Large-scale stable isotope alteration around the hydrothermal carbonate-replacement Cinco de Mayo Zn-Ag Deposit, Mexico

Carbonate-hosted hydrothermal deposits typically show narrow visible mineralogical and textural alteration halos, which inhibit exploration targeting. In contrast, hydrothermal modification of the country rock’s stable isotope composition usually extends far beyond the limited visible alteration. Hence, stable isotope studies should be an effective tool to aid exploration for carbonate-hosted deposits. Here we present new insight into the development of a large stable isotope alteration halo based on 910 O and C isotope analyses of carbonate veins and hydrothermally altered limestone hosting the Cinco de Mayo Pb-Zn-Ag (Au, Cu) carbonate replacement deposit (CRD), in Chihuahua, Mexico. Our results demonstrate that stable isotope alteration is consistent with reactive, magmatic fluid flow into unaltered limestone and represents a powerful tool for the characterization of these hydrothermal ore systems. Synmineralization veins are texturally and isotopically distinct from those formed during pre- and postmineralization diagenesis and fluid flow and show distinct gradients along the direction of mineralizing fluid flow: this appears to be a promising exploration vectoring tool. Downhole variations in wall-rock isotope values reveal aquifers and aquicludes and outline the principal hydrothermal flow paths. Furthermore, wall-rock δ¹⁸O_VSMOW systematically decreases toward mineralization from ~23‰ to <17‰ over a distance of ~10 km, providing another vectoring tool. The extent of the stable isotope alteration halo likely reflects the overall fluid volume and areal extent of a fossil hydrothermal system, which may be expected to scale with the mineral endowment. This suggests that constraining the size, shape, and degree of isotopic alteration has direct application to mineral exploration by outlining the system and indicating the potential size of a deposit.