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Geology, geochemistry and geometallurgy of the Productora Cu-Au-Mo deposit, Chile

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posted on 2023-05-27, 10:41 authored by Escolme, AJ
The Productora Cu-Au-Mo deposit is hosted by a hydrothermal breccia complex in the Coastal Cordillara of Region III, northern Chile. Mineralisation at Productora extends discontinuously over 8 km in a northeast-oriented corridor. The current resource, which includes the neighbouring Alice porphyry Cu-Mo deposit, is estimated at 236.6 Mt grading 0.48 % Cu, 0.10 g/t Au and 135 ppm Mo. The district is characterised by hydrothermal mineralisation associated with Mesozoic arc magmatism along the convergent South American plate margin. The Andean tectonic cycle led to the formation of orogen-parallel metallogenic belts, each representing discrete epochs of mineralisation characterised by distinctive deposit styles and a stepped eastward younging of the deposits. During the Mesozoic, a range of deposit styles formed under extensional conditions in central and northern Chile, including small porphyry Cu, large iron oxide-Cu-Au (IOCG), magnetite-apatite, manto-Cu and precious metal epithermal deposits. The Mesozoic mineral systems in Northern Chile formed within a convergent tectonic regime where evolving subduction style, magmatic arcs and associated back-arc basin stratigraphy, plutonic complexes and the crustal scale syn-arc strike-slip Atacama Fault Zone (AFZ) evolved in response to changes in the prevailing geodynamic conditions. The Productora deposit is hosted within a thick sequence of broadly coeval rhyolite to rhyodacite lappili tuffs (`128.67 ¬¨¬± 1.29 Ma; U-Pb_(zircon)`) and breccias between two major intrusions; the Cachiyuyito tonalite (129.8 ¬¨¬± 0.1 Ma) and Ruta Cinco granodiorite batholith (`92.05 ¬¨¬± 1 Ma; U-Pb_(zircon)`). Several phases of intermediate- and mafic-dykes cross cut the deposit. The deposit is hosted in a domain of structural complexity defined by major northwest-striking faults (normal obliqueslip with dextral strike-slip and northeast side up dip-slip) that crosscut major north to northeast-striking faults (normal oblique-slip with sinistral strike-slip, and east-side up dip-slip movement). Hydrothermal breccias, tectonic breccias, veins and alteration assemblages observed on two cross sections at Productora have been separated into five paragenetic stages. Stage 1 produced quartz - pyritecemented hydrothermal breccia with muscovite alteration. Stage 2 formed a chaotic matrix-supported tectonic breccia with kaolinite - muscovite ‚Äö- pyrite alteration. Stage 3 tourmaline ‚Äö- pyrite ‚Äö- chalcopyrite ¬¨¬± magnetite ¬¨¬± biotite-cemented hydrothermal breccias are associated with K-feldspar ¬¨¬± albite alteration. Stage 4 veins contain chalcopyrite ¬¨¬± pyrite ¬¨¬± sericite, illite, epidote and chlorite, and stage 5 veins contain calcite. The stage 3 breccias have been further subdivided into five facies based on their mineralogy. The Productora hydrothermal system crosscuts earlier-formed sodic-calcic alteration and magnetite-apatite mineralisation associated with the Cachiyuyito stock. The breccia complex formed as a result of at least two stages of hydraulic fragmentation (stage 1 and stage 3) and one tectonic breccia event (stage 2), which shows evidence of multiple episodes of reactivation. Alteration minerals are consistent with moderate temperature (<300¬¨‚àû‚Äö-400¬¨‚àûC) weakly acidic fluids during stage 1, and moderate to high temperature (>300¬¨‚àûC) alkaline fluids during stage 3. Lower temperature (<300¬¨‚àûC), weakly acidic fluids prevailed during stages 4B and 4C, and alkaline fluids predominated during stages 4A, 4D and 5. Main stage mineralisation is associated with the stage 3 hydrothermal breccia (average grade 0.34‚Äö-0.62 % Cu, 0.8‚Äö-0.14 g/t Au, 66‚Äö-128 ppm Mo) and stage 2 tectonic breccia (average grade 0.8 % Cu, 0.21 g/t Au, 141 ppm Mo). Chalcopyrite is the dominant hypogene Cu-sulphide mineral and occurs predominantly as stage 3 breccia cement and syn-breccia veins with pyrite in equal proportion. Chalcopyrite and pyrite are disseminated in the stage 2 breccias. Pyrite is elevated in the south compared to the north, and chalcopyrite:pyrite ratios are ~1.00 and <0.25 respectively. Gold and Cu are strongly associated spatially, and synchrotron XRF and LA-ICP-MS analyses indicate that Au occurs as micron to sub-micron grains on pyrite and chalcopyrite grain boundaries. In this study, mineralisation at Productora was dated using Re-Os on stage 3 molybdenite at 130.1 ¬¨¬± 0.6 Ma. The Alice Cu-Mo porphyry deposit is situated 400 m to the west of Productora. Mineralisation occurs as disseminated chalcopyrite and quartz ‚Äö- pyrite ‚Äö- chalcopyrite ¬¨¬± molybdenite vein stockwork hosted by a granodiorite porphyry stock (121.1 ¬¨¬± 2.1 Ma). Potassic alteration (biotite ¬¨¬± actinolite replacing hornblende) is associated with quartz ‚Äö- sulphide veins. Mineralisation was dated by Re-Os on molybdenite at 124.1 ¬¨¬± 0.6 Ma (within error of the porphyry stock). The margins and deeper parts of the system are overprinted by albite ¬¨¬± epidote ¬¨¬± sericite alteration, which locally caused destruction of biotite and chalcopyrite. The Alice porphyry is spatially associated with the Silica Ridge lithocap, which is characterised by massive textureless quartz-altered rock above domains of alunite, pyrophyllite and dickite. At Productora, `˜í¬•^(34)`\\(S_{sulphide}\\) values range between -8.5 and +2.2 ‚ÄövÑ‚àû. This is consistent with a magmatic sulphur source and fluids evolving under oxidising conditions with no significant input from evaporateor seawater-sourced fluids. Stage 3 tourmalines (n = 8) have average initial Sr of 0.70397, consistent with Cretaceous intrusive rocks and mantle-derived Sr. One stage 4 epidote sample returned a more radiogenic initial Sr value of 0.70525, indicating fluid mixing or fluid- wall rock interaction. An ˜í¬µNd value of +5.2 (n = 1, tourmaline) is consistent with the Cretaceous and Jurassic igneous rocks of the region. LA-ICP-MS analysis on stage 3 pyrite (n = 8) for Pb isotopes and trace elements indicate low Pb content (<5 ppm) with significant thorogenic and uranogenic Pb isotope contamination. Early pyrite grains have elevated Pb (4.6 ppm), Cu (47 ppm), U (4.0 ppm), Th (2.6 ppm) and Au (0.06 ppm) which also all co-vary. Late-stage pyrite that has replaced chalcopyrite does not contain significant Pb, U, Th, As or Au, but does have elevated Cu (>530 ppm). Supergene mineralisation in the north of Productora (chrysocolla, malahcite and Cu-wad) is indicative of a geochemically mature weathering environment developed under near neutral to alkaline pH. In the south, the supergene assemblage is less mature (chalcocite/digenite) and indicates in-situ weathering of chalcopyrite. Quantitative and predictive geometallurgical models have been developed to integrate geological findings with geometallurgical data in order to advance ore body knowledge at Productora. To improve the understanding of mineralogical variability across the deposit, whole rock geochemistry (33-element ICP-MS) has been converted to mineral proportions through calculated mineralogy using linear programming for each assay interval in the deposit. Calculated mineralogy results for major minerals, including quartz, K-feldspar, albite, pyrite, iron oxides, chalcopyrite and molybdenite, are excellent (`R^2 > 0.8`) when compared with the measured mineralogy by quantitative X-ray diffraction. A new sample classification scheme for dominant Cu-species (oxide, transitional-oxide, transitional-sulphide, sulphide or insoluble) was also developed based on sequential leach data and S wt%. The new scheme enabled domains of weak acid insoluble Cu-wad to be identified. Machine learning algorithms were used to predict Cu-species class using a series of nine proxies (sample depth in drill hole, Ca %, Cu %, Fe %, K %, Mn ppm, S %, Ln(Cu/S) and the logged regolith term). The optimum algorithm was Bagging-REPTree with five iterations. Applied to the training set with ten-fold cross validation, the model is 67.6 % accurate. The model is most successful at recognising sulphide, transitional-sulphide and insoluble samples with accuracy of 70.2 %, 87.5 % and 67.8 % respectively against the training set. Based on textural, mineralogical, stable and radiogenic isotope data, the Productora breccia complex is inferred to be a magmatic-hydrothermal breccia complex formed as a result of explosive volatile fluid release at depths causing brecciation and alteration of the overlying rock mass. Metal-bearing fluids were of magmatic affinity and evolved under oxidising conditions. Despite sharing many similarities with the Andean IOCG clan (strong structural control, regional sodic-calcic alteration, local U), fluid evolution at the Productora Cu-Au-Mo deposit is consistent with that of a porphyry magmatic hydrothermal breccia (sulphur-rich, acid alteration assemblages and relatively low magnetite, <5 wt%). The Productora camp provides an excellent example of the close spatial association of Mesozoic magnetite-apatite, porphyry (Alice) and a magmatic-hydrothermal breccia mineralisation styles, a relationship seen throughout the Coastal Cordillera of northern Chile.


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