Rebecca Price MSc thesis abstract

Thesis Title: 
Mineralogy and petrology of the Good Hope carbonatite complex, Marathon, Ontario
Rebecca Lynne

The Good Hope carbonatite is located northwest of Marathon, Ontario (49° 02’ N, 86°43’ W). It occurs along the northwest margin of the Prairie Lake complex. The objectives of this research are to characterize the carbonate and other minerals of the Good Hope carbonatite, use mineral compositions and textural associations to discuss the petrogenesis, and compare the mineralogy of the Good Hope and Prairie Lake carbonatites to assess potential relationships.

The major rock-forming minerals in the Good Hope carbonatite are calcite, ferroan dolomite, and apatite. The apatite occurs predominantly as elongated aggregates and clasts that define heterogeneous banding within the carbonatite. Syenite xenoliths are centimeter scale, entrained within the carbonatite, composed of K-feldspar, biotite-phlogopite, magnesio-arfvedsonite, aegirine, quartz, and carbonate. The minor minerals within the carbonatite include dolomite, K-feldspar, quartz, chlorite, magnetite, barite, biotite – phlogopite, magnesio-arfvedsonite, aegirine, pyrochlore, albite, and fluorite. The accessory minerals include pyrite, synchysite-(Ce), rutile, siderite, strontianite, thorite, parisite-(Ce), bastnaesite-(Ce), burbankite, and zircon.

The sequence of formation began with the crystallization of the syenite from an unknown alkaline parent magma, which was followed by the first pulse of carbonatitic magma that disaggregated and entrapped the syenite xenoliths and began crystallizing the Ca-Na pyrochlore and apatite. Subsequent evolution of the initial carbonatitic magma resulted in the alteration of the pyrochlore as well as the dissolution-reprecipitation of the apatite. The second pulse of carbonatitic magma resulted in the resumption of magmatic crystallization of the Ca-Na pyrochlore and apatite together with the crystallization of the alkaline silicates, aegirine and magnesio-arfvedsonite. The apatite + pyrochlore ± magnesio-arfvedsonite accumulate and are subsequently disaggregated with the clasts being deformed by turbulent flow of the third carbonatitic magma pulse. Groundmass calcite and dolomite with increasingly Fe-rich compositions were formed as crystallization progressed. The system becomes increasingly influenced by hydrothermal and carbothermal processes with the crystallization of the late ferroan dolomite with associated late-hydrothermal apatite ± pyrochlore. The precipitation of the hydrothermal phases and Fe-overgrowths are the last stages of crystallization.

The close spatial association with the Prairie Lake carbonatite complex and the crystallization sequence consistent with other alkaline rock-carbonatite complexes may support a genetic association. However, the mineralogy of Good Hope is distinctly different from that of Prairie Lake and as yet there is insufficient mineralogical or geological evidence to permit formulation of any simple genetic relationships between the two complexes.