GEOCHEMICAL CHARACTERIZATION, ELEMENTAL GAIN-LOSS, AND GEOCHRONOLOGY OF IGNEOUS ROCKS ALONG THE GETCHELL TREND, NORTHERN NEVADA

The Getchell trend, located in north central Nevada, is a northeast-trending alignment of Carlin-type gold deposits of probable Eocene age. Spatially associated with gold are a variety of igneous rocks. The upper and lower plates of what has been mapped as the Roberts Mountains thrust contain a series of Paleozoic basalts and mafic sills cut by the Cretaceous Osgood Mountains granodiorite stock and related dikes.

Felsic rocks include calc-alkaline volcanic arc granites (VAG) (granodiorite, porphyritic dacites, fine-grained dacite, and rhyolite), and a calc-alkaline within plate granite (WPG) (trachyte). Alteration assemblages are primarily propylitic and argillic. Alteration of the dacites resulted in gains of Ag, W, Ni, Cu, P2O5, K2O, Zn, Rb, and Ba, and losses of MnO, MgO, CaO, Sn, Co, Pb, Sr, and Na2O. 40Ar/39Ar and U/Pb dating of the volcanic arc granites yielded ages of ~85 – 114 Ma suggesting a protracted (~30 My) Cretaceous igneous event. A preliminary U/Pb age of ~ 350 Ma is indicated for the trachyte.

Mafic rocks consist of andesite-basalt, gabbro, and peridotite. Geochemical analyses indicate formation in three tectonomagmatic environments and the rocks can be classified as alkalic ocean island basalts (OIBs), tholeiitic normal-type mid ocean ridge basalts (N-MORBs), and tholeiitic plume-type mid ocean ridge basalts (P-MORBs). Alteration includes chlorite, serpentine, and albite assemblages. On the eastern side of the study area the upper plate contains N-MORB pillows, whereas the lower plate contains a thick OIB package predominantly comprised of sills. On the western side the upper plate contains P-MORBs above OIBs and the lower plate contains N-MORBS. These stratigraphic relations are not easily correlated across the study area and further emphasize the complexity of the Getchell trend.