Maggie Hays Mine Geology
The Maggie Hays orebody sits upon the upper surface of an overturned sequence of komatiitic ultramafic cumulates dipping west at between 65 and 75 degrees. The ultramafic body is in the form of a 'keel', with a southward plunging folded surface pinching out the northern end of the resource, where it enters a heterogeneously sheared structure. The ultramafic sequence is folded at its down-dip termination, where it enters a shear, with remobilised sulfides forming a discontinuous remobilised stringer ore zone of low grade.
A proposed 'shelf fault' terminates the down dip portion of the ultramafic in the south of the MAggie Hays portion of the CUU, however it is equally likely that a porpoising hinge to the syncline could equally well explain the apparent truncation of the depth of the orebody in the south.
The ultramafic unit is a massive serpentinite, likely of mesocumulate to adcumulate composition. Peak metamorphic assemblages were in the amphibolite facies, with development of metamorphic olivine-anthophyllite-pyroxene assemblages. Retrograde metamorphism has occurred, leaving a serpentine-anthophyllite/cummingtonite-talc assemblage, with some areas of talc-carbonation.
The internal structure of the ultamafic includes some interesting features, notably a so-called "chill-zone" of assumed orthocumulate composition, which occupies the marginal facies of the ultramafic unit. This chill zone may represent a chill zone which infers an intrusive origin for the ultramafic, or it may represent a zone of metasomatism and/or contamination.
The orebody itself is composed of 2 to 6 metres of massive nickeliferous sulfides, usually banded and foliated pentlandite-pyrrhotite-pyrite. Dodecahedral pyrite crystals to 20 cm are formed within the massive sulfide zones. The massive zone is overlain by a matrix ore zone composed of the above sulfide assemblage and coarse jackstraw textured bladed olivine, now retrogressed to black serpentinite. This is in turn overlain by stringy-beef textured recrystallised disseminated ore zones containing retrogressed metamorphic olivine and distinctive bladed anthophyllite.
The structural overprint of the ultramafics and orebody by deformation during prograde metamorphism is a matter of debate, however the ductile nature of the deformation has affected the ultramafic CUU heterogenously and contrasts with the felsic footwall. The felsic footwall is subject to a pronounced stretching lineation which increases in intensity to the north. The lineation orientation is a uniform 65 degrees toward 120 degrees. Approaching the CUU body, the lineation breaks down into a zone of heavy boudinage and oblique shear with a pronounced C-S shear fabric, especially in the remobilised massive sulfides and at the leading edge of the keel structure. This is interpreted to occur due to competency contrast between the felsic footwall and the ultramafic unit.
Read more about this topic: Emily Ann And Maggie Hays Nickel Mines