|Abstract or Summary
- This dissertation is informally divided into three major sections. In the first section (Chapter 2) I use data from field mapping, isotopic geochronology, whole rock geochemistry and trace element concentrations in zircons to examine the petrology, geochemistry and ages of the Haquira East porphyry copper deposit of southern Peru. In the second section (Chapters 3 - 6) I investigate the timing, temperature, zonation and lifespan of the magmatic-hydrothermal alteration and Cu-Mo mineralization by applying field mapping, whole rock geochemistry, petrography, geothermometry, spectral, X-ray and cathodoluminiscense imaging of rock samples. The timescales of the magmatic-hydrothermal system was constrained by isotopic dating, diffusion of titanium in quartz and the novel approach of oxygen diffusion via δ¹⁸O of quartz analyzed by secondary ion mass spectrometry. These data are also used construct cross-sections and 3D models. In the final section (Chapter 7) I describe the production and evaluation of a calibration for portable X-ray spectrometers that could potentially be applied to further investigate porphyry copper deposits.
Haquira East is a moderate grade porphyry copper-molybdenum-gold deposit (688 Mt ore at 0.59 wt. % Cu, containing 4.7 Mt Cu, 37,000 t Mo and 0.9 M oz Au) in the Eocene-Oligocene Andahuaylas-Yauri porphyry belt of southern Peru. Based on a Re/Os age of molybdenite, the copper and molybdenum mineralization and hydrothermal alteration at Haquira East formed at ~33.85 Ma. These ores are associated with dominantly granodioritic intrusions that range from 34.5 to 33.5 Ma in age and represent some of the youngest intrusions of the Andahuaylas-Yauri batholith between 40 and 32 Ma. In this study, new U-Pb zircon ages of these intrusions ages were determined via laser ablation inductively coupled mass spectroscopy (LA-ICP-MS¬) and sensitive high resolution ion microprobe in reverse geometry (SHRIMP-RG). At Haquira East, the regional Eocene-Oligocene shortening of the Incaic orogeny resulted in the folding of Jurassic-Cretaceous meta-sedimentary rock into the northwest-trending and northeast-overturned Tocone syncline that is associated with northwest-striking and southwest-dipping thrust faults. The magmatism was synchronous with the deformation and began with the ~34.5 Ma andesitic to dacitic Lahuani sills and was followed by the ~34.2 to 33.5 Ma Haquira granodiorite stock and the slightly younger, narrow and subvertical Haquira porphyry dikes of similar composition. Several sets of Haquira porphyry dikes were emplaced synchronously with the porphyry copper and molybdenum mineralization, veining and K-silicate hydrothermal alteration. At the waning stages of the magmatism (~33.5), the dacitic Pararani porphyry dikes were emplaced along a north-south strike and closely followed in time with D veins, sericitic halos and sericitically-altered hydrothermal breccias.
Whole-rock trace element modeling indicates that the Haquira East magmas originated in a garnet-bearing basaltic-andesite to andesite zone of melt-assimilation-storage-hybridization (MASH) in the lower crust below ~25 km depth. Andesite melts from the MASH zone were injected into an inferred granodiorite magma chamber at ~10km depth beneath Haquira East, where the oxidized and water-rich magma fractionated hornblende and small amounts of plagioclase, together with traces of titanite, apatite, zircon and magnetite. Intrusions sourced in this chamber have enrichments in Sr/Y (>60) and V/Sc (>12), depletions in the light rare earth elements (REE) and high mid-REE/heavy-REE ratios inherited from melts derived from the MASH zone where garnet is residual. Similarly, the intrusions that were emplaced closely in time with the mineralization have high Eu[subscript N]/Eu[subscript N]* (0.4 to 0.8) and Ce[subscript N]/Ce[subscript N]* (200 to 5000) in zircon compared to older intrusions (LA-ICP-MS, and SHRIMP-RG) further evidencing the high oxidation state and water content of the magma.
New core logging observations, ICP-MS whole rock geochemical data and short wave infrared spectroscopy data document the sequence and spatial distribution of veins, hydrothermal alteration zones and Cu-Mo-bearing sulfide ore zones and are summarized in cross-sections and three-dimensional models. From oldest to youngest, the sequence of veins consists of biotite veins/micro-breccias, aplite dikes, deep quartz (DQ) veins, actinolite veins with plagioclase halos and epidote veins, early dark micaceous (EDM) halos with bornite-chalcopyrite, Cu-sulfide±quartz veinlets with chalcopyrite and/or bornite, banded molybdenite-quartz (BMQ) veins, B-type quartz-bornite-chalcopyrite veins, D-type pyrite-quartz-sericite veins with sericitic halos, and fractures with green and white intermediate argillic halos containing mixtures of illite-smectite-chlorite-kaolinite ± pyrite. The areas with the highest density of EDM halos constitute the high-grade copper ore. Scanning electron microscopy imaging (QEMSCAN) have been used to identifiy mineralogy, textures and zoning of hydrothermal alteration zones. The EDM halos are formed by hydrothermal biotite, muscovite, K-feldspar, with rare quartz and corundum replacing magmatic plagioclase and hornblende and carry up to 10-15 volume % disseminated bornite and chalcopyrite. Later BMQ veins host the high-grade molybdenum ore, whereas late B veins contribute both copper and molybdenum to the ores. The copper ore shell forms a continuous high-grade ore zone (>0.5 wt. %) in the Haquira stock, but is lower grade where it projects into the relatively non-reactive meta-sedimentary, mostly quartzite, wall rock, likely as a result of a limited supply of iron to enable copper-iron sulfide precipitation. In contrast, molybdenite mineralization forms a roughly symmetric shell overlapping both the stock and quartzite with an axis of symmetry located along the southwest flank of the Haquira granodiorite stock.
The abundance of EDM halos, lack of A veins and the dearth of unmixed fluid inclusion assemblages in quartz veins and prescense of two phase (liquid + vapor) fluid inclusions (P>1.5 kbar) suggest that Haquira East was emplaced at significant depths. The emplacement depth of Haquira is estimated at ~10 km, at greater depth than shallower and outcropping Acojasa intrusion to the south, emplaced at ~ 8 km according to hornblende barometry (~2.0 ± 0.5 kb). The veining, hydrothermal alteration and mineralization formed at this depth magmatic hydrothermal fluid was released from an inferred magmatic cupola, hydro-fractured the wall rock and depressurized from ~3 kb at lithostatic pressure to >1.2 kb at close to hydrostatic pressure without unmixing into a vapor and brine phase.
The temperature evolution of the magmatic-hydrothermal system was estimated by mineral phase equilibria and the application of different geothermometers. For TitaniQ geothermometry, Ti-in-quartz of veins and dikes was measured by electron microprobe (EMP) and LA-ICP-MS (1 ppm to > 100 ppm) on samples that were previously imaged by secondary electron microscope cathodoluminescense, Temperatures were also estimated through Ti-in-zircon (2 ppm to 13 ppm, via SHRIMP-RG), Ti-in-biotite (~1.0 to 2.3wt.%, via EMP) and the δ³⁴S compositions of pyrite-anhydrite and chalcopyrite-anhydrite pairs (-1.4 ‰ to -0.1‰ and 7.5 ‰ to 10.2‰ respectively). Porphyry dikes, aplites and DQ veins formed first between ~720ºC and 650ºC. Copper was introduced later in EDM halos at ~500ºC and was followed by molybdenite in BMQ veins at ~650ºC during a thermal reversal. A second stage of copper and molybdenum was introduced in B veins at ~550°C. Late D veins with pyrite and sericite formed first at ~450ºC and later at ~350ºC. Diffusion modeling of both Ti (EMP) and δ¹⁸O gradients (via secondary ion mass spectrometry, ~10.7 ‰ to ~12.7‰) across initially abrupt quartz growth boundaries yields a calculated maximum lifespan for the Haquira East porphyry of 170,000 years for the period from initial fluid injection at >650 ºC to cooling below ~ 350 ºC. However, the high-grade copper and molybdenum ore formed relatively rapidly in less than 30,000 years.
In the last chapter of this dissertation two calibrations were produced and tested for a Bruker Tracer IV portable X-ray fluorescence portable spectrometer (pXRF). Concentrations of Al₂O₃, CaO, FeO*, K₂O, MnO, TiO₂ and, in lesser degree, SiO₂ were accurately reproduced together with Cr, Cu, Nb, Ni, Sr, Y, Zr and less accurately V in powdered basaltic samples, and Nb, Pb, Rb, Sr, Y, Zn, Zr and, less accurately, Ba in powdered rhyolitic samples. The pXRF is particularly reliable for measuring relatively immobile elements (e.g. Nb, Zr, Ti and Y), which are often resistant to hydrothermal alteration and weathering. Therefore pXRFs may be useful for lithogeochemical mapping of hydrothermally altered rocks that are zoned around porphyry copper deposits.