Geochemical Characteristics And Significances Of The Dadao Erji Chromite Deposit Ore-forming Rock In Gansu Province

The Dadao Erji chromite ore-forming rock locates in the western QilianMountains，the combination of the Middle Qilian block and the Northern Qilianorogenic zone. The Dadao Erji rock body consists of cumulate complex and mantlemetamorphic peridotites. The cumulate complex is the major rock unit, and theore-bearing rock as well. There is a suite of andesite-basic lavas in the south of therock body， from north to south, forming banded distribution： the mantlemetamorphic peridotitesâ†'containing chromium mafic-ultramafic cumulatecomplexesâ†'andesite-basic lavas，and they constitute a typical ophiolite section.The Dadao Erji chromite ore-forming rocks are cumulate complexes，whichincludes three cyclic cumulate. The most developed one is the third cyclic cumulate，and most industrial orebodies are distributed in this cycle. The bottom of each cycliccumulate is Ultramafic Dunite，and upward is mafic and ultramafic laminatedcomplex, composed of dunite，diopsidite and gabbro, the upper is only mafic gabbro,which form a cyclothems occurring continuously. The studies of petrochemistraly,mineralogy of the cumulates and geochemical characteristics indicate that thesecrystals are of tholeiitic magma origin from fractional crystallization of MgO-andCr2O3-rich in an opening system under a higher pressure.Pyroxene-peridotites are picked out in the containing chromium mafic-ultramafic cumulate complexes, which Sm/Nd isotopic age is441±58Ma, itrepresents the emplacement age of the ophiolite suite. In addition, a precise zirconLA-ICP-MS U-Pb concordia age from adamellite that invased cumulate complexesobtained was462.9+1.4Ma, and the comparisons of regional geological datas, Iconcludes the Dadao Erji mafic-ultramafic cumulate complexes formed in the EarlyOrdovician. The crust lavas of the Dadao Erji ophiolite belong to basaltic andesite，samplesare the tholeiitic basalt. Geochemical characteristics of these rocks indicate that theDadao Erji ophiolite derived from a moderate depleted mantle source，but mightundergo the contamination of crust-derived material, and it shows geochemicalcharacteristics of N-MORB and tholeiitic basalt. All the geochemical characteris-tics indicate that The Dadao Erji ophiolite might form in the back-arc setting.The industrial chromite deposit in the Dadao Erji rock body occurs in the duniteand dunite-bearing-pyroxene on the bottom of the third cycle cumulates，and fewchromite are seen in the mantle peridotite. The ore-bodies of chromite consist mainlyof various disseminated chromite ores, while the country rocks are all dunites. Theaccessory spinel is similar to ore-forming spinel in composition. All of these featuressuggest that the chromite deposit in Dadao Erji rock body is part of the cumulusdeposit formed in the orthomagmatic stage. The genetic mechanism of the cumuluschromite deposit is characterized by fractional crystallization of an ultramafic mag-ma. The author hence hold the opinion that the Dadao Erji chromite deposit is atypical chromite deposit related to the cumulate of ophiolite in China and isobviously different from podiform chromite of the mantle chromite.