Assessing the origin of some representative Archean-Paleoproterozoic porphyry Cu ± Au deposits
Date
2021-06-04
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Abstract
Most known porphyry Cu ± Au deposits are exclusively associated with oxidized and sulfurrich Phanerozoic arc-related magmas, but Precambrian analogues are rare. The genesis of some
rare examples of such deposits (e.g., Tongkuangyu in Trans-North China Orogen) remains
debated, and in particular, it remains unclear as to whether similar metallogenic processes that
typify the Phanerozoic deposits also operated in the Precambrian. To resolve these issues, three
study areas were chosen to variably characterize relevant aspects of these deposits in the
context of porphyry-type metallogenies (i.e., the nature of mineralization, the redox states and
volatile element abundances of the causative magmas): (1) the ~2.1 Ga Tongkuangyu deposit
in Trans-North China Craton; (2) the ~1.88 Ga Haib porphyry Cu deposit, Southern Namibia;
and (3) representative porphyry-type Cu ± Au deposits (Côté Gold, St-Jude, and Croxall) in the
~2.7 Ga Abitibi subprovince.
At the Tongkuangyu deposit, remapping of the distribution of the host rocks, alteration, and
mineralization support a porphyry-type model where ore formation occurred at ~2.1 Ga. This
age is significantly later than the host granodiorite porphyry (~2.18 Ga) and schists (~2.5–2.2
Ga) and indicates that porphyry-type mineralization need not be linked directly to a causative
magma exposed at surface. Instead, the latter is interpreted to be represented by subjacent
porphyritic stocks and dikes.
At the Haib deposit, the host calc-alkaline plutonic rocks and mineralization are dated to ca.
1886–1881 Ma. Estimations of magmatic fO2 and S, based on zircon geochemistry, apatite µXANES spectra, and apatite S analysis, demonstrate that oxidized, sulfur-rich arc magmas
associated with porphyry Cu mineralization already existed in the late Paleoproterozoic. In addition, zircon Hf-O isotopes and whole-rock geochemistry support magmas of mantlederivation with minor crustal contributions that experienced amphibole ± plagioclase
fractionation. These features of the Haib magmas are thus similar to those for Phanerozoic
porphyry Cu deposits.
For the ~2.7 Ga TTG (tonalite-trondhjemite-granodiorite-diorite)-related porphyry-type Cu ±
Au deposits in the Abitibi subprovince, whole-rock geochemistry, zircon Hf-O isotopes, apatite
S contents, and multiple oxybarometers suggest that variable magmatic origin and fO2 for the
causative magmas. The rarity of porphyry-type Cu-Au deposits in these older settings may be
attributed to either location restriction of these favorable metallogenic conditions and/or
preservation, or exploration bias.
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Keywords
: Porphyry Cu systems, Archean, paleoproterozoi, Trans-North China Orogen, Richtersveld Magmatic Arc, Abitibi greenstone belt, Tongkuangyu, Haib, Côté Gold,, St-Jude, Croxall, Magmatic oxygen fugacity, Zircon, Apatite, µ-XANES, zircon Hf-O isotopes