Active Components of the Subcontinental Lithospheric Mantle for Crustal Metallogenic Endowment in Off-Craton Regions

Abstract

Ore deposits are rock volumes with anomalously high concentrations of selected elements that enable their extraction with economic profit. Metallic ore deposits may form in a wide spectrum of geological conditions, although they mostly occur in the magmatic and hydrothermal environments. Despite extensive investigation aimed to clarify the genesis and distribution of magmatichydrothermal ores, major controversies still concern the processes that drive the transfer and storage of metals from background levels in their source region to exploitable amounts in the crust. A key unresolved aspect regards whether the ore-productive magmas inherit their metallogenic fertility from pre-enriched domains of the mantle source, or they attain high metal concentrations due to favorable fractionation conditions during magma percolation and ascent. This issue is especially important for targeting mineral camps, as it constrains the specific geological conditions and environments that enhance the metallogenic endowment of the continental crust. To this regard, a better knowledge of the mechanisms and repositories that control the mobility and storage of metals in mantle rocks and magmas is essential. The general aim of this Ph.D. thesis is to assess the geological factors that govern the mobilization, transport, and concentration of chalcophile metals in the subcontinental lithospheric mantle (SCLM), emphasizing their impacts on the formation and distribution of magmatic-hydrothermal ore deposits in the overlying crust. To achieve this main goal, the thesis investigates the mineralogy, geochemistry and petrology of mantle peridotite xenoliths and associated volcanic rocks from mineralized regions in Central Mexico and Southeast Spain, focusing on base-metal sulfides (BMS), platinum-group minerals (PGM) and nano-tomicrometer- sized metal-rich particles as mineralogical-geochemical tracers of the behavior of precious and semi-metals in the SCLM. For this purpose, this study combines traditional analytical methods (e.g., transmitted/reflected light optical microscopy, scanning electron microscopy, electron microprobe analysis) with advanced techniques for the in-situ characterization of minerals (e.g., laser ablation-(multicollector) inductively coupled plasma-mass spectrometry, highresolution transmission electron microscopy). The structure of the thesis comprises a general introduction to the research topics, followed by the description of aims and methods. Subsequently, the first chapter of the results section (Chapter 4) presents a mineralogical and geochemical study of BMS and silicates in mantle peridotite xenoliths from the Santo Domingo, Ventura-Espíritu Santo and Durango volcanic fields (Central Mexico). The obtained results support that these rocks record low degrees of partial melting in the stability fields of garnet (⁓ 2%) and spinel peridotites (⁓ 2- 4%), and interaction with hydrous alkaline melts possibly during the Basin and Range extensional tectonism since Late Oligocene. Enclosed and intergranular grains of monosulfide solid solution (mss) in Santo Domingo peridotites are residues after the extraction of 0.1-0.5 fractions of Ni-Cu-rich sulfide melt during mantle melting events. On the other hand, globular sulfides (pentlandite ± chalcopyrite) hosted in glass veinlets in the Ventura-Espíritu Santo and Durango peridotites crystallized from Ni-Cu-rich droplets of sulfide melt, immiscible in the Quaternary silicate magmas that brought the xenoliths to the surface. Rheniumdepletion model ages extrapolated by Re-Os isotope analysis of these sulfide populations indicate that part of the SCLM of the Oaxaquia terrane (beneath Santo Domingo and Ventura-Espíritu Santo) originated in the Archean-Paleoproterozoic as the southernmost extension of the Laurentia craton, and was assembled with the Central terrane (beneath Durango) during the Grenville orogeny (⁓ 1.0 Ga). Early Paleozoic (⁓ 500 Ma) model ages common to sulfides from the peridotite xenoliths of the three volcanic fields suggest that the Oaxaquia-Central composite block split away from North America during the Rodinia break-up and experienced the Pan-African-Brasiliano orogeny that led to Gondwana assembly. During the Cenozoic, the reactivation of translithospheric faults bounding this composite old SCLM provided preferential pathways for focusing the ascent of ore-productive magmas/fluids associated with the subduction-related metallogeny of the Pacific active margin of Mexico. The second chapter of the results section (Chapter 5) examines a suite of spinel peridotite xenoliths (plus one plagioclase-bearing) hosted in alkaline basalts from the Tallante volcanic field (Southeast Spain). Mantle metasomatism in fertile lherzolites from Tallante caused the crystallization of clinopyroxene + orthopyroxene + spinel clusters due to the percolation of Miocene subalkaline melts during the westward migration of the subduction front in the western Mediterranean. In the Pliocene, heat and volatiles provided by alkaline hostmagmas triggered very low melting degrees of metasomatic pyroxene-spinel assemblages, generating melt quenched to silicate glass and reactive spongy coronae around clinopyroxene and spinel. The refertilization event documented by the Tallante peridotites involved the precipitation of abundant base-metal sulfides currently included as spherical droplets within metasomatic clino- and orthopyroxene. These sulfide inclusions consist of pentlandite ± chalcopyrite ± bornite aggregates with homogeneous compositions in terms of major elements (Ni, Fe, Cu) and semi-metals (Se, As, Te, Sb, Bi), but with wide variability of platinum-group elements (PGE) fractionation (0.14 < PdN/IrN < 30.74). Furthermore, several grains of Pt-Pd-Sn-rich platinum-group minerals (PGM) and/or Au-particles were detected as euhedral nano-to-micrometer-sized inclusions within the BMS. The heterogeneous geochemical and mineralogical distribution of precious metals within the Tallante sulfides cannot be explained by conventional models of chalcophile partitioning from sulfide melt. Rather, these characteristics reflect the possible incorporation of distinct populations of BMS, PGM, and metal nanoparticles (especially of Pt, Pd, and Au) during mantle melting and/or melt percolation. Therefore, these results support that Miocene subalkaline melts released by asthenosphere upwelling upon slab tearing of the Iberian continental margin effectively produced metal-rich metasomatized domains within this sector of SCLM. Remarkably high Au concentrations in Tallante BMS (median 1.78 ppm) support that these metasomatized domains provided a fertile source of metals, especially gold, for the ore-productive Miocene magmatism of the westernmost Mediterranean. The third chapter of the results section (Chapter 6) reports the presence of Pb-rich nanoparticles enclosed in BMS from metasomatized mantle peridotite xenoliths (Tallante volcanic field) and spatially/genetically associated lamproite dykes (Fortuna volcanic field) from the Neogene Volcanic Province of Southeastern Spain. These BMS document the journey of sulfide melt droplets extracted by partial melting of the SCLM and transported in silicate alkaline magmas through the lithosphere. Nanoinclusions of galena in BMS have microstructural features that are inconsistent with their origin by low-temperature exsolution of PbS previously dissolved within the BMS crystal-lattice. Rather, these minerals likely crystallized from Pb(-Cu)-rich nanomelts originally immiscible within the sulfide liquid. This first mineralogical evidence of immiscibility between metal(Pb)-rich nanomelts and sulfide liquids challenges the traditional models of metal and sulfide transport in silicate magmas based solely on chemical solubility and chalcophile partitioning. Instead, these results require evaluating the role of surface-energy physical relationships between nanophases, sulfide liquids and silicate magmas in the transport and concentration of mantlederived metals in the crust.