iii gold deposition in geothermal environments.pdf
TRANSCRIPT
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The following presentation was prepared for a short course toMinera Meridian Peru SAC,
Cuzco, Peru, 14 December, 2008.
Most of the images and graphs are the property of Stuart F Simmons or his colleagues and the slides are made available for the sole purpose of reviewing
information delivered during the short course.
Please obtain permission from Stuart F Simmons to use any of these images for publication or presentation (email: [email protected]).
The main references for this presentation are Simmons and Browne, 2000 & Simmons et al., 2004.
Simmons, S. F. and Browne, P. R. L., 2000, Hydrothermal minerals and precious metals in the Broadlands-Ohaaki geothermal system: Implications for understanding low-sulfidation epithermal environments; Economic Geology, v. 95, p. 971-999 .
Simmons, S. F., Mauk, J.L. and Rowland, J. R., 2004, Active and extinct hydrothermal systems of the North Island, New Zealand: Society of Economic Geologists Field Guide, 124 pp.
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Gold deposition in active epithermal environments in New Zealand
Stuart F Simmons
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El camino por iluminacion es largo y difcil, es por lo cualyo te pido traer sandwiches y un cambio de ropa
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Simmons & Browne, 2000
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phase diagram: Hayashi & Ohmoto, 1991Geochim. Cosmochim. Acta
Maximum goldsolubility
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Broadlands-Ohaaki
52 geothermal wells
400-2700 m depth
1-2 ppb Au deep water
1 million oz Au / 6000 yrs
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Br-Oh Trace MetalsSubsurface Environment
Well precipitates: elevated Au-Ag-Au-SbAu-As correlate poorly
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Colloform amorphous silicaw- Au-Ag-rich sulfides
Well silencer-Kawerau
0.5 cm
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Br-Oh Trace MetalsSubsurface Environment
Altered rockselevated As-Sb (~10:1)Au-As nil to modest
correlation
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altered rocks
WK data (Mitchell, 1989)
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Simmons & Browne, 2000
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Champagne Poolchloride waters
collapse cratersacid sulfate waters
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NG4 NG13 N G11S ESea le velUmawera SouthF ault Maungamutu Fault
Wai parera Fault FaultWairoaNW Shear Zone(extrapolated)Shear ZoneSprings(ex trapoltaed)Shear ZoneShear Zone
1 kmVertical ex aggeration X4
Se a level20 0 m
- 400m
-1200m- 800m
Mangatawai
A B
b)
Cretaceous-Permiansediments Greywackebasement
Depth below sea level (meter)
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Waiotapu Trace MetalsSurface Environment
Chloride waters: elevated Au-As-Sb (filled circles)
Acid-sulfate waters: very low Au-As-Sb (open circles)
Au deposits due to absorption on to colloids
As-Sb-S deposit due to mixing and cooling
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Simmons & Browne, 2000
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Key points
High concentrations of gold and silver deposit from boiling in geothermal wells: analogue for epithermal veins
Gold and silver deposit as low grade, sub-economic disseminations in country rock, strongly associated with As and Sb.
Gold and silver deposit in hot springs with As, Sb, Hg, Tl, and are incorporated into silica sinter. These occurrences do not form epithermal ore bodies because they are too small and rare.