Controls of fault mechanics on mineral precipitation in gold-bearing fault-veins, Indiana Deposit, Northern Chile

Many epithermal ore deposits form in fault-veins that channel large volumes of vertical fluid flow synkinematic with fault slip. A key genetic process is the interplay among fluid flow, fault activation, and mineral precipitation; however, significant questions remain regarding the mechanics of fault slip under high fluid-flux conditions and its impact on mineralisation. A fundamental question is whether ore deposition is coseismic, post-seismic, or interseismic—specifically, whether pressure drops during seismic rupture are the dominant trigger for mineral precipitation, or whether mineralisation occurs during longer-lived, aseismic creep and sealing cycles. A deeper understanding of these processes is essential for predicting ore grade and spatial distribution.The Cretaceous Indiana deposit, located in the Coastal Cordillera of northern Chile, is a Cu–Au (Mo–Co) fault-vein system composed of several subvertical NW- or ENE-striking fault veins, with lengths ranging from 300 m to 2 km. Artisanal tunnels provide access to multiple structural levels in oxides and sulphides mineralization, offering exceptional three-dimensional exposure. NW-striking fault-veins host Au–Cu-Fe–Co-rich mineral assemblages associated with pyrite, chalcopyrite, magnetite, actinolite, albite, garnet, epidote, quartz, tourmaline, and late jarosite, clay and hematite. These are cross-cut by ENE-striking fault-veins containing Au–Cu–Mo-Co mineralisation in pyrite and chalcopyrite paragenetically associated with garnet, epidote, actinolite, quartz, and less sericite. High-grade ore shoots commonly occur in dilatational jogs and at the intersections between these two structural sets.Fault-veins are 1–3 m wide and display complex internal structures. Fault zones of variable thickness occur along the vein margins and mainly record strike-slip motion, expressed as thin (