at Gongchangling, Anshan, China can be divided into twogenetic types on the basis of their modes of occurrence: biogenic and inorganic; theformer 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 orig-inal rock is a volcanic tuff-bearing clayey siltstone. Graphite disseminates fairly uni-formly in the schist. Chemical analysis of 20 samples of graphite yields an average con-tent of 0.29±0.22%. The average σ^(13)C value of 4 samples is -26.6±0.6‰ (PDB). Rich magnetite ore bodies occur in the form of lens and layer within the bandedmagnetite quartzite, and wall-rock alteration is also noticed. Graphite-bearing rich ma-gnetite ore is composed of magnetite, maghemite and minor graphite. Late chlorite andsiderite are recognized locally. Disseminated graphite is generally distributed in scalyaggregates interstitial to the grains of magnetite, occasionally found within the grainsof magnetite. It is non-uniformly distributed in the horizon of shoot, mainly in thecore of the shoot. No graphite is found in the outer part of the shoot, poor ore in thesame horizon, wallrock near the shoot and altered rock, indicating that graphite hasa great bearing on the shoot. Chemical analysis of 15 samples gives an average gra-phite content of 0.89±0.51%. The average σ^(13)C value of 18 samples is -4.7±2.1‰(PDB). This kind of graphite seems to have been formed by the following reaction:6 FeCO_3=2 Fe_3O_4+5 CO_2+C in the primary sedimentary siderite under condition ofamphibole-facies regional metamorphism.