Structural control of mineralization by dilatancy due to differential thermal expansiveness (in disseminated deposits) and fault flexures (in veins): review and working hypotheses

The generation of dilating spaces in the host rocks is essential to facilitate the circulation and accumulation of mineralizing fluids with economic potential. In disseminated deposits, the volumetric and morphological changes in rocks with quartz and feldspar, due to their coefficients of relative differential thermal expansion (contraction), determine dilatancy, making them porous and permeable to fluids. The contrasting thermal expansion constants for quartz and feldspar in thermo-increasing and thermo-decreasing processes are reviewed by volumetric modeling of the effect of a significant variation of the quartz transition alpha-beta around 575 degrees C. This phenomenon could vary from 0.4 % to 1.2 % for granite with different quartz content during the cooling of a typical porphyry boxwork. In the case of vein bodies, faulting produces dilation by transtensive bending with an increase in thickness when they show irregular, stepped and/or bended shapes. According to Anderson's Law, portions of these faults will be dilatant when the principal stresses are subparallel (gamma = the angle between sigma(1) and the fault surface). The morphostructural and kinematic knowledge of the mechanisms that generate the controlling dilatancy of mineralization constitutes a valuable and practical work methodology that contributes to the location forecast of the best mineralization sectors in terms of volume and quality during prospecting and exploration activities. Both described physical mechanisms that generate dilation in the host rocks are factors of great importance in the economic definition of the mineralizations control.