The increasing recognition that many, possibly all, Carlin-type
deposits in Nevada are Eocene has led to considerable recent study
of Eocene magmatism. Eocene magmatism began ~43 Ma in northeastern
Nevada and swept southward across the region of Carlintype
deposits at 20-30 km/Ma. This sweep is part of a regional pattern
that includes older Eocene rocks in Washington, Idaho,
Montana, and Wyoming, and contemporaneous Eocene rocks in
Utah. Eocene igneous centers, which are widespread in northeastern
Nevada and northwestern Utah, are dominated by andesite and
dacite or granodiorite; these rock types are present in all centers.
Late rhyolitic intrusions are common in most; ash-flow tuffs are also
widespread, but only one caldera has been found. Rocks are high-K
calc-alkalic, becoming less alkalic southward in Nevada.
New detailed mapping, published maps, and aeromagnetic data
indicate that the largest Eocene centers in Nevada are in an area
extending from the Tuscarora volcanic field southwest to Battle
Mountain and Cove, and east to the Railroad district and Robinson
Mountain. This area covers all of the Carlin trend and many other
major gold deposits. Most Eocene centers outside this area are small.
The northern Carlin Trend-Emigrant Pass igneous complex (NCEP),
which extends from the Humboldt River to the northern Carlin trend,
is the largest center; its size is best indicated by the corresponding
700-km2 aeromagnetic anomaly. The NCEP was active between 40
and 36 Ma in several distinct pulses that were contemporaneous with
episodes of mineralization in the trend. 40Ar/39Ar dating demonstrates
that magmatism around the trend swept southward, similar to
regional patterns. In the northern Carlin trend, Eocene rocks consist
only of numerous dikes; extrusive rocks may never have been present,
but the dikes require the existence of nearby stocks. Around
Welches Canyon in the middle of the NCEP, subvolcanic vents are
common, but coeval lavas are completely eroded. Near Emigrant
Pass, andesitic to dacitic lavas and vents are abundant.
The NCEP is distinctive in several ways that may be significant
for the origin of deposits of the Carlin trend. It was the largest and
most long-lived (³4 Ma) Eocene igneous center in Nevada.
Magmatism transferred immense amounts of heat to the upper crust,
which was then available to generate hydrothermal systems. Rocks
are notably hornblende rich (up to 12% hornblende phenocrysts)
indicating H2O-rich magmas. Pyroclastic rocks are absent, and possibly
no lavas erupted in the northern Carlin trend; therefore, magmatic
volatiles and any contained metals were likely trapped in the
subsurface and would have been available to generate deposits.
Numerous questions remain about Eocene magmatism and its
relation to Carlin-type deposits. Eocene rocks are present but not well
known near Getchell and Twin Creeks, despite the ~42 Ma age of
these deposits. Did magmas contribute metals and fluids or only
heat? What was the contribution of mantle and crust to magmas? Was
magmatism related to subduction or some other tectonic setting?