THE CONTRIBUTION OF RE-OS ISOTOPE GEOCHEMISTRY TO DE-CONVOLUTING ORE GENESIS

Pick up any recent journal in economic geology and scan the table of contents. Chances are you’ll see “Re-Os” in the titles of several papers. In fact every single paper in a recent issue of Mineralium Depositahas “Re-Os” in the title. No doubt, Re-Os has revolutionized the field of economic geology. The ability to directly date ore-forming sulfide minerals has profoundly redirected our thinking, not only at the deposit scale but also at the whole earth scale where the ultimate source, transport, and concentration of metals are becoming processes that can be quantified. In this talk, a brief history of bringing Re-Os technology to the field of ore geology will be presented, followed by an analysis of the skills needed for the next leap forward in economic geology.

Rhenium and osmium have highly contrasting chemical characteristics, but used together, they are proxies for the behavior of both platinum group elements and less refractory metals transported in volatile-rich systems. Also, Re has the unsurpassed advantage of significant residency in only a very few minerals. Nearly all the world’s Re is housed in molybdenite. The resilience of molybdenite, even under weakly oxidizing conditions where other sulfides may breakdown, is also an advantage for the Re-Os clock. Important is that the Re-Os chronometer is chemically-controlled, not temperature controlled, which also presents an advantage in sulfide-stable metamorphic environments.

With Re-Os safely stowed in the economic geologist’s tool kit, the next big leap forward will likely come from those with keenly honed observational skills – a trait best derived from field observation, logging core, and petrography and a wide knowledge of whole earth processes (atmosphere-hydrosphere-lithosphere). We now have the opportunity to catalog the entirety of the ore-forming process – not just timing, but source and processes of metal capture, release, and migration.

Re-Os chronology provides options where other chronometers may carry muddled signals. On the other hand, muddled Re-Os results provide an exciting platform for new understanding of how metals gather and disperse through the crust, and ultimately converge to form ore deposits.