Evidence for metasomatic mantle carbonatitic magma extrusion in Mesoproterozoic ore-hosting dolomite rocks in the middle Kunyang rift, central Yunnan, China

The Kunyang rift lying on the western margin of the Yangtze platform is a rare Precambrian Fe-Cu mineralization zone. Wuding–Lufeng basin that is an important part of the zone is located on the west edge in the middle of the rift. The most important ore-hosting rocks are Mesoproterozoic dolomite rocks in the basin controlled by a ring fracture system, which is a fundamental structure of the basin. Plenty of silicate minerals and acicular apatite, feldspar phenocrysts and small vesicular, flown line and flown plane structures, melt inclusion and high temperature fluid inclusion found in most ore-hosting dolomites suggest that this kind of rocks could not be sedimentary dolomite, marble or hydrothermal carbonate rocks. The Zr/Hf and Nb/Ta values of the rocks are identical with those of associated mantle-derived rocks, and vary widely. For the monomineral dolomite, δ¹⁸O_SMOW‰ = +5.99 to +18.4 and δ¹³C_PDB‰ = −3.01 to +0.94, which fall within the range for all carbonatitic volcanic rocks of the world. As for the accessory minerals, the values of δ¹⁸O_SMOW‰ of magnetite (=+3.47 to +5.99‰) are close to that of the mantle (<5.7%), and the δ³⁴S‰ values of sulfides (−5.09 to +5.78, averaging +1.50) are close to that of meteorite. For all the ore-bearing dolomite rocks, ε_Nd = +0.19 to +2.27, and the calculated I_Sr = 0.699143, while for the associated mantle-derived rocks, ε_Nd = +3.18 to +3.72. All the data suggest that the mineral assemblage is not only igneous but also of metasomatic mantle origin. And the presence of acicular apatite indicates that the rocks were formed by magma rapidly cooling. And the phenocryst texture and vesicular, flown and ropy and pyroclastic structures suggest that the igneous rocks were extrusive. Therefore, the ore-bearing dolomite rocks are carbonatitic volcanic rocks. This conclusion implies that most iron and copper ore deposits hosted in the dolomite rocks should be of the carbonatite type.