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GMEG - Geology, Minerals, Energy, & Geophysics Science Center

Antimony in and around the Yellow Pine deposit, central Idaho

 

Issue:

Antimony is a critical mineral commodity for advanced technological uses and the US imports over 70% of antimony consumed domestically. Most of our imported antimony comes from China, which is beginning to reduce its production. Understanding of the origin of this deposit, the largest known antimony deposit in the US, would aid in future exploration for undiscovered deposits of this type, both domestically and internationally.

Images of the Yellow Pine area, ID. Access courtesy of Midas Gold, Inc. Select a thumbnail for a larger image.

View looking down into the Yellow Pine antimony-gold deposit

View looking down into the Yellow Pine antimony-gold deposit

Geologists observing the West End pit where gold ore was mined during the 1980’s and early 1990’s.  Photograph by Austin Zinsser.

Geologists observing the West End pit where gold ore was mined during the 1980’s and early 1990’s. Photograph by Austin Zinsser.

Boulder of folded calc-silicate beds from the Stibnite pit, which was mined for gold from 1995 to 1997.  Photograph by Austin Zinsser.

Boulder of folded calc-silicate beds from the Stibnite pit, which was mined for gold from 1995 to 1997. Photograph by Austin Zinsser.

Geologists finding a mercury-bearing mineral called cinnabar at Fern Mine, which is a small, historic mine.  Photograph by Austin Zinsser.

Geologists finding a mercury-bearing mineral called cinnabar at Fern Mine, which is a small, historic mine. Photograph by Austin Zinsser.

Retired crusher that was used to crush ore-bearing rocks near the historic Fern Mine, which produced mercury.  Photograph by Austin Zinsser.

Retired crusher that was used to crush ore-bearing rocks near the historic Fern Mine, which produced mercury. Photograph by Austin Zinsser.

Rock with abundant scheelite that fluoresces blue under shortwave UV light. Scheelite is calcium tungstate (CaWO4) and is the ore mineral from which tungsten is produced.  Photograph by Niki Wintzer.

Rock with abundant scheelite that fluoresces blue under shortwave UV light. Scheelite is calcium tungstate (CaWO4) and is the ore mineral from which tungsten is produced. Photograph by Niki Wintzer.

Objectives:

Document the origin of the Yellow Pine gold-antimony deposit and, by extension, the origin of this type of deposit. Our goal is to understand the structural, tectonic, and magmatic setting of the deposit, the character of the ore-transporting fluids, the conditions of ore deposition, and the regional stratigraphic framework and geochemical ore controls of metasedimentary packages that host some of the ore bodies. These studies should lead to a clearer model of this type of antimony deposit.

This will be a 3-pronged study of the Yellow Pine deposit aimed at understanding the controls of this important mineralization.

  1. Investigate the structure of the apparently controlling Meadow Creek fault, as well as the parallel Johnson Creek shear zone (which also hosts antimony prospects) 10 km to the west and the unnamed caldera-bounding fault 5 km to the east.
  2. Closely study the ore deposit geometries, their chemistry and mineralogy, and their alteration haloes to document the conditions of ore deposition. We also plan to analyze ore fluids extracted from fluid inclusions in ore and gangue minerals within the deposit to document the chemical and isotopic composition of the ore transporting fluids prior to ore deposition.
  3. Investigate metasedimentary roof pendants that host some of the antimony ore bodies, and correlate their stratigraphy, petrography, structural, and metamorphic histories with other pendants that surround the district and occur regionally in west-central Idaho. We believe this information will help us understand what stratigraphic features may control ore deposition within these metasedimentary packages.
Images of the Yellow Pine area, ID. Access courtesy of Midas Gold, Inc. Select a thumbnail for a larger image.

USGS contractor Shyla Hatch takes a strike/dip reading on foliated rocks within the antimony-gold deposits near Stibnite, ID

USGS contractor Shyla Hatch takes a strike/dip reading on foliated rocks within the antimony-gold deposits near Stibnite, ID

Geologist examining key minerals in the metasedimentary rocks northeast of Stibnite, Idaho.  Photograph by Austin Zinsser.

Geologist examining key minerals in the metasedimentary rocks northeast of Stibnite, Idaho. Photograph by Austin Zinsser.

Structural, stratigraphic and ore deposit studies at the Yellow Pine antimony-gold deposits at Stibnite, Idaho

Structural studies will focus on the geometry, style, offset features, kinematics, age constraints, pressure-temperature constraints, and structural evolution within the four major fault zones in the study area: the ore-controlling Meadow Creek fault, the ore-controlling West End fault, the parallel Profile Gap-Johnson Creek shear zone (which also hosts antimony prospects) 10 km to the west, and the unnamed fault bounding the Eocene Thunder Mountain caldera 5 km to the east. New geochronological analyses will be used to constrain movement on these faults, as well as the age of mineralization.

Ore deposit studies will first work with the mining company staff to document the known geometries of the ore bodies, their chemistry and mineralogy, their alteration haloes and the existing age constraints on ore deposition. A sampling program will be developed utilizing exposures in the mining area as well as available drill core. Samples will be selected which preserve fluid inclusions in ore and gangue minerals within the deposit. We also plan to analyze Ore fluids will be extracted and analyzed for their chemical and isotopic composition to document the nature of the ore transporting fluids prior to ore deposition. Further characterization of ore and gangue mineralogy may also be warranted.

Investigation of metasedimentary roof pendants that host some of the antimony ore bodies will focus on four topics:

  • The stratigraphic units, stratigraphic succession, and ages of the metasedimentary rocks present in the four roof pendants of the greater Yellow Pine district.
  • The appropriate correlations for the metasedimentary rocks in the roof pendants and the regional extents of the stratigraphic packages or units, including which units host ore along the Johnson Creek-Profile Gap shear zone.
  • The metamorphic and structural history as recorded in the roof pendants and the extent these structures controlled regional exposures of the stratigraphic packages.
  • Implications for local structural, rock composition, and (or) stratigraphic controls of the ore deposits at the West End ore deposit and the many other prospects in the Yellow Pine district roof pendants.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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