Configuration of Columbia, a Mesoproterozoic Supercontinent

A-type granites in northeastern China: age and geochemical constraints on their petrogenesis

What do experiments tell us about the relative contributions of crust and mantle to the origin of granitic magmas

Lithotectonic elements of the Grenville Province: review and tectonic implications

A hybrid origin for the Qianshan A-type granite, northeast China: Geochemical and Sr–Nd–Hf isotopic evidence

Nd isotope evidence for crust-mantle interaction in the generation of A-type granitoid suites in Labrador, Canada

The metal concentrations of sulfide liquids depend upon the mass of silicate
magma that they are able to process for metals. Some high-grade magmatic Ni-Cu
(±PGE) deposits and many magmatic platinum group element (PGE) (±Ni-Cu)
deposits demand very high magma/sulfide ratios that seem improbable in the light
of physical and kinetic constraints. Recent models for some high-grade deposits
invoke multistage upgrading processes, in which an early-formed sulfide liquid
reacts with multiple later batches of silicate magma. Quantitative models of
this open-system process demonstrate that it is indeed more efficient than a
closed system. However, it is likely that most later magmas in such a system
will be sulfur undersaturated and will thus partly redissolve preexisting
sulfide liquids, further increasing metal concentrations in the remaining
sulfide liquids. This combined process is termed "multistage-dissolution
upgrading," and quantitative models show that it could reduce the mass of
silicate magma that must be processed by as much as two orders of magnitude.
Furthermore, if sulfide liquids are extensively dissolved during enrichment,
their base metal concentrations will generally stabilize at a limiting value,
whereas their PGE concentrations will generally increase without limit. This
divergence could account for unusually high PGE concentrations and high PGE/base
metal ratios in many PGE-dominated deposits. The multistage-dissolution
upgrading model is tested, using reasonable degrees of dissolution, in the
context of natural deposits in the Noril’sk area and the Bushveld intrusion.
The models reproduce the observed sulfide compositions significantly better than
previous models and are also consistent with other aspects of the geology of
these deposits. Thus, dynamic sulfide-forming magmatic systems may be
intrinsically self destructive, but this apparently undesirable attribute could
play an important role in forming high-grade deposits. However, dissolution could lead to the complete destruction of sulfide
liquids and the return of all metals to later magma batches. In this case, no
sulfide deposit would remain, but metal-depletion signatures indicating sulfide
liquid segregation would be preserved in rocks that crystallized from early
magmas. Caution is therefore advised in the use of magmatic depletion signatures
to infer overall mineral potential or to estimate the possible sizes of
undiscovered magmatic sulfide deposits.

Self-Destructive Sulfide Segregation Systems and the Formation of High-Grade Magmatic Ore Deposits

Abstract The Makkovik Province of Labrador represents the extension of the Ketilidian Mobile Belt of south Greenland into mainland North America; it exhibits a threefold division into a foreland region, a fold-and-thrust belt, and an interior magmatic zone. The Kaipokok Domain is dominated by Archaean basement rocks that form an extension of the North Atlantic Craton, but Proterozoic reworking is recorded by the reorientation of a c. 2230 Ma dyke swarm. Supracrustal rocks, consisting of shallow-marine sedimentary rocks overlain by greywackes and mafic volcanic rocks, rest unconformably upon Archaean basement, but towards the interior of the belt the basal unconformity is eradicated by northwest-directed thrusting. In the Aillik Domain, highgrade supracrustal rocks of similar aspect to those of the Kaipokok Domain are separated from the basement by mylonite zones, in a thick-skinned fold-and-thrust belt (Kaipokok Bay Structural Zone), believed to record significant northwest-directed translation. The Aillik Domain also contains abundant felsic volcanic rocks that lack typical arc-like geochemical signatures. The Cape Harrison Domain is dominated by plutonic rocks, including suites of 1840 Ma, 1800 Ma, 1720 Ma and 1650 Ma age, but gneissic inliers apparently represent ‘juvenile’ Proterozoic crust. The dominant 1800-1720 Ma plutonic suites are late-orogenic to post-orogenic, siliceous, potassic granitoid rocks, which resemble Phanerozoic post-collisional suites, rather than subduction-related arc batholiths. Nd isotopic variations position the eastern edge of the North Atlantic Craton close to the boundary between the Aillik and Cape Harrison Domains. The structural evolution of the Makkovik Province records a shift from pre- 1800 Ma northwest-directed thrusting to post-1800 Ma tight upright folding, also northwest-verging. However, there is also evidence for earlier (pre- 1890 Ma) events in the Kaipokok Domain. Major unresolved problems include the timing of early sub-horizontal deformation (perhaps related to collisional events), the age relations and setting of supracrustal sequences, the location of suture zones, the absence of clear arc-like magmatic assemblages, and the nature and antiquity of the eastern juvenile crustal block.

The Makkovik Province: extension of the Ketilidian Mobile Belt in mainland North America

The importance of late- and post-orogenic crustal growth in the early Proterozoic: evidence from SmNd isotopic studies of igneous rocks in the Makkovik Province, Canada

Magmatic sulfide mineralization is present in three of the four compositional groupings in the Mesoproterozoic Nain Plutonic Suite and in compositionally similar Paleoproterozoic rocks. Anorthosite-hosted mineralization appears to be spatially linked to small gabbronorite bodies, where sulfide liquids separated from silicate magmas to variable degrees, depending on the wall-rock conditions. Sulfide metal contents are low (0.2–1.0% Ni) and the Ni/Cu ratio is variable, although sulfide Ni values remain fairly consistent within individual occurrences. Pyroxenite-hosted mineralization is essentially restricted to minor intrusions, and locally, has higher sulfide metal contents (up to 2% Ni) and less variable Ni/Cu. There are similarities between pyroxenite- and anorthosite-hosted mineralizations, and they may form part of a broad continuum. Ferrodiorite-hosted disseminated sulfide mineralization is associated with abundant iron oxides, and typically, has low amounts of Ni in the sulfide (<0.5%), and Ni/Cu <<1. Gabbro-troctolite–hosted mineralization contains the most variable and metal-enriched sulfides, containing from 1 to 7 percent Ni, and showing consistent Ni/Cu ≪1. The best examples are associated with the Voisey’s Bay intrusion (dominantly troctolite) and the Pants Lake intrusion (dominantly olivine gabbro). Significantly, these are the only Nain Plutonic Suite mafic intrusions that were emplaced into or in close proximity to sulfide-bearing paragneisses; exploration of gabbroic and troctolitic rocks within Archean orthogneisses has not yet revealed significant mineralization. The Pants Lake intrusion has intriguing geologic similarities to the Voisey’s Bay intrusion and supports some aspects of genetic models for the latter. However, it has lower sulfide Ni contents (around 2%) and relatively elevated Cu and Co, suggesting differences in the source magmas and/or conditions of sulfide segregation.

The different styles of mineralization can be integrated with petrogenetic models for the Nain Plutonic Suite and similar plutonic suites. Anorthosite-hosted sulfide mineralization is interpreted to be derived from residual mafic magma that coalesced and migrated during ascent of semicrystalline anorthositic magmas, and ferrodiorite-hosted mineralization is interpreted to be derived from late Fe-enriched differentiates of deep-seated mafic chambers. In both cases, the source magmas were fractionated, which limits their potential for metals. Crustal sulfur from paragneisses may have played a role in sulfide saturation in both cases but does not appear to be a critical ingredient. Some pyroxenite-hosted mineralization is akin to anorthosite-hosted types, but other examples may be linked to silicic contamination of more mafic magmas. Gabbro-troctolite–hosted mineralization is associated with mafic magmas that rose from deep crustal chambers prior to extensive fractionation; these were richer in Ni, Cu, and Co when they ascended into the upper crust. Sulfide-bearing paragneisses apparently played a critical role in inducing sulfide saturation in the magmas. The lessons from the search for nickel in Labrador are mostly familiar from other areas; the regional metallogeny of the Nain Plutonic Suite underlines the critical roles of a relatively unevolved host magma and crustal contamination processes, possibly involving addition of externally derived sulfur. It emphasizes the importance of identifying and prioritizing these two key factors in the exploration of similar anorthosite-dominated plutonic suites elsewhere in the world.

/pdf/threading-the-eye-of-the-needle-lessons-from-the-search-for-5af1hny7ca.pdf

Threading the Eye of the Needle: Lessons from the Search for Another Voisey’s Bay in Labrador, Canada

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