The Aihai magnesite deposit near Haicheng (Liaoning Province, NE China) is part of the Yingke magnesite ore belt with numerous giant magnesite and talc deposits. This ore belt consists of early Proterozoic metamorphic rock series of Mg-rich carbonate formations which occur stratabound in the third section of the Dashiqiao Formation of the Liaohe Group. The Liaohe Group represents a low grade metamorphic (greenschist - lower amphibolite) volcano-sedimentary facies embedded in the Paleoproterozoic Jiao-Liao-Ji Belt at the eastern margin of the North China Craton. The North Liaohe Group is conventionally divided into four formations. The uppermost Dashiqiao formation can be sub classified into three sections. The third section consists dominantly of dolomite marbles intercalated with minor carbonaceous slates and hosting the largest magnesite deposits in the world. The magnesite ore bodies are distributed in an area of 100 km in length 4 km width. The NE trending Aihai ore body is underlain by mica schists and intercalations of thinly bedded dolomitic marble. The ore displays metasomatic transitions to the dolomite host rocks. Occasionally the ore is interbedded with siliceous green marble. In the late Jurassic lamprophyre dyke swarms intruded the Aihai magnesite deposit, but show no concomitant alteration of the magnesite ore. The deposit is crosscut by faults hosting younger magnesite sinter. The sparry magnesite varies in colour from white, white-pink, white-yellow, white-grey and grey. The mineral crystal sizes are finely grained (< 0,5 cm) to coarsely grained (1,0 - 5,0 cm). In special parts of the deposits exceptional giant crystals up to 15cm are found. Associated mineral phases are talc, Mg-chlorite, tremolite, diopside, pyrite, graphite, and others. Selectively mined magnesite has 46.89 wt. % of MgO, 0.93 % of CaO, 0.99 % of SiO2, 0,44 % of Fe2O3 (total) and an LOI of 50.78 wt. %. The lowest MgO contents occur in the transition zones to the dolomitic marbles and in younger brittle shear zones. In turn CaO shows highest values at the contacts to the dolomitic marbles. The variation of CaO is explainable by late redolomitization and relics of dolomite inclusions. Redolomitization is mainly caused by descending surface water through young brittle shear zones. The SiO2 variations are caused by talc/Mg-chlorite in tension gashes and some quartz in micro joints. The silicate hosted elements Zr, Y, V, Si and Al are positively correlated. Cr and Ni are below the detection limit of 20 ppm in all magnesite samples. Most of the Aihai open pit meta-carbonates have atypical seawater-like REE+Y patterns with depleted HREE, abundant LREE and unincisive Ce anomalies. SiO2 derives from metamorphic fluids. Precipitation and biogenic sedimentation generated the carbonate host rocks in the Dashiqiao Formation. They underwent hydrothermal metasomatism during the early stage of the Lüliang movement (orogeny). The mineralizing fluids came from lagoonal or diagenetic brines, which percolated down along fractures and faults to replace under-laying permeable platform carbonates. The impermeable metasediment bedrock (2nd Dashiqiao Section) caused the limited occurrence of magnesite in the third section of the Dashiqiao Formation. During the Lüliang movement the Liaohe Group was folded, regionally metamorphosed at greenschist to amphibolite facies conditions transforming the carbonates to dolomitic marble and coarsely grained magnesite marble. In the metamorphic epoch, late in the early Proterozoic, minerogenic buried seawater and brines were mobilized too. They metasomatized the Mg-rich carbonates to form some new pure magnesite bodies and veins.