Geology Reference
In-Depth Information
Fig. 19.2
Simplified Precambrian geology of Central Africa with its known mineral deposits. (For details of the geology see Chap.
2
, this Topic)
comprise predominantly of Precambrian rocks that have been
severely deformed and metamorphosed during late Neo-
proterozoic Pan African thermo-tectonism (ca. 550-650 Ma;
for more details see de Wit et al.
2008
)
The Gondwana mineral data consists of more than 20 000
deposits covering fifteen commodities (chemical elements)
of which 7,571 are in Africa (Thiart and de Wit
2006
). In all
cases the data were obtained from the literature and/or from
geological surveys between 1994 and 2006; and especially in
1999, when our database was merged with that of the BRGM
(Bureau de Recherche Geologiques et Minieres, France).
The main features in the mineral layer of the database
consist of geographic position (latitude, longitude), deposit-
id, deposit-name, commodity and size of commodity. The
commodity reported in the database is the major chemical
element at that location, whereas size of commodity is reported
as an ordinal interval scale. Regular data updates have been
well described elsewhere (Wilsher et al.
1993
; de Wit et al.
1999
; Thiart and de Wit
2006
; Thiart and Stein
2013
).
For our Metallogenic fingerprint analysis we have selected
11 elements and these are divided into five groups, in part
based on their geochemical affinities (e.g. lithophile, chalco-
phile, etc., where appropriate; Thiart and de Wit
2006
). The
five groups are cross tabulatedwith the simplified Precambrian
geology for each of the three regions (Table
19.1
): within
Central Africa (CA; Fig.
19.2
and Table
19.1a
): within the
Central Africa Shield (CAS) (Fig.
19.3
and Table
19.1b
)and
the Congo Shield (CS; Fig.
19.3
and Table
19.1c
). Note that the
numbers reflected for the CAandCAS exclude those of the CS.
19.3 Methodology
We have previously defined
of
fragments of continental crust by their spatial association
with a combination of element groups, and have applied this
at three scales (cratons, continents, and super-continents; de
Wit and Thiart
2005
). Here we use 3 different local scales as
on Table
19.1
; viz: (a) Central Africa (CA); (b) Central
African Shield (CAS) and (c) Congo Shield (CS).
The measure of spatial association (normalized to area)
is referred to as the spatial coefficient (Mihalasky and
'
metallogenic fingerprints
'
Search WWH ::
Custom Search