SEG 45 Final_qx4 - Society of Economic Geologists

14 SEG NEWSLETTER No 63 • OCTOBER 2005
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Exploring for Deposits Under Deep Cover Using Geochemistry (Continued)
WEST Cu Soil
Anomalies
EAST
Water Table
Gravel
Andesite
Basement
Saline
Water
minerals, or held within secondary
minerals, such as carbonates or iron
and manganese oxides. The objective is
to maximize the anomaly/background
contrast. For both Spence and other
sites in Chile, and for Mike the response
of five selective leaches: deionized
water, Enzyme Leach, MMI, ammonium
acetate, and hydroxylamine, plus
one non-selective leach, aqua regia,
were compared (Cameron et al., 2004).
At the Chile sites selective leaches give
better anomaly/background contrast
than aqua regia. This is consistent with
Fracture Zone
Cu Deposit
Porphyry
Meteoric
Water
FIGURE 7. Conceptual model for groundwater flow and generation of soil anomalies over
the Spence deposit. Saline formation waters from depth migrate up a fracture zone coincident
with the deposit. In the deposit and gravel aquifer they mix with meteoric waters
derived from precipitation in the Andes to the east. During earthquakes, saline waters are
pumped to the surface entraining Cu from the deposit. Salts derived from the evaporation
of water reaching the surface are largely preserved in the hyper-arid climate of the
Atacama desert.
the model that anomalous elements,
such as Cu, have been transferred to
the surface dissolved in water, and soil
forming processes have yet to incorporate
these in more resistant minerals
that are dissolved by aqua regia. At
Mike, the contrast shown by selective
leaches and aqua regia are similar.
Given the locus of the anomalies at
Mike—on the interpreted scarp slope
of the Nebulous fault zone—the soil
anomalies may have been derived from
mineralized fault rock, not directly
from water.
ACKNOWLEDGEMENTS
At Mike, Robert Jackson of Newmont
Inc. provided a great deal of help and
advice. Our sampling there was carried
out with Mary Doherty and Kevin Creel.
John Norby kindly provided much additional
information on the geology of
Mike. At Spence, Ollie Bonham and
Jack Currie of RioChilex provided many
courtesies. EMC and MIL were assisted
in the field in Chile with enthusiasm
and humour by Daniel Salinas and
Alexi Ramirez. George Steele of Rio
Tinto generously gave logistical support
for the Chile work. We thank CAMIRO
and 26 company sponsors for their support
and encouragement throughout
the project.
REFERENCES
Aravena, R., Suzuki, O, Pena, H., Pollastri,
A., Fuenzalida, H. and Grilli, A., 1999,
Isotopic composition and origin of the precipitation
in northern Chile: Applied
Geochemistry, v. 14, p. 411–422.
Bawden, T.M., Einaudi, M.T., Bostick, B.C.,
Meiborn, A., Wooden, J., Norby, J.W.,
Orobono, M.J.T. and Chamberlain, C.P.,
2003, Extreme 34 S depletions in ZnS at the
Mike gold deposit, Carlin trend, Nevada:
Evidence for bacteriogenic supergene sphalerite:
Geology, v. 31, p. 913–916.
Briggs, R.C. and Troxell, H.C., 1955, Effect of
Arvin-Tehachapi earthquake on spring and
stream flow, in Oakeshott, G.B., ed.,
Earthquakes in Kern County, California,
during 1952. Part 1: California Department
of Natural Resources, Division of Mines
Bulletin, v. 171, p. 81–98.
Cameron, E.M. and Doherty, M.E., 2001,
Geochemical data from a soil traverse
across the Mike gold-copper deposit, Carlin,
Nevada: Canadian Mining Industry
Research Organization (CAMIRO) Report,
17 p.
JOSEPH R. ANZMAN
Exploration Geophysicist
• consulting
• interpretation
• project management
• geophysical surveys
• domestic & foreign
P.O. Box 370526 303-337-4559
Denver, Colorado 80237 telephone/fax
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