Research On The Characteristics And Ore-bearing Property Of The Caldera At Taipusiqi

The caldera at Taipusiqi belongs to the western section of Mesozoic Yanshan volcanic mobile belt, located in the binding site of the Inner Mongolia axis(Paleo-Uplift) and the stratigraphic traps in the edg of HuaDe group. The caldera which deposits are mainly rhyolitic welded tuff is producted in BaiYingaolao period. The calderawhich appears typical double round mountain system and semiorbicular water system showed clearly in the remote sensing image is approximately equant. The outer ring is low and gentle mountains which are comprised of lava from overflows and extrusive facies; the inside ring is aerial moutains which are comprise of pumice flow deposits from explosive facies. The center is taken up by the granite porphyry body from intrusion facies.The△T anomalies of the high accuracy magnetic survey reflects clearly the ring characteristic of the caldera at Taipusiqi.Lithofacies spread annularly in the caldera and the pumice flow deposits from explosive facies is the most developed, in addition, there is overflow facies, eruption-sedimentary facies, volcanic conduit facies, extrusion facies and volcanic-intrusion facies developed in the region. Pumice flow deposits develop typical eruption units, flow units, colling units. There is slightly ground surge facies and ash-cloud surge facies following pumice flow movement and present separately at the top and bottom of the flow. The extrusion facies emerged generally in the later period of volcanic cycles develops classic forth which is the sign of reconstruction of volcanic edifice and dividing volcanic cycles. Ring and radial faults are developed in the caldera and it is characterized by having multiplex ring factures. The outer ring controlled the distribution of overflow facies and parasitic craters, the inner ring controlled the formation of caldera and intrusive body in center.In the region, the volcanic rocks strata is made up of Baiyingaolao formation which is divided into four members: the first member consists of pyroclastic rocks facies. At the bottom, it is Plinian air-fall deposits and then shows ground surge facies, pumice flow facies, ash-cloud surge facies upward in turn. On the top, it is co-ignimbrite air-fall deposits.The second member consists of rhyolites,etal. from overflow facies. The third member consists of sediment-tuff, tuffaceous sandstones and conglomerates from eruption- sedimentary facies. The fourth member consists of slightly trachyandesites distributing along the ring fracture. The evolution of the caldera has gone through the evolutionary process in various ways including a sudden intense burst—the sinking of caldera—the formation of crate—the activity of the parasitic volcano—the resurgence of the caldera,etal. The model of action and the composition of magma have a great difference in the different stages. The process of magma evolution shows that acid—intermediate—relatively basic—acid.Pb, Zn, Ag, Cu,etal. polymetallic deposits are controlled by the extrusive-intrusive activity in the center in the resurgence stage of caldera. The mineralization locates in the contact zone of granite-porphyry of intrusive facies and froth of extrusice facies that superposed by the volcanic faults in the center of caldera. There are multiple factors of ore-controlling, such as east-west basement faults, caldera tectonics, porphyries body in the center, faults formed after the eruption,etal.