Geophysics HM

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Geophysical methods are used in mineral exploration and mining for targeting and mapping anomalous zones with potential for economic mineralization. The field surveys are generally medium to large scale investigations designed and oriented to targeting anomalous concentrations of minerals with outstanding physical property differences compared to the background geological formations or matrix.

The interpretation results are used for proposing drill targets, delineate ore deposits and correlate the measured geophysical magnitudes with known mineralization for targeting similar responses elsewhere.

Lode Gold Deposits

A lode Au deposit is a hydrothermal deposit whose principal commodity is Au. Dubé and Gosselin (2007) provide a comprehensive synthesis of this deposit type and its various subcategories including 1) shear- and fault-zone-related deposits, principally greenstone-hosted quartz-carbonate vein deposits (orogenic, mesothermal, lode gold, shear-zonerelated quartz-carbonate or gold-only deposits) associated with collisional tectonics, 2) intrusion-related deposits associated with felsic plutons of subaerial, oceanic, and continental setting, and 3) epithermal deposits (high- and low-sulphidation) associated with subaerial and shallow-marine environments (Lydon et al., 2004).

Mississippi Valley Pb-Zn-Cu-U

Mississippi Valley-type (MVT) Pb-Zn deposits are typically stratabound, some are prismatic pipe-shaped bodies, hosted by limestone or dolomite in platform carbonate sequences (Sangster, 1995) and occur in clusters. Sphalerite and galena are the dominant ore minerals that characteristically occupy open spaces in carbonate breccias; replacement of host rocks is relatively rare.

Most deposits or MVT districts occur below unconformities or nonconformities, related probably to minor uplift or warping, but few deposits have been affected by subsequent deformational events. Dimensions of the one hundred known orebodies in the Pine Point district vary from 60 to 2000 metres in length, 15 to 1000 metres in width, and 0.5 to 100 metres in thickness (Hannigan, 2007).Additional information on various aspects of MVT deposits may be found in Dewing et al. (2007), Hannigan (2007), Paradis and Nelson (2007), and Paradis et al. (2007).

Porphyry Copper Deposits

Porphyry deposits are a major source of production for Cu, Mo, and Re, and an important source forAu,Ag, and Sn. Sinclair, (2007) provides a comprehensive synthesis of porphyry deposits. Porphyry-style base and precious metal mineralization is spatially and genetically related to high level, epizonal and mesozonal felsic to intermediate porphyritic intrusions and adjacent host rocks.

Mineralization may be in the form of stockwork quartz veins and veinlets, fractures, disseminations, and replacements containing pyrite, chalcopyrite, bornite, and magnetite. Deposit forms are quite variable and range in size from hundreds to thousands of metres laterally and with depth, and are commonly zoned with barren cores and generally concentric metal zones surrounded by barren pyritic haloes.

Alteration associated with porphyry deposits is typically zoned from an inner potassic (biotite and/or K-feldspar) alteration zone, closely associated with mineralization, to a more extensive propylitic alteration zone consisting of quartz, chlorite, epidote, calcite, albite, and pyrite, which surrounds the inner potassic zone. Zones of phyllic and argillic alteration may occur between and overlap with the inner potassic and outer propylitic alteration zones. Minerals relevant to geophysical detection include magnetite, pyrite, chalcopyrite, biotite, K-feldspar, and sericite.

Sedimentary Ehalative Sulphide Deposits

SEDEX deposits (Pb-Zn) are found in sedimentary basins, usually in the form of conformable to semiconformable sheets or tabular lenses of stratiform sulphides (Lydon, 1995; Goodfellow and Lydon, 2007). Such bodies have typical aspect ratios of 20, maximum thicknesses of 5 to 20 m (Lydon, 1995), and may extend over a distance of more than 1 km (Goodfellow and Lydon, 2007).

The principal ore inerals are sphalerite and galena. Chalcopyrite is sometimes concentrated in feeder zones of SEDEX deposits, but only rarely attains concentrations of economic interest. Pyrite is the most abundant sulphide, and pyrrhotite may be common.

The physical and chemical properties of SEDEX deposits make them amenable to detection by several geophysical techniques. In the Purcell Basin, southeastern British Columbia, sulphide mineralization associated with the Sullivan and smaller North Star and Stemwinder Pb-Zn-Ag deposits is reflected by strong finite conductors and positive magnetic anomalies (Lowe et al., 2000).

Uranium Deposits

Unconformity-related uranium deposits are the most significant high-grade, low-cost source of uranium in the world (Jefferson et al., 2007). In Canada, notable targets for exploration are the mid-Proterozoic sedimentary Athabasca and Thelon basins in the northwestern Canadian Shield.

The deposits occur typically along or near unconformities between metamorphic basement rocks, commonly containing graphitic pelitic units, and overlying undeformed sedimentary successions consisting mainly of quartzose sandstone. Graphitic units provide a lithological control on mineralization (Ruzicka, 1995), acting as a reductant.

Volcanogenic Massive Sulphides

Volcanic massive sulphide (VMS) deposits form by discharge of hydrothermal solutions onto the seafloor, commonly near plate margins. A comprehensive synthesis of VMS deposits is provided by Galley et al. (2007). VMS deposits typically develop in the form of a concordant lens that is underlain by a discordant stockwork or stringer zone comprising vein-type sulphide mineralization located in a pipe of hydrothermally altered rock.

VMS deposits typically have density, magnetic, conductivity, and acoustic velocity properties that differ significantly from those of their host rocks.

There is, therefore, enormous potential for direct detection of orebodies using geophysical methods that measure these properties.

The most common sulphide mineral in VMS deposits is pyrite, which may be accompanied by subordinate pyrrhotite, chalcopyrite, sphalerite, and galena (Galley et al., 2007). Magnetite, hematite, and cassiterite are common nonsulphide metallic minerals, and the gangue mineral barite may also be present. Densities of these minerals range from 4.0 to 7.5 g/cm3

River Lode Gold Prospecting

The purpose of carrying out geophysical surveys is to find out something about the rocks in the survey area. Geophysical methods all depend on measuring a physical property of rocks. Geophysical surveys generally look for concentrations of anomalously high values of the property being measured. The results of the survey are used to identify a target of interest, or to correlate the spatial variation of values of the property with variations in the geology. The primary reasons to do geophysics are to get information on geology, and possibly to find targets of economic interest (W.J. Scott, Ph.D., P.Eng., P.Geo, 2014).

Geophysics HM

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MINERAL EXPLORATION GEOPHYSICS

Geophysical Methods Used for Mineral Exploration

Applications and Solutions

Base Metals & Minerals

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