Description

Location: Western Folklife Center, Elko, NV

Contact: justin.b.milliard@gmail.com

The GSN Elko Chapter’s October Meeting will be held on the SECOND Thursday instead of the usual third Thursday to accommodate scheduling conflicts.  The meeting location is the Western Folklife Center, 501 Railroad St., Elko, NV.  Food and Drinks @ 6 pm, Talk begins @ 7:00 p.m.  The meeting will also be available on Zoom again this month!  Zoom login will be emailed to members prior to the meeting.

Title: The Robertson Deposit: Eocene Intrusion Related Gold Deposit in the Northern Shoshone Range, Nevada

 Presenter:  Neal Mankins, Center for Research in Econ. Geology

Authors: Neal E. Mankins,1 John L. Muntean,1 Matthieu Harlaux,1 and Robert A. Creaser2 

1Ralph J. Roberts Center for Research in Economic Geology, University of Nevada Reno, Reno, Nevada

2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada

Abstract: 

The Robertson deposit, formerly known as “Tenabo”, is located in Lander County within the Bullion mining district about 7 km north of Pipeline, a 20Moz+ Carlin-type gold deposit in the northern Shoshone Range, Nevada. Robertson is an Eocene intrusion related gold deposit primarily hosted in Paleozoic sedimentary rocks within the Roberts Mountain allochthon known as the “upper plate.” Historically, gold was discovered in the Tenabo area in 1905, when a series of gold-bearing quartz veins and shear zones were found, producing nearly 10,000 ounces of gold from 1906-1912. Gold mineralization is spatially and temporally related to an intrusive complex, “the Tenabo stock”, which is composed of successive intrusive phases including an early diorite, polyphase ilmenite-bearing granodiorite, and later dikes that vary in composition from dioritic to rhyolitic. Most of the previously defined gold mineralization occurred near the contacts of intrusions and is hosted by hornfels in the overlying lithologies. Ongoing exploration has discovered significant free visible gold both within the stock and in more distal settings within metasedimentary host rocks. Intrusion-related gold deposits in Nevada are not well documented and Robertson may provide new insights into this style of mineralization supplementing other studies at Bald Mountain, Buffalo Valley, and Copper Canyon.

The purpose of this study is to characterize gold mineralization as it relates to intrusive phases, alteration styles, and structural controls by combining fieldwork, petrography, geochemistry and geochronology. This study is the first in depth examination of Robertson focusing on gold mineralization within intrusive phases and in distal settings, presenting six new U/Pb LA-ICP-MS ages of intrusive phases, and one molybdenite Re-Os age, constraining emplacement ages of igneous units and associated alteration and mineralization. A paragenetic sequence of mineralization and alteration events at Robertson was constructed based on field relationships, drill core logging, assays, and structural data.

The emplacement of the Tenabo stock produced contact metamorphism of the host siliciclastic lithologies yielding quartz, biotite, and calc-silicate hornfels aureole. As a result of field mapping, core logging, and applied petrography, successive hydrothermal alteration phases have been identified, including potassic alteration, endo/exoskarn, sericitic, and chloritic alteration. Previous studies along with this work indicate that gold occurs as disseminations and in quartz veins within intrusive units and host metasedimentary rocks. Gold consists of native particles that range in size from 2 to 200 microns having strong associations with As-Bi-Te-Se-S sulfosalts, spatially adjacent to or on the surface of arsenopyrite and loellingite grains. Ongoing petrographic observations of veins at Robertson indicate the succession of three main stages referred as 1) pre-gold “early,” 2) syn-gold, and 3) post-gold “late.” The pre-gold stage includes barren quartz veins, potassic alteration veins comprised of biotite (EDM veins and/or secondary K-feldspar), quartz – chalcopyrite ± pyrrhotite, and quartz – molybdenite veins. The syn-gold stage is composed of quartz – gold ± bismuth – loellingite – arsenopyrite veins. Finally, the post-gold veins encompass quartz – pyrite, quartz – calcite – sulfide “base metal veins” (Pb-Zn-Sb-Ag-Sn bearing sulfides), and calcite veins.

Cross-cutting relationships along with our geochronological data indicate that gold mineralization at Robertson resulted from the emplacement of a reduced ilmenite-bearing igneous stock into siliciclastic sedimentary lithologies forming a high-tonnage, low-grade gold deposit characterized by a Au-Bi-Te-Se-S metal association. Based on petrographic evidence, we infer that mineralizing conditions for gold precipitation were under relatively low sulfur fugacity and oxidation state leading to preferential formation of minerals such as native Bi, arsenopyrite, and loellingite, typical of low-sulfidation assemblages.

Please see their websites for more information:

Timberline Drilling Inc. – EVENT SPONSOR

Incorporated in 1996, Timberline Drilling, Inc. is headquartered in beautiful Coeur d’Alene, Idaho and supported by our field office in mining hub Elko, Nevada. We are a leading underground and surface core-drilling contractor in the United States.  Our ability to deliver quality core samples, on time and on budget, from meticulously clean and safe drill sites, has made us the go-to contractor for several clients. Our goal is to create long-term relationships with every customer we have the pleasure the serve.  https://timberline-drilling.com/

 

Ralph J. Roberts Center for Research in Economic Geology (CREG)

CREG is a partnership between the Mackay School of Earth Sciences and Engineering, the Nevada mining industry, and the U.S. Geological Survey. These partners pool their resources to fund research that is both fundamental in understanding the genesis of mineral deposits and applicable to the discovery and production of mineral deposits. The original mutually supportive goals of the CREG program continue to be to:

  1. Prepare well-educated professionals for careers in the Mining Industry and Research by exposing them to a wealth of expertise within the School, the industry, and the USGS.
  2. Produce and publish high-quality research results on topics of critical interest to the Nevada mining industry.

The CREG program is a center of study and learning that is leading to a greater understanding of the geology, geochemistry, geophysics, genesis and exploration of mineral deposits, especially Nevada’s famous Carlin-type gold deposits, which make the U.S. the third leading gold producer in the world. The pooled resources within CREG are mainly used to fund graduate students to complete Masters and PhD theses in the Department of Geological Sciences and Engineering at UNR.  http://www.nbmg.unr.edu/CREG/

Details

05/19/2022 18:00:0005/19/2022 20:00:00America/Los_AngelesGSN Elko Chapter MeetingThe GSN Elko Chapter's October Meeting will be held on the SECOND Thursday instead of the usual third Thursday to accommodate scheduling conflicts.  The meeting location is the Western Folklife Center, 501 Railroad St., Elko, NV.  Food and Drinks @ 6 pm, Talk begins @ 7:00 p.m.  The meeting will also be available on Zoom again this month!  Zoom login will be emailed to members prior to the meeting.

Title: The Robertson Deposit: Eocene Intrusion Related Gold Deposit in the Northern Shoshone Range, Nevada

 Presenter:  Neal Mankins, Center for Research in Econ. Geology

Authors: Neal E. Mankins,1 John L. Muntean,1 Matthieu Harlaux,1 and Robert A. Creaser2 

1Ralph J. Roberts Center for Research in Economic Geology, University of Nevada Reno, Reno, Nevada

2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada

Abstract:  The Robertson deposit, formerly known as “Tenabo”, is located in Lander County within the Bullion mining district about 7 km north of Pipeline, a 20Moz+ Carlin-type gold deposit in the northern Shoshone Range, Nevada. Robertson is an Eocene intrusion related gold deposit primarily hosted in Paleozoic sedimentary rocks within the Roberts Mountain allochthon known as the “upper plate.” Historically, gold was discovered in the Tenabo area in 1905, when a series of gold-bearing quartz veins and shear zones were found, producing nearly 10,000 ounces of gold from 1906-1912. Gold mineralization is spatially and temporally related to an intrusive complex, “the Tenabo stock”, which is composed of successive intrusive phases including an early diorite, polyphase ilmenite-bearing granodiorite, and later dikes that vary in composition from dioritic to rhyolitic. Most of the previously defined gold mineralization occurred near the contacts of intrusions and is hosted by hornfels in the overlying lithologies. Ongoing exploration has discovered significant free visible gold both within the stock and in more distal settings within metasedimentary host rocks. Intrusion-related gold deposits in Nevada are not well documented and Robertson may provide new insights into this style of mineralization supplementing other studies at Bald Mountain, Buffalo Valley, and Copper Canyon. The purpose of this study is to characterize gold mineralization as it relates to intrusive phases, alteration styles, and structural controls by combining fieldwork, petrography, geochemistry and geochronology. This study is the first in depth examination of Robertson focusing on gold mineralization within intrusive phases and in distal settings, presenting six new U/Pb LA-ICP-MS ages of intrusive phases, and one molybdenite Re-Os age, constraining emplacement ages of igneous units and associated alteration and mineralization. A paragenetic sequence of mineralization and alteration events at Robertson was constructed based on field relationships, drill core logging, assays, and structural data. The emplacement of the Tenabo stock produced contact metamorphism of the host siliciclastic lithologies yielding quartz, biotite, and calc-silicate hornfels aureole. As a result of field mapping, core logging, and applied petrography, successive hydrothermal alteration phases have been identified, including potassic alteration, endo/exoskarn, sericitic, and chloritic alteration. Previous studies along with this work indicate that gold occurs as disseminations and in quartz veins within intrusive units and host metasedimentary rocks. Gold consists of native particles that range in size from 2 to 200 microns having strong associations with As-Bi-Te-Se-S sulfosalts, spatially adjacent to or on the surface of arsenopyrite and loellingite grains. Ongoing petrographic observations of veins at Robertson indicate the succession of three main stages referred as 1) pre-gold “early,” 2) syn-gold, and 3) post-gold “late.” The pre-gold stage includes barren quartz veins, potassic alteration veins comprised of biotite (EDM veins and/or secondary K-feldspar), quartz – chalcopyrite ± pyrrhotite, and quartz – molybdenite veins. The syn-gold stage is composed of quartz – gold ± bismuth – loellingite – arsenopyrite veins. Finally, the post-gold veins encompass quartz – pyrite, quartz – calcite – sulfide “base metal veins” (Pb-Zn-Sb-Ag-Sn bearing sulfides), and calcite veins. Cross-cutting relationships along with our geochronological data indicate that gold mineralization at Robertson resulted from the emplacement of a reduced ilmenite-bearing igneous stock into siliciclastic sedimentary lithologies forming a high-tonnage, low-grade gold deposit characterized by a Au-Bi-Te-Se-S metal association. Based on petrographic evidence, we infer that mineralizing conditions for gold precipitation were under relatively low sulfur fugacity and oxidation state leading to preferential formation of minerals such as native Bi, arsenopyrite, and loellingite, typical of low-sulfidation assemblages. Please see their websites for more information: Timberline Drilling Inc. - EVENT SPONSOR Incorporated in 1996, Timberline Drilling, Inc. is headquartered in beautiful Coeur d’Alene, Idaho and supported by our field office in mining hub Elko, Nevada. We are a leading underground and surface core-drilling contractor in the United States.  Our ability to deliver quality core samples, on time and on budget, from meticulously clean and safe drill sites, has made us the go-to contractor for several clients. Our goal is to create long-term relationships with every customer we have the pleasure the serve.  https://timberline-drilling.com/   Ralph J. Roberts Center for Research in Economic Geology (CREG) CREG is a partnership between the Mackay School of Earth Sciences and Engineering, the Nevada mining industry, and the U.S. Geological Survey. These partners pool their resources to fund research that is both fundamental in understanding the genesis of mineral deposits and applicable to the discovery and production of mineral deposits. The original mutually supportive goals of the CREG program continue to be to:
  1. Prepare well-educated professionals for careers in the Mining Industry and Research by exposing them to a wealth of expertise within the School, the industry, and the USGS.
  2. Produce and publish high-quality research results on topics of critical interest to the Nevada mining industry.
The CREG program is a center of study and learning that is leading to a greater understanding of the geology, geochemistry, geophysics, genesis and exploration of mineral deposits, especially Nevada’s famous Carlin-type gold deposits, which make the U.S. the third leading gold producer in the world. The pooled resources within CREG are mainly used to fund graduate students to complete Masters and PhD theses in the Department of Geological Sciences and Engineering at UNR.  http://www.nbmg.unr.edu/CREG/
Reno, NV
Event StartsEvent Ends
05/19/202205/19/2022
All Day Event
6:00pm8:00pm