Page 146 - 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. 147

Chapter 6

ANALYSIS OF GEOLOGIC CONTROLS ON MINERAL OCCURRENCE

INTRODUCTION
Occurrences (or locations) of mineral deposits of the type sought are themselves
significant geochemical anomalies and are samples of a mineralised landscape. The
spatial distribution of such samples is invariably considered by many geoscientists,
especially mineral explorationists, to be non-random because of the knowledge that an
inter-play of certain geological processes has controlled their occurrence. It follows then
that such samples of a mineralised landscape are associated spatially, as well as
genetically, with certain geological features. It is for these reasons that mineral
prospectivity for the type of deposits sought is, in many cases, modeled by way of
probabilistic techniques. That is to say, the probability or likelihood that mineral deposits
of the type sought are contained or can be found in a part of the Earth’s crust is
considered greater than would be expected due to chance.
We recall from Chapter 1 that, in any scale of target generation, modeling of mineral
prospectivity usually starts with the definition of a conceptual model of mineral
prospectivity for mineral deposits of the type sought (Fig. 1-2). Analysis of the spatial
distribution of occurrences of mineral deposits of the type sought and analysis of their
spatial associations with certain geological features are useful in defining a conceptual
model of mineral prospectivity of the type sought in a study area. Analysis of the spatial
distribution of occurrences of mineral deposits of the type sought can provide insights
into which geological features have plausibly controlled their localisation at certain
locations. In addition, the analysis of spatial associations between occurrences of mineral
deposits of the type sought and certain geological features is instructive in defining and
weighting relative importance of certain geological features to be used as spatial
evidence in predictive modeling of mineral prospectivity.
This Chapter explains techniques for analysis of the spatial distribution of
occurrences of mineral deposits of the type sought and techniques for the analysis of
spatial associations between occurrences of mineral deposits of the type sought and
certain geological features. These techniques are demonstrated by using a map of
occurrences of epithermal Au deposits in the case study Aroroy district (Philippines)
(Fig. 3-9) in order to define a conceptual model of prospectivity for this type of mineral
deposit in that district. Models of multi-element anomalies, which are derived by
applications of the methods explained and demonstrated in Chapters 3 to 5, are examined
further here in terms of their spatial associations with the known occurrences of