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Title: Zinc metallogenesis, and indium and selenium distribution at the neves corvo deposit, iberian belt, Portugal
Author: Carvalho, João Ricardo Silva, 1980-
Advisor: Relvas, Jorge Manuel Rodrigues de Sancho, 1960-
Keywords: Teses de doutoramento - 2016
Defense Date: 2016
Abstract: The Neves Corvo deposit stands out among the VHMS deposits of the Iberian Pyrite Belt (IPB) for its richness in terms of metal grades and total metal contents, and for its multisourced regime of metal supply. Previous studies suggested that the metallogenesis of the tin and copper-rich ores in the deposit resulted from a long-lived magmatic-hydrothermal system. Hence, the understanding of the geology of zinc, indium and selenium within the overall context of the Neves Corvo ore-forming processes constitutes an unsolved challenge and an opportunity to assess the occurrence of indium and selenium in the deposit. This study focuses therefore on the zinciferous ores at the Neves Corvo deposit, and on the indium and selenium abundance, distribution and mineral allocation in the deposit. The aims of this study consisted in (1) characterizing the hydrothermal patterns and geochemical signatures related with the zinc-rich mineralization; (2) determining the textural, mineralogical, and geochemical features of the zinciferous ores; (3) assessing the abundance, distribution and mineral allocation of indium and selenium in the deposit; and finally (4) proposing an integrated metallogenetic model for the deposit. The Lombador orebody represents the largest and zinc-richest orebody in the deposit, comprising over 106.2 Mt of massive sulfides and a total amount of zinc resources of 73.5 Mt @ 5.86% Zn, which represent about 63% of the mine total zinc resources. Extensive log and petrographic characterization of selected drill holes indicate that the Lombador orebody immediate footwall sequence is characterized by a much more pronounced lateral and vertical variation of lithofacies than that commonly recognized in other areas of the deposit. This is though to result from variable paleomorphology of the basin where the mineralization developed. The Lombador orebody comprises a huge, stratabound stockwork system that extends across the entire length of the orebody, and that is better developed in the central and central-W sectors of the orebody, where the axis of the feeder system locates and chalcopyrite-rich veins dominate. As elsewhere in the IPB, the hydrothermal alteration pattern at the Lombador orebody comprises a chlorite-dominated innermost alteration zone (Type I alteration) that grades outwards into a K-sericite-dominated (Type IIa alteration) peripheral alteration zone. The Na-sericite-bearing alteration (Type IIb alteration) was only found in some coherent rhyolite lenses tectonically emplaced in a hanging wall position. Moreover, the intense footwall carbonatization was found to constitute a distinctive feature relative to the hydrothermal pattern recognized at the Corvo orebody. Microprobe analysis confirmed the occurrence of iron-rich ripidolite, donbassite, sericite and siderite as the main alterationrelated mineral phases. Also, the overall abundance of iron-rich chlorite, siderite, pyrite, ironrich sphalerite, arsenopyrite and, to a lesser extent, pyrrhotite in the Lombador ores indicate that these formed by iron-rich, low pH, CO2-enriched, and high-temperature ore-forming solutions under moderate to strongly reduced conditions. However, although the chlorites from the Lombador feeder system are more aluminous than those reported in typical IPB stringer systems, its iron and aluminum contents, as well as its estimated average peak formation temperature do not reach values as high as the ones found in the Corvo copper-rich stockwork system. This implies less intense ore-forming conditions at the zinc-rich Lombador stockwork system than those at the copper-rich Corvo stockwork system. The petrography and geochemistry of the zinc-rich Lombador ores indicate that these have formed from a combination of prominent sub-seafloor replacement and episodic direct exhalation onto the seafloor. The mineralization develops around, and was controlled by, a NNW-SSE trending sub-vertical fault zone located in the central-W and south sectors of the orebody. Sub-seafloor replacement phenomena prevailed in this part of the orebody, and caused large scale enrichment in iron and depletion in silica and alkalis in Type I altered rocks due to intense chlorite and sulphide formation, and the enrichment in silica and alkalis towards the peripheral Type II altered zones. Contrastingly, the presence of banded, semi-massive, carbonate-rich zinciferous ores alternating with sediment-rich facies in the northernmost peripheral areas of the orebody indicates that part of the zinc-rich ores formed by episodic and tectonically triggered exhalation of ore fluids onto the seafloor. The same inference can be made for part of the zinc-rich ores from the remaining Neves Corvo orebodies as these exhibit similar ore petrographic and geochemical features. Factors such as a porous and chemically reactive footwall host sequence, temperature decrease, variations in the pH, mixing of fluids, and boiling should have promoted and controlled the extensive alteration and ore deposition. The investigation of the abundance and distribution of the ore metal contents, metal zonation, and ore petrographic and geochemical features suggest that the various Neves Corvo orebodies display distinct degrees of hydrothermal maturity, in close parallel with present-day submarine hydrothermal systems. The large predominance of zinc-rich ores over low grade copper ores at the Lombador orebody, coupled with ore-forming signatures that denounce a sustained, low to moderate temperature (<300 ºC) hydrothermal circulation regime, strongly suggest that this orebody is hydrothermally more immature than the remaining orebodies in the deposit. Local formation of high-temperature, copper-rich ores should have occurred during focused, short-lived pulses of higher temperature hydrothermal upflows. In turn, the high copper, tin, indium and selenium ore grades, abundance of copper-rich ores over zincrich ones, and the well developed copper-rich ores at the Corvo (in particular) and Graça orebodies indicate that these orebodies represent hydrothermally more mature mineralization within the Neves Corvo hydrothermal field. The development of the high-temperature, copperrich ores in both these orebodies should have occurred through a massive influx of high| temperature (>300 ºC) copper-rich hydrothermal fluids and sustained, long-lived hydrothermal activity, and should have involved additional metal contributions of probable magmatic affiliation as previously indicated by their radiogenic isotopic signatures. The indium and selenium distribution, abundance and mineral allocation at the Neves Corvo deposit is complex, and resulted from the combined effects of long-lasting ore-forming processes and late tectono-metamorphic remobilization, specially in the most deformed domains. Indium and selenium are mainly associated to the copper-rich ores at the Neves Corvo deposit, in particular in the MCZ ores, which account for more than 60% of the estimated indium and selenium metal content in the deposit. This argues for the joint transport and co-precipitation of copper, indium and selenium from high-temperature, acidic, reduced, saline, copper-rich ore fluids. Also, the abundance, distribution and mineral allocation of both indium and selenium agree well with the degree of hydrothermal maturity shown by the various Neves Corvo orebodies. In addition, the zinc-rich ores may constitute as well a potential source for indium and selenium, in particular in ore zones affected by stronger tectono-metamorphic deformation, where significant indium and selenium enrichment occurs.Sphalerite, chalcopyrite and minor stannite are the main indium-carrier ore sulphides in the deposit, whereas galena constitutes the main selenium-carrier. Nevertheless, replacement of the early-formed low-temperature ore types, zone refining, recrystallization, annealing, and tectono-metamorphic remobilization were responsible for the occurrence of minor amounts of roquesite, naumanite, roquesite-sakuraiite intermediate phases, and unknown Bi-Se and Pb- Bi-Se phases. Strontium, neodymium and lead isotope data indicate that the footwall rocks and, in particular, the PQ Group sequence constitute the main source of zinc and copper for the metal budget of the deposit. Nevertheless, some highly radiogenic lead and neodimium isotopic signatures, as well as the high indium and selenium abundances depicted by some of the copper-rich ore types, suggest that as tin and part of the copper metal contents in the Neves Corvo deposit, a significant fraction of the indium and selenium may have resulted from direct magmatic contributions. A sustained, long-lived, multi-stage and multi-sourced hydrothermal ore-forming system, developed within a highly dynamic rift basin environment, was envisaged to model the overall metallogenetic process at the Neves Corvo deposit. Nevertheless, further investigation is needed to fully understand the unique metal association, mineral assemblage and ore geochemistry of the Neves Corvo deposit , and to fully constrain its complex, multi-stage and multi-sourced metallogenesis.
Description: Tese de doutoramento, Geologia (Metalogenia), Universidade de Lisboa, Faculdade de Ciências, 2016
Designation: Doutoramento em Geologia
Appears in Collections:FC - Teses de Doutoramento

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