The Rock Physical Property Database of British Columbia, and the distinct petrophysical signature of the Chilcotin basalts^,

Abstract

Rock physical properties provide the link between geophysical surveys and their geological interpretation. The British Columbia rock physical properties database, compiled by the Geological Survey of Canada, now comprises 3876 values of density, 930 values of electric resistivity, 12 356 values of magnetic susceptibility, and 2576 values of magnetic remanence and Koenigsberger ratio. The measurements are linked to sample locations, lithologies, and geological formation or unit. Maps, histograms, and biplots are used to demonstrate useful links between lithology and physical properties, and serve as a background for future rock physical properties studies. As expected, density and resistivity are controlled mostly by porosity and mineralogy. Magnetic susceptibility has a bimodal distribution (maxima at 4 × 10 −4 and 2 × 10 −2 SI) controlled by magnetite concentration. Magnetic remanence is shown to be more important than usually considered in magnetic survey analysis, with Koenigsberger ratios greater than unity in 42% of the samples. A case study of the Chilcotin Group basalts is highlighted, as they form a significant barrier to mineral exploration in central British Columbia. These rocks are magnetically distinct from other basalts in British Columbia, distinguished by magnetic susceptibilities having a range of values concentrated around 3 × 10 −3 SI and falling in the valley between the two susceptibility modes. These basalts are also characterized by very high Koenigsberger ratios (96% above unity), probably caused by a preponderance of fine-grained single-domain magnetite. The database provides a wealth of petrophysical properties that can help constrain analysis of several types of geophysical surveys and, in particular, modelling of anomalies in the quest to determine the three-dimensional distribution of rock units. Résumé :Les propriétés physiques des roches constituent le lien entre les levés géophysiques et leur interprétation géologique. La base de données de propriétés physiques des roches de la Colombie-Britannique, compilée par la Commission géologique du Canada, comprend maintenant 3876 valeurs de densité, 930 valeurs de résistivité électrique, 12 356 valeurs de susceptibilité magnétique et 2576 valeurs de rémanence magnétique et de rapport de Koenigsberger. Les mesures sont reliées aux sites d'échantillonnage, aux lithologies et aux formations ou unités géologiques. Des cartes, des histogrammes et des diagrammes de double projection sont utilisés pour démontrer des liens utiles entre la lithologie et les propriétés physiques; ils servent aussi en tant que cadre de référence pour de futures études des propriétés physiques des roches. Tel qu'attendu, la densité et la résistivité sont contrôlées principalement par la porosité et la minéralogie. La susceptibilité magnétique a une distribution bimodale (maximums à 4 × 10 −4 and 2 × 10 −2 SM) contrôlée par la concentration en magnétite. La réma-nence magnétique est démontrée être plus importante que normalement considérée dans les analyses de levés ma...

The Henkel Petrophysical Plot: Mineralogy and Lithology from Physical Properties

“…The existence of unaltered volcanic rocks which carry atypically low magnetic susceptibility (~10 -3 SI), such as the Miocene Chilcotin basalts of central British Columbia (Enkin, 2014), helps test our model justifying the predominance of volcanic rocks with susceptibilities above 10 -2 SI lying on the Henkel plot Magnetite Trend. The Chilcotin Basalts are within-plate, dry, glassy, mafic, olivine bearing volcanic rocks (Dostal et al, 1996, Bevier, 1983.…”

“…Furthermore, since the compilation started as a database for the province of British Columbia (Enkin, 2014), it has a geographic bias in a region containing rocks of relatively low metamorphic grade. Nevertheless, the features present in the CRPPD Henkel plot are apparent in the global compilation of Henkel plots (appendix of Enkin, 2018), and correspond to common rock and mineral forming processes, so we contend that the detailed analysis and interpretations of the CRPPD Henkel plot are valid for most geological settings.…”

The Henkel Petrophysical Plot: Mineralogy and Lithology From Physical Properties

“…The existence of unaltered volcanic rocks which carry atypically low magnetic susceptibility (~10 −3 SI), such as the Miocene Chilcotin basalts of central British Columbia (Enkin, ), helps test our model justifying the predominance of volcanic rocks with susceptibilities above 10 −2 SI lying on the Henkel plot magnetite trend. The Chilcotin basalts are within plate, dry, glassy, mafic, and olivine bearing volcanic rocks (Bevier, ; Dostal et al, ).…”

“…The data set is biased by the research goals of the geologists, mostly involved with mineral exploration, who have collected and submitted samples and measurements. Furthermore, since the compilation started as a database for the province of British Columbia (Enkin, 2014), it has a geographic bias in a region containing rocks of relatively low metamorphic grade. Nevertheless, the features present in the CRPPD Henkel plot are apparent in the global compilation of Henkel plots (appendix of Enkin, 2018) and correspond to common rock and mineral forming processes, so we contend that the detailed analysis and interpretations of the CRPPD Henkel plot are valid for most geological settings.…”

Correcting airborne gravity data for overburden thickness using airborne transient electromagnetic data

“…The overburden conceals large areas of bedrock which has complicated mineral exploration efforts. This specific problem exists in the Interior Plateau of British Columbia (Levson and Giles 1997, Mihalynuk 2007, Enkin et al 2008, Enkin 2014).…”