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.
This content will become publicly available on September 27, 2023
Variable Modes of Formation for Tonalite–Trondhjemite–Granodiorite–Diorite (TTG)-related Porphyry-type Cu±Au Deposits in the Neoarchean Southern Abitibi Subprovince (Canada): Evidence from Petrochronology and Oxybarometry
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 more »
- Award ID(s):
- 1924142
- Publication Date:
- NSF-PAR ID:
- 10357801
- Journal Name:
- Journal of petrology
- ISSN:
- 0022-3530
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Understanding the behavior of chalcophile elements during the evolution of arc magmas is critical to refining models for the formation and distribution of porphyry copper deposits used in mineral exploration. Because magmas in continental arcs undergo copper depletion during their early differentiation, a widely held hypothesis posits that the removed copper is locked at the base of the crust in copper-rich cumulates that form due to early sulfide saturation. Testing this hypothesis requires direct evidence for such copper-rich reservoirs and a comprehensive understanding of the mechanisms driving sulfide saturation. Interaction between oxidized magmas and reducing crustal material in island arcs has been shown to be an efficient process causing sulfide saturation. However, the extent to which crustal assimilation impacts the flux of chalcophile elements during magmatism in thick continental arcs remains to be established. Here, we provide a deep perspective into these problems by studying a suite of subarc cumulate rocks from the Acadian orogen, New England (USA). These cumulates record the imprint of subduction zone magmatism and represent the residues left behind during the genesis of intermediate to evolved Acadian magmas (ca. 410 Ma). We find that the most primitive Acadian cumulates are enriched in copper (up tomore »
-
Iron oxide copper-gold (IOCG) deposits are major sources of Cu, contain abundant Fe oxides, and may contain Au, Ag, Co, rare earth elements (REEs), U, and other metals as economically important byproducts in some deposits. They form by hydrothermal processes, but the source of the metals and ore fluid(s) is still debated. We investigated the geochemistry of magnetite from the hydrothermal unit and manto orebodies at the Mina Justa IOCG deposit in Peru to assess the source of the iron oxides and their relationship with the economic Cu mineralization. We identified three types of magnetite: magnetite with inclusions (type I) is only found in the manto, is the richest in trace elements, and crystallized between 459° and 707°C; type Dark (D) has no visible inclusions and formed at around 543°C; and type Bright (B) has no inclusions, has the highest Fe content, and formed at around 443°C. Temperatures were estimated using the Mg content in magnetite. Magnetite samples from Mina Justa yielded an average δ56Fe ± 2σ value of 0.28 ± 0.05‰ (n = 9), an average δ18O ± 2σ value of 2.19 ± 0.45‰ (n = 9), and D’17O values that range between –0.075 and –0.047‰. Sulfide separates yieldedmore »
-
The oxidation state of the Earth’s mantle and its partial melting products exert a key control on the behavior and distribution of sulfur and chalcophile and siderophile elements between the mantle and crust, underpinning models of ore deposit formation. Whether the oxidized nature of magmas is inherited from the asthenospheric mantle source or acquired during ascent and differentiation is vigorously debated, limiting our understanding of the mechanisms of extraction of sulfur and metals from the mantle. Here, we focused on the redox-sensitive behavior of sulfur in apatite crystallized from quenched alkaline basaltic melts preserved within a peridotite xenolith from the El Deseado Massif auriferous province in southern Patagonia. We took advantage of this unique setting to elucidate the redox evolution of melts during their ascent through the subcontinental lithospheric mantle (SCLM) and grasp the inner workings of the Earth’s mantle during gold metallogenesis. Our data reveal that an initially reduced silicate melt (1FMQ 2.2 to 1.2) was oxidized to 1FMQ between 0 and 1.2 during percolation and interaction with the surrounding peridotite wall-rock (1FMQ 0 to C0.8). This process triggered changes in sulfur speciation and solubility in the silicate melt, boosting the potential of the melt to scavenge ore metalsmore »
-
Magnetite is the most important iron ore in iron oxide-apatite (IOA) deposits which represent the Cu-poor endmember of the iron oxide-copper–gold (IOCG) clan. Magnetite chemistry has been used as a petrogenetic indicator to identify the geological environment of ore formation and as a fingerprint of the source reservoir of iron. In this study, we present new textural and microanalytical EPMA and LA-ICP-MS data of magnetite from Carmen, Fresia, Mariela and El Romeral IOA deposits located in the Cretaceous Coastal Cordillera of northern Chile. We also provide a comprehensive summary and discussion of magnetite geochemistry from Andean IOAs including Los Colorados, Cerro Negro Norte, El Romeral (Chilean Iron Belt) and the Pliocene El Laco IOA deposit located in the Central Volcanic Zone of the Chilean Andes. Microtextures coupled with geochemical data were used to define and characterize the occurrence of different magnetite types. Magnetite exhibits a variety of textural features including oscillatory zoning, colloform banding, re-equilibration textures, exsolution lamellae and symplectites. The magmatic vs. hydrothermal origin of the different magnetite types and the evolution of IOA deposits can be assessed using diagrams based on compatible trace elements. However, magnetite is very susceptible to hydrothermal alteration and to both textural and compositionalmore »
