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 (<10 cm) clay-rich gouge bands or thicker (20–80 cm) foliated cataclasites. Ore-bearing veins commonly occur adjacent to these zones and display varied widths, textures, and mineral assemblages. Gold is hosted by quartz or amorphous silica, either free or in pyrite. Brecciated and banded veins record multiple mineralisation events, whereas comb quartz textures with 2–5 cm euhedral crystals indicate slow growth in open space.

Microstructural analyses document multiple episodes of quartz deposition in the form of subparallel and superposed veins that cross-cut clasts of the andesitic host rock. Some brecciated bands contain spherical clasts completely surrounded by concentric cement bands, forming cockade-like structures that suggest fluidised conditions in which cement precipitation occurred while clasts were suspended.

This preliminary evidence indicates the coexistence of long-lived mineralisation processes and cyclic, short-lived deposition events, likely linked to repeated fault activation. Ongoing analyses integrating microstructural observations with mineral chemistry aim to constrain the fault-slip mechanisms responsible for specific mineralisation styles.