Gold mineralization at Croagh Patrick, western Ireland, occurs in quartz veins associated with three synchronous oblique tensile shear systems. The veins evolved by progressive thickening of isolated, en echelon, lensoid tensile fractures that show a power-law relation between their length and thickness. A change in power-law exponent marks the linking of the en echelon arrays by large, oblique-tensile veins. Fluid-inclusion data show that gold was deposited due to unmixing of an H2O-CO2-NaCl fluid at 320–240 °C and 160–30 MPa primarily due to decreases in fluid pressure. Evidence for phase separation is only observed in the large linking veins and is abundant close to intersections of the vein arrays. These sites are where high gold grades occur. The distribution of gold is believed to reflect both spatial and temporal variations in fluid pressure fluctuation during seismic fracture propagation, controlled by the vein growth mechanism and zones of maximum dilation.

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