PHANEROZOIC OOLITIC IRONSTONE IN CHINA

The Phanerozoic oolitic ironstone in southern and eastern China occurs in the strata ranging in age from Middle Ordovician through Middle and Late Devonian to Early and Middle Jurassic times, especially in the Late Devonian strata where almost 70.5% of the ore reserves have been explored and confined to the subenclosed shallow-marine lagoons. The ooids deposited in the Ordovician and Devonian sea water vary in diameter from 0.7 to 2 mm and from 0.2 to 0.8mm, respectively, whereas those deposited in the Jurassic lake water from 0.15 to 0.66mm. The oolitic girdles were formed in the marine and lacustrine water. The water body energy impelled the ferruginous colloids or suspended matter to rotate around quartz fragments or the preexisting microgranular iron minerals as euhedral crystals to form the ooids (with nuclei or without nuclei). The oolitic girdles and matrix consist mostly of euhedral iron minerals or carbonates, while the oolitic nuclei are composed either of quartz fragments, or of euhedral iron minerals or clastic iron minerals.