STABLE AND RADIOGENIC ISOTOPES IN THE EXPLORATION FOR VOLCANOGENIC MASSIVE SULFIDE DEPOSITS

Radiogenic (e.g., U-Th-Pb, Rb-Sr, Sm-Nd, Lu-Hf), traditional stable (e.g., S, O, H, C, He, B, and Li), and non-traditional metal stable (e.g., Se, Fe, Zn, Cu, Mo, and Tl) isotopes are increasingly being recognized as powerful geochemical tools for understanding metal and sulfur sources, depositional ages, ambient basin redox, area selection, and vectoring in the exploration for volcanogenic massive sulfide (VMS) deposits. Volcanogenic massive sulfide deposits are metal sulfide deposits that are globally economically important sources of base metals (Zn, Cu, Pb), and, in some deposits, precious (Au, Ag) and by-product critical (e.g., As, Bi, Co, Ge, In, Sn, Sb, Ga, Se, and Te) metals. These deposits are closely associated in space and time with submarine volcanism. Seawater is drawn down into the subsurface, magmatically heated by magma chambers and/or subvolcanic intrusions, and chemically modified during hydrothermal circulation; metals and other solutes in the rocks are leached along the flow path and precipitated at or near the seafloor in response to strong physicochemical gradients between the mineralizing fluid and cold, ambient seawater at the depositional site. Isotope data are not routinely used in mineral exploration due to perceived cost and complexity in interpretation; however, advances in analytical technologies and techniques, refinements to VMS deposit genetic and exploration models, and data integration and reduction algorithms have facilitated the practical potential application of isotope data in exploration for many mineral deposits, including VMS. New technologies have resulted in lower cost, facile analysis, and better understanding of the processes that govern isotopic fractionation, in particular for non-traditional (metal) isotope systems. Radiogenic isotopes are primarily used to date the lithologies (e.g., U-Pb, Ar-Ar) that host the VMS mineralization or the mineralization itself (e.g., Re-Os, Rb-Sr), or hydrothermal alteration products (clays, white micas) (e.g., Ar/Ar). Radiogenic isotopes (e.g., Pb, Sr, Nd, Hf) are also used as tracers to understand metal and solute sources (e.g., mantle versus crust) and processes. Stable isotopes are also used to elucidate metal and solute sources such as seawater, igneous, sedimentary (e.g., S, C, O, H, He), mineralizing processes such as boiling/phase separation (e.g., O, H), and to track hydrothermal fluid-rock interaction (e.g., O). Non-traditional metal isotopes (e.g., Fe, Cu, Zn, Se, Mo, Ni, Hg) have only been applied to studies of VMS deposition for the last decade or so. For these isotope systems, there are still only limited data for ancient VMS. However, most VMS deposits have isotopic values that are essentially similar to mantle values, only rarely showing more extreme fractionation. Several isotope systems (e.g., O, H, C, Sr) are commonly used to recognize and quantify water-rock interactions attendant with hydrothermal alteration associated with VMS mineralizing processes. The most well-studied isotope system in this regard is O, with large isotopic exchange occurring as a function of increasing temperature and water/rock interaction. Strontium isotopes are also useful for understanding alteration around VMS deposits, with potentially significant differences in Sr-87/Sr-86 as a function of composition of the magma feeding the hydrothermal system, the isotopic composition of extant seawater, and the age and composition of the footwall lithologies.