The timing and duration of Archean ‘porphyry-type’ deposits

: insights from Archean magmatic-hydrothermal mineralised systems

Abstract

Phanerozoic porphyry Cu ± Mo ± Au deposits are generally linked to subduction zones, where volatiles are brought into the mantle by subduction and hydrous, mineralising magmas can be generated. Recent high-precision geochronology has provided insights into the timescale of the formation of these deposits. By contrast the formation of Archean magmatic-hydrothermal Cu ± Mo ± Au deposits is less well understood, although some are suggested to be comparable to Phanerozoic porphyry deposits. However, evidence relating the formation of
these deposits to subduction is difficult to trace.

This thesis combines field relations, petrology and with a particular emphasis on high-precision geochronology to provide new insights in the formation processes of Archean magmatic-hydrothermal Cu ± Mo deposits from Western Australia and Finland. The deposits are located in two cratons with contrasting cratonic formation models, representative of vertical and horizontal Archean tectonics. Zircon U-Pb geochronology by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MC-ICPMS) and Chemical Abrasion Isotope
Dilution Thermal Ionisation Mass Spectrometry (CA-ID-TIMS) is used to reconstruct the timing and maximum time scales of Archean magmatic-hydrothermal systems. Additionally, deposit formation is linked to geological
processes associated with craton formation.

The Mätäsvaara deposit, Finland, is interpreted to be formed in a primitive arc setting and has metallogenic features and timescales similar to Phanerozoic porphyry deposits. Similarly, the magmatic-hydrothermal Cu ± Mo deposits of the Eastern Pilbara Terrane, Western Australia, have metallogenic features and in the case of the Gobbos Cu-Mo prospect time scales of the magmatic and hydrothermal systems similar to Phanerozoic porphyry deposits. Magmatic-hydrothermal deposit formation in the EPT took place over a period of ~7 Ma and is related to the early phases of the magmatic Emu Pool Supersuite. However, the geological process triggering supersuite magma generation cannot be simply linked to vertical or horizontal tectonics and therefore questions remain to
the nature of these Paleoarchean magmatic-hydrothermal systems.

Molybdenites were analysed for Re-Os geochronology from deposits with clear geologically and zircon geochronological cross-cutting relationships. Developments in a pre-screening technique for the molybdenites
using Scanning Electron Microscope (SEM) and Electron Microprobe (EMP) analyses, allow the effective detection of open-system behaviour and micro-textures affecting the Re-Os isotopic system. The formation of the secondary mineral powellite associated with the molybdenite consistently produces erroneously old Re-Os dates.

Date of Award	23 Jan 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Ian J Parkinson (Supervisor), Tim Elliott (Supervisor) & Simon Tapster (Supervisor)