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: stuart@hotsolutions.co.nz).

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.

Gold deposition in active epithermal environments in New Zealand

Stuart F Simmons

stuart@hotsolutions.co.nz

El camino por iluminacion es largo y difcil, es por lo cualyo te pido traer sandwiches y un cambio de ropa

Simmons & Browne, 2000

phase diagram: Hayashi & Ohmoto, 1991Geochim. Cosmochim. Acta

Maximum goldsolubility

Broadlands-Ohaaki

52 geothermal wells

400-2700 m depth

1-2 ppb Au deep water

1 million oz Au / 6000 yrs

Br-Oh Trace MetalsSubsurface Environment

Well precipitates: elevated Au-Ag-Au-SbAu-As correlate poorly

Colloform amorphous silicaw- Au-Ag-rich sulfides

Well silencer-Kawerau

0.5 cm

Br-Oh Trace MetalsSubsurface Environment

Altered rockselevated As-Sb (~10:1)Au-As nil to modest

correlation

altered rocks

WK data (Mitchell, 1989)

Simmons & Browne, 2000

Champagne Poolchloride waters

collapse cratersacid sulfate waters

p

h

o

t

o

:

L

l

o

y

d

H

o

m

e

r

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)

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

Simmons & Browne, 2000

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.