Correlation, Method of Least Squares, Linear Regression and the General Linear Model - Geomathematics: Theoretical Foundations, Applications and Future Developments - page 118

4.3.1 Abitibi Copper Deposits Example

Stepwise regression was used in the study (Agterberg et al. 1972 ) to relate
occurrences of large copper deposits to lithological and geophysical variables in
the Abitibi area on the Canadian Shield (Figs. 4.3 and 4.4 ). This region was selected
in 1967 for various projects in the former Geomathematics Program of the Geo-
logical Survey of Canada on the systematic quantification of geoscience data for
larger regions and to correlate the points of occurrence of mineral deposits to the
resulting digitized versions of the geological framework. Initially the grid shown in
Fig. 4.4 was used but later a grid of equal-area (10 km

10 km) cells was projected
on various geological maps for the area at scales 1 in. to 4 miles (approx. 1:
250,000) or larger. This grid (see, e.g., Fig. 4.5 ) corresponds to the Universal
Transverse Mercator map projection used for Canadian topographic maps at scales
1:250,000 and larger.
Traditionally, the Abitibi area was important for its gold deposits, but in the
1960s gold-mining was becoming uneconomical because the price of gold had been
kept artificially low (at $35.00 US per ounce) and mining shifted from lode gold to
volcanogenic massive sulphide deposits that also occur abundantly in this region.
Most large sulphide bodies are lenticular, massive to disseminated, stratabound
deposits enclosed by volcanic and sedimentary rocks of Archean age. They contain
mixtures, in various proportions, of pyrite, pyrrhotite, sphalerite, chalcopyrite, and,
in some instances, galena. Copper from chalcopyrite is economically most signi-
ficant; zinc from sphalerite is recovered from a number of deposits, in some of
which significant copper is absent. The volcanogenic massive sulphide deposits
also may contain significant (minable) amounts of gold and silver.
The Abitibi area in Fig. 4.4 consists of 814 cells measuring 10 km on a side. For
644 cells, values for the following ten attributes were determined: (1) granitic rocks
(acidic intrusives and gneisses); (2) mafic intrusions; (3) ultramafics; (4) acidic
volcanics; (5) mafic volcanics; (6) Archean sedimentary rocks; (7) metamorphic
rocks of sedimentary origin; (8) combined bedrock surface length of layered iron
formations; (9) average Bouguer anomaly; and (10) aeromagnetic anomaly at cell
center after removal of effect of Earth's total magnetic field. The geophysical
variables were corrected for their overall regional means so that below-average
values became negative. More explanatory details on these variables are provided
in Agterberg et al. ( 1972 ).
A geological environment is not only characterized by individual attributes but,
more importantly, by the different types of coexistences of attributes. Some exam-
ples of pairwise coexistences are: (1) coexistence of Archean sedimentary rocks
and iron formations in a cell may define another sedimentary facies; (2) coexistence
of acidic volcanic rocks and relatively high Bouguer anomaly may indicate an
ancient volcanic centre where rhyolites and tuffs cap a relatively thick pile of
andesites and basalts; and (3) coexistences of different rock types indicate presence
of contacts between these rock types in a cell (various types of mineral deposits tend
to occur at or near contacts between specific rock types). In order to consider