MINING GEOLOGY/Exploration 617 Figure Geological mapping in the Farallon Negro district, Argentina All rocks show intense hydrothermal alteration related to a major porphyry copper molybdenum gold deposit called Agua Rica rock, and, in some cases, water, vegetation, or air (Table 2; Figure 3) Analytical results (‘anomalies’) may suggest prospective areas Geochemical techniques are applicable only where the rock that hosts the ore, or material derived directly from it, is accessible Sampling of overlying transported material (e.g., glacial debris) is not effective unless the source of the transported material is known, or unless chemical elements related to mineralization have migrated up through the transported cover In areas of shallow cover, drilling may be used to collect samples from beneath the cover Proper orientation surveys are essential for effective geochemical surveys These involve testing the response of known mineralization in a similar nearby area to verify the effectiveness of the method under the prevailing conditions, before using it routinely to look for unknown mineralization Geophysical Techniques Geophysical techniques rely on variations in the physical properties of the mineralized rocks or the rocks that surround them to indicate the location of mineralization directly or indirectly (Table 3; Figure 4) The physical response may be a direct property of the rock (e.g., its density, natural radioactivity, magnetic properties, or resistivity), or an induced response produced by exposing the rock to a physical stimulus such as an electrical or magnetic field Geophysical techniques may indicate mineralization directly, or may allow the search area to be reduced to a favourable host rock or a favourable structural environment With the increasing availability of large-scale surveys, often involving more than one technique, the data from geophysical surveys can be interpreted to produce geological maps and to identify favourable settings for mineralization, exactly as they would be from conventional geological maps A major advantage of geophysical techniques is that they can detect responses from mineralization buried several hundred metres below the ground surface, at depths too great to be reflected in the surface geology or detected by geochemical surveys Recognizing Targets Sometimes orebodies are exposed and their surface expression can be recognized by explorers, but as exploration continues, new surface targets become fewer and fewer, and increasingly there is a need to look for more subtle targets That typically involves the use of multiple exploration tools The data from all the techniques used are compiled onto maps, and the strength and location of anomalies in each technique are compared Areas that stand out with coincident anomalies from more than one technique are usually priority targets for drill testing One additional geological tool universally used by modern explorationists is the ore deposit model There are models available for a wide range of ore deposit types, and new ones are regularly developed These models summarize all the important geological characteristics (including spatial relationships) of a particular deposit type Models include large-scale aspects such as the tectonic and structural settings,