Copper isotope fractionation in sedimentary copper mineralization ...

242 D. Asael et al. / Chemical Geology 243 (2007) 238–254
Cambrian Timna basin. The TIC is unconformably overlain
and onlapped by a sequence of sedimentary Cambrian
rocks consisting of the fluvial sandstones [Amudei
Shelomo (Solomon's Pillars) Formation], the Lower
Cambrian Timna Formation, and Middle Cambrian
continental sandstones of the Shehoret Formation.
The Timna Formation consists of the lower Member
(Hakhlil) overlain by the upper Member (Sasgon), which
formed in a marine lagoonal–sabkha environment
during a marine transgression. Copper mineralization
predominantly occurs in the sandstones, dolomites and
shales of the Sasgon Member (Fig. 2b), as well as in the
Shehoret Formation sandstones (Bentor, 1956; Wurzburger,
1967; Garfunkel, 1970; Bartura and Wurzburger,
1974; Segev, 1986; Shlomovitch et al., 1999). Lower
Cretaceous Formations, which unconformably overlie
the Cambrian sediments, also feature minor copper
mineralization, most probably derived by reworking of
the Cambrian deposits (Kedar, 1984). These Lower
Cretaceous cupriferous sandstones are not studied in this
work, which is restricted to the Cambrian and Precambrian
rocks.
Intense erosion and peneplainization of the Timna
igneous rocks has left few exposures of the copper
porphyry rocks. Primary igneous copper minerals in the
exposed rocks have undergone secondary temperature
alteration (most probably supergene), and only minute
relics of igneous chalcopyrite are found (Wurzburger,
1967).
Sedimentary Cu-sulphides occur in dolomite rocks of
the Timna Formation as dispersed mm to cm sized
spherules (Fig. 3a and b). The dolomites are strictly
mangano-dolomites with up to 2.8 wt.% manganese as
Mn 2+ in the dolomite lattice (Bar-Matthews, 1987;
Segev and Sass, 1989). The Cu-sulphides are interpreted
to have formed in reducing conditions during early
diagenesis (Shlomovitch et al., 1999). These Cusulphides
were then replaced by Cu(II) minerals in
two stages: 1) alteration to malachite (Cu 2 (OH) 2 CO 3 ),
which armours and sometimes completely replaces the
sulphide minerals and is the only time that this mineral
formed in the Cambrian sedimentary sequence; 2) development
of paratacamite (Cu 2 (OH) 3 Cl) as veins
emanating from the sulphides and cross-cutting the
dolomitic rocks. Copper silicates are principal minerals
of the sandstones and shales of the Timna and Shehoret
Formations. Optical microscope and scanning electron
microscope (SEM) studies show that these minerals
formed by the reaction of quartzo-feldpathic minerals
with Cu(II) solutions (Charach et al., 1976; Shlomovitch
et al., 1999; Fig. 3d). The high abundance of copper in
sandy lithofacies of the Timna Formation has also been
attributed to in-situ copper pre-concentration resulting
from karstic dissolution of the carbonate fraction of
precursor sandy dolomites (Segev and Sass, 1989).
Early studies proposed that deposition occurred
through stratiform syngenetic sedimentation in lagoons
(Bentor, 1956; Wurzburger, 1967; Bartura and Wurzburger,
1974). Charach et al. (1976) later proposed that
weathering of the Precambrian porphyries transported
copper into the lowermost clastic sandstone sequences,
and that subsequent mobilization of copper into the
overlying sediments was induced by migrating groundwater.
Mixing of groundwater with saline marine water
during the marine transgression would provide the
solutions necessary for the transport of copper, either as
Cu(II) chloride complexes in relatively oxidized conditions
or as CuCl 3 2− complex in intermediate redox conditions
(Renfro, 1974; Rose, 1976; our own calculations
using The Geochemist's Workbench version 4.03).
The Cu-bearing rock samples studied in this work
are: 1) Cambrian dolomites, containing spherules of Cusulphides,
partially or completely altered to malachite
and paratacamite (Fig. 3a and b); 2) Cambrian sandstones
and shales, containing veins of Cu-silicates
(Fig. 3c and d); and 3) copper porphyry from the TIC
containing secondary supergene Cu-sulphides and Cu
(II) minerals (Fig. 3e and f). Because primary copper
minerals are not found in the Timna Valley igneous
rocks, six Cu-sulphide samples (chalcopyrite, bornite,
covellite) from the Copper porphyry ore deposits at
Butte, Montana were analyzed for comparative
purposes.
3. Analytical methods
3.1. Sample preparation
Dolomite and copper porphyry samples were first
sawn into thin slices in order to expose as many Cusulphides
as possible. The rock slices were then drilled
using a 1 mm diameter diamond drill in the dark sulphiderich
cores and blue-green envelopes (Cu(II) minerals)
(Fig. 3a and e). Other samples, such as paratacamite veins
in dolomites, and veins of Cu-silicates, were separated
from their host rock by crushing and hand picking.
Mineral phase identification was performed using X-ray
powder diffraction (XRD) using a Bruker D8 Advance X-
ray diffractometer at the Hebrew University Center for
Nanoscience and Nanotechnology and by SEM at the
Geological Survey of Israel (GSI) (JEOL-840 combined
with LINK-10000 energy dispersive spectrometer). The
elemental chemical composition of samples was determined
at the GSI with a Perkin Elmer Optima Inductively