Trace element and stable isotope dispersion halo around a high sulfidation epithermal Au-Ag deposit emplaced in Quarzitic Sandstone: The Lagunas Norte Deposit, Northern Peru
The Lagunas Norte high sulfidation epithermal Au-Ag deposit is, with 11.2 Moz of total resources, the most important ore deposit discovered recently in Peru. It is largely hosted by folded, Cretaceous quartzitic sandstone of the Chimu Formation, but ~30% of the ore was emplaced in the Miocene Calipuy Group volcanic rocks overlying the quartzites, as well as in two phreatomagmatic diatreme breccia bodies cutting the Cretaceous units. Due to the low reactivity of the quartzites, the alteration halo around the deposit is obscure so that the deposit eluded discovery until 2001. However, stable-oxygen isotope geochemistry combined with multi element data can vector to ore. The oxygen isotopic composition of quartz reflects hydrothermal alteration beyond the halo visible in the relatively pure (mostly >95% SiO2) sandstones. Regional background δ18O values of unaltered sandstones are between 13.2‰ and 13.8‰ whereas much higher v18O values between 16.5‰ and 16.8‰ in finegrained yellowish quartz veins in the sandstones (silice parda) and vuggy quartz in the volcanic rocks record the influence of the mineralizing fluid. The quartzitic sandstones affected by hydrothermal fluids near the deposit show δ18O values between 14‰ and 15‰ and similar elevated values are observed near faults to the north of the deposit. The highest δ18O values in the silica parda correlate with high Ti and Te contents but low Ga and Pb contents relative to those of the unaltered quartzites. Mobile elements like Na and K are commonly below detection limit (ALS Chemex - ICP-MS ME-ICP06 & ME-MS61), even in the unaltered sandstones. We conclude that stable isotope geochemistry is a potentially powerful tool when defining the extent of hydrothermal fluid flow in non-reactive host rocks
