Structural Setting and Genesis of Gold Deposits in the Bald Mountain-Alligator Ridge Area, East-Central Nevada

The Bald Mountain-Alligator Ridge area is on the southern
extension of the Carlin trend in east-central Nevada. Gold deposits
at Bald Mountain are predominantly disseminated along high-angle
structures in the Jurassic Bald Mountain stock and surrounding
Cambrian and Ordovician sedimentary rocks. Gold deposits in the
adjacent north-trending Mooney basin, Alligator Ridge, and Yankee
areas, of which Alligator Ridge is the largest, are mostly stratabound
in the Devonian-Mississippian Pilot Shale. Previous studies concluded
that the Devonian-Mississippian deposits are Carlin-type and
the Bald Mountain deposits pluton-related. However, the later stages
of mineralization and more distal deposits at Bald Mountain exhibit
a number of characteristics, such as gold-bearing arsenian pyrite,
marcasite, or arsenopyrite, high Au/Ag ratios, argillic alteration, or
sulfidation of igneous dikes, that are suggestive of Carlin-type mineralization.
Despite these characteristics, our petrographic, geochemical,
and isotopic work on these stages and deposits confirm
that they too are pluton-related and that there is no evidence of a
Carlin-type overprint at Bald Mountain. The Au endowment of Bald
Mountain is much greater than any other Jurassic pluton-related district
in eastern Nevada.
Quadrangle mapping and ore deposit studies indicate the Bald
Mountain deposits, centered around a Jurassic pluton, and the northtrending
Devonian-Mississippian deposits formed in different tectonic
settings. The Bald Mountain deposits are interpreted as forming
in a dilational zone along a west- to northwest strike-slip fault
during Jurassic compression, and at depths of 3-6 km. The northtrending
Devonian-Mississippian deposits formed at much shallower
depths during Eocene-Oligocene northwest-southeast extension
that reactivated Mesozoic structures. The Mooney basin-
Alligator Ridge-Yankee system appears to have extended up into the
hot spring environment as silicified middle Eocene fluvial sandstones
have d18O values (18-22ä) similar to those of ore grade
jasperoids (15-21ä) in underlying Devonian-Mississippian rocks.
Both types of deposits are disrupted by middle Miocene and
younger faults. The RBM deposit on the western edge of Bald
Mountain was originally in the upper part of the Bald Mountain system,
and was subsequently downdropped along the Ruby listric fault
to its present position. More than 1 km of gold-bearing rock between
the RBM deposit and present day exposures of the originally underlying
Top deposit was removed by erosion.