geology, petrology, geochemistry and platinum-group element-gold-copper-nickel ore asselmblage [i.e. assemblage] of the roby zone, lac des iles mafic-ultramafic complex, northwestern ontario
abstract
the archean lac des lies complex is a mafic to ultramafic intrusion emplaced into gneissic
tonalite. the lac des lies complex is the largest of several mafic to ultramafic intrusions that
form a circular outcrop pattern approximately 30 kilometers in diameter. the lac des lies
complex is composed of two ultramafic intrusions exposed at lac des lies and a gabbroic
intrusion located south of the lake. the gabbroic rocks contain the economically significant
roby zone pge-au-cu-ni deposit.
the roby zone deposit is composed of two texturally- and compositionally-distinct portions.
the northern portion of the deposit is composed of a relatively unaltered layered gabbroic
sequence consisting of leucogabbro, gabbronorite, gabbro and clinopyroxenite. field data,
including the orientation and type of geolgic contacts, indicate that the layers represent an
intrusion of magma into a largely crystallized mush. in-situ fractionation was identified within
individual layers. the southern portion of the roby zone consists of a lithologically and
texturally complex unit containing numerous rounded and angular fragments varying in
composition from leucogabbroic to pyroxenitic and grain size ranging from medium-grained
to pegmatitic. these rocks have experienced pervasive deuteric alteration that modified the
original magmatic textures and compositions. numerous pegmatitic dikes and patches occur
throughout the heterolithic gabbro.
pge-au-cu-ni mineralization within the northern layered sequence often forms net-textured
sulphides and represents primary magmatic mineralization. within the heterolithic gabbro,
pges occur as primarily sulphides and tellurides. these pge minerals occur as blebs within
pegmatitic pods and as fine-grained inclusions and streaks within secondary silicates suggesting that deuteric fluids have concentrated and deposited the metals within the
heterolithic gabbro. within the southern portion of the roby zone, higher pge
concentrations are associated with altered areas.
the model for the development of the roby zone and its attendant pge-au-cu-ni
mineralization consists of 1) fractionation of tholeiitic magma in lower magma chamber and
exsolution of immiscible sulphide liquid with associated pge-au-cu-ni, 2) intrusion of
fractionated magma into roby zone and subsequent in-situ fractionation, 3) prior to
complete solidification of the layers, a volatile-rich gabbroic magma injected the roby zone
resulting in brecciation of the layered sequence and formation of the heterolithic gabbro
composed of rounded and angular fragments within a gabbroic matrix, 4) partially solidified
rounded fragments and partial melting of some of the remaining fragments by the gabbroic
magma triggered liquid immiscibility, 5) deteuric fluids percolated through the fragmented
gabbroic rocks modifying the original magmatic textures and compositions and concentrated
and redeposited metals within the heterolithic gabbro. subsequent regional deformation
tilted the roby zone to the east and shearing occurred within a portion of the
clinopyroxenite. late-stage local suiting and hydrothermal fluids further modified the original
magmatic textures, compositions and pge-au-cu-ni mineralization.
collections
- retrospective theses [1604]