Geological characteristics and diagenetic geochronology of the Huangyangshan graphite deposit, Xinjiang

The Huangyangshan deposit is a superlarge crystalline graphite deposit recently discovered in Xinjiang. The reserves of crystalline graphite in the deposit are estimated at least 72.64 Mt. The deposit is hosted in granite, and 90% of graphite is of spherulitic structure. The longest diameter of the spherulite can reach 20 cm. It is extremely rare in the world. Through drill core logging, prospecting trench logging and petrographic and zircon U-Pb geochronologic studies, the authors investigated mineralization, mineral assemblage and diagenetic age of the deposit, and discussed its ore genesis. The results reveal that the diagenetic age of the deposit is (306±4) Ma, which is Late Carboniferous. Graphite spherulite and matrix have the same lithology of alkali-feldspar granite. However, biotite, hornblende and clinopyroxene are relatively more concentrated in the spherulite. Metallic minerals associated with graphite are pyrrhotite, chalcopyrite, ilmenite and hematite. Due to strong reducibility of graphite, these metallic minerals are mainly distributed within graphite spherulite, forming typical zoning texture. Graphitization could be divided into two periods, namely magmatic hydrothermal period and hydrothermal superimposition period. The former was the principal oreforming period, producing spherulitic and disseminated graphite, while the latter produced vein graphite. Crystals of graphite are of flaky and colloform texture. The flaky graphite is in acicular form along the section and has preferred orientations. Graphite ores have low negative bulk-rock carbon isotopic composition, which implies that the carbon consisting of graphite was derived from organic matters in strata. Magma assimilated organic matters in strata during its ascending. At the late stage of magma evolution, with the separation of the melt and liquid phase, carbonaceous matters were incorporated into the liquid phase. When the temperature and pressure decreased, carbon precipitated from the magmatic hydrothermal to form graphite. Medium-grained arfvedsonite granite and fine and medium grained biotite granite contain graphite spherulites, and hence Huangyangshan pluton has considerable ore-prospecting potential.