Gold mineralization assisted by crustal melting: the case of the Maevatanana mesothermal gold deposit, Madagascar

The genesis of mesothermal Au-quartz vein deposits, which are among the more economically important type of Au deposits, is still under debate. There are two main theories, proposed in the literature, for the subsequent formation of these deposits in suitable structure involved: (1) Au-bearing ore fluid was derived from magmatic intrusions; or (2) Au was extracted from a greenstone belt by metamorphic fluids. In this paper, we report geochemical data from the mesothermal Maevatanana Au deposit in Madagascar, from which we deduce, based on synthesis of these data with field observations and with data from previous studies, that Au-quartz veins and silicified gneisses in the footwall of the granite dike were derived from both greenstone rocks and magmatic intrusions. From this interpretation, we propose a two-stage fertilization model for the mesothermal Maevatanana Au deposit. The first stage involved recycling of metal-fertilized lower crust. The second stage involved emplacement of granitic magma into the greenstone belt, resulting in increase in temperature of the host rock, and subsequent low-degree partial melting of the crust produced volatile-rich high-silica melts. Subsequently, Au-rich fluid was extracted from the granitic magma and convection in the magma reservoir caused by ascent of the less-dense volatile-rich high-silica melt and settlement of crystals (e.g., plagioclase) resulted in the formation of ore-forming fluids at the top of the magma reservoir. Finally, the Au-rich fluids were carried by magmatic dikes and transported from the magma reservoir to shallow host structures to form the mesothermal Maevatanana Au deposit.