Multiple Sulfate Isotopic Evidence on the Formation of Oxide Copper Ore at Spence, Atacama Desert, Northern Chile

In the Atacama Desert of northern Chile, one of the world's richest metallogenic provinces, porphyry copper deposits are characterized by the unique occurrence of atacamite in their oxidized zones. The origin and formation of the oxide zone of these copper deposits is, however, controversial. It was proposed that Cl-rich deep formation water pumping-up events along faults by earthquakes, after onset of the hyperaridity, were required (Cameron et al., 2007). Their model would imply that supplies of saline deep formation water from fractures to the surface should have left behind a homogeneous or fracture-controlled salt profile from surface down to the oxide zone. While no excluding the deep formation water model in other deposit, here we propose that, in our sampling region, the alternative saline source, which is critical for atacamite formation, could be locally evaporated groundwater, Cl-rich salts leached from arid surface by meteoric water, or brines from eastern salar basins at a time when the climate in northern Chile was changing from arid to hyperarid. At this climate transition, arid- requiring minerals such as atacamite in the oxide zone were formed and, more importantly, preserved upon evaporation beneath the surface alluvial deposits. Since salt accumulation at the surface remain active during hyperarid condition, our model would predict that water-soluble salt profile from surface to the oxide zone should have a characteristic pattern: salts with an atmospheric component on the surface gradually transitioning to salts of the oxide ore zone on the bottom and a mixing zone in between. To test these two alternative models, we focus on sulfate salts, one of the common water-soluble salts in arid environments. An added advantage is that sulfate accumulated on desert surface has a secondary atmospheric component that bears a unique triple oxygen isotope signature, easily distinguishable from sulfate formed by the oxidation of sulfide minerals at the oxide ore zone. Samples were collected from a drill core that extends from surface soil to an oxide zone where gypsum and jarosite coexist with atacamite at Spence, a supergene enriched copper porphyry deposit located between Calama and Antofagasta. We found that at 15 to ~100 m depths, the Δ17O and δ34S both decrease while the δ18O increases steadily with depths, suggesting a binary mixing of two distinct sulfate sources, with the surface sulfate having Δ17O, δ34S, and δ18O at +0.55‰, +5.80‰, and +10.80‰, while the deep oxide-ore- zone sulfate at -0.23‰, +3.6‰, and+19.8‰, respectively. The surface sulfate has reached a maximum depth of ~ 50 meters, as marked by the disappearance of positive Δ17O signals at that depth. The intact preservation of this transitional sulfate mixing profile supports our model, a model that does not require a deep formation water source for atacamite formation in oxide zone of Spence copper porphyry deposit.