Geoscience Reference
In-Depth Information
11.5.1 Gejiu Mineral District Example
Where the landscape permits this, stream sediments are the preferred sampling
medium for reconnaissance geochemical surveys concerned with mineral explora-
tion (Plant and Hale
1994
). During the1980s and1990s, government-sponsored
reconnaissance surveys covering large parts of Austria, the Canadian Cordillera,
China, Germany, South Africa, UK, and USA were based on stream sediments
(Darnley et al.
1995
). These large-scale national projects, which were part of an
international geochemical mapping project (Darnley
1995
), generated vast amounts
of data and continue to be a rich source of information. Cheng and Agterberg (
2009
)
applied singularity analysis to data from about 7,800 stream sediment samples
collected as part of the Chinese regional geochemistry reconnaissance project
(Xie et al.
1997
). For illustration, about 1,000 stream sediment tin concentration
values from the Gejiu area in Yunnan Province were used. This area of about
4,000 km
2
contains 11 large tin deposits. Several of these, including the Laochang
and Kafang deposits, are tin-producing mines with copper extracted as a by-product.
These hydrothermal mineral deposits also are enriched in other chemical elements
including silver, arsenic, gold, cadmium, cobalt, iron, nickel, lead, and zinc. Appli-
cations to be described here are restricted to tin, arsenic, and copper. Tin and copper
are the ore elements of most interest for mineral prospecting whereas arsenic is a
toxic element. Water pollution due to high arsenic, lead, and cadmium concentration
values is considered to present one of the most serious health problems especially in
underdeveloped areas where mining is the primary industry such as in the Gejiu area.
Knowledge of the characteristics of spatial distribution of ore elements and associated
toxic elements in surface media therefore is helpful for the planning of mineral
exploration as well as environmental protection strategies.
The Gejiu mineral district (Fig.
11.23a
) is located along the suture zone of the
Indian Plate and Euro-Asian plates on thesouthwesternedgeoftheChina
sub-plate, approximately 200 km south of Kunming, capital of Yunnan Province,
China. The Gejiu Batholith with outcrop area of about 450 km
2
is believed to have
played an important role in the genesis of the tin deposits (Yu
2002
). The ore
deposits are concentrated along intersections of NNE-SSW and E-W trending
faults. Stream sediment sample locations in the Gejiu area are equally spaced at
approximately 2 km in the north-south and east-west directions. Every sample
represents a composite of materials from the drainage basin upstream of the
collection site (Plant and Hale
1994
). Regional trends are captured in a moving
average map of tin concentration values from within square cells measuring
26 km on aside (Fig.
11.23c
). Several parameters had to be set for this use of
the inverse distance weighted moving average method (Cheng
2003
). In this
application, each square represents the moving average for a square window
measuring 26 km on a side with influence of samples decreasing with distance
according to a power-law function with exponent set equal to
2. Original sample
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