Geology and gold deposits of the Twin Creeks Mine, Humboldt County, Nevada

The Twin Creeks Mine, with pre-mine reserves of ~500 t (+16 Moz), consists of
Mega, Vista, West, and Disco pits that exploit Carlin-type orebodies hosted in rocks
ranging in age from late Cambrian to Cretaceous. The Roberts Mountains thrust
placed Ordovician Valmy basalt on Cambrian-Ordovician siltstone, silty limestone,
mafic tuff, and intrusions of the Comus Formation, the main ore host. Rocks of the
Valmy and Comus Formations are unconformably overlain by coarse clastic and carbonate
rocks of the Pennsylvanian-Permian Battle and Etchart Formations, which in
turn are overlain by diverse rocks of the Havallah Formation along the Golconda
thrust. Cretaceous porphyritic dikes, with associated hornfels, are abundant in the
northern half of the mine.
The Comus Formation at Twin Creeks is broken into four units, from uppermost
C1 to lowest C4. Thin-bedded to laminated mafic tuff, siltstone, and silty limestone
that grades downward to laminated siltstone, are present in C1 and C2, whereas more
thickly bedded calcareous and locally tuffaceous siltstone and silty limestone characterize
C3 and C4, the primary host rocks at Twin Creeks. Abundant mafic sills, nearvent
volcanic debris flows, and lesser basalt lava comprise ~40% of the formation.
These alkalic igneous rocks have markedly different mineralogy and geochemistry
from mid-ocean ridge-type basalt (MORB) of the Valmy Formation. Phlogopite from
two sills in the Comus yield 40Ar/39Ar ages of 500.7 ± 4.1 and 481.6 ± 2.4 Ma, indicating
that at least part of the Comus is late Cambrian or older. At the nearby Pinson
and Getchell Mines, the Comus Formation contains much less igneous rock but otherwise
shows similar stratigraphic relationships.
The Etchart Formation at Twin Creeks is broken into three units, upper, middle,
and lower. Thin-bedded to laminated siltstone and silty limestone characterize upper
Etchart, quartzite pebble conglomerate occurs with silty limestone of the middle
Etchart, and sandy thin- to medium-bedded limestone comprise the lower Etchart,
the most important gold host at Vista pit.
Gold deposits are aligned north-south and are generally associated with structural
highs, including anticlines and/or uplifted fault blocks. NS- and NE-striking
faults are important for localization of ore at the deposit scale. Mineralization occupies
sequentially higher stratigraphic levels northward in part because of a regional
NNW stratigraphic dip. The bulk of mineralization in the Comus Formation is stratiform
and stratabound. Comus sills are an important ore control that enhanced
hydrothermal flow into adjacent sedimentary rocks, particularly in and near the
hinge of the overturned NE-vergent Conelea anticline. Gold grade in some ore zones
is as much as two orders of magnitude greater in sedimentary rocks versus sills, resulting in selective mining opportunities. In contrast, mineralization in basalt of the
Valmy Formation is fracture controlled, whereas that in the Etchart limestone is
stratiform and largely restricted to decalcified lower Etchart. In all deposits, highangle
faults commonly contain higher gold grades. Two pulses of gold mineralization
are recognized at Twin Creeks. Early quartz veins containing Cu-Pb-Zn-Ag ± Au are
related to 114 ± 2 Ma felsic dikes and a nearby buried stock. The early veins are crosscut
by fine-grained, sooty Fe-sulfide related to major Au deposition at ~41 Ma.