From ambient to metamorphic conditions: Deciphering fluid evolution and ore-forming processes through mineralogy, geochemistry, and thermodynamic modelling

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Dokumentart: PhDThesis
Date: 2021-12-02
Language: English
Faculty: 7 Mathematisch-Naturwissenschaftliche Fakultät
Department: Geographie, Geoökologie, Geowissenschaft
Advisor: Markl, Gregor (Prof. Dr.)
Day of Oral Examination: 2021-08-18
DDC Classifikation: 550 - Earth sciences
Keywords: Thermodynamik , Arsen , Arsenide , Kinetik , Modellierung , Lagerstätte , Fluid , Erzmineralien , Gold , Silber
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The formation of hydrothermal ore deposits depends on both the fertility of the involved fluids and the precipitation mechanism. The groundwork for understanding these two aspects is best laid by a detailed mineralogical and geochemical investigation, and supplemented by a geological-framework-considering qualitative model. However, in order to fully understand the involved processes, and to verify the qualitative model, an at least semi-quantitative thermodynamic model is essential. This requires the time-consuming task of compiling, and estimating where appropriate, thermodynamic data for all important mineral species involved. In this thesis, this approach is applied to various aqueous settings from ambient to metamorphic conditions. The thesis is subdivided to cover four different settings: (I) groundwater brines, (II) the fertility and evolution of basement brines and the process of fluid mixing, (III) As- and Ni- bearing hydrothermal brines and the process of fluid reduction and (IV) metamorphic fluids and the process of hydrothermal Ni-remobilization under reducing conditions as well as cooling of Sb-rich fluids. (I) Surficial water can drastically change its fluid composition through fluid-rock interaction. This is an important process that occurs anywhere between surficial and deep-seated crustal settings. Furthermore, ligands such as Cl play a vital role in the ability of such fluids to mobilize elements. At Buggingen, SW Germany, groundwater partially dissolved evaporitic salts and subsequently reacted with mafic magmatic dikes. Progressive desiccation due to H2O consumption by clay mineral formation and swelling, the fluid ultimately reached the point of halite saturation. (II) Surficial fluids can infiltrate deeper into the crust and form fertile hydrothermal fluids, and if certain conditions are met, form hydrothermal veins. In the study area of the Black Forest, this occurred over the past ~300 Ma in the basinal and rift-related setting of SW Germany. The trace element distribution between primary and alteration mineral reveal that basement aquifer fluid-rock interactions can liberate both major and trace elements into the fluid during host rock alteration. Temporally resolved fluid inclusion analyses by microthermometry and LA-ICP-MS on three hydrothermal veins which, in total, formed over a period of 150 Ma, were done to attain a better understanding of the basement aquifer evolution. This revealed a transition with depth and time from a NaCl to a CaCl2-richer basement aquifer. Based on aquifer host rock mineralogy, Na Ca K thermometry, Na-K thermometry, compositional trends, and thermodynamic modelling, this evolution and transition with depth can be traced back to the preferred alteration and consumption of Ca-rich feldspar during infiltration of the basement by saline fluids. Fluid mixing from various depths along this transition results in the formation of the Triassic to Cretaceous fluorite-quartz-barite veins in the Black Forest. The formation of the base metal sulfides further requires an influx of sulfide into this otherwise binary mixing scenario. The shift in hydrothermal regime to the subsequent post-Cretaceous veins due to the opening of the Upper Rhine Graben reflects a gradual transition. This has been observed for the siderite-chalcopyrite-gersdorffite veins in the Black Forest. (III) The Black Forest also presents a rare but characteristic occurrence of native As±native Ag±arsenide±antimonide-bearing ore shoots that formed cogenetic with the Triassic to Cretaceous veins. The same mineralization type, typically known as native element-arsenide, five element, or Bi-Co-Ni association, is also found as independent veins worldwide. Based on both a local study of the ore shoots and a comprehensive global literature compilation, the previously proposed formation mechanism by reduction is reinforced and refined. Novel thermodynamic modelling of the Ni-Co-Fe-As-Sb-Ag-Bi system reveals that reduction suffices to produce the characteristic and nearly ubiquitous general mineral sequence: native Ag / native Bi  Ni-arsenides  Co-arsenides  Fe-arsenides  native As. The relative abundance of the precipitated minerals is correlated to the fluid composition. When Sb is present in the fluid, the Ni-mono- and -sulfantimonide quickly become stable over their respective As counterpart. Textural information, sulfur isotopic investigation, and experimental literature data supplemented by thermodynamic modelling reveal a redox disequilibrium between sulfate and sulfide which enables the fluid to maintain a low sulfide activity during reduction. This is required for the arsenides and native As to form. The formation of native As is further aided by low temperatures and the presence of sulfide-binding elements such as Pb or Zn. Thus, a reduction of a moderately oxidized fluid (increased solubility of most elements involved) forms these arsenide-bearing associations. (IV) However, it is also possible to mobilize minor amounts of Ni in reduced, low-saline, near-neutral fluids, if higher temperature/metamorphic conditions prevail. This was studied at Kambalda, Western Australia, where the Ni mobilization at these conditions from primary magmatic Ni-sulfide bodies resulted in the rare formation of several types of hydrothermal pentlandite-rich veins. The “more typical” regional metamorphic hydrothermal fluid is, however, not rich in Ni. A more common scenario is the presence of reduced, Sb-, Fe-, and sulfide-rich fluids, that occasionally are Au-bearing. The formation of hydrothermal veins from such fluids is found in veins all over the Black Forest. Mineralogical, textural, fluid inclusion analysis, and oxygen isotopes augmented by thermodynamic modelling reveal a formation of these Sb-Pb-Ag±Au veins by cooling of a metamorphic fluid and subsequent remobilization stages due to the influx of new basinal brines. Concluding, the studies presented in this thesis show the applicability and fruitfulness of fluid-mineral system thermodynamic modeling from ambient to metamorphic conditions. Specifically, when it is based on a well-founded mineralogical and geochemical groundwork. Although this multimethod approach is not novel, it is not commonly applied to such an extent as shown here.

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