Page 172 - Introduction to Mineral Exploration

8

Exploration

Geochemistry

Charles J. Moon

Geochemistry is now used in virtually every commodity sought and its location. In the same
exploration program, if only to determine the way as geological and grade–tonnage models
grade of the material to be mined. However are generated (seesection 4.1.3) modeling can
exploration geochemistry has evolved from its be extended to include geochemical factors,
early origins in assaying, to using the chem- summarized in Barton (1986). Thus the geolo-
istry of the environment surrounding a deposit gist will start with a knowledge of the elements
in order to locate it. This particularly applies to associated with a particular deposit type, an
the use of surficial material, such as soil, till, idea of the economic size of the deposit to
or vegetation, that can be used in areas where be sought, the mineralogical form of the ele-
there is little outcrop. The object is to define a ments, and the probable size of the elemental
geochemical anomaly which distinguishes the anomalies around it. The outline of a deposit is
deposit from enhancements in background and defined by economic criteria and the mineable
nonsignificant deposits. This chapter explains material is surrounded by lower concentra-
how geochemistry may be employed in the tions of the mined elements which are how-
search for mineral deposits. Further details of ever substantially enriched compared with
the theory behind exploration geochemistry unmineralized rock. This area of enrichment is
are given in the exhaustive but dated Rose et al. known as the primary halo, by analogy with
(1979) and Levinson (1980). the light surrounding the outline of the moon,
Exploration geologists are likely to be more and the process of enrichment as primary dis-
directly involved in geochemistry than with persion. In addition ore-forming processes con-
geophysics which is usually conducted by con- centrate or deplete elements other than those
tractors and supervised by specialist geophy- mined. For example, massive sulfide deposits
sicists. A geochemical program can be divided often contain substantial arsenic and gold
into the following phases: in addition to the copper, lead, and zinc for
1 Planning; which they are mined. A summary of typical
2 Sampling; elemental associations is shown in Table 8.1.
3 Chemical analysis; The geologist’s problem is then to adapt
4 Interpretation; this knowledge of primary concentration to the
5 Follow-up. exploration area. The geochemical response at
The field geologist will probably carry out the surface depends on the type of terrain and
phases 1, 2, 4, and 5, while analysis is normally especially on the type of material covering the
performed by a commercial laboratory. deposit as shown in Fig. 8.1. The response in an
area of 2 m of residual overburden is very differ-
ent from that of an area with 100 m deep cover,
8.1 PLANNING or if the overburden has been transported. Also
elements behave differently in the near-surface
The choice of the field survey technique environment from that in which the deposit
and the analytical methods depends on the formed. For example, in cases where copper,