A DISCUSSION ON THE GENESIS OF BIF OF THE PRECAMBRIAN HUOQIU GROUP
Qi Runzhang ( IGMR, Nanjing )
The iron-bearing formation(BIF) of the Precambrian Houqiu Group is the ore belt consisting of several large scale metamorphic ferrite deposits. Genetically, it obviously differs from other Precambrian BIF in China. According to the data of Rb/Sr isochron (2680-2790Ma), it belongs to late Archean. The BIF of the Precambrian Huoqiu Group has been reformed by the metamorphism of high temperatures and middle pressures. Its representative rock assemblage is sillimanite-kyanite gneiss, garnet-biotite gneiss, schist, dolomitic marble and banded quartz magne- tite. Its initial (87)Sr/(38)Sr ratios range from 0.7038 to 0.7084,averaglg 0.705. various primary rocks have been recovered by means of the study of petrology, petrochemistry and geochemistry. For instance, granulites, gneisses and plagioamphibolites in the schist, most zircon are round or axiolitic, their crystal faces and edges are obviously fretted, Originated sedimentary rocks The concentiaticns of Cr, Ni Sr, Ba are lower in the chorismites, schist, gneisses and amphibolites. The average of Sr/Ba ratio is 1.15 in the amphibolites, within the range of paraamphibolites (0.1-1.4). The kki values of siderophile elements (Cu, Zn, Ni, Cr etc) are 11.008-6.51, less than those of shale and clay which are equivalent to paraamphibolites. The Rfm values of sicerophile elements range from 2.26 to 1.30. Chondrite-normalized REE patterns of amphibolites show that the fractionations of LREE and HREE are not evident, there is no anomaly of Eu, and the curve is flat, and it can be compared with the REE model of Archean sedimentary rocks from Yilgarn Block. On the bases of above information, it is evident that the paraamphibolites are of mainly sedimentary origin, which might be mixed up with cortain amount of discal basic rock dehitus. The BIF of Huoqiu Group occured in the epicontinental trough basin. It is dominated by epicontinental clastic sedimentary formation and characterized by the ingressive sequence of argillaceous rock, carbonate and siliceous of chemical sediments. Sedimentary cycle and rhythem are clear in the BIF. So banded structures are common. The formative mechanism of banded structure is complicated. It resulted from the variation of pHand Eh caused by season change and flood entering into the basin, which was the mainly reason for siliceous iron sedimentation. The BIF of Huopiu Group differs from BIF of Archean Greenstone belt, by absence of basic volcanic rock series at the bottom of iron ore belt. The Fe might come from the adjaceent region, by weathering and erosion of mafic rocks. The iron was brought into the basin in the form of Fe(3+) or hydroxyl Fe(OH)3 in acid solution. Based on the study of mineral assemblage and metamorphic typomor-phic minerals-biotite, garnet, amphibole, staurolite, kyanite, sillimanite etc, the calculated P, T values are P = 5.5±0.5 kb, T = 650-750℃, which give rise to high grade amphibolite facies. To sum up, the BIF of Huoqiu Group had been a typical sedimentary deposit. Later, it was reformed by the metamorphism of high temperatures and middle pressures. It is similar to the BIF of Superior type in North American platform in some ways.