The Bajo de la Alumbrera porphyry ore deposit, NW Argentina occurs in the Late Miocene Farallón Negro Volcanic Complex. Dacitic intrusions associated with Cu-Fe sulfide and Au mineralization are cotemporal with part of the host volcanic rocks. Observations made at Bajo de la Alumbrera, including those documenting the concentric zonal pattern of hydrothermal alteration, are included in the accepted genetic models of porphyry ore deposits. Geological mapping and field observations (~18 km of measured stratigraphic sections, and ~75 km2 detailed structural mapping), combined with new U-Pb zircon geochronology and whole-rock geochemistry of the Farallón Negro Volcanics have reinterpreted the structure, architecture, and development of the volcanic complex that hosts the Bajo de la Alumbrera deposit.
The Late Miocene Farallón Negro Volcanic, which conformably overlie a generally thin Miocene red-bed sequence, has been dismembered by a series of major younger steep reverse faults, such that crystalline basement is now exposed on three sides of the complex. Broadly a volcanic complex, the district is dominated (particularly in the basal parts) by thick volcaniclastic accumulations with lesser lavas and associated breccias. This volcanosedimentary pile (~1.5 km exposed above the Miocene basement) is characterized by strong lateral facies changes. The lower sedimentary breccia dominated sequence is principally basaltic andesite to andesite in clast composition and associated coherent facies (e.g., lavas and sills). A prominent disconformity with strong local relief separates this lower mafic-volcanic dominated sequence from an upper silicic and more lava dominated sequence. A thin conformable red-bed formation overlies the andesitic volcanics and this is in turn disconformably overlain by Pliocene and younger sediments. This later disconformity has removed the tops of major intrusions across the district and possibly a large part of the most recent volcanic and volcaniclastic deposits.
The occurrence of thick sedimentary debris and hyperconcentrated flood flow breccias along with peperitic intrusive contacts throughout the andesite volcanic stratigraphy are inconsistent with the widely accepted interpretation of a single large (25 km in diameter and up to 4 km high) stratovolcano. Rather, sedimentation occurred into an intermontane basin that formed during the Early to Middle Miocene, synchronous with the uplift of the Puna-Altiplano of NW Argentina and NE Chile. Porphyry-related mineralization occurred beneath a multi-vent volcanic complex, a setting common to many Au-rich, porphyry Cu deposits of the circum-Pacific.
The results of ELA-ICP-MS, U-Pb zircon geochronology show that volcanism in the complex began at about 8.5 Ma and persisted for a little over 1.5 m.y. Porphyry-related Cu- Au mineralisation began during the early volcanism. This differs from most other Andean porphyry districts where ore-related intrusions are emplaced at the culmination of a protracted history of tens of millions of years.
At Bajo de la Alumbrera, hydrothermal alteration developed in and around multiphase porphyries. These intrusions are high-K, calc-alkaline, biotite(hornblende)-phyric dacites, which commonly contain mafic inclusions rich in clinopyroxene-hornblende-magnetite-plagioclase-apatite and some biotite. New petrologic and geochemical studies of the intrusions and volcanic rocks of the Farallón Negro Volcanic Complex reveal that at the time of porphyry mineralization the associated dacitic magmas became dramatically enriched in sulfur. This is corroborated by unusually high sulfur contents (up to 0.6 wt% as SO3 ) in apatite phenocrysts. In addition, geochemical evidence shows that mingling and/or incomplete mixing was important in the petrogenesis of the Farallón Negro Volcanics. The abundance of mafic inclusion in dacite intrusions associated with porphyry ore deposits (e.g., Bajo de la Alumbrera and El Durazno), combined with sieve textures and the presence of internal resorption discontinuities in plagioclase are, in part, evidence for magma mixing and/or mingling. The importance of the mafic magma mixing in a compositionally evolved magma chamber is as a source of sulfur and metals, as previously described in a number of porphyry ore deposits.
In the Bajo de la Alumbrera porphyry Cu-Au deposit, intrusions are multiphase, occurring at two discrete intervals. The earliest at 8 Ma (7.99±0.12 Ma) is believed to be associated with minor Cu-Au ore while the later intrusions at 7 Ma (7.12±0.04 Ma) are emplaced synchronously with the bulk of the mineralization and hydrothermal alteration. Comparison of these intrusion ages with previously published40Ar/39Ar ages for potassic and phyllic alteration shows that the magmatic-hydrothermal system was probably sustained for a few to several hundred thousand years. A prolonged period of formation is inconsistent with accepted mechanisms of formation as the small intrusions typically associated with porphyry Cu deposits cannot solely sustain a hydrothermal system more than 100,000 yr. before the invasion of meteoric water occurs, as shown by numerical modelling. Consequently, the protracted development of hydrothermal alteration is cause by continued fluid exsolution and/or magma degassing of an underlying batholith.
Hydrothermal alteration at Bajo de la Alumbrera is concentrically zoned from a central quartz-magnetite barren core outwards through potassic (biotite-K-feldspar±quartz) and propylitic (chlorite-illite-epidote-calcite) assemblages. Potassic alteration developed during two stages: the first was a K-feldspar-dominant assemblage associated with only minor mineralization. The bulk of the Cu-Fe sulfide and Au ore was emplaced during the second biotite-dominant phase. Intermediate argillic alteration (chlorite-illite±pyrite), which also accompanies significant mineralization, overprints the later biotite alteration and is zoned outwards into phyllic (quartz-muscovite-illite±pyrite) alteration across the top of the deposit and extends downwards along the periphery of the potassic zone. Significant amounts of Cu-Fe sulfides are found in these zones. Late basemetal-rich fault and fracture-fills (carbonate-galena-sphalerite±quartz-anhydrite-montmorillonite-nontronite) occur as the final phase in the deposit.
Fluid inclusion studies and stable isotope geochemistry on material from Bajo de la Alumbrera show that the earliest potassic alteration developed from high-temperature (300 to 700°C), saline (32 to 68 wt.% NaCl equiv.) fluids, whereas the successive biotite-dominant assemblages decline in temperature (300 to 550°C) and overall salinity (32 to 53 wt.% NaCl equiv.). Calculated and measured δ18O and δD compositions of fluids (+3.8 to +10.4‰ δ18O and –37.8 to –76.2 ‰ δD) confirm a primary magmatic origin for the potassic alteration phase. The depleted D compositions measured in the late potassic alteration are similar to those measured in other porphyry ore deposits (e.g., Butte and Copper Canyon, North America; and Granisle and Bell, British Columbia). It is often thought the depleted D compositions of potassic alteration implies a component of meteoric water in its formation; however, at Bajo de la Alumbrera, the isotopic composition of the potassic alteration is consistent with late degassing and volatile exsolution of the underlying batholith. Similar mechanism can explain the isotopic composition (+5.1 to +8.4‰ δ18O and –23.6 to –81.4‰ δD) of the lower temperature (300 to 390°C) and less saline (less than 15 wt.%, but averaging 4.1 wt.% NaCl equiv.) phyllic alteration, whereby the alteration developed from magmatic fluids, not meteoric water mixed with magmatic aqueous fluids.
The Bajo de la Alumbrera deposit is the result of the conjunction of several critical processes, including the protracted period of hydrothermal alteration formation (probably 2 to 4 m.y.) developed on multiple dacitic intrusions emplaced one million years apart. Volatiles (along with sulfur) were exsolved from a huge underlying pluton, which was being replenished by new batches of mafic magma. Magmatic aqueous fluids with initial high temperatures (up to 700°C) and salinities (greater than 32 wt. % NaCl equiv.) formed potassic (K-feldspar-biotite) alteration associated with the bulk of the Cu-Au. Lower temperature (below 400°C) and less saline (less than 4.1 wt.% NaCl equiv.) magmatic fluids formed the overprinting mineralized phyllic (illite-muscovite-pyrite-quartz) alteration. Although this differs from the traditional porphyry model, where phyllic alteration is peripheral to mineralized potassic alteration, it is not unique. Cu-Fe sulfide- and Au-bearing phyllic (and intermediate argillic) alteration within and not peripheral to the potassic altered core, has been documented at a number of porphyry ore deposits. These observations challenge the widely used genetic models in which phyllic alteration in porphyry ore deposits results from the ingress of meteoric fluids and refocuses attention on the evolution of the underlying batholith, from which the exsolved fluids and metal are derived.