Abstract:

 The metallogenic systems related to shallow magmatic intrusions are the very important and complicated dynamic systems. In such a system, fluid overpressure caused by the shallow intrusion and consequent rock fracturing are critical dynamic processes for mineralization. This paper presents systemic analysis on the importance of fluid overpressure for mineralization related to shallow intrusions, the mechanisms for generating overpressure and the constraints of overpressured fluids on mineralization, with examples of skarntype mineralization from the TonglingAnqing district. The close association of skarn ore deposits with breccias and the characteristics of such orerelated breccias indicate the important role played by hydraulic fracturing in mineralization. The mechanisms for generating fluid overpressure associated with ore formation are very complex, including magma exsolution and degassing, expansion of pore fluids sealed in the country rocks due to heating by magma, and metamorphic reactions of the carbonate rocks with the magma. The contribution of the overpressured fluid to mineralization is realized through hydraulic fracturing followed by a series of dynamic events, mainly including the decompression of the fluids and consequent phase separation as well as mixing and reactions of fluids from different sources. The fluid overpressuringhydraulic fracturing and  the fluid decompressioncrack sealing, the two opposite processes, are coupled with fluid flow through changing the porosity and permeability of the rocks. These processes, governed by the magmatic dynamic input and tectonic stress field, selforganized the system towards a higher order state, leading to mineralization. Because rock fracturing is caused both by fluid pressure and tectonic stress, and is critical for ore formation, the combination of fluid pressure and stress field control not only the patterns and mechanisms of fracturing, but also the styles of ore formation.

Key words:  fluid overpressure, fluid decompression, hydraulic fracturing, shallow intrusion, ore formation, dynamics