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TitleLinking porphyry deposit geology to geophysics via physical properties: six porphyry deposits in British Columbia, Canada
 
AuthorEnkin, R JORCID logo; Mitchinson, D E
Source23rd World Mining Congress 2013, proceedings; by World Mining Congress; 2013 p. 1-11
Image
Year2013
Alt SeriesEarth Sciences Sector, Contribution Series 20130330
PublisherWorld Mining Congress
Meeting23rd World Mining Congress 2013; Montreal; CA; August 11-15, 2013
Documentbook
Lang.English
Mediadigital
File formatpdf
ProvinceBritish Columbia
NTS93
AreaPrince George; Williams Lake; Terrace
Lat/Long WENS-128.0000 -120.0000 56.0000 52.0000
Subjectsigneous and metamorphic petrology; metallic minerals; geophysics; magnetic susceptibility; porphyries; alteration; mineralization; porphyry deposits; magnetic anomalies; argillization; resistivity; porosity; sulphides; geophysical interpretations; Mount Milligan deposit; Takla Group; Huckleberry mine
Illustrationsanalyses
ProgramTargeted Geoscience Initiative (TGI-4) Methodological Development
AbstractModern mineral exploration depends on multidisciplinary 3-dimensional integration of geological and geophysical data. Identification of the geological sources of geophysical anomalies requires knowledge of the physical property fingerprint of rock types, formations and their alteration products. Understanding how and why physical rock properties vary within the Earth's subsurface ensures that appropriate geophysical exploration methods can be chosen, and allows for more efficient and effective surveys to be designed. Physical property knowledge is integral to geophysical modeling - measured data or values estimated based on geological understanding are used to produce realistic forward calculations and to constrain inversion models.
Mira Geoscience and the Geological Survey of Canada, in collaboration with industry and academe, are building a Rock Properties Database System (http://rpds.mirageoscience.com). It compiles the most valuable physical properties for mineral exploration (density, porosity, magnetic susceptibility and remanence, electrical resistivity and chargeability) linked with location and lithological metadata.
A critical knowledge gap, addressed in this study, concerns the dependence of physical properties on alteration and mineralization processes. We report on the results of a Geoscience British Columbia project, "Integrated Geological & Geophysical Porphyry Models: Adding Value to Geoscience BC Geophysical Data". As part of the QUEST and QUEST-West geophysical initiatives, Geoscience BC focused detailed geophysical surveys on a suite of six known porphyry deposits: Mount Milligan, Endako, Huckleberry, Bell, Granisle, and Morrison. Physical properties for a set of 272 samples of characteristic host rocks and their altered equivalents were measured. The causes of physical property variations were investigated through their correlations with ore and alteration mineralogy determined from thin-section observations and geochemical measurements.

Physical properties of variably altered host-rocks and intrusive rocks vary significantly between different BC porphyry deposits. No specific unifying geophysical model exists that can be uniformly applied during exploration. Knowledge of local background geology and local physical property variations is necessary as host-rocks and intrusive rock compositions vary depending on magmatic affinities, and alteration styles will vary reflecting magmatic affinities, crustal depth and influence of meteoric water. A district-scale exploration strategy requires location of intrusive bodies, which are commonly magnetic, resistive and low in density. Correlations should not be expected between density and magnetic susceptibility, because alteration may lead to magnetite destruction or to secondary magnetite development. Deposit-scale ground geophysics might image potassic alteration zones that can be magnetic in both alkalic and calcalkalic systems. Low magnetic susceptibilities, resistivities and densities might aid in locating the typically more porous phyllic and argillic zones. All geophysical data must be interpreted with background knowledge of local rock types and in light of the expected deposit model and associated magmatic and hydrothermal processes.
The most effective means to building physical property knowledge prior to geophysical investigation remains collecting physical rock property measurements. The cost of rock property data collection is low, relative to the overall costs of running a mineral exploration program, and can provide an important framework for design of geophysical surveys, survey method selection, geophysical modelling, and data interpretation.
Summary(Plain Language Summary, not published)
The Targeted Geoscience Initiative 4 (TGI-4) is a collaborative federal geoscience program that provides industry with the next generation of geoscience knowledge and innovative techniques, which will result in more effective targeting of buried mineral deposits. British Columbia holds the majority of Canada¿s porphyry mineral deposits which produce most of the country¿s copper. Exploration for this deposit type, especially in deep or remote settings, is done using geophysical surveys. In order to link geophysical survey analysis to the rock types which form these deposits, samples were collected from 6 porphyry deposits in British Columbia. The physical properties and mineralogy were measured on the same samples to reveal the mineralogical control on the physical properties. Recommendations of which geophysical tools are effective in different settings are proposed.
GEOSCAN ID293333

 
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