METALLOGENY AND MAGMATISM IN THE EARLY YELLOWSTONE HOTSPOT

We present new geochronologic and geochemical data to show that precious metals in many of the epithermal, bonanza-type Au-Ag deposits of the northern Great Basin (among the world's most valuable) were extracted from mantle-derived mafic magmas of the early Yellowstone hotspot. Such a relationship between metallogeny and magmatism in a Large Igneous Province (LIP) provides clues to metal transfer between crustal and mantle reservoirs that are important to our understanding of LIPs and have significant applications in the exploration for economic mineral deposits. Laser ⁴⁰Ar/³⁹Ar ages of adularia from several epithermal, Bonanza-type Au-Ag veins of northern Nevada and the Owyhee mountains in southwestern Idaho range from a maximum of 16.54±0.04 Ma (the Jumbo deposit of northern Nevada, 95% confidence level) to ca. 15.5 Ma. Single-crystal incremental heating plateau ages for adularia vary by up to 300 ka for single veins, which is interpreted to reflect some loss of radiogenic argon from less retentive crystals. These veins occur over a region broadly coincident with the early Columbia River province flood basalts of Steens Mountain, Oregon and the McDermitt caldera. We interpret the maximum age and duration of vein formation to correlate closely with the initiation and duration of Steens magmatism recently proposed (Brueseke et al., 2007). Trace quantities of lead alloyed with gold in these veins were analyzed with high precision MC-ICP-MS, and found to have isotopic ratios unlike those of host country rocks but very similar to those of the Steens basalt and related Mid-Miocene basalts in the region. We interpret our results to indicate that primitive, mantle-derived magmas in the early Yellowstone hotspot transported Au and Ag from the mantle to epithermal, vein-forming systems. We do not presently see evidence for a northeastern decrease in age along the track of the Yellowstone hotspot. Instead, the data can be interpreted to indicate discrete, local vein-forming events from ca. 16.5-15.5 Ma throughout the region of the earliest volcanic centers of the Yellowstone hotspot.