Page 8 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Geochemical Anomaly and Mineral Prospectivity Mapping in GIS
by E.J.M. Carranza
Handbook of Exploration and Environmental Geochemistry, Vol. 11 (M. Hale, Editor)
© 2009 Elsevier B.V. All rights reserved. 3

Chapter 1

PREDICTIVE MODELING OF MINERAL EXPLORATION TARGETS

INTRODUCTION
Mineral exploration endeavours to find mineral deposits, especially those with
commercially viable concentrations of minerals or metals, for mining purposes. It has
four phases, namely (1) area selection, (2) target generation, (3) resource evaluation and
(4) reserve definition. Area selection defines permissive regions where mineral deposits
of the type sought plausibly exist based on knowledge of environments at or near the
surface of the Earth’s crust where the geological processes (e.g., plate tectonics) are or
were favourable for mineral deposit formation (Singer, 1993). Target generation
demarcates, within permissive regions, prospective areas for further investigations until
mineral deposits of interest are discovered based on exploration models for the deposit-
type sought and on relevant thematic geoscience (geological, geochemical and
geophysical) data sets. Resource evaluation estimates grade and tonnage of specific
minerals or metals in discovered mineral deposits based largely on systematic drilling.
Reserve definition classifies the various parts of mineral deposits as ore reserves
(proved, probable) or mineral resources (measured, indicated, inferred) based on
economic and technical feasibility analysis. This volume is concerned with only the
target generation phase in mineral exploration.
Target generation is a multi-stage mapping activity from regional-scale to local-scale.
Every scale of target generation involves collection, analysis and integration of various
thematic geoscience data sets in order to extract pieces of spatial geo-information,
namely (a) geological, geochemical and/or geophysical anomalies associated with
mineral deposits of the type sought and (b) prospective areas defined by intersections of
such anomalies. An example of a geological anomaly is hydrothermal alteration,
although it may not necessarily be accompanied by mineral deposits. A geophysical
anomaly is a variation from normal background patterns of measured physical properties
of the Earth’s upper crust (e.g., magnetism), which can be attributed to localised near-
surface or subsurface materials such as metallic mineral deposits. A geochemical
anomaly is a departure from the geochemical patterns that are normal for a given area. It
can represent either geogenic (i.e., natural) or anthropogenic (i.e., industry-induced)
enrichment in one or more elements in Earth materials. In mineral exploration,
geochemical anomalies associated with mineral deposits are called significant
anomalies, whereas geochemical anomalies associated with other natural processes or
anthropogenic processes are called non-significant anomalies. Because not every