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TitleLinking porphyry deposit geology to geophysics via physical properties: adding value to geoscience BC geophysical data
AuthorMitchinson, D E; Enkin, R J; Hart, C J R
SourceGeoscience BC, Report 2013-14, 2013, 116 pages
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20130336
PublisherGeoscience BC
Mediaon-line; digital
File formatpdf; xls
ProvinceBritish Columbia
NTS93N/01; 93K/03; 93E/11; 93L/16; 93L/16; 93M/01
AreaMount Milligan; Endako; Huckleberry; Granisle; Bell; Morrison
Lat/Long WENS-127.5000 -124.0000 55.2500 53.5000
Subjectseconomic geology; geophysics; mineral occurrences; mineral deposits; porphyry deposits; geophysical surveys; geophysical interpretations; magnetic properties; electrical properties; Cache Creek Terrane; Quesnel Terrane; Stikine Terrane
Illustrationslocation maps; tables; histograms; geophysical images
ProgramTargeted Geoscience Initiative (TGI-4), Methodological Development
As part of the QUEST and QUEST-West geophysical initiatives, Geoscience BC focused detailed electromagnetic and magnetic surveys on a suite of six porphyry deposits to attempt to identify characteristic geophysical footprints for these economically important deposit types. The deposits chosen for investigation included Mount Milligan, Endako, Huckleberry, Bell, Granisle, and Morrison. To aid the interpretation of the collected geophysical data, a project was initiated to investigate physical rock properties of the hosting and mineralized rocks associated with the porphyry deposits surveyed. The primary objectives of the research were to 1) identify physical property data distributions and ranges for characteristic host rocks and their altered and mineralized equivalents, 2) understand the causes of physical property variations, and 3) provide a synopsis of physical property criteria defining altered and mineralized rocks in specific BC porphyry settings to help reduce risk during geophysical exploration within these regions.
The BC porphyry deposits studied share features typical of calc-alkalic and alkalic porphyry deposits. The deposits are generally characterized by an association with porphyritic intrusions, by overprinting potassic, phyllic, and propylitic alteration, and by sulfides controlled by fracture and vein systems. Despite these similarities, geophysical responses vary from one porphyry deposit to another. Whereas some differences in geophysical response are expected between calc-alkalic and alkalic porphyry systems due to differing alteration mineralogy and deposit geometries, dissimilarities between similar porphyry types can also be expected. Physical property analyses on sample suites from BC porphyry deposits has shown that changes in physical properties between background and mineralized rock arise predominantly from mineralogical and textural variations.
The effect of porphyry-related alteration on magnetic susceptibility and magnetic signatures varies. At Mount Milligan, secondary magnetite forms in association with potassic alteration increasing magnetic susceptibilities of altered rocks proximal to mineralized zones. This is also true at Bell, Granisle, and Morrison. Although secondary magnetite was also formed as a result of alteration and mineralization processes at Huckleberry, magnetite has additionally been introduced to host rocks here as a result of hornfelsing related to contact metamorphism. The pervasive hornfelsing obscures magnetic anomalies which may have identified potassically altered mineralized zones at Huckleberry. At Endako, primary magnetite in hosting monzonitic rocks is destroyed by overprinting phyllic and argillic alteration. Phyllic alteration also partially overprints potassic alteration at deposits in the Babine porphyry district, locally destroying magnetite.
Sericite and clay dominated alteration can bring about increases in porosity and correlated decreases in density. Alteration of feldspars to form sericite and clay causes the rock to become friable and weak, allowing increased fluids to permeate the rock, lowering electrical resistivities. Phyllic and argillic alteration is related to decreased resistivities at several of the porphyry deposits studied, including Endako, Bell, Granisle, and Morrison. At Mount Milligan and Huckleberry, phyllic and argillic alteration is not prominent; the rocks are generally low porosity and high resistivity.
Sulfide abundance and distribution also affect electrical rock properties. If sulfides are disseminated, the rock may be chargeable, but not necessarily conductive. Connected networks of sulfides will enhance conductivities, but may not result in high chargeabilities. The variety of sulfide and oxide minerals which occur will also influence electrical rock properties, with some minerals being inherently more chargeable or conductive than others.
Although knowledge of the geology of the area being explored will allow physical property trends to be anticipated to some degree, the most effective means of building physical property knowledge prior to geophysical investigation remains collecting physical property measurements. Ideally data is collected at a range of scales, from hand sample to outcrop. 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 (TGI-4) is a collaborative federal geoscience program that provides industry with the next generation of geoscience knowledge and innovative techniques to better detect buried mineral deposits, thereby reducing some of the risks of exploration. 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.