Dan Thompson MSc Thesis Abstract
The Lundmark Akow area is located in the south-central portion of the South Rim assemblage of the North Caribou Greenstone Belt. The mineralization consists of several base metal-bearing massive sulphide horizons hosted in a sequence of garnetiferous staurolite mica schists in the southern portion of the study area, to a northern host rock sequence dominated by volcanic and intrusive mafic to felsic rocks. The igneous host rock portion of the study has been dated between 2973 to 2980 Ma through zircon U/Pb geochronology. The meta-sedimentary sequence which hosts the mineralization is built upon a basement of intrusive and volcanic rocks which formed in an oceanic plateau through plume magmatism before impinging upon a subduction zone. Negative high field strength element (HFSE) anomalies show that arc related magmatism built upon the oceanic plateau. Sm/Nd isotope values from the mafic to felsic volcanic and intrusive rocks show a spread in εNd from -1.53 to 3.07 suggesting that melts were derived from both depleted mantle and plume sources, with some melts being contaminated by an older crustal basement.
The characteristics of the massive sulphide horizons, as well as the host meta-sedimentary rocks are consistent with them having formed through distal VMS processes including hydrothermal particulate fallout from buoyant plumes combined with pooling of dense sulphide-rich fluids in topographic lows on the sea floor. The garnet-rich meta-sedimentary rocks show Fe and Mn enrichment when normalized to immobile Al and Ti, consistent with addition through hydrothermal plume particulate processes. The characteristics of the garnet layers suggest they formed through metamorphism of a sediment derived from the intermixing of hydrothermal and terrigenous particulates. The garnet compositions show prograde growth with a Mn- and Ca-rich core to a Fe- and Mg-rich rim, supporting their formation as a result of metamorphic conditions post exhalative activity.
The massive sulphide horizons occur in lenses of 5 to 50 cm in width and do not show any clear alteration zonation leading up to the mineralization, suggesting they formed distal to the vent likely from a dense bottom-hugging sulphide-rich brine. Tourmalinite occurrences associated with the massive sulphide horizons formed through metasomatic interactions between the plume fluids and the underlying seafloor sediments, forming continuous stratigraphic beds. The tourmaline composistions were very Fe-rich with base metal concentrations similar to the host massive sulphide, supporting the formation of the tourmaline coevally with the sulphides in an Fe rich hydrothermal plume.
The sedimentary nature of the host rocks to the mineralization did not allow for the evaluation of hydrothermal alteration through methods of Rb/Sr ratios or alteration index/ chlorite carbonate pyrite index alteration box plots. An evaluation of SWIR data on white micas showed the Al-OH band occurring at around 2200 nm, suggesting formation due to regional metamorphism rather than through a high heat and fluid/rock interaction associated with proximal VMS mineralization.
Efforts to vector to mineralization through mineral compositions were not successful. The presence of Mn-rich garnet was more dependent on host rock composition rather than proximity to massive sulphide. Zinc enrichment in staurolite was limited and no values greater than 2 wt.% were observed which suggests there is no massive sulphide mineralization in the vicinity of all tested staurolite. Evaluating a change in XFe in ferromagnesian silicates (garnet, biotite, staurolite, chlorite) in proximity to metamorphosed massive sulphide horizons due to the formation of a sulphidation oxidation halo showed that the composition of garnet, biotite, and chlorite were modified. However, the extent of this halo was limited to within a few meters of a metamorphosed massive sulphide horizon and as such the usefulness of this as a vector is limited.
Geothermometry using a Ti in biotite and garnet biotite composistion as well as a garnet, biotite, plagioclase muscovite barometer was applied to show an increasing metamorphic grade from the north to the south across the belt. Temperatures range from 573 ± 32.2° C in the north to 651 ± 32.6° C in the southern portion of the Lundmark Akow area. Pressure variation likewise varies from 3.0 Kbar in the north to 5.3 Kbar in the southern most sample. Calculating peak metamorphic conditions based on calculated theoretical mineral assemblages from whole rock data through the use of a pseudosection is broadly consistent with the geothermobarometry, with a stable field for the coexistence of biotite garnet and staurolite occurs from between 510-600 °C and pressure between 2-5 Kbars.