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The Maoniuping REE deposit, located about 22 km to the southwest of Mianning, Sichuan Province, is the second largest light REE deposit in China, subsequent to the Bayan Obo Fe-Nb-REE deposit in the Inner Mongolia Autonomous Region. Technically, it is located in the transitional zone between the Panxi rift and the Longmenshan-Jinpingshan orogenic zone. It is a carbonatite vein-type deposit hosted in alkaline complex rocks. The bastnaesite-barite, bastnaesite-calcite, and bastnaesite-microcline lodes are the main three types of REE ore lodes. Among these, the first lode is distributed most extensively and its REE mineralization is the strongest. The δ34SV-CDT values of the barites in the ore of the deposit vary in a narrow range of +5.0 to +5.1‰ in the bastnaesite-calcite lode and +3.3 to +5.9‰ in the bastnaesite-barite lode, showing the isotopic characteristics of magma-derived sulfur. The δ13CV-PDB values and the δ18OV-SMOW values in the bastnaesite-calcite lode range from −3.9 to −6.9‰ and from +7.3 to +9.7‰, respectively, which fall into the range of “primary carbonatites”, showing that carbon and oxygen in the ores of the Maoniuping deposit were derived mainly from a deep source. The δ13CV-PDB values of fluid inclusions vary from −3.0 to −5.6‰, with −3.0 to −4.0‰ in the bastnaesitecalcite lode and −3.0 to −5.6‰ in the bastnaesite-barite lode, which show characteristics of mantle-derived carbon. The δDV-SMOW values of fluid inclusions range from −57 to −88‰, with −63 to −86‰ in the bastnaesite-calcite lode and −57 to −88‰ in the bastnaesite-barite lode, which show characteristics of mantle-derived hydrogen. The δ18OH2OV-SMOW values vary from +7.4 to +8.6‰ in the bastnaesitecalcite lode, and +6.7 to +7.8‰ in the bastnaesite-barite lode, almost overlapping the range of +5.5 to +9.5‰ for magmatic water. The 4He content, R/Ra ratios are (13.95 to 119.58)×10−6 (cm3/g)STP and 0.02 to 0.11, respectively, and 40Ar/36Ar is 313 ± 1 to 437 δ 2. Considering the 4He increase caused by high contents of radioactive elements, a mantle-derived fluid probably exists in the inclusions in the fluorite, calcite and bastnaesite samples. The Maoniuping deposit and its associated carbonatite-alkaline complex were formed in 40.3 to 12.2 Ma according to K-Ar and U-Pb data. All these data suggest that large quantities of mantle fluids were involved in the metallogenic process of the Maoniuping REE deposit through a fault system.

On the basis of the geological data from 220 intrusions and isotope ages of 115 rock bodies, the paper discusses the distribution of 18 alkaline intrusive rock belts along deep faults and their relations to the plate tectonic activity. In view of the presence of a large amount of Variscan alkaline rocks of deep-source character, it is inferred that mantle diapirism was very strong in China during the Variscan. Besides anorogenic and postorogenic alkaline granites, there also occur alkaline granites formed in the terminal orogenic stage when the regional tectonic activity was of compressional nature. Finally, the paper discusses the tectonic environment of the Maoniuping nordmarkite in Mianning county, Sichuan province, and the authors consider it to be a compressional environment originating after rift closing.

Mianning-Dechang Himalayan REE Metallogenetic Belt in Sichuan Province lies along the western margin of the Yangtze Craton. We have conducted detailed mineralogical studies on ore minerals collected from Maoniuping and Dalucao, the two largest deposits in the belt. With optical microscope, SEM/EDS, and EPMA, three rare intermetallic minerals, i.e. , zinccopperite (Cu 2 Zn), Snbearing native copper, and Cu-bearing jupiter were found to occur in the main ore along with barite, fluorite, apatite, sulfide and bastnaesite. Since the conditions under which zinccopperite and Sn- bearing native copper formed are quite unique, finding of these minerals, for the first time in domestic REE deposits, has significant implications for the genesis of the ore deposits in which they occur. In comparison with Cu-Zn intermetallic minerals in other occurrences, we propose that the formation of this REE metallogenetic belt is associated with fast upwelling of the Himalayan magma from deep source.

Geotectonically, the Bayan Obo deposit is located on the northern margin of the North China platform. When the Proterozoic Bayan Obo Group was formed, the area was probably a rift tectonic environment. Dolomite, the main wall rock (or host rock) of the orebody of the Bayan Obo REE deposit, shows a distinct stratified structure and is remarkably similar to REE-rich and Nb-rich carbonatite rocks elsewhere in the world in mineral constituents, petrochemical composition and incompatible element composition. K-feldspar rock, another host rock of the orebody, may have been formed as a result of volcanic exhalation. The Sm Nd whole rock (whole ore) isochron age of the Main orebody of the Bayan Obo deposit is 1.58 Ga, and the Sm Nd model age is 1.61 Ga, with eNd(t) being +6.1. The data suggest that the ore deposit was formed in the Middle Proterozoic, and that the ore-forming material came from the mantle. Middle Proterozoic seems to be the time when the Bayan Obo Group was deposited; the REE metallization occurred together with the deposition of the strata. The isochron age is close to the model age, implying that the ore-forming material was precipitated to form the ore deposit immediately after its separation from the mantle, without much contamination by crustal substances. Beside a Middle Proterozoic metallogenic age, Caledonian and Hercynian ages have also been recorded: for the former time interval, the Rb Sr isochron age of the Kuangou carbonatite vein determined by the authors is 433 Ma; for the latter time interval, the Th Pb age of aeschynite from the ore deposit and the Rb Sr isochron age of biotite granite from the southern part of the ore district were already determined by previous workers. These data indicate that the ore-forming process was polyphasic. In addition, δ18Omagnetite values of massive REE-Fe ore are −2.99 to +3.55%, and δ18Omagnetite values of riebeckite REE-Fe ore are +0.76 to +1.92%. These values, together with the S isotope data obtained by previous workers, also suggest that the ore-forming material for the ore deposit was derived from deep sources. The ore-forming process of the Bayan Obo deposit was, as indicated above, polyphasic, but the major metallization took place in the Middle Proterozoic. The ore-forming material was brought upward in the form of volcanic exhalations from the mantle, and then through sedimentation formed an ore deposit which was genetically of alkalic rock-carbonatite type. In the Caledonian time interval, with the intrusion of carbonatite veins, small amounts of ore-forming material might have been brought upward from depths, leading to extensive metasomatism. In the Hercynian time interval, with the intrusion of granite in the south, the ore deposit was transformed and the metasomatism further developed; nevertheless, little REE ore-forming material was brought into the orebody from depths or from the outside at that time.

Host rocks to the Fe-Nb-REE orebodies at Bayan Obo, Inner Mongolia, China, include dolomitic marble and K-rich slate. Microphotographic evidence demonstrates that these rocks formed by autometasomatic atic phenomena. The protolith of the dolomitic marble is magmatic carbonatite, whereas the protolith of the K-rich slate is trachytic volcaniclastic rock or volcanic tuff.