The Garson Ni-Cu-PGE deposit is located on the South Range of the 1850 Ma Sudbury
structure along the contact between the Sudbury Igneous Complex (SIC) and the
underlying metasedimentary and metavolcanic rocks of the Paleoproterozoic Huronian
Supergroup. It comprises four ore bodies that are hosted by E-W-trending shear zones
that dip steeply to the south. The shear zones formed as south-directed D1 thrusts in
response to flexural-slip during regional buckling of the SIC. They imbricated the ore
zones, the SIC norite, the underlying Huronian rocks and they emplaced slivers of
Huronian rocks and anatectic breccia into the overlying Main Mass norite. Coexisting
garnet-amphibole pairs yielded syn-D1 amphibolite facies metamorphic temperatures
ranging from ~550°C to 590°C. The shear zones were coeval with the moderately southdipping
South Range and Thayer Lindsley shear zones, which formed to accommodate
the strain in the hinge zone as the SIC tightened with progressive D1 shortening. The SE
limb of the SIC was overturned together with the D1 thrusts, which were then reactivated
as steeply south-dipping reverse shear zones during syn-D2 greenschist metamorphism.
Syn-D2 metamorphic titanite yield a U-Pb age of ca. 1849 ± 6 Ma, suggesting that D1 and
D2 are part of a single progressive deformation event that occurred immediately after
crystallization of the SIC during the Penokean Orogeny.
The ore bodies plunge steeply to the south parallel to the colinear L1 and L2 stretching
mineral lineations. Ore types consist mainly of pyrrhotite-pentlandite-chalcopyrite breccia
ores, but also include pyrrhotite-pentlandite-chalcopyrite disseminated sulfide
mineralization in norite, and syn-D2 quartz-calcite-chalcopyrite-pyrrhotite-pentlandite
iv
veins. In the breccia ores, matrix sulfides surround silicate rock fragments that have a
strong shape-preferred orientation defining a pervasive foliation. The fragments are
highly stretched parallel to the mineral lineations in wall rocks, suggesting that the ore
bodies are zones of high strain. Pyrrhotite and chalcopyrite occur in piercement
structures, in boudin necks between fragments, in fractures in wall rocks and in fold
hinges, suggesting that the sulfides were mobilized by ductile plastic flow. Despite
evidence of high strain in the ore zones, the sulfide matrix in D1 and D2 breccia ores show
little evidence of strain as they consist predominantly of polygonal pyrrhotite aggregates,
suggesting that they recrystallized during, or immediately after D1 and D2. However, rare
elongate pyrrhotite grains aligned parallel to S2 are locally preserved only in D2 breccia
ores. Exsolution of pentlandite loops along grain boundaries of elongate pyrrhotite
formed S2-parallel pentlandite-rich layers in D2 breccia ores, whereas the pentlandite
loops are multi-oriented in D1 contact breccia as they were exsolved along grain
boundaries polygonal pyrrhotite. Because exsolution of pentlandite post-date D1 and D2,
and that individual pentlandite grains neither have a shape-preferred orientation nor show
evidence for cataclastic flow, the sulfides reverted to, and were mobilized as a
homogeneous metamorphic monosulfide solid solution (mss) during D1 and possibly D2.
This is in agreement with predictions from phase equilibria as the average Garson
composition plots within the mss field in Fe-Ni-S ternary diagram at temperatures above
~400°C.
Disseminated and breccia ores at Garson have similar mantle-normalized multi-element
chalcophile patterns as undeformed contact-type disseminated and massive ore,
v
respectively, at the well known Creighton mine in the South Range. This suggests that the
Garson ores are magmatic in origin and that their compositions were not significantly
altered by hydrothermal fluids and deformation. The lack of variations in Ni tenors
between the disseminated and breccias ores suggest that the R-factor was not the process
controlling metal tenors because the disseminated sulfides do not consistently have higher
metal tenors than the breccia ore. The breccia ores are enriched in Rh-Ru-Ir and are
depleted in Cu-Pd-Pt-Au, in contrast to footwall-type ore at the nearby Garson Ramp
mine which is enriched in the same metals. When Ni100, Rh100, Ir100, Pt100 and Pd100 are
plotted against Cu100, the breccia and footwall-type ore analyses plot along model mss
fractionation and sulfide melt model curves, suggesting that these two ore types are
related by mss fractionation.
In summary, the Garson breccia ores are mss cumulates that settled quickly at the base of
the SIC via a gravity filtration process, and were mobilized as a metamorphic mss by
ductile plastic flow during D1 and D2. Despite minor local hydrothermal mobilization of
some metals, the study confirms findings from other studies that highly deformed Ni-Cu-
PGE deposits, such as the Garson deposit, can provide important information on the
genesis of the deposits.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:OSUL.10219/2029 |
| Date | 31 July 2013 |
| Creators | Mukwakwami, Joshua |
| Publisher | Laurentian University of Sudbury |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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