Petrographic and cathodoluminescence characterization of apatite from the Sue-Dianne and Brooke IOCG mineralization systems, Great Bear magmatic zone, Northwest Territories
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Abstract:
Apatite is the main REE-bearing phase in 19 thin sections from two mineralized systems and their host rocks, the Cu-Ag-(Au) Sue-Dianne IOCG deposit and the nearby Brooke Zone, located within the southern Great Bear magmatic zone, Northwest Territories, Canada. The thin sections show a range of IOCG-related alterations and igneous protoliths. Apatite occurs in all thin sections, in amounts ranging between traces to 5-10%. It can be divided into four populations on the basis of size, cathodoluminescence (CL) response and petrographic association: 1) small euhedral apatite (20-40 µm), occurring in clusters of 5 to 20 grains, generally free of inclusions, found in the volcaniclastic samples; 2) single apatite crystals (60-80 µm) with pitted edges, numerous fluid inclusions and sometimes monazite and hematite inclusions; 3) large single apatite crystals (100-300 µm) within epidote veins, sometimes associated with scheelite; and 4) large fractured apatite crystals (0.5 to 4 mm) with heavy hematite staining associated with hydrothermal breccias. The first 3 populations have a green to yellow CL response and the third population shows some irregular zoning. The fourth population shows a blue CL tint, with irregular green zones throughout grains or near the crystal rims. In that fourth population, mineral inclusions with an orange CL response are likely calcite. Group 3 and 4 also show an heterogeneous REE distribution within crystals under EDX. Given the overprinting of multiple alteration types in many of the specimens, additional work is needed to relate these apatite populations to specific alteration stages associated with IOCG-type mineralization. However, the presence of apatite coarse enough to be picked from glacial sediment samples and showing distinctive CL signatures in rocks related to IOCG mineralization suggests potential as an indicator mineral method.Keywords:
Cathodoluminescence
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Abstract Iron oxide–copper–gold (IOCG) and iron oxide–apatite (IOA) deposits are important sources of Cu and Fe, respectively. They contain abundant Fe-oxides and may contain Au, Ag, Co, rare earth elements (REEs), U, Ni, and V as economically important by-products. In Peru, the Mina Justa IOCG deposit is located next to the giant Marcona IOA deposit. Constraining the timing of Fe and Cu mineralization at Mina Justa is fundamental to understanding the duration and type of processes that generated this mineral deposit, and ultimately to testing the genetic link with other deposits in the area. Previous authors used alteration minerals to indirectly date Cu mineralization at Mina Justa at around 100 Ma. We report Ar/Ar dates of actinolite, U-Pb dates of magnetite, apatite, and titanite collected by in situ laser-ablation–multicollector–inductively coupled plasma–mass spectrometry, and Re-Os thermal ionization mass spectrometry dates for sulfides. These results indicate that Cu mineralization at Mina Justa occurred at ca. 160 Ma and that Fe mineralization is older and coeval with the neighboring Marcona IOA deposit, consistent with Cu mineralization overprinting IOA-style mineralization at Mina Justa.
Geochronology
Overprinting
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Prospecting
Iron ore
Ore genesis
Gold ore
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