Abstract Title: Origin of the Carlin and Battle Mountain Eureka Trends

Abstract Author(s): Muntean, John (Placer Dome Exploration) 

Abstract: The Carlin and Battle Mountain-Eureka trends are the two best-known 
alignments of Carlin-type gold deposits Ð the most productive type of gold 
deposit in the United States. These two northwest trends have been attributed 
to basement features for many years (Robert, 1966) and have been demonstrated 
to correspond with gross geophysical and isotopic features (cf. Grauch, 1998), 
including gradients in basement gravity (Grauch et al., 1995), zones of 
electrical conductivity (Rodriguez, 1998), and initial strontium and lead 
isotope ratios of Mesozoic and Tertiary igneous rocks (Wooden et al., 1998; 
Tosdal et al., 2000). I will present evidence, both stratigraphic and 
structural, that the Carlin and Battle Mountain-Eureka trends correspond to 
reactivated Paleozoic normal fault zones that likely had their origins in the 
Proterozoic during rifting of the western margin of North America, as suggested 
by Tosdal et al. (2000). Within the trends, many Carlin-type deposits are 
controlled by 330-350¡ (NNW) and 290-310¡ (WNW) trending faults and folds that 
are interpreted to have been localized by the fault fabric established during 
Proterozoic rifting (Marshak et al., 2000; Timmons et al., 2001). Stratigraphic 
evidence for syn-sedimentary faulting includes: 1) abrupt facies and thickness 
changes, 2) growth sequences, 3) sedimentary breccias and soft-sediment 
deformation, and 4) unconformities. Structural evidence for inversion of rift 
faults during Antler and younger compressional events includes: 1) fault 
propagation folds and monoclines, 2) Òshort-cut thrustsÓ and Òfloating islandÓ 
geometries, 3) radiating arrays of faults similar to Òflower structuresÓ, and 
4) anomalous trends of fold axes. The basement rift faults likely have a deeper 
crustal extent relative to later structures and are critical to Carlin-type 
gold deposits in that they serve as collecting points and conduits for 
hydrothermal fluids sourced from deeper crustal levels. From the point of view 
of the mining industry in Nevada, I would like to see Earthscope projects 
designed to help resolve the origin of these mineralized trends. This may 
include setting up seismic arrays in patterns that would optimize detecting 
northwest-trending basement features. There should be follow-up on the MT 
surveys that the USGS initiated across the two trends. From a larger viewpoint, 
the mining industry needs a better view of the crustal controls on the 
formation of Carlin-type deposits. It is now well accepted that Carlin deposits 
formed during a brief period in the late Eocene between ~40 and 36 Ma. What is 
special about the late Eocene in northern Nevada? From a better understanding 
of the crustal and tectonic evolution of northern Nevada, we need to derive the 
reservoirs and pathways of magma, volatiles, and heat through the entire crust 
during the late Eocene. And in the context of these reservoirs, what are the 
reservoirs and sinks of specific elements?