The Cobar Basin developed as four deep-water troughs bordered by shallow-water shelves. The southern parts of the Basin, where the May Day deposit is located, are covered by the Mount Hope and Rast Troughs which were filled with sediments, volcaniclastics, and volcanics of bi-modal nature. May Day is thought to be a deformed VMS deposit.
The mineralised zone can be found in proximity to the contact between the sediments (the Upper Amphitheatre Group) and the volcanics (the Mount Halfway Volcanics) within highly sheared-altered (talc-sericite-chlorite-carbonite) zones. The primary sulphides are pyrite-sphalerite-galena-chalcopyrite. Assays received to date show strong, continuous, and wide gold-polymetallic intercepts, confirming substantial true width (~25m) at relatively shallow depths. A high-grade core proximal to the hanging wall is evident with an approximate true thickness of up to ~12m. Results compliment previously received assays which also confirmed good down-dip continuity (minimum 180m down dip continuity from the base of the pit) of the May Day mineral system.
May Day, The Upper Amphitheatre Group
The Upper Amphitheatre Group contains interbedded shales-siltstones-sandstones of thin to moderate bed thicknesses. Locally the sediments can host tephra, with rhyolitic to rhyodacitic tuffs compositions, these are considered to be the stratigraphical equivalent to the Mount Halfway Volcanics. The depositional environment for this group is interpreted to have been deep marine environment turbidites.
May Day, The Mount Hope Volcanics
The Mount Halfway Volcanics at May Day are predominately made up of rhyolitic to rhyodacitic lavas with porphyritic tuffs. Localised sedimentary based units of interbedded sandstones and siltstones can be commonly found too. The depositional setting has been interpreted to have been deep marine with the rocks having been deposited as pyroclastic ashflows with interbedded turbidites. The Mount Halfway Volcanics are conformably overlain by and interfinger with the Upper Amphitheatre Group.
The Shear Zone
Due to the steeply plunging nature of the structures at May Day, it is likely that the deformation which lead to the shearing is associated with the Cobar deformation. It has been observed that the highly mineralised intervals contain intensely sheared zones, where the initially lithology can frequently no-longer be distinguished due to the level of shearing and associated synchronous alteration. Mineralised quartz veins were likely developed coevally with the aforementioned deformation, and it is considered that the veins likely formed steeply plunging shoots.