| dc.description.abstract | The Quetico Intrusions are a group of small,
geochemically primitive, locally Cu-Ni-PGE-rich, mafic to
ultramafic intrusions closely associated with the margins of
the Quetico Batholithic Complex. They are composed of
distinctively textured, sometimes layered, hornblende-rich,
cumulate rocks that range from hornblende wehrlite to quartz
diorite. The ultramafic rocks characteristically contain
numerous large, interstitial, poikilitic hornblendes up to 3
cm in diameter. The mafic lithologies are often coarsely
hornblende-phyric. All rock types contain texturally
distinct intermediate to mafic patches and veins that
exhibit large, ragged, cored, prismatic to acicular
hornblendes referred to as appinites. This study examined
fifteen of the Quetico Intrusions, concentrating on those
containing mineralized zones. Three intrusions were mapped
in detail.
Petrographic examination suggests a crystallization
order of: olivine, olivine + clinopyroxene, clinopyroxene
(locally orthopyroxene), clinopyroxene + hornblende,
hornblende, hornblende + plagioclase. Biotite, apatite and
titanite are present within most rock types.
Alteration and the cumulus nature of the rocks
precludes the use of whole rock data to show definitive
fractionation trends. Instead, emphasis was placed on the
quantitative mineral chemistry of olivine and clinopyroxene.
Olivine is rich in MgO and exhibits a narrow forsterite
range of Fo77.55-80.89 Clinopyroxene is generally diopsidic
with a compositional range of Wo44.60-50.53' En39.03-49.02' Fs5.35-
11.31. Mineral chemistry data, plus the presence of
myrmekitic intergrowths of magnetite with clinopyroxene,
suggests that the Quetico clinopyroxenes crystallized from a
magma of sub-alkaline, possibly calc-alkaline, affinity.
Nine intrusions host mineralized zones containing
copper, nickel and platinum-group element mineralization.
This mineralization consists of chalcopyrite, pyrrhotite,
pentlandite, and magnetite with accessory pyrite and
nickeliferous cobaltite, minor cubanite, altaite,
empressite, breithauptite, and rare bismuthinite. Violarite
and covellite occur as minor alteration products of
pentlandite and chalcopyrite, respectively. The major
sulphide and accessory phases probably crystallized from a
PGM-bearing Ni- and Cu-rich monosulphide phase trapped
interstitially to surrounding silicates. The presently
observed textures are mainly due to subsolidus
reequilibration and to the formation of lower temperature
phases as the intrusions slowly cooled.
The platinum-group minerals (PGM) were identified using
semi-quantitative EDS techniques and are generally less than
5µm in diameter. They include michenerite, hollingworthite, froodite, sobolevskite, and sperrylite.
Complex PGM intergrowths are common. Palladian gold is the only PGE bearing alloy observed. A few
PGM are interstitial to, or contained within, silicate minerals, suggesting some form of late-stage
remobilization.
A comparison of the Quetico lithologic, textural, petrographic, whole rock, and mineral chemistry
data with that of the appinite suite and the Alaskan-type Complexes suggests that the Quetico
Intrusions are Archean equivalents of the Caledonian-age appinite suite of Great Britain, Ireland,
and elsewhere. They may have crystallized from a water-rich sub-alkaline olivine basalt liquid,
possibly of calc-alkaline affinity, at a relatively high PHW and P . They must have crystallized
under a relatively high PHW in order for hornblende to remain stable over such a wide range of
magmatic conditions. Assimilation of metasedimentary wall-rock may have resulted in the
hybridization of some of
the magma. | |
