Most known porphyry Cu±Au deposits are associated with moderately oxidized and sulfur-rich,
calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched
tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized
to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation.
Here, we test this hypothesis by interrogating the causative magmas for the ∼2·7 Ga TTG-related
Côté Gold, St-Jude, and Clifford porphyry-type Cu±Au deposit settings in the Neoarchean southern
Abitibi subprovince. New and previously published geochronological results constrain the age
of emplacement of the causative magmas at ∼2·74 Ga, ∼2·70 Ga, and∼2·69 Ga, respectively.
The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a
plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for
tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford
igneous rocks yielded εHf(t)±SD values of 4·5±0·3, 4·2±0·6, and 4·3±0·4, and δ18O±SD values
of 5·40±0·11 , 3·91±0·13 , and 4·83±0·12 , respectively. These isotopic signatures indicate
that, although these magmas are mantle-sourced with minimal crustal contamination, for the St-
Jude and Clifford settings the magmas or their sources may have undergone variable alteration by
heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite,
and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist
facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values,
reported relative to the fayalite–magnetite–quartz buffer as FMQ, using zircon geochemistry
indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from
FMQ –0·3±0·6 to FMQ +0·8±0·4 and to FMQ +1·2±0·4, respectively. In contrast, amphibole
chemistry yielded systematically higher fO2 values of FMQ +1·6±0·3 and FMQ +2·6±0·1
for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray
absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ S) in
primary apatite yielded ≥ FMQ−0·3 and FMQ+1·4–1·8 for St-Jude and Clifford, respectively. The
magnetite–ilmenite mineral pairs from the Clifford tonalite yielded FMQ +3·3±1·3 at equilibrium
temperatures of 634±21 ◦C, recording the redox state of the late stage of magma crystallization.
Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to
0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed
to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results
to indicate that the deposits at Côté Gold and Clifford formed from mildly (∼ FMQ +0·8±0·4)
to moderately (∼ FMQ +1·5) oxidized magmas where voluminous early sulfide saturation was
probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of
externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma
chamber (∼ FMQ +0). Furthermore, we conclude that variable modes of formation for these
deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may
be attributed to either local restriction of favorable metallogenic conditions, and/or preservation,
or an exploration bias.
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Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga
Most known porphyry Cu deposits formed in the Phanerozoic and are exclusively associated
with moderately oxidized, sulfur-rich, hydrous arc-related magmas derived from partial
melting of the asthenospheric mantle metasomatized by slab-derived fluids. Yet, whether
similar metallogenic processes also operated in the Precambrian remains obscure. Here we
address the issue by investigating the origin, fO2, and S contents of calc-alkaline plutonic
rocks associated with the Haib porphyry Cu deposit in the Paleoproterozoic Richtersveld
Magmatic Arc (southern Namibia), an interpreted mature island-arc setting. We show that
the ca. 1886–1881 Ma ore-forming magmas, originated from a mantle-dominated source with
minor crustal contributions, were relatively oxidized (1‒2 log units above the fayalitemagnetite-
quartz redox buffer) and sulfur-rich. These results indicate that moderately oxidized,
sulfur-rich arc magma associated with porphyry Cu mineralization already existed in
the late Paleoproterozoic, probably as a result of recycling of sulfate-rich seawater or sediments
from the subducted oceanic lithosphere at that time.
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- Award ID(s):
- 1924142
- NSF-PAR ID:
- 10294820
- Date Published:
- Journal Name:
- Nature communications
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
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