Thrusting Folds To The North Of The Giant Bayan Obo Ore Deposit

In recent years, we have did some detailed field geological research on the fold and thrust zone in the north east mining area of Bayan Obo, focused on the Jianshan to Sai Wusu southwest、Gaowei to Chaganchulu two north-south geological section. In the range of nearly 60 square kilometers, we studied a total of 219 geological point, collected a large number of first-hand data. On the deposition of some relevant phenomena(foreset laminae、olistostromes, etc.) and structural characteristics(pressure shadows、ductile shear zone、echelon veins, etc.) research, deepen the understanding of the sedimentary environments and tectonic evolution. According to field observations, calculated the thickness of most strata roughly, drawed geological profile and stratigraphic column map of Jianshan to Sai Wusu southwest、Gaowei to Chaganchulu two north-south geological section. Through field investigation and the microscopic thin section observation, stratigraphic sequence from down to up in the study area is roughly: pebbly coarse sandstone �' fine sandstone �' slate �' dolomite �' limestone �' dolomite�' slate�' fine sandstone�' pebbly coarse sandstone. Basically represents the sedimentary water from shallow to deep to shallow, sedimentary environment from continental facies to marine facies to continental facies, first transgression then regression, a complete sedimentary cycle. The attitude of typical foreset laminae in sandy limestone indicate the direction of palaeocurrent is from south east to north west, has the high south east and low north west palaeogeomorphology pattern, namely the ancient ocean center is in the north. According to the olistostromes found in the Upper Member of the Halahuogete Formation, We believe that the olistostrome should be deposited at the bottom of a hemipelagic-pelagic ocean basin adjacent to the margin of carbonate platform. A series of thrust sheet from north to south composed the main tectonic framework of our study area, the thickness of a single thrust sheet is about 580 meters. The repeated part of the rocks in thrust sheet may be one important factor leading to the formation of previous calculated value is too large. Researches on the ductile shear signs(sheath folds、stretching lineations、pressure shadows,etc.) in Chaganchulu ductile shear zone outcrops show that: the broad valley near Chaganchulu is indeed a strong compression sinistral ductile shear zone, the depth of ductile deformation was about 10-15 km or deeper below the surface. The echelon quartz veins near Jianshan are the products of deformation in semi-brittle shear zones in the process of the uplifting. Based on the above findings, we have discussed the tectonic evolution of the study area briefly.