21 ledge history report
Post on 29-Jan-2018
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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
Geologic Map of the 4100 Level Homestake Mine showing Ore Ledges, workings (cyan) and Homestake Fm (red) - grid 1000 feet green
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 Ledge Fold Plunge Line
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
21 Ledge Fold Plunge Line
gold ore body
gold ore bodygold ore body
21 Ore Ledge 5.2 million oz Au
Geologic Map of the 5900 Level Homestake Mine showing Ore Ledges, workings (cyan) and Homestake Fm (red) - grid 1000 feet green
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
Geologic Map of the 6800 Level Homestake Mine showing Ore Ledges, workings (cyan) and Homestake Fm (red) - grid 1000 feet green
North Drift
21 Ledge
Yates Member
Deep Main Ledge Drill Program
25 LedgeDrill Program
15 Ledgedrill program
Enlarged view of 21 Ledge 6800 Level Homestake Mine showing workings (cyan), Homestake Fm (red) and Ore Bodies (orange)
grid 1000 feet green
ore body
ore body
ore body
6 Shaft
7 Shaft
21 Ledge
Ellison Fm
Ellison Fm
Ellison Fm
ef
ef
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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
23 Ledgedrill program
15 Ledgedrill program
Deep Main Ledgedrill program
25 Ledgedrill program
Enlarged view of 21 Ledge 7400 Level Homestake Mine showing workings (cyan), Homestake Fm (red) and Ore Bodies (orange)
grid 1000 feet green
Poorman Formation
Poorman Formation
Ellison Formation
Ellison Formation
ore body
ore body
ore body
21 Ledge
Geologic Map of the 8000 Level Homestake Mine showing Ore Ledges, workings (cyan) and Homestake Fm (red) - grid 1000 feet green
Yates Member
21 Ledge
Deep 21 Ledgedrill program
Deep 25 Ledgedrill program
Deep Main Ledge drill program
15 Ledgedrill program
Enlarged view of 21 Ledge 8000 Level Homestake Mine showing workings (cyan), Homestake Fm (red) and Ore Bodies (orange)
grid 1000 feet green
21 ledge
21 ledge
Deep 21 Ledgedrill program
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AB
B'
AB
B'
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