GSN Regular Membership Meeting—May 17

 

“The location is again at Great Basin’s Taps & Tanks, 1155 S. Rock Blvd. Reno, NV. Drinks @ 6:00 pm, APPETIZERS @ 6:30 pm, Talk @ 7:15 pm. Speaker, Mike Ressel, NBMG/UNR. Title: “Carlin-type Deposits As Part of the Great Basin’s Eocene Metallogeny“. DRINKS SPONSORED BY: FALCON DRILLING AND HARRIS EXPLORATION DRILLING! Appetizers only for $15—prepay online or pay at the door. GSN students are free. For more info contact Laura Ruud at gsn@gsnv.org or 775-323-3500. Details on page 3. To PREPAY for appetizers please click on this link: https://squareup.com/store/GSNV

No reservations required but avoid the line by pre-paying!”

Speaker: Mike Ressel, NBMG

Topic: Carlin-type Deposits As Part of the Great Basin’s Eocene Metallogeny

Mike Ressel, Curtis Johnson, Elizabeth Hollingsworth, Christopher Henry, and Philipp Ruprecht, Nevada Bureau of Mines and Geology and/or University of Nevada, Reno

Abstract: Late Eocene (~42-34 Ma) ore deposits have contributed a majority of the Great Basin’s precious- and base-metal production through 2015, including about 6.2 Kt Au (199 Moz), 27.6 Kt Ag (887 Moz) , 17.5 Mt Cu, and 0.5 Mt Mo. The production of these four metals alone from Eocene deposits would have a present-day value of over $400 billion (USD), ~64% of which is attributable to gold, 29% to copper. Eocene ore deposits are restricted to the northern half of the Great Basin, coincident with arc magmatism and the earliest stages of Cenozoic crustal extension. Curiously, metal production from Eocene deposits is strikingly polarized, with 79% of Au production derived from mines in north-central Nevada, and 98% of Cu and 86% of Ag derived from mines in north-central Utah. Most Nevada Au production is from three major Au belts that host giant sedimentary rock-hosted Carlin-type Au deposits: Carlin, Battle Mountain-Eureka, and Getchell. Three major Utah districts produced the bulk of Cu, Mo, Au, and Ag (Oquirrh, Park City, and Tintic), with the vast majority of Cu, Mo, and Au coming from the Bingham Canyon porphyry system. In addition, Eocene deposits in northern Nevada contain more than 1.68 Kt of Au (54 Moz) and 0.67 Mt Mo in reserves, and Bingham Canyon contains reserves of about 2.72 Mt of Cu and 0.15 Mt of Mo.

Perhaps more than any other deposit type in the Great Basin, Carlin-type gold deposits are enigmatic. After more than five decades of mining more than 150 Moz of gold from Carlin-type deposits in north-central Nevada, we continue to debate what one is, resorting to terms such as Carlin-like to qualify our uncertainty. As a result, the rest of the exploration world is even more baffled by them. Uncertainty has hindered exploration beyond the Great Basin because of lack of a well-constrained exploration model that incorporates not only elements of existing descriptive models but also the syn-mineral regional geologic framework of type deposits in Nevada.

We assess findings contributing toward a global Carlin-type deposit exploration model. Key regional components of the model based on Great Basin geology are: 1) age and age progression of Carlin-type gold mineralization, 2) spatial and temporal association of Carlin-type deposits with other gold-rich deposits, and 3) the regional geologic setting of gold deposition including crustal architecture and syn-mineral tectonism and magmatism. The progressive change in Eocene ore deposits from Au- to Cu-dominant across the central to eastern parts of the northern Great Basin is extraordinary and has major exploration implications. This change in metals parallels changes in the geochemical and isotopic character of magmatism across the Eocene arc, although rocks are grossly similar in terms of their mineralogy and bulk composition. We preliminarily interpret west to east variations in Eocene igneous chemistry and styles of metallization as interrelated, with contrasting chemistries reflecting major differences in architecture and composition of the crust through which magmas traveled.

Thus, the range of Eocene sedimentary rock-hosted, disseminated gold deposits (SHDGs), including Carlin-type deposits, in northern Nevada defines a distinct intrusion-related gold metallogeny that contrasts with the “classic” deposits formed in many other continental arc settings, including the eastern Great Basin in Utah, which are typified by porphyry Cu and Cu-Mo, polymetallic skarns and replacements, and high- and intermediate-sulfidation epithermal Au-Ag deposits. The reduced mineralogy and geochemistry of ores and Au-dominant or Au-only character of SHDGs in northern Nevada infer overall reduced ore fluids that fundamentally differ from highly oxidized fluids indicated for porphyry-related systems. We suggest that emplacement of the Eocene arc far inboard of the plate margin and into kilometers-thick carbonaceous slope and basinal rocks of the Neoproterozoic through Paleozoic passive margin progressively modified and chemically reduced mid-crustal magmas from mafic to silicic compositions through assimilation of reduced crust.

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