McCurdy and 9 co-authors reported in 2022 that the trace element composition of detrital magnetite grains recovered from six local streams around the Casino highgrade porphyry Cu–Au–Mo deposit, west-central Yukon, is compared with igneous and magmatic-hydrothermal magnetite recovered from mineralized and unmineralized host rocks at the deposit.
Linear discriminant analysis of 12 elements (Mg, Al, Ti, V, Mn, Co, Cr, Ni, Cu, Zn, Ga and Ge) and plots of Ti v. Ni/Cr are used to discriminate between magmatic-hydrothermal magnetite from the potassic alteration zone and igneous magnetite from granodiorite and quartz monzonite hosting the deposit.
Magmatic-hydrothermal magnetite with a trace element composition similar to that from the potassic alteration zone at Casino is identifiable in stream sediments draining the deposit.
Copper in magmatic-hydrothermal magnetite, present as minute inclusions of sulfide minerals such as chalcopyrite or substituted within the magnetite crystal lattice, is a strong indicator of Cu mineralization. We show that the chemical compositions of magnetite recovered from stream sediments can be used to explore for porphyry systems.
The authors concluded that magnetite originating from granodioritic host rocks and from the potassic alteration zone of the Casino deposit can be identified in local stream sediments. A combination of linear discriminant analysis based on 12 variables (Mg, Al, Ti, V, Mn, Co, Ni, Ga, Ge, Cu, Cr and Zn) in samples from known sources with a Ti v. Ni/Cr plot (Dare et al. 2014) effectively classifies igneous and magmatichydrothermal magnetite from the Casino deposit. Further refinement of the methodology, identifying Cu contents above the detection limits of LA-ICP-MS in magmatic-hydrothermal magnetite, is effective in classifying magnetite grains from a mineralized source.
The presence or absence of apatite inclusions and trellis textures are useful for identifying magnetite from other igneous and magmatic-hydrothermal sources, as are relative abundances of Ti and Cr in igneous sources and Cu in magmatic-hydrothermal magnetite. Alteration, oxidation, exsolution, and re-equilibration processes can each affect the petrographic characteristics and chemical composition of both igneous and magmatic-hydrothermal magnetite, and must be accounted for when attempting to classify grains and establish provenance.
Analysis of different size fractions of magnetite grains recovered from stream sediments is recommended: an examination of the chemical composition of magnetite grains that are <0.25 mm could improve the efficiency of this exploration method. Further work is required to establish an optimum grain size for recovery and chemical analysis.
The authors recommended that a minimum of 200 magnetite grains from each stream site be analysed by LA-ICPMS for robust multi-variate analysis.
Comparing and combining magnetite mineral chemistry data from different locations must be carried out with extreme care. Factors such as bulk rock and fluid composition, temperature, fugacity, pressure, and cooling history affect the composition of magnetite and can vary even within hand samples. Different instrument parameters and calibration methods could affect LAICPMS analytical results and must be taken into consideration when
using data from other locations.
Petrographic and compositional characterization of magnetite from potential bedrock sources is key to tracing the bedrock source of magnetite in streams. Applying this knowledge to mineral exploration using magnetite in stream and other detrital sediments could be further refined by characterizing magnetite from all potential bedrock sources in a survey area.
These strategies would be of practical use in the exploration for porphyry systems in Canada, especially when combined with abundance data for other indicator minerals present in the same stream sediment samples.
Managed & Built by I2M Consulting, LLC | © 2010-
