|We are excited to announce that Mr. Nick Hillemeyer will be giving the second virtual presentation for the Winnemucca and Elko Chapters on his MS research at the Center for Research in Economic Geology at the University of Nevada, Reno.
October 15, 2020 – 7:00 PM Pacific Time (US and Canada)
“Controls on Epithermal Gold-Silver Mineralization and Alteration at the
Gravel Creek Deposit, Elko County, Nevada”
Abstract. Western Exploration’s Gravel Creek project is a recently discovered Miocene low-sulfidation epithermal Au-Ag deposit in northern Elko County, Nevada. The deposit is primarily hosted by stratabound mineralization with intervals of high-grade veins within an Eocene ash flow tuff known as the Frost Creek volcanics. Mineralization is primarily controlled by the thickness and permeability of the Frost Creek as well as two major structural trends: north-northwest striking normal faults that dip steeply to the east and a more indiscreet set of northeast-trending, near vertical strike-slip structures. Alteration proximal to high-grade zones is quartz-sericite-pyrite grading out to intense silicification of the Paleozoic basement stratigraphy (Schoonover Sequence) below and strong smectite and illite bearing clay alteration of the overlying Eocene Mori Road Formation and the Miocene Jarbidge Rhyolite with illitic alteration extending to the surface on the hanging-wall of the north-northwest striking normal faults. Gold in high grade intervals occurs in chaotic quartz-sulfide breccias and banded quartz-sulfide veins as fine-grained high-silver electrum commonly enclosed in pyrite±marcasite and arsenopyrite overgrowths. Silver mineralization is associated with coarse pyrargyrite, naumannite, and a selenium bearing phase of stephanite. Although mineralization is focused in the Eocene volcanic strata, significant portions of mineralization occur in the underlying Schoonover as well as within the overlying Jarbidge Rhyolite. Mineralization in the Schoonover is hosted as chaotic breccias like those found in the Frost Creek with fewer banded veins. Jarbidge Rhyolite mineralization is hosted as both high and low angle veins primarily in the hanging-wall of the major normal faults but situated along the pervasive N-E trending strike-slip structural corridor. Mineralization that occurs within the bulk of the deposit is located approximately 1,500 feet below the surface and extending down beyond 3,200 feet at the deepest core holes. The most striking feature throughout the deposit is the pervasive nature of marcasite throughout the majority of the mineralized zones. Surficial exposure of hydrothermal breccia veins within the Jarbidge Rhyolite is similar texturally, geochemically, and mineralogically to those that have been intercepted by drilling at depth. A distinct lack of true low-sulfidation epithermal textures (e.g. colloform-crustiform banded quartz, bladed-quartz after calcite, adularia, etc.) suggests that the system never experienced intense boiling events like those corresponding with bonanza grades at Fire Creek, Midas, or Hishikari but new evidence suggests the presence of a high-grade feeder vein masked by post-mineral fault movement along the north-northwest striking normal faults. Sulfur isotope data was collected from a suite of samples representing the different mineralization styles, hosts, and various spatial relationships for 29 samples. Marcasite and Pyrite samples returned an average δ34SVCDT (‰) value of 9.5±1.1 excluding two extreme outliers suggesting a homogenous source for sulfur throughout the entire system likely partially contributed by disseminated diagenetic sulfur within the Schoonover Sequence. Gravel Creek offers the unique opportunity to view a nearly one-kilometer vertical section of a Nevada low-sulfidation epithermal system from silicified ash fall tuffs at the surface representing the paleosurface at the time of mineralization to beneath the mineralized zone in three dimensions. Increasing understanding of the interplay between depth, host rock physical properties, host rock geochemistry, and mineralization at Gravel Creek may prove to have importance for further mineral exploration beneath the vast Jarbidge Rhyolite in northeastern Nevada as well as a greater understanding of the role permeable host rocks can have in forming stratiform ore zones in epithermal systems.
Topic: Elko & Winnemucca – Oct 15th Meeting – Nick Hillemeyer
Time: Oct 15, 2020 7:00 PM Pacific Time (US and Canada)
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Meeting ID: 878 0749 1061
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Meeting ID: 878 0749 1061