Geoscience Reference
In-Depth Information
10 m bench height) and an underground mine (Estefanía, most-
ly cut-and-fill with 5 m lifts) as of 1999. The mine is located
approximately at 900 m above sea level. Figure
14.49
shows
the location of the district in northern Chile.
The district geology shows a very significant strato-vol-
canic sequence, called La Negra Formation, regionally dip-
ping 30 ° to the NW, and composed of a series of andesites
and volcanic breccias of different characteristics. The andes-
ites vary from afanitic to porphyritic, intermixed with the
volcanic breccias. Mineralization is hosted in this volcanic
sequence, where the porous breccias are the most favorable
mineralization hosts (Ferraris and Di Biase
1978
).
The resulting mineralized bodies (“mantos”) are ellipsoi-
dal in shape, of variable dimension and grades, and gener-
ally concordant with stratification. It is difficult to predict
the existence and the size of each manto, as well as its grade
distribution. Generally, the mantos are small, 4-5 m thick,
with length and widths of up to 40 or 50 m, and many being
less than 25 m. Mineralization is mostly Atacamite, a Cu hy-
droxide, with some Chrysocola, as well as sulfide Cu miner-
alization at depth: chalcocite, covellite, bornite, and chalco-
pyrite. Grades within the mantos are typically 1 to 5 % Total
Cu (TCu), with up to 10 % TCu. The cathode plant receives
a head grade of 1.6 % Cu. Declared resources in late 1999
were, at a 0.5 % Cu cutoff, approximately 63 million tons
with a 1.44 % TCu and 0.86 % Soluble Cu (SCu) grades. This
is distributed in various zones and at different depths, and
includes all drill-indicated resources to that date.
The main subzones within the deposit are separated into
amenable to open pit (Lince), or underground (Estefanía).
In addition, within the open pit mine there are several min-
ing areas, such as Lince, D4 Zone, and Hilary; within the
underground mine, zones are delimited by mining extraction
sequence, and are named using letters and numbers, such as
A1, B2, D3, and so forth. There are at least 17 areas of inter-
est within the open pit and underground mines. Resources
were classified following the guidelines of the 1999 JORC
Code, and result in about 21 % of the total resources being
classified as measured, 64 % classified as indicated, and the
remaining 15 % classified as inferred.
In early 2000 an infill drilling campaign was completed,
and the existing drillhole database updated. The geologic
model was updated, and a new resource block model was
obtained. The grade model was obtained using multiple in-
dicator kriging, and providing an e-type estimate for each
block. Following the completion of the Resource Model and
its classification into Measured, Indicated, and Inferred, a
conditional simulation model was implemented to assess un-
certainty and risk. This conditional simulation model was a
Sequential Indicator Simulation (SIS, Alabert
1987
) model.
The simulation model focused on areas and phases to be
mined in the upcoming 5 years (both from the open pit mine
and for the underground mine). The simulated realizations
Fig. 14.49
Location map of the Michilla district and mines
geologic region of the deposit allows for a better understand-
ing and use of the classification scheme implemented.
In addition, basic questions such as “how different is
measured from indicated?”, or “does measured mean 0 %
error?”, or “how different is measured in Zone A compared
to measured in Zone B?” can have a quantitive answer, as
provided by the simulation model.
14.5.1
The Lince-Estefanía Mine
The Lince-Estefanía mine is located within the district of the
same name, some 120 km to the north of Antofagasta, Chile. The
mine is operated by Minera Michilla S.A., producing 50,000 t
of cathode Cu per year from both an open pit (Lince, using a
