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El Teniente porphyry coppermolybdenum deposit, Central Chile

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Cannell, JB (2004) El Teniente porphyry coppermolybdenum deposit, Central Chile. PhD thesis, University of Tasmania.

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Abstract

El Teniente occurs in the late Miocene-early Pliocene metallogenic belt of central
Chile. It is the world's largest known copper resource, containing 94.4Mt of fine
copper, and 2.5Mt of fine molybdenum. The ore deposits formed during the final
stages of a period of compression and crustal thickening initiated approximately 15
m.y. ago due to subduction of the Juan Fernandez Ridge.
El Teniente is hosted by the Miocene Farellones Formation and is located at the
intersection of two major faults. The NNW trending Codegua Fault is interpreted to
have formed from reactivation of a basement Triassic rift and has localised late
Miocene volcanism. The NNE-trending Teniente Fault Zone controlled the
emplacement of the 8.9 to 7 Ma Sewell Diorite complex. The Teniente host sequence
is a strongly altered package of mafic to intermediate sills, stocks, extrusives and
volcaniclastic rock. Early, widespread, barren magnetite-Ca-plagioclase alteration of
the host sequence occurred, prior to emplacement of the late Miocene - early Pliocene
calc-alkaline Teniente intrusive complex.
Copper-molybdenum ore at El Teniente is hosted in veins and subordinate breccias,
and is associated with extensive zones of hydrothermal alteration. Sulfide minerals are
zoned from bornite (core) through chalcopyrite to pyrite (deposit periphery). The 0.5
%copper contour defines a 2.6 km long and up to 2.0 km wide wedge shape, broadly
centred on the Teniente intrusive complex. The timing of vein and breccia formation
has been constrained temporally by nine new Re-Os dates (5.9 to 4.7 Ma) obtained
from molybdenite. The grey porphyry (diorite), dacite pipes, and NNW-trending,
multiphase dacite porphyry dyke intruded the host sequence during the Late Magmatic
(LM) stage (5.9 to 4.95 Ma). Multiple generations of quartz-anhydrite-chalcopyritebornite
veins and anhydrite-sulfide-biotite breccias formed at this time. These
structures host approximately 60% of the copper at El Teniente. Na-K-feldspar
alteration occurred within and around some of the dacite intrusions, grading out to
intense, texturally destructive biotite alteration and distal chlorite-stable propylitic
alteration assemblages.
Chalcopyrite-rich veins with phyllic (sericitic) alteration halos formed in the
Principal Hydrothermal (PH) stage (4.95 to 4.85 Ma). Despite the short duration of this stage and the low vein densities, these veins host approximately 30% of the total
copper resource at El Teniente. No coeval intrusive phase has been identified.
The Late Hydrothermal (LH) stage (4.85 to 4.40 Ma) is a second stage of phyllic
alteration and veining, which is related to intrusion of the 1200 m wide, funnel-shaped
Braden pipe and also to the emplacement of late dacite dykes (4.8 Ma). The pipe is
composed of an inner, unmineralised, rock flour matrix brec~cia facies, and an outer,
tourmaline-chalcopyrite-anhydrite-cemented marginal facies. LH stage veins have a
diverse ore and gangue mineralogy, including base metal sulfides, sulfosalts,
tourmaline, and carbonates. Late post-mineralisation and alteration hornblende dykes
(3.8- 2.8 Ma) are the youngest rocks in the deposit.
LM and PH veins are orientated mostly concentrically and radially around a
postulated deep-seated magma chamber, interpreted to have sourced the upper crustal
intrusions, stresses, heat, metals, and fluids for the Teniente deposit. In contrast, the
LH veins are orientated steeply-inward dipping, concentric to the magma chamber,
implying that they formed during a stage of magma withdrawal. Other paragenetically
late veins and faults are NE trending, which formed when far field stresses associated
with the TFZ exceeded the stresses localised around the magma chamber.
Abundant liquid-rich opaque) low to moderate salinity fluid inclusions occur in
LM veins, which are interpreted to have trapped a one-phase magmatic-hydrothermal
fluid at 5oo·c ± 1 oo·c. Sporadic decompression of this fluid resulted in generation of
brine and vapour phases. The brine phase cooled and was diluted as it migrated
laterally away from the dacites. Proton induced X-ray emission (PIXE) analyses
detected several weight percent copper in both high and low salinity fluids in the
centre of the deposit. One fluid inclusion analysed from the propylitic zone contains
only 0.01 wt % copper. During the PH and LH stages the hydrothermal fluids were
boiling at temperatures between 450- 3oo·c. Hydrostatic pressure estimates indicate a
depth below the palaeowater table of less than ~2,500m for the PH stage and less than
~I, 700m for the LH stage. Salinity arrays provide support for fluid mixing as a
potential depositional mechanism at El Teniente.
Oxygen and deuterium isotopic analysis indicates the predominance of magmatichydrothermal
fluids (o18
0fluid +5.7o/oo to +8.2o/oo) in most stages at El Teniente, even
at the deposit periphery. The exception is LH carbonates (o18
0fluid = +2.4o/oo to +9.1%o)which have a significant meteoric water component. oD11uid values for LM stage (-39%o
to -56%o) overlap with the felsic magmatic fluid values, whereas deuterium
enrichment in PH and LH stage fluids (-38%o to -6%o) imply the involvement of
volcanic vapours. Sulfur isotope values for sulfides at El Teniente are between -5.9%o
and +2.4%o and for sulfates are+ 10.0%o to+ 13.4%o. These values are consistent with a
bulk sulfur isotopic composition of 6%o. The most negative values from LM stage
sulfides occur close to and within the dacites, grading out to values around zero on the
deposit periphery. This zonation can be explained by an oxidized fluid (S02/H2S = 6)
being progressive reduced as it migrated outwards from the dacites. The vertical
zonation of sulfur isotope values from the PH and LH stages can be modelled by
cooling an oxidised fluid (S02/H2S = 2-3) from approximately 475"C to 325"C over
1,000m elevation, indicating a vertical temperature gradient of 15"C/100m. This
gradient is too large to be explained simply by conductive cooling or phase separation
and requires either fluid mixing or thermal disequilibrium in the system.
Strontium and neodynium isotopes for anhydrite from all the vein stages are from
0.70396 to 0.70404 and 0.51276 to 0.51281, respectively. These values overlap with
the local wall rock compositions from which they were most likely sourced. In
contrast, lead isotopic values for the same anhydrites vary widely, for example
206PbP04Pb values are between 17.490 and 18.559. These values are depleted compared
to the sulfide ores, which have the same lead isotopic composition as the host rocks.
Lead isotopic values in anhydrite are zoned, with most enriched values occurring in the
centre of the deposit to most depleted values at the deposit periphery, which may have
been derived from an unidentified exotic source of lead.
Ore deposition during the LM stage at El Teniente is believed to have occurred
mainly due to sulfate reduction and cooling of lithostatically-pressured, magmatichydrothermal
fluids. Secondary magnetite has been altered to biotite, implying that a
very effective reductant interacted with both the mineralising fluid and the wallrock.
The transition to the PH stage involved a change to brittle conditions and hydrostatic
pressures, possibly due to rupturing of a lithostatic seal as the deposit was exhumed to
depths shallower than 2,500m. High uplift rates (-2.4mm/yr) have been calculated for
the short-lived PH and LH stages. Eventually magmatic and fluid pressures exceeded
the confining lithostatic pressures (possibly facilitated by phreatomagmatic explosion),and explosive brecciation and fluidization of the rock mass occurred. This resulted in
the emplacement of the Braden Pipe. Ore deposition in the PH and LH stages was most
likely related to phase seperation-induced cooling, coupled with meteoric fluid mixing,
at least during the LH stage.
Apart from its anomalous stze, El Teniente is a typical porphyry copper-
I
molybdenum deposit, in terms of its alteration and sulfide assemblage, zonation,
association with felsic intrusions, and predominance of quartz vein-hosted copper
mineralization. Mineralogical, isotopic, and fluid inclusion datasets at El Teniente
indicate that a complex interplay of processes occurred during ore formation, including
fluid mixing, cooling, oxidation-reduction, phase separation, and water-rock
interaction. The combination of these processes resulted in the formation of this giant
porphyry copper deposit.

Item Type: Thesis (PhD)
Date Deposited: 25 Jun 2012 08:28
Last Modified: 11 Mar 2016 05:54
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