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Geophysical Signatures of Mineral Deposit Types in Finland Edited by Meri-Liisa Airo Geological Survey of Finland, Special Paper 58, 9–70, 2015

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Geophysical Signatures of Mineral Deposit Types in Finland Edited by Meri-Liisa Airo Geological Survey of Finland, Special Paper 58, 9–70, 2015 Geophysical signatures of mineral deposit types in Finland Edited by Meri-Liisa Airo Geological Survey of Finland, Special Paper 58, 9–70, 2015 GEOPHYSICAL SIGNATURES OF MINERAL DEPOSIT TYPES – SYNOPSIS by Meri-Liisa Airo GENERAL ISSUES A mineral deposit, as an anomalous unit of metal- 2. Form of an ore deposit (e.g., size, shape, orien- liferous minerals, contains minerals with quite tation, depth; ore mineral distribution and tex- different physical properties to those of country ture). rocks. Pyrite, pyrrhotite or magnetite are common • The size, orientation and depth extent of a minerals in ore deposits, all of which have distinc- mineral deposit are the main factors with re- tive physical properties and may greatly affect the gard to geophysical expressions. geophysical response. In addition to petrophysi- • A great depth suppresses geophysical signa- cally relevant ore minerals, other geological or tures. geometrical factors or environmental conditions • Gravity and magnetic methods only detect influence the geophysical expressions of ore de- lateral contrasts in density or magnetization, posits or mineralized systems. The main factors are but in contrast, electrical and seismic meth- gathered below. This list is inspired by a summary ods can detect vertical, as well as lateral, con- of the geochemical expressions of ore deposit types trasts of resistivity and velocity or reflectivity. presented by McQueen (2005): • In the case of sulphide mineralization, the shape of the deposit may affect the magnet- 1. Composition of the ore deposit and the con- ic signature by strengthening the remanent tained elements. magnetization in the direction of the long • Density depends on the elementary compo- axis of the deposit. sition of minerals; many metals have high • The electrical conductivity of a rock is a specific densities. function of many factors, among which the • Magnetically, the most distinctive are the mineral texture (galvanic structure) and po- ferrimagnetic minerals magnetite and the rosity (with contained water) have a signifi- monoclinic form of pyrrhotite. cant role. • All metals are electrically conductive in a broad sense, but the conductivity of an ore 3. Associated geological structures. deposit primarily lies with sulphides or • Most of the mineral deposits are structurally graphite. controlled; mineral occurrences are often re- • The radioactivity of rocks is based on radio- stricted to structural elements such as faults, active elements, mainly potassium (K), ura- shear zones and lithological unconformities; nium (U) and thorium (Th). some deposits form stratiform bodies, while 9 Geological Survey of Finland, Special Paper 58 Meri-Liisa Airo others are formed in a specific stratigraphic 5. Non-ore element component. interval as a stratabound formation. • Sometimes, chemical alteration of host rocks • Knowledge of the structure interrelationship produces detectable geophysical signatures and stratigraphic units is essential for min- if it produces minerals having anomalous eral exploration. Seismic methods are able to physical properties. Although the petrophys- produce high-resolution images of the geo- ical properties of different host rocks or ore logical structure. deposits may be well studied, there is a lack of information on how the physical proper- 4. Associated host rocks. ties are related, for instance, to proportional • The association of particular ore types with alteration of various kinds. particular host rock assemblages broadly • Extensive fluid-related alteration of the host reflects the geological environment and pro- rocks may have a significant effect on geo- cesses that have formed the ore, e.g. meta- physical signatures: sulphidization or pyriti- morphosed graphitic shales (black schists) zation (electrical properties), sericitization in Finland are distributed along all major (potassium radiation), chloritization, car- crustal boundaries. As sensitive and highly bonate alteration or tourmalinization (e.g. reactive, reducing rocks, they may host or magnetic properties). be associated with mineralization, and their • Mineralogical changes associated with the geophysical properties related to chemical formation or emplacement of mineralization composition can be used as indicators of (such as hydrothermal alteration haloes). In the geological settings in which they formed regional geophysics, the expressions of alter- (Airo & Loukola-Ruskeeniemi 2004). ation haloes may be minor, but the detailed • Mineralization tends to accumulate along study of radiometric or hyperspectral analy- plate boundaries. The composition of sedi- ses permits the mapping of key minerals. If mentary rocks along these boundaries may highlighted by more detailed investigations, reveal information on the crustal conditions these haloes may also be recognized by high- and processes at the time of mineralization. resolution airborne surveys. Regional geophysical data sets Airborne geophysical data sets provide full cov- borne geophysical concept of GTK has been de- erage of Finland and form the basic material for scribed in detail by Hautaniemi et al. (2005). regional investigations, particularly greenfield ex- ploration. The use of regional data sets in an auto- Airborne magnetics mated approach to characterizing areas containing known deposits and seeking similar areas else- The magnetic method utilizes small variations where, or similarity analysis of certain geophysi- in magnetic mineralogy among rocks (magnetic cal key signatures, benefits from high-resolution, iron and iron-titanium oxide minerals, including multivariate geophysical datasets. Concerning magnetite, titanomagnetite, titanomaghemite and more detailed investigations, airborne geophysi- titanohematite, and some iron sulphide minerals, cal data also can motivate applications that require including pyrrhotite and greigite). Magnetic rocks improved spatial resolution and accurate position- contain various combinations of induced and re- ing. The integration of different geophysical data manent magnetization, depending on the Earth’s sets is a current theme in geophysical and geologi- primary field. The magnitudes of both induced and cal interpretation, and there are now more soft- remanent magnetization depend on the quantity, ware tools available to facilitate this. However, as composition and size of magnetic-mineral grains. stated by Thomson et al. (2007), although image The magnetic method gives a coherent picture of analysis may often seem intuitive, simple image- the distribution of magnetization of the crust and based assessments of data are not a substitute for is not disturbed by lakes, waterways or soils that proper geologically supported interpretation. may cover the bedrock. In Finland, exposed bed- Specifications and general uses of geophysical rock hardly makes up more than 3% of the sur- methods are outlined in the following. The air- face. The aim of the magnetic method is to detect 10 Geological Survey of Finland, Special Paper 58 Geophysical signatures of mineral deposit types – Synopsis magnetically anomalous source bodies, but also to the use of airborne radiometric data as a uranium determine structural trends. Detailed magnetic in- exploration tool in southern Lapland. vestigations on magnetic mineralogy complement the regional picture of magnetic anomaly source Airborne electromagnetics rocks. Studies on remanent magnetization and the anisotropy of magnetic susceptibility (AMS) are Airborne electromagnetic (EM) methods are used gaining increasing interest as a mineral explora- to screen large areas and provide information for tion tool (Willliams 2009). Palaeomagnetic studies targeting ground surveys. They are capable of di- may be important for the timing of the mineraliz- rectly detecting conductive base-metal deposits. ing fluids or the alteration. Discussion of the mag- The traditional application of EM methods in netic mineralogy responsible for magnetization mineral exploration has been in the search for effects is presented in this Special Paper volume in low-resistivity (high-conductivity) massive sul- the chapter on Au deposits in southern Finland by phide deposits. The wide whole-country cover- Mertanen & Karell (p. 89). age of frequency-domain EM data in Finland is unique in the world and allows mapping of the Airborne radiometrics regional distribution of bedrock conductivity, also supporting structural interpretation. GTK Gamma-ray methods identify the presence of the used a fixed-wing multi-frequency survey system natural radioelements potassium (K), uranium that is better suited to relatively near surface ap- (U), and thorium (Th) in rocks. Gamma ray plications than deeper investigations (down to 100 penetration is only of the order of half a metre, m). Electromagnetic survey data are vulnerable so that in regions with poor exposure due to gla- to non-geological noise, but also to conductivity cial, largely transported overburden, the meas- anomalies due to soil properties and moisture. The urement of natural radioactivity due to K, U and noise is worth filtering out in the case of mineral Th may not be very useful. In Finland, the use exploration. The interpretation of electromagnetic of radiometrics is frequently limited by the wide data may be demanding, and 3D interpretation coverage of glacial soil, with a thickness vary- methods would greatly strengthen the use of the ing from 0 to 100 m and an average of <10 m. airborne electromagnetic method. An example of In southern Finland, cultivated land dominates
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