| Abstract | dc.description.abstract | El Teniente porphyry copper deposit, the world’s
greatest intrusion-related Cu–Mo ore body, is hosted within
basaltic–andesitic volcanic and gabbroic rocks (mafic complex).
This ore body is strongly affected by multiple events
of alteration/mineralization with pervasive potassic and
chloritic alteration and coetaneous with associated copper
mineralization. We present paleomagnetic results obtained
from oriented samples at four locations within the mine and
from two drill cores, 200 and 400 m long, respectively.
Samples are representative of all the main hydrothermally
altered rock units, with emphasis on the mafic host rock and
dacitic (Teniente dacite porphyry) and dioritic porphyry
intrusions. Magnetic experiments [hysteresis loop, isothermal
remanent magnetization (IRM), k–T curves, thermal,
and alternating field demagnetization] show the presence of
prevailing magnetite. Microscope and SEM observations
show two families of magnetite, (a) large multidomain
magnetite grains, associated with biotite and chlorite of
various different hydrothermal alteration events, and (b)
abundant small to medium grain-size magnetite (<10 μm)
contained within plagioclase, either related to an early Na–
Ca–Fe alteration or included within plagioclase during
magmatic crystal growth. While the Teniente dacite
porphyry and the quartz diorite–tonalite have low magnetic
susceptibility (<0.0005 SI) and low natural remanent
magnetization (NRM, 10−4–10−3 Am−1), the mineralized
mafic host rocks have usually high susceptibility (>0.01
and up to 0.2 SI) with NRM in the range 0.1–2 Am−1.
Most mafic complex rock samples have univectorial
magnetizations during alternating field or thermal demagnetization.
Within the mine, the magnetic polarity is
spatially distributed. In the northern part of the deposit,
the Teniente dacite porphyry, the associated hydrothermal
breccias, and the hosting mafic complex record a reverse
polarity magnetization, also observed in the El Teniente
sub-6 mine sector immediately to the east and southeast. In
the eastern part of the deposit, a normal polarity is observed
for samples of the mafic complex from the two long drill
cores. There is no evidence for superimposed magnetizations
of opposite polarities in samples of the mafic
complex. Anhysteretic remanent magnetization (ARM) in a
DC field of 40 μT and NRM have similar magnitude and
comparable behavior upon alternating field demagnetization.
The well-defined strong remanent magnetizations
associated with high unblocking temperatures (>500°C)
indicate an acquisition of remanent magnetization during
mineralization by circulating high temperature fluids related
with ore deposition. Paleomagnetic results and the recorded
polarity zonation suggest multiple mineralization events
occurred at El Teniente, each one with its own evolution stages, superimposed within the district. These results
indicate that a simplified broad four-stage model for El
Teniente, as presented and overly employed by many
authors, divided in (1) late magmatic, (2) main hydrothermal,
(3) late hydrothermal, and (4) posthumous stage, does
not recognize various short-lived single mineralization
events, some superimposed and some distinctly separated
in time and space. There is no paleomagnetic evidence for
post-mineralization deformation. | en_US |
