Lithospheric conductors reveal source regions of convergent margin mineral systems.

The clean energy transition will require a vast increase in metal supply, yet new mineral deposit discoveries are declining, due in part to challenges associated with exploring under sedimentary and volcanic cover. Recently, several case studies have demonstrated links between lithospheric electrical conductors imaged using magnetotelluric (MT) data and mineral deposits, notably Iron Oxide Copper Gold (IOCG). Adoption of MT methods for exploration is therefore growing but the general applicability and relationship with many other deposit types remains untested. Here, we compile a global inventory of MT resistivity models from Australia, North and South America, and China and undertake the first quantitative assessment of the spatial association between conductors and three mineral deposit types commonly formed in convergent margin settings. We find that deposits formed early in an orogenic cycle such as volcanic hosted massive sulfide (VHMS) and copper porphyry deposits show weak to moderate correlations with conductors in the upper mantle. In contrast, deposits formed later in an orogenic cycle, such as orogenic gold, show strong correlations with mid-crustal conductors. These variations in resistivity response likely reflect mineralogical differences in the metal source regions of these mineral systems and suggest a metamorphic-fluid source for orogenic gold is significant. Our results indicate the resistivity structure of mineralized convergent margins strongly reflects late-stage processes and can be preserved for hundreds of millions of years. Discerning use of MT is therefore a powerful tool for mineral exploration.

Oxygen isotopes trace the origins of Earth's earliest continental crust.

Much of the current volume of Earth's continental crust had formed by the end of the Archaean eon1 (2.5 billion years ago), through melting of hydrated basaltic rocks at depths of approximately 25-50 kilometres, forming sodic granites of the tonalite-trondhjemite-granodiorite (TTG) suite2-6. However, the geodynamic setting and processes involved are debated, with fundamental questions arising, such as how and from where the required water was added to deep-crustal TTG source regions7,8. In addition, there have been no reports of voluminous, homogeneous, basaltic sequences in preserved Archaean crust that are enriched enough in incompatible trace elements to be viable TTG sources5,9. Here we use variations in the oxygen isotope composition of zircon, coupled with whole-rock geochemistry, to identify two distinct groups of TTG. Strongly sodic TTGs represent the most-primitive magmas and contain zircon with oxygen isotope compositions that reflect source rocks that had been hydrated by primordial mantle-derived water. These primitive TTGs do not require a source highly enriched in incompatible trace elements, as 'average' TTG does. By contrast, less sodic 'evolved' TTGs require a source that is enriched in both water derived from the hydrosphere and also incompatible trace elements, which are linked to the introduction of hydrated magmas (sanukitoids) formed by melting of metasomatized mantle lithosphere. By concentrating on data from the Palaeoarchaean crust of the Pilbara Craton, we can discount a subduction setting6,10-13, and instead propose that hydrated and enriched near-surface basaltic rocks were introduced into the mantle through density-driven convective overturn of the crust. These results remove many of the paradoxical impediments to understanding early continental crust formation. Our work suggests that sufficient primordial water was already present in Earth's early mafic crust to produce the primitive nuclei of the continents, with additional hydrated sources created through dynamic processes that are unique to the early Earth.

No evidence for high-pressure melting of Earth's crust in the Archean.

Much of the present-day volume of Earth's continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5 GPa (>50 km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40 km average thickness of modern continental crust.