GSN membership meeting-March 16, 2012

Reservations Are Required – Please Cancel if You Are Unable to Attend


Please call 775-323-3500, Fax 775-323-3599 or e-mail by NOON on Thursday, March 15, 2012.
Social Hour: 6:00 PM – Dinner: 7:00 PM – Speaker: 8:00 PM
$17.00 per person

Location: Elks Lodge, 597 Kumle Lane, Reno, NV
Directions: across (W) from the Reno-Sparks Convention Center
(S. Virginia Street, behind the Les Schwab Tire Center)

Prepaid dinner reservations will only be accepted for the current monthly meeting. Cancellations must be received two days before the meeting in order for your money to be refunded.

“Mesozoic-Cenozoic Magmatism and Mineralization in the Greater Cortez Area: An Example of NBMG Framework Studies”

Chris Henry and John Muntean (Nevada Bureau of Mines and Geology)
David John and Joe Colgan (US Geological Survey)

NBMG and collaborators are studying major areas of mineralization to understand the basic geology that led to the mineralization, with the intent that others can use this information to better explore for more deposits. Our work is based on geologic mapping, integrated with structural analysis, geochemistry, geochronology, alteration studies, and lots more.

The greater Cortez part of the Battle Mountain–Eureka trend, including the Cortez and northern Shoshone and Toiyabe Ranges, has undergone multiple episodes of Jurassic, Cretaceous, Miocene, and especially Eocene magmatism and mineralization. The major Jurassic episode was emplacement of the composite Mill Canyon granodiorite–quartz monzonite and related lamprophyre and quartz porphyry dikes. 40Ar/39Ar and U-Pb dating indicates intrusion occurred over several million years at ~160 Ma. The major Cretaceous episode was emplacement at 104 Ma of two quartz monzonites, neither of which crops out—the Gold Acres body just west of Pipeline and a similar intrusion in a drill hole ~4km west of Cortez Hills. Both intrusions have a wide metamorphic aureole.

Multiple episodes of mostly silicic to intermediate magmatism began about 40 Ma and continued at least to 32 Ma. Intrusion was probably nearly continuous over this time, but associated volcanism was more limited. Rocks of some igneous episodes are not exposed but are inferred from indirect evidence. Mineralization is unequivocally associated with episodes at about 40-39 Ma (Hilltop, Tenabo) and 35.7 Ma (Cortez Hills, Tenabo?).

Granodiorite stocks intruded at 39.5 Ma at Granite Mountain, Hilltop, and Tenabo. A similar stock probably underlies the Cortez area, based on a single dike outcrop and the presence of 39-40 Ma zircons in younger Eocene igneous rocks. Published ReOs dates on molybdenite at Hilltop and Tenabo and our 40Ar/39Ar dates on sericite near Hilltop and sericite and adularia at Tenabo show that some mineralization was contemporaneous with these intrusions.

The big event was intrusion of abundant, 35.7-35.8 Ma rhyolite dikes, centered around Cortez Hills, but continuing in a north-northwest zone from at least Tenabo on the north to the north end of the Simpson Park Mts on the south. This belt of rhyolite dikes requires one or more large plutons to have fed them. The dikes at Cortez Hills were contemporaneous with mineralization but are generally not ore. They were unreactive and, probably most importantly, commonly had glassy margins that quickly altered to montmorillonite making them impermeable to hydrothermal fluids. However, some dikes contain marcasite, pyrite, and locally arsenian pyrite, orpiment, and realgar. Mafic minerals and feldspars are commonly altered to clay, mainly montmorillonite but locally illite and kaolinite. The dikes at Tenabo and rhyolite domes in the Simpson Park Mts are indistinguishable in phenocrysts, composition, and age with the dikes at Cortez Hills. Whether they are associated with similar mineralization remains to be seen.


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