Geochemical Characteristics of the Paleozoic Sedimentary Rocks in the Winnemucca Quadrangle, Nevada, U.S.A.—Background Values for Gold Exploratio

Exploration & Mining Geology, Vol. 4, No. 3, 1995

L.C. HSU, H.F. BONHAM, JR., J.G. PRICE, L.J. GARSIDE, M.O. DESILETS and P.J. LECHLER, Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, U.S.A

Geochemical background levels have been established for unmineralized sedimentary rocks that are elsewhere the hosts of Carlin-type gold deposits in the Getchell Trend. The Paleozoic rocks in the Winnemucca Quadrangle consist predominantly of the lower Paleozoic siliceous and clastic sequences that were carried eastward along the Roberts Mountains thrust during the Late Devonian and Early Mississippian Antler Orogeny and the late Paleozoic Havallah and Pumpernickel sequences brought along the Golconda thrust eastward during early Mesozoic Sonoma Orogeny. Over 260 rock samples were collected from the Paleozoic sequences. Major and trace elements were determined by instrumental methods best suited for the speci?c elements. The general geochemical features of the Paleozoic sedimentary rocks are discussed in detail with reference to the data of average abundances of elements in sedimentary rocks. The background and threshold values of certain trace elements in sandstone, shale, and carbonate rock of one formation, as well as background levels for the entire sequence were obtained. The characteristic geochemical features of the Paleozoic rocks in this area include: consistently high Ba and low Au contents (1.1 ppb, 1.5 ppb, 1.1 ppb Au for average sandstone, shale and carbonate, respectively); generally low contents of those elements which may be of environmental concern such as As, Cd, Hg and Pb. Plots of major and trace elements suggest that the majority of the Paleozoic clastic rocks are related to a passive margin tectonic setting. Samples with gold values higher than threshold are considered to have been affected by hydrothermal alteration related to gold mineralization. Most of these samples generally also have higher values of Sb, Hg, V and/or As. These observations support the use of such elements as path?nders for gold, although V has not been identi?ed as such before. Ba is not effective for gold exploration in this area due to the generally high background value of Ba, even though it may be an important path?nder elsewhere.

Mots Clés: Geochemistry, Sedimentary rocks, Gold, Gold exploration.