Timing and Duration of Mineralisation at the Bajo de la Alumbrera Porphyry Copper Deposit and the Petrogenetic Link to Explosive Volcanism

This thesis comprises three papers describing the formation of the world class Bajo de la Alumbrera porphyry copper deposit in NW Argentina. Combining high precision zircon geochronology with geochemistry we have constrained the timescales of porphyry copper deposit formation and established a temporal and genetic link to nearby explosive volcanism. Finally, this link to explosive volcanism provides a unique opportunity to study the volcanic deposits to reveal key insights into the formation of a world class porphyry copper deposit. Chapter 1 provides a introduction and overview to the objectives of the thesis and details of the current state of research in both geochronology and the genesis of porphyry copper deposits. Chapter 2: From a long-lived upper-crustal magma chamber to rapid porphyry copper emplacement: Reading the geochemistry of zircon crystals at Bajo de la Alumbrera (NW Argentina), presents new high precision zircon U-Pb geochonological data in order to constrain the timing and duration of porphyry copper deposit formation. We calculate the duration of ore-formation by dating zircons from pre-, syn- and post-mineralisation porphyries based on intersection relations between successive intrusion and vein generations, using high precision chemical-abrasion isotope-dilution thermal ionisation mass spectrometry (CA-ID-TIMS). Based on the youngest assemblages of zircon grains, which overlap within analytical error, all four intrusions were emplaced within 29 ka, which places an upper limit on the total duration of hydrothermal mineralisation. Re/Os dating of hydrothermal molybdenite fully overlaps with this high-precision age bracket. By examining the geochemistry of zircon antecrysts, which record ~200 ka of magmatic evolution, we show that that the four porphyry intrusions record a common geochemical and thermal history, and that they were derived from the same upper-crustal magma reservoir. During this upper crustal magmatic evolution, the presence of low cathodoluminescent zones ("dark-zones") in some zircon crystals record zircon crystallisation during a transient event of rapid growth. This rapid growth resulted from mafic magma injection into the base of the magma chamber, releasing a CO2-rich vapour phase into the dacitic crystal mush. This vapour phase then migrated upwards to the apical part of the magma chamber from where it was expelled, together with successive batches of magma, to form the porphyry copper deposit within a short time-span of less than a few 10,000 years. The short duration of host rock emplacement, hydrothermal alteration and mineralisation presented in this study provides critical constraints on fluid storage in magma chambers and the genesis of large porphyry copper deposits. This chapter is published in Earth and Planetary Science Letters. Chapter 3: Zircon petrochronological evidence for a plutonic-volcanic connection in porphyry copper deposits, presents new high-precision U-Pb zircon geochronological evidence linking the formation of the Bajo de la Alumbrera porphyry copper deposit to nearby explosive volcanism. The results indicate that the magmatic-hydrothermal deposit and 245 m of explosive volcanism were derived from a common magma reservoir that evolved over a minimum duration of 217 ± 25 ka before the fi nal eruption. A time-gap of 19 ± 12 k.y. between porphyry mineralization and the onset of explosive volcanism indicate that these distinct events occurred as the result of a common magmatic process. We propose that deposit formation and volcanic eruption were both triggered by the same injection of a volatile-saturated primitive magma into the base of the magma chamber. Catastrophic loss of volatiles by explosive volcanism terminated the ore forming capacity of the upper crustal magma chamber, as evidenced by the intrusion of a syn-eruptive barren quartz-feldspar porphyry. Our results demonstrate that porphyry copper deposits provide critical information to understand how volatiles control the fate of hydrous magmas, between pluton formation and explosive volcanism. This chapter is published in Geology. Chapter 4: Magmatic petrogeneseis leading to the formation of the Bajo de la Alumbrera porphyry copper deposit: Insights from cogenetic explosive volcanism, builds on the findings of Chapter 3 by examining the magmatic petrogenesis of the explosive volcanism that terminated the ore formation at the Bajo de la Alumbrera porphyry copper deposit. We examined amphibole and apatite crystals from the entire volcanic sequence to track the magmatic evolution during the time preceding ore formation. The presence of two distinct generations of amphibole crystals from the same volcanic rocks reveals records both a deep (15 - 25 km) and shallow (~10 km) crustal evolution of porphyry forming magmas. The upper crustal evolution is revealed from amphibole and apatite crystals, which indicate the co-crystallisation of plagioclase, amphibole, biotite, apatite, titanite and zircon in an upper crustal magma reservoir, which evolved over ~200 ka. During the upper crustal evolution, the magma was H2O saturated and exsolving a magmatic fl uid. The melt compositions in equilibrium with apatite reveal the upper crustal magma was not enriched in S, relative to other arc magmas. Conversely, amphibole compositions reveal low Cu concentrations (~10 ppm) in the melt at the time of eruption, which suggest either the upper crustal magma reservoir had inherently low Cu concentrations or the formation of the Bajo de la Alumbrera porphyry copper deposit (~19 ka prior to explosive volcanism) effectively stripped the upper crustal magma reservoir of Cu. The lower crustal record is preserved in mafic crystal clots that appear towards the upper parts of the 7 Ma volcanic sequence. The presence of olivine pseudomorphs, which are replaced by clinopyroxene and amphiboles, together with the absence of plagioclase corroborates their crystallisation in the lower crust under hydrous conditions. High fO2 (ΔNNO > 3), revealed from the amphibole and Cr-rich spinel compositions, likely resulted from extended olivine crystallisation. Injection of this oxidised, hydrous, magma into the upper crustal magma reservoir at a late stage is recorded by abundant dissolution and reverse zoning in the major phenocryst phases towards the upper parts of the 7 Ma volcanic sequence. Our results demonstrate that the interplay between the injection of oxidised magmas from the lower crust and high Cl contents in the upper crustal magma chamber, promoted the efficient extraction of Cu and S from the melt to form the world class Bajo de la Alumbrera porphyry copper deposit. Chapter 5 presents the general conclusions of this thesis including the potential for future work