豫西铝土矿沉积环境初探

THE PRELIMINERY STUDY ON THE SEDIMENTARY ENVIRONMENT OF BAUXITE DEPOSITS IN WESTERN HENAN

含铝岩系沉积时期,豫西地区东北部为滨、浅海环境,形成粘土页岩和碳酸盐岩;中部为临滨、滨外环境,产铝土矿;南部为滨岸环境,铝土矿沉积时期为前滨、临滨环境,产铝土矿,铝土矿的富矿区与其中的半封闭海湾、泻湖环境有关.海侵初期形成铁质粘土岩,铝土矿形成于海侵最大时期.

The bauxite-bearing rock sense in the western Henan exists in Middle or Upper Carboniferous, and covers discordant upon the carbonate rock of Middle Ordovician or Upper Cambrian.This paper deals with the vertical , lateral changes and their distribution of facies, mineral associations, chemical composition , texture and structure, trace metals, size fraction and the determination of gravel orientation of the ore-bearing rock series. Vertically, the ore-bearing series makes up a circle of transgression to regression which is composed of the secondary circle in the initial stage of transgression of the Carboniferous.It is shown that the ferrous clay rocks below is fresh water or brackish water deposit, formed in the backshore mudflat area in the early peroid of the transgression; the bauxite deposits belongs to the normal sea water or freshening sea water deposit, and formed in the foreshore and nearshore district in the largest period of transgression, but meanwhile there forms clay deposits in the north of western Henan; the clay shale above dcposiged in the freshening seawater circumstance in the backshore mudflat district in the period of regression; sequently, somewhere developed swamps and formed battie with coal line / coal bed; afterward the seawater withdrawn from the region temporarily. Laterally, in the sedimentary period of bauxite deposits, the relief feature in this area was lower in northeast and higher in southwest, the sedimentary basin behaved trianglarly and opened towards east, there developed a series of ancient islamds, which in some degree segrated the sedimentary environment.One tributary of the seawater flow, being larger than that one, flowed northeasterward from Hebi and Anyang district, then turning to west; the other one came from Yongcheng in the eastern area.The sedimentary environment controlled the distribution of the ore deposit and rocks.There mainly developed two layers of carbonate rocks in Hebi and Anyang district, which formed in the shelf sea; in the northern area-Jiaozuo-Kaifeng there is the nearshore and offshore environment, in which formed clay rocks and fine fragmental clay deposits; in the southern area there is the shore circumstance, in foreshore and nearshore deposited bauxite. Because of the segretion of relief feature, the southern shore enviroSment could be subdvided into five subfacics, which are Yanshi-Xinyang -Gong County open shore subfacies, Songshan-Qishan lagoon and bay sudfacies between islands, Sanmenxia-Shan County-Mianchi bay subfacies, Xinan semi-closed lagoon subfacies, and Ruyong Lushan-Baofeng lagoon subfacies. The richer bauxite deposits lie mainly in the semi-closed lagoon and bay subfacies, where exist a stronger karstificution. In some areas the bauxite deposit is characterised by grading bedding, massive bedding, parallel cross bedding and basement-sustaining texture, which indicates that the bauxite belongs to dense-flow sediments. And through the bauxite appeared in the shore circumstance, possessing fragment-pisolite-oolite hybrid texture, pseudo-thin wrapper pisolite-oolite texture, rolling bedding, lenticular bedding and depending on grading analysis, we have a conclusion that some bauxite may be a sort of nearshore storm sediment. The bauxite formed upon the weathering crust which had as long as 150 million years time developping.It had simple mineral association ( diaspore, kaolinite, illinite, etc.) and chem ical composition. The ore- forming material was tranaported in fine fragment and colloidal state, and then deposited in the sedimentary basin and again Transported mechanically Based on size fraction, it is indicated that most bauxite deposited in a torrential dynamical condition. All these processes happened in the steady tectonic background-platform stage. The bauxite formed in a specific mud coast, and had little land-derived fragment (quartz, feldspar, mica) contaminating .From Jurassic, because the district entered a new tectonic stage-diwa (geodepression) stage, the ore-bearing rock series was raised up near land surface, and