Abstract: A marine sedimentary sepiolite deposit was for the first time discovered in Middle and Upper Proterozoic strata in the Jixian area of Tianjin Municipality through. 1:50000 regional surveys. This is a major breakthrough in mineral prospecting in this area. The sepiolite deposit occurs in dolomite of the middle-upper part of the 8 th member of the Wumishan Formation of the Jixian System, yielding abundant stromatolites. The ore-bearing rock series. 35 m thick, is stratigraphically peisistent. This deposit includes two ore beds, which respectively are 1.91m and 2.21 m thick and have average grades of 25.6% and 13.%%. The ore-bearing dolomite was formed in an environment of neritic-littoral carbonate rock platforms.