21 Ledge History Report

History of Discovery 21 Ledge and West Ledge System

Homestake Gold Mine - South Dakota

The Homestake Mine over its 125-year history produced 40 million ounces of gold. The gold was hosted exclusively in an iron-formation and produced preferentially from synclinal fold structures known as ledges. The deposit was discovered in 1876 and outcropped at the surface near Lead City, Dakota Territory in a fold structure that eventually became known as Main Ledge. Main Ledge provided almost all of the gold production for the first sixty years of mine’s operation. In the early 1930’s a second gold discovery occurred with 9 Ledge which was discovered during the sinking of the Ross Shaft at the 3050 Level. The 9 Ledge ore system lies west of Main Ledge and is not economic where the fold structure, up plunge, outcrops at surface. Main Ledge would in the end be 20 million ounces and 9 Ledge, 10 million ounces, first and second largest ledges in the mine.

The 9 Ledge discovery led to additional systematic exploration of ledge fold structures further to the west. During the 1950’s major underground exploration development was extended to the west on the 2600 Level with a 600-meter-long cross-cut and then 3500-meter-long on strike drift down the center of 17, 19, and 21 Ledges. Diamond drilling was conducted from the drift on 65 meter centers in an east and west direction. Minor gold mineralization was noted in the form arsenopyrite and very low gold.

The West Ledge exploration program was continued in the 1960’s on the 4100 level with similar cross-cuts, strike drifts, and the 65-meter center diamond drilling like that of the 2600 Level. Weak gold mineralization was discovered. Parallel exploration in time continued, jumping down to the 5900 Level and then the 6800 Level when the major ore bodies in 21 Ledge were identified in 1969. Mine development and definition drilling continued on 21 Ledge and the other west ledges through the 1970’s and early 1980’s. The center of mining focused on these new gold discoveries which were very high grade – 21 Ledge plus 10 g/t and large. There were some VCR stopes (bulk mined) in 21 Ledge that you could hang glide in – 200 meters long, 75 meters wide and 150 meters vertical.

During this time the mine had five production divisions – 1 through 5 with Division 5 focused on 21 Ledge and was producing the highest-grade ores in the mine until it close in 2001. The remaining divisions largely producing from remnant portions of the other ledges. 21 Ledge would eventually produce 5 million ounces of gold and in longitudinal section was quite compact coming and going in 400 vertical meters (Homestake’s third largest ledge). Its termination below the 7700 Level was acquit, expressed by a trail of arsenopyrite and weak gold mineralization into the Poorman Formation (foot wall stratigraphy). This has been hypothesized as the fluid path for gold mineralizing fluids to the ore bodies up plunge.

Viewed in plan, it should be noted that where the 21 Ledge synclinal fold becomes complicated by ductile shear intersection and interaction (6800 Level) the ore bodies in 21 Ledge reach their maximum. This is the case for all ledges, but the geometry of west ledges are unique

Geologic Map of the 2600 Level Homestake Mine showing Ore Ledges, workings (cyan) and Homestake Fm (red) - grid 1000 feet green

1950's West Ledge exploration drift with drill stations 65m centers

1950's West Ledge exploration drift

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21 Ledge

21 Ledge

Yates Member


1960's West Ledge exploration drift with drill stations 65m centers

1980's15 Ledge drill program

21 Ledge

Yates Member

Geologic Map of the 4850 Level Homestake Mine showing Ore Ledges, workings (cyan) and

Homestake Fm (red) - grid 1000 feet green

Yates Member

21 Ledge

WhitewoodCreek

Gold Run

Lead CityHomestake Open Cut SoutheastNorthwest

5900 Level

8000 Level

21 Ledge Fold Plunge Line

gold ore body


21 Ore Ledge5.2 million ounces gold

Longitudinal Section through 21 Ledge Homestake Mine South Dakota showing gold ore bodies

Phanerozoic coverUnconformity

lies 800 meters east

(8000 feet below surface)

Enlarged View of Longitudinal Section through 21 Ledge Homestake Mine South Dakota showing gold ore bodies and fold plunge line

5900 Level

8000 Level


gold ore body

gold ore bodygold ore body

21 Ore Ledge 5.2 million oz Au


Yates Member

21 Ledge

Enlarged view of 21 Ledge 5900 Level Homestake Mine showing workings (cyan), Homestake Fm (red) and Ore Bodies (orange)

grid 1000 feet green

21 Ledge Syncline

21 Ledge Syncline

ore bodies

6 Shaft

Poorman Formation

Poorman Formation

Ellison Formation


North Drift

21 Ledge

Yates Member

Deep Main Ledge Drill Program

25 LedgeDrill Program

15 Ledgedrill program



ore body

ore body

ore body

6 Shaft

7 Shaft

21 Ledge

Ellison Fm

Ellison Fm

Ellison Fm

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Poorman Fm

Poorman Fm

Geologic Map of the 7400 Level Homestake Mine showing Ore Ledges, workings (cyan) and Homestake Fm (red) - grid

1000 feet green

Yates Member



Deep Main Ledgedrill program




Poorman Formation

Poorman Formation

Ellison Formation

Ellison Formation

ore body

ore body

ore body

21 Ledge


Yates Member

21 Ledge

Deep 21 Ledgedrill program


Deep Main Ledge drill program




21 ledge

21 ledge


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Documents

21 Ledge History Report