Constraining Cu-(Co) mineralisation in sediment-hosted copper deposits using rutile, apatite and carbonate geochemistry

Transitioning to a green economy requires unprecedented metal supply rates, highlighting the need for the discovery of new ore deposits. Sediment-hosted Cu deposits (SSHC’s) are attractive, high-grade deposits however, despite decades of research, lack indicator and vectoring mineral exploration tools relative to other deposit types whilst controversy still surrounds the timing of mineralisation. To address these issues, this thesis explores the potential of rutile, apatite, and calcite as effective U-Pb geochronometers to constrain mineralisation in the Zambian Copperbelt (ZCB) and Neuquén basin. Furthermore, trace element geochemistry and machine learning are used to assess the potential of rutile and apatite as indicator minerals for SSHC deposits.

Rutile is intergrown with chalcopyrite-bornite, defining a broad window of mineralisation from ~570-520 Ma, supporting previous molybdenite Re-Os constraints. No evidence for syndiagenetic mineralisation is observed and Cu mineralisation is interpreted as syn-orogenic, linked to K-Mg metasomatism caused by circulation of high temperature, potassic basinal brines during the Lufilian Orogeny. A 2nd hydrothermal event between 490-470 Ma is constrained by rutile and apatite, aligned with post-orogenic uplift of the Lufilian Orogeny and existing Re-Os bornite constraints. Late-stage Ca-Na metasomatism, caused by infiltration of halite undersaturated brines derived from evaporite dissolution is invoked to explain this stage, indicating mineralisation in the ZCB is episodic rather than one single event.

Similar, syn-orogenic mineralisation is constrained by calcite U-Pb geochronology in the Neuquén basin at ~27 Ma, linked to Oligocene-Miocene basin inversion during Andean tectonism. Earlier calcite generations at ~95 Ma and ~77 Ma align with diagenesis and hydrocarbon and formation water migration respectively, showcasing calcite U-Pb geochronology as an effective tool, defining different stages of basin evolution. Overall, a model linking Cu mineralisation with redox reactions involving hydrocarbons and oxidised brines is invoked, with the Neuquén basin potentially representing a younger analogue to the ZCB.

Textural and trace element geochemical characteristics support a metasomatic origin for rutile and apatite. Hydrothermal rutile is W, Ta, Sn, Th and potentially Cu enriched but Zr, Hf and Mo poor compared to metamorphic and detrital rutile, offering distinction between hydrothermal and metamorphic environments. Orebody apatite at Nchanga and Mindola North display convex upward, MREE enriched profiles and are separated from most metamorphic apatite using principal component analysis, based on LREE depletion and low Sr/Y ratios. However, neither mineral displays definitive criteria distinguishing SSHC deposits from other ore deposit types. Hence, the ability for these mineral phases to act as indicator minerals is limited on a global basis but may demonstrate more success on a deposit scale.

University of Southampton

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