Geoscience Reference
In-Depth Information
Table 14
.11
Indicator classes and class mean, Haz-Hi, Haz-Lo, and SAP domains
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rock has been logged as altered, but it is only a necessary, not
sufficient, condition for ore grade mineralization to occur.
Two-meter down-the-hole composites were generated
without restriction to lithology or alteration. Short compos-
ites may preserve better the high grade tail of the distribu-
tion, and thus are preferred in this case to avoid as much as
possible grade smearing and dilution when compositing.
The following criteria must be met for any interval to be de-
fined as part of the low-grade (HAZ-Lo) alteration zone:
1. Metasandstone lithology with pellite lenses.
2. In the conglomerate and basement lithologies the altera-
tion boundary must be defined by the presence of sulfide.
The low grade mineralization envelope must contain all
the high HAZ zones. Low HAZ envelope boundaries
must be steeply dipping or vertical.
3. Intercept length must be three samples or 6 m with one
or more acceptable index (metasandstone lithology with
pellite lenses, or presence of sulfide).
Figure
14.31
shows the histogram and basic statistics for all
2 m Au composites, as well as the defined domains. There are
30,546 2 m Au composites in the database, with a positively
skewed Au distribution and few samples representing a sig-
nificant high-grade population. The average overall Au grade
is 0.260 g/t, with a standard deviation of 2.78 g/t, and a coef-
ficient of variation (CV) of 7.88. Approximately 75 % of the
composite data is below 0.21 g/t (which is below the projected
economic cutoff), and only 10 % of the data is above 0.573 g/t.
The HAZ-hi envelope has an average grade of 0.81 g/t,
although with still a significant proportion of low grade com-
posites, while the SAP zone shows a lower average grade at
0.47 g/t Au, but a larger proportion of medium- and high-
grade composites, and therefore a lower CV. Finally, the
HAZ low-grade envelope shows an even higher CV, with a
0.23 g/t Au average grade, and only 10 % of the composites
above 0.37 g/t Au.
Figure
14.32
shows a cross section with interpreted al-
teration envelopes and corresponding drill holes. The 2 m
composites within these wireframes were used in grade es-
timation.
14.2.2
Geologic Modeling
The São Francisco geologic model is used to define estima-
tion domains, and consists of three main elements, as delin-
eated by three-dimensional wireframes: the high-grade alter-
ation envelope (HAZ-Hi), a low-grade envelope (HAZ-Lo),
and the saprolite (SAP) zone, which was interpreted from
drill hole logs.
The definition of the HAZ-Hi envelope was based on
certain indicators of mineralization as described in the geo-
logic logs, which are: (1) medium to high (>50 %) content
of quartz, (2) medium to high (>50 %) sulfide (pyrite) con-
tent, (3) gold assay greater or equal than 0.40 g/t (this value
corresponds to the 60 % cumulative probability of the entire
Au distribution), (4) the presence of coarse gold (nugget),
kaolin, or hematite, and (5) the intercept length.
The following criteria must be met for any interval to be
defined as part of the HAZ-Hi zone:
1. Three samples or 6 m length minimum with two or more
acceptable indexes (one if it is high sulfide or high gold
grade), in a mineralized zone which has at least 2 inter-
cepts at maximum 35 m apart. The structural orientation
of the mineralized zones must be shallow dipping to NE
in the open pit mineralization (extensional veins zone) or
steeply dipping to NE in the Deep South mineralization
(extensional + shear veins zone).
2. Six samples or 12 m length minimum with two or more
acceptable indexes (one if it is high sulfide or high gold
value) to create discontinuous ore zone. Discontinuous
ore zones may be extruded to half distance to the next
section. Structural orientation must be the same as for the
continuous mineralized zone defined above.
14.2.3
Class Definition for Multiple Indicator
Kriging
The definition of the thresholds (or indicators) and corre-
sponding classes chosen to perform the MIK varied accord-
ing to the estimation domain considered. Table
14.11
shows
the class defined by the Au Indicators, the corresponding de-
