Jamestown Gold Mine, CA: Mother Lode Exposed in Open Pit

 

In April 1992, I toured the Jamestown mine that is adjacent to Highway 49 in the Mother Lode district of California.  I snapped a photo of the southern highwall of the pit.  The mine was called the "Harvard Pit."



USGS 71/2' topo of the Jamestown area.

 

 

Google Earth image of the Jamestown area (Image May 2017)

I snapped this photo in 1992 of the highwall of the pit.  The mine was called the "Harvard Pit."

Harvard pit looking southeasterly.

Geologic sketch of the Harvard pit showing the pit lake.  Source:  Savage, Ashley, Bird, 2000 +/-, Geology of the Jamestown mine, Mother Lode gold district CA, and geochemistry of the Harvard mine pit lake, USGS.
Source:  Link to PDF of Presentation

 Harvard, Crystalline, and Alabama mines

• Discovered in 1848 placers and 1850 lodes.
• Major production from 1890 to 1916
• Gold produced was 110,000 to 160,000 troy ounces (grade 1.0 to 1.4 opt)
• Ore processed 330,000 to 1,100,000 tons

 

Jamestown mine (Harvard pit)

 

• Exploration periods 1938 to 1942 with the increased gold price and 1974 to 1985.
• Mining and processing from 1986 to 1994.
• 660,000 troy ounces produced.
• Ore processed 10,500,000 tons.
• Recovered grade 0.063 opt.
• Stripping ratio >4.15:1
• Largest "nugget" gold  recovered 876 troy ounces (lode gold not placer).

Source:  Savage, et.al.



 

Hanging wall:  Graphitic slate, metavolcanics. Quartz, albite, chlorite, sericite, graphite, ferroan dolomite, pyrite.

Ore Zone:  Sheared fragments of hang wall and footwall rocks and quartz veins.  Quartz, albite, mariposite, chlorite, sericite, talc, ferroan dolomite, magnesite, calcite, arsenian pyrite, native gold, sulfosalts, Au-Ag tellurides.

Footwall:  Altered serpentinite, diorite, gabbro.  Talc, tremolite, actinolite, chlorite, chromite, magnesite, gersdorffite, niccolite, millerite, colbaltite.

Gossan:  Goethite, magnesiocopiapite, jarosite, gypsum, hexahydrite.

Source:  Savage et.al.