W-Sn Ore-Mineral Geochronology: New Ages Improve Genesis Models

Direct ore-mineral U-Pb geochronology of scheelite (CaWO₄), cassiterite (SnO₂), and wolframite ([Fe,Mn]WO₄) using recently-developed reference materials led to new ore-genesis insights for multiple worldwide W-Sn/rare metal deposits. Scheelite from the Yellow Pine epithermal Au-W-Sb deposit in Idaho, USA was age dated using U-Pb via isotope dilution thermal ionization mass spectrometry (TIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). These analyses provided both the first age constraints on the tungsten mineralization (ca. 57 Ma) and a scheelite U-Pb reference material (NMNH-107667; 57.52 ± 0.22 Ma). The data reveal ore mineralization occurred in numerous discrete pulses during crustal uplift, which contrasts with the previous two-mineralization-event model.

The Yellow Pine scheelite reference material enabled U-Pb scheelite geochronology via LA-ICP-MS for multiple other deposits worldwide; namely, the polyphase stratabound scheelite-ferberite mineralization hosted within Fe-rich magnesite zones and marbles in two locations around Mount Mallnock, Austria. Two unexpected but geologically meaningful age dates (294 ± 8 Ma) for Mallnock West and (239 ± 3 Ma) for Mallnock North revealed for the first time that ore mineralization occurred during an extensional geodynamic setting as part of the breakup of Pangea, as opposed to the previous model invoking the older compressional tectonics of the Variscan orogeny.

Combining direct-ore geochronology methods for several ore minerals was particularly powerful for Sn- and W-bearing deposits in southeast Australia. A U-Pb cassiterite age date (435 ± 2 Ma) revealed the tin-bearing lithium pegmatites of the Dorchap Dyke Swarm are ca. 15 Ma older than some previous estimates suggesting mineralization was related to the earliest magmatic activity recorded in the Wagga-Omeo Metamorphic Belt. Additionally, a new U-Pb wolframite age date (395 ± 5 Ma) for the Womobi polymetallic (W-Mo-Bi) deposit is ca. 21 million years younger than the host Thologolong granite, suggesting the granite was a passive host that was mineralized by a concealed intrusion. Both instances revealed mineralization ages that were significantly different than previously accepted. More widespread application of these increasingly diverse, direct-ore geochronology methods stand to replace uncertain spatial or textural associations, thereby providing an opportunity to significantly improve ore genesis models.