SEG 45 Final_qx4 - Society of Economic Geologists

10 SEG NEWSLETTER No 63 • OCTOBER 2005
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Exploring for Deposits Under Deep Cover Using Geochemistry (Continued)
the strong anomalies for Zn and Cd
were enigmatic. Why would these elements
be strongly anomalous over an
Au-Cu deposit? Further drilling by
Newmont described by Norby and
Orobona (2002) revealed the presence of
a deposit-wide blanket of sphalerite concentrated
in the upper 60 m of the sulfide
zone, approximately 500 m below
surface. The secondary Zn blanket, containing
1 to 4% Zn, contains submicrometer-sized
framboids of sphalerite
that have extremely low values for δ 34 S,
down to –70‰, indicative of bacterial
reduction of sulfate during supergene
alteration (Bawden et al., 2003).
Cadmium is a ubiquitous constituent of
sphalerite, with contents typically in the
range 0.1 to 0.8% (Piatak et al., 2004).
Newmont is currently measuring the Cd
content of the sphalerite-rich zone.
Preliminary results give average Cd/Zn
ratios above that obtained from crustal
abundance data for these elements.
Norby and Orobona (2002) provide
additional structural detail for the
deposit. The interpreted post-Carlin
Formation fault B, where the strongest
geochemical anomalies are found at the
surface, corresponds to the Nebulous
fracture zone, which forms the boundary
structure for the West Mike secondary
mineralization, including the Zn-rich
zone (John Norby, pers. commun., 2003).
Its location in the basement was recognized
by gravity contrast caused by offsets
along the basement unconformity.
This fault has a westerly dip (Norby and
Orobona, 2002), and thus the surface
trace (fault B) shown by the topographic
relief is east of the gravity expression in
the basement (Figure 1a). Similarly, fault
C is the surface expression of the westerly
dipping North-Pointing Dog fault of
Norby and Orobona (2002). Fault A may
be the surface topographic expression of
a westerly dipping D-Day fault or a
down-faulted block between the
D-Day and Hillside faults. North-northeast–striking
faults, such as the
Nebulous, locally control gold mineralization
and also down-drop mineralization
and the base of oxidation. There are
landslides along the southwest projections
of the Nebulous and D-Day faults
(John Norby, pers. commun., 2003),
which may reflect recent earthquakes.
We interpret the anomalies at the
surface intersection of the Nebulous
fracture zone to be the result of mobilization
of metals where the permeable
fault zone cuts the ores, and pumping
of the resulting metalliferous fluids up
the fault. It is likely that it was the constituents
of the sulfide zone that were
the most amenable to oxidation and
mobilization. In addition to Au, Cu, Cd,
and Zn, other elements such as Ag, As,
Ba, Hg, Mo, Ni, Sb, Se, and V are
anomalous along the surface intersection
of the Nebulous fault. Major elements
such as K and Na, which might
indicate hydrothermal alteration, are
not enhanced. Isotopic studies by
Dublyansky et al. (2003) at the Yucca
Mountain nuclear waste disposal site,
also in Nevada, have shown that fluids
of deep-seated origin have moved up
several hundred meters along a permeable
fault through a thick vadose zone.
SPENCE DEPOSIT
Spence is a supergene-enriched copper
porphyry deposit located between
Antofagasta and Calama in the
Atacama Desert of northern Chile.
RioChilex discovered the deposit in
1996 by reconnaissance drilling.
Porphyry intrusion and hypogene mineralization
took place during the
Palaeocene. Following supergene
enrichment, the deposit was covered by
50 to 100 m of piedmont gravels of
Miocene age. The gravels are indurated
and for the most part are poorly sorted
with a fine grained matrix that makes
them relatively impermeable, except
where fractured or in better sorted layers,
as near their base. Copper minerals
are atacamite and brochantite within
the oxide zone, and chalcocite and covellite
in the enriched zone. The primary
sulfides comprise chalcopyrite, bornite,
molybdenite, tennantite, and pyrite.
Reserves recoverable by open-pit mining
are 79 Mt of oxide ore at 1.18% Cu
and 231 Mt of sulfide ore at 1.13% Cu.
The long axis of the deposit and the
porphyry intrusions trend north-northeast,
similar to the orientation of a
prominent lineament that runs through
the area. We carried out sampling of
soils and groundwaters within and
around the deposit in 1999 and 2000;
results are described by Cameron et al.
(2004) and Cameron and Leybourne
(2005).
In this region, groundwater flows
southwest. Over most of the deposit, the
water table lies within the basal gravels,
which act as an aquifer, but in the
south of the deposit it lies below the
unconformity. Contents of Cl in the
groundwaters (Fig. 3) show two distinct
types of groundwater: low-salinity water
east of the long axis of the deposit, and
saline water west and downstream from
the axis. There is an order of magnitude
difference in the Cl content of the two
waters, which average 1,300 mg/L and
11,600 mg/L, respectively. The maximum
for the saline water is 21,200
mg/L, compared to seawater with
19,000 mg/L Cl. The saline water is distinguished
by high contents of a number
of elements, most notably As (Fig. 3)
and Se, but also, B, Br, Ca, I, K, Li , Mg,
Sr, and Rb (Cameron and Leybourne,
2005).
The two waters are also distinguished
by differences in their isotopic composition.
On a δ 2 H vs. δ 18 O plot (Fig. 4), the
low-salinity waters plot near the global
meteoric water line (GMWL), whereas
the saline waters plot well to the right
(Cameron and Leybourne, 2005). Formation
waters recovered from deep sedimentary
basins are different from meteoric
waters, both in their higher salinity
and the deviation in δ 2 H and δ 18 O values
from the GMWL. The saline groundwaters
from Spence plot within the field
of formation waters indicating that the
waters found on either side of the axis
of the deposit are of different origins:
those to the east are meteoric waters,
those to the west are formation waters,
with mixing of the two as they flow
down-gradient towards the southwest.
The Atacama Desert is hyper-arid;
rainfall may occur only once every few
years. Recharge for the groundwater
that lies beneath the desert floor is in
the Andes mountains and foothills ca.
120 km to the east, and precipitation is
significant only above altitudes of 3,000
m (Spence lies at 1,700 m). The δ 18O
composition of precipitation varies with
the altitude of the land surface, becoming
increasingly negative with greater
altitude. In northern Chile, Aravena et
al. (1999) found values of δ 18O in the
range –5 to –7 ‰ at 2,500 m altitude,
decreasing to –20 ‰ above 4,000 m.
The least saline of the Spence groundwaters,
which are interpreted to be of
meteoric origin, range in δ 18 O from –8
to –11 ‰, consistent with derivation
from precipitation at higher altitudes
east of Spence.
Over the deposit, Cu is enriched in
both saline and meteoric waters (Fig. 5),