Mineralization, Alteration, and Breccias of the El Niño Au-Ag Deposit, Elko County, Nevada

El Niño is an underground section of the Arturo mining operation at the northern
end of the Carlin Trend, Elko County, Nevada. The mineralization is unusual for a
Carlin-type gold deposit in having a high Ag:Au ratio of about 5.37:1 (average total
Arturo project). The major hosts of Au and Ag at El Niño are multiple types and phases
of tectonic, hydrothermal, and dissolution collapse breccias of the generally argillitic
calcareous Rodeo Creek and underlying massive limestone Bootstrap formations in the
footwall of the Roberts Mountains Thrust. The thickest and most continuous breccia
is the Roberts Mountains thrust tectonic breccia (Bx1). Bx1 is a deformed mélange of
carbonaceous and metalliferous limey mudstone clasts and matrix that spans the entire
thickness of the Rodeo Creek Formation in most areas of this deposit. Hydrothermal
breccias are found in the lower section of the Rodeo Creek Formation, at the contact
with the Bootstrap Formation and are differentiated by their cement and relative timing. Crackle to chaotic breccias with pyrite fill (Bx3b) cross-cut early silica flooded
breccias of at least two phases, which in turn are cut by a late pyrite fill breccia with
bleached selvages (Bx3a). Collapse breccias (Bx5) are the second most prevalent breccia type at El Niño and are confined to the upper contact of the Bootstrap Formation.
The spatial distributions and geometries of Au mineralization (+As, Tl and Re)
and Ag mineralization (+Te, Se, and Hg) differ in that Au tends to have the highest
concentrations above a zone of intense silicification, partly within the silicified body
and partly in the overlying weakly silicified Bx1. The highest concentrations of Ag are
typically below Au, completely within the hydrothermal and collapse breccias, especially associated with Bx3a, but patches of Ag enrichment are also widespread above
the Roberts Mountains Thrust.
Emplacement of Au and Ag is interpreted to have been in two phases. The first
was Eocene Carlin-type Au mineralization in which fluids rose along faults and spread
out into volumes of high permeability such as splay faults, bedding contacts, porous
lithologies and breccias. The second phase of mineralization is interpreted to be an
epithermal event similar to those at nearby Ivanhoe district, Midas, Mule Canyon,
and Storm mines that enriched the deposit in Ag. The widespread distribution of Ag
adjacent to a late high-angle normal fault related to Basin and Range extension suggests that most Ag was emplaced much later than the Au chemical suite. Much of the
Ag mineralization appears to be spatially related to the Bx3a hydrothermal breccia,
which contains ammonium illite. Spectral maturity of ammonium illite suggests that
it precipitated from a higher temperature fluid (≥ 300°C), compared to non-ammonium-bearing micas spatially related to Au mineralization that had an approximate
temperature range of 100–200°C.