STRUCTURES AS PATHS OF LEAST RESISTANCE FOR HYDROTHERMAL FLUID FLOW: INTERPRETATIONS OF SOME LARGE, GEOMETRICALLY COMPLEX ORE SYSTEMS

Many large structurally-controlled hydrothermal ore deposits, particularly those of low-sulfidation epithermal and orogenic gold classes, have complex internal geometries in which the majority of ore is in discrete narrow lodes and veins of multiple orientations within larger but finite volumes of rock. The lodes or veins are in most cases hosted in shear or fault zones, of one or more sets, but may include systematically orientated dilational fracture veins. Typically, some shear and fault zones present in the rock volume at the time of hydrothermal fluid flow remain unmineralized. Deposit geometries, including which structures become mineralized, can be modelled as being results of high flux fluid flow at low effective stresses along 'paths of least resistance' through complexly deformed and fractured crystalline rocks in regional stress fields. Forcing of fluid flow by tectonic displacement is not required. (1) Permeability was dominated by fractures, which may be extension fractures, through-going planes in shear or fault zones, foliation surfaces, or contacts. (2) Permeable fractures may either be reactivated structures that first formed during earlier deformation phases or be structures that formed at the time of mineralization, e.g. as a result of hydrofracture under conditions of low or negative effective stresses. (3) Pre-existing structures are weaker than intact rock, with lower but not zero tensile strength and shear strength. (4) The regional stress field, the strength of pre-existing structures, and the orientation of these structures relative to the regional stress field together control whether fluid flow takes place along the pre-existing structures or whether new fractures form, and control also which sets of pre-existing structures are reactivated and hence mineralized.