Formation and genesis of super large carbonatite deposits

of rare-earth elements

N.V. Vladykin

Institute of Geochemistry after А.P.Vinogradov SB RAS, E-mail: vlad@igc.irk.ru

 

               The largest TR deposits have carbonatite origin. This is the Mountain Pass in the USA, Bajun-Obo in China and the ore of the Tomtor massif. Some of the largest deposits are known in the Southern Gobi of South Mongolia. It was proved these deposits consist of carbonatites, however what formations of alkaline rocks they belong to is still not known. The detailed petrological-geochemical study of these complexes verified that the complexes belong to the formation of K-alkaline rocks.

              The Mountain Pass lies in California, the SW part of the USA. The complex consists of stocks and dykes of K-alkaline rocks: shonkinites, syenites and granites. Carbonatites penetrate all rocks. They are mainly composed of calcite, to minor extent Fe-dolomite, ankerite and siderite.  The latest formations are composed of quartz-calcite containing up to 60% of bastnesite. The earlier carbonatite formations contain about 10% of bastnesite and parasite, as well as barite, celesite and quartz. The age of the complex reaches 900-1000 Ma.

             The Bajun-Obo deposit is located in Inner Mongolia, about 100 km from the Mongolian border, being opposed on the other side by the alkaline volcanic complex Mushugay-Khuduk, Lugingol massif of pseudoleucite syenites and Ceden occurrence.  The deposit is located within the large «lense» of metamorphic (?) dolomites containing 1% TR. It involves some stocks of magnetite-fluorite-bastnesite rocks and silicate-carbonate rocks. The content of bastnesite reaches 30%. Over 100 minerals have been discovered in the ores, some containing rare metal elements and new minerals. The deposit is Precambrian, but in places there are some stocks of K-syenites and granites of Paleozoic age. In the deposit no silicate rocks are the case, with which ore occurrences could be genetically associated. They are possibly obliterated by metamorphic processes.

       The volcanic area of Munshugay-Khuduk in Mongolia is made of flows of leucite melanephelenites, trachytes, phonolites, small stocks of alkaline and nepheline syenites, which are penetrated by numerous (up to 1000) thin dykes of small and fine-grained  subvolcanic calcite and calcite-phluorite carbonatites nearly devoid of TR. There are some exposures of volcanic craters filled with the breccia of K-Fsp – rocks cut by the stockwork of quartz-calcite-fluorite  rocks holding 3 % TR. Prior to carbonatites there was the volcanic crater  quenched by monomineral trachyte apatite rocks containing about 15% TR, in the center bearing single mineral magnetite rocks with phlogopite. The age of the complex is  150-120 Ma.

      The Lugingol massif is located east of the Mushugay-Khuduk deposit. It is composed of pseudoleucite and nepheline syenites involving a large outlier of K-Fsp pyroxenites. Small xenoliths of micaceous shonkinites are the case. All rocks are penetrated by the veins and dykes of calcite-bastnesite-synchysite carbonatites (about 30% TR). In places they contain plentiful limonitic pyrite and fluorite, and they are very much similar to the Bajun-Obo ore. The age of Lugingol massif is 280 Ma.

      The Ceden occurrence is located between the Khan-Bogda and Lugingol massifs. It contains the stock of К- shonkinites with mineral and chemical composition indistinguishable from shonkinites OF Mountain Pass. The hosting rocks include a small dyke of calcite-bastnesite carbonatites  (about 10% TR).

     The Tomtor volcano pluton is located in the East Anabar shield and in the Udzhinsky uplift.

The area of the massif is 250 км2, the age is defined by zircon in syenites as 650 Ma. The massif is up of 70%  К-alkaline and nepheline syenites (12% К2О и 3-4% Na2O) in the center with a large stock of carbonatites. The massif also incorporates ijolites, olivine and leucite lamproites,  К-picrites, Ap-Mgt rocks and xenoliths of pyroxenites and Bt pyroxenites. The carbonatites are colcitic, dolomitic and ankeritic. In the center of the massif there is a caldera of subsidence filled with explosive tuff formations  of phosphorus Fe carbonate-silicate formations (ores) and siderite carbonatites containing 10% TR and 15% Nb. Some hypotheses on ore formation were put forward, but isotopic (Sr-Nd, C,O) studies confirmed the endogenous nature of ores, being analogous to that of carbonatites. This completely rejects their sedimentary genesis and secondary redeposition. The charts of numerous TR spectra (over 200) and spider diagrams of rare elements  also provide evidence on the genetic affinity of tuff-generating ores and carbonatites.  All described deposits are referred to the formation of К-alkaline volcano-plutonic complexes. As indicated by isotope data, they have deep-seated mantle sources. The P-T geochemical research conducted by Andreeva I. at the Mongolian deposits showed they were derived from the residual magma from silicate melts, i.e. fluids saturated with gas-fluid (F,CO₂, H₂O, P₂O₅) modifiers with high initial temperatures of crystallization.

The work has been implemented under RFBR financioal support, grants 09-05-00116, 10-05-00122 and 10-05-90400.