Abstract:

The distinction between ore bearing intrusions and non ore bearing intrusions is the basis for the understanding of the formation of magma type deposits, and it provides also the evidence for deep prospecting prediction. In this article, the author takes the Qingpi Village Miyi mafic intrusion, Sichuan as an example, using petrography and mineral composition profile and quantitative texture analysis to clarify the solidification process of non ore bearing intrusions, then compares it with ore bearing intrusions of Panzhihua to explain the difference in the formation process between them. Petrographic analysis shows that rock forming minerals of medium grain gabbro from Qingpi Village intrusions can be divided into 4 generations: ①coarse grained plagioclase→②medium grained plagioclase+clinopyxene→③biotite+Fe Ti oxide→④very coarse grained plagioclase, demonstrating the solidification process of the closed system. Adding up the mineral association: albite+clinopyxene+amphibole+apatite from the very coarse grained gabbro veins, we can divide the minerals of Qingpi Village intrusions into 5 generations and 4 crystal populations: channel crystal, chamber crystal, matrix crystal and fluid crystal. The mineral composition profiles indicate that the channel crystal and chamber crystal have the normal zoning, showing the feature of cooling crystallization of closed system, and the growth of the channel crystal is accompanied by the decompression effect; that the matrix crystal initially has the normal zoning and finally has the opposite zoning, demonstrating the aggregation and escape of residual fluids; and that the appearance of fluid crystal shows the phase separation and venting of super critical fluids. Quantitative texture analysis reveals the coarsening process in the later period of solidification, which is the important evidence for the solidification process of closed magma system, consistent with the petrographic and crystal composition profile analysis. In addition, the Fe Ti oxide content in Qingpi Village intrusion is only 4 Vol% which is unlikely to accumulate forming the mineralization. Compared with the Panzhihua intrusion, the solidification process of Qingpi Village intrusion is lack of the dissolution structure of previous existing crystals, which is considered as the result of influx of high temperature ore bearing fluid. Accordingly, this article proposes that whether there is the external orebearing fluid(transmagmatic fluid) input determines the capacity of oreproducing of mafic magma intrusions; that furthermore, it is considered that the fluid input leads to magmatic differentiation, instead of magmatic differentiation inducing the formation of ore bearing fluid.

Key words: Panzhihua type iron deposit, magmatic differentiation, crystal population, quantitative texture analysis, trans magmatic fluid