17.1 ZACORO RESOURCE ESTIMATE
17.1.2 Resource Estimation Procedure
Geological modelling of the Macocozac skarn deposit was carried out in a systematic process, beginning with a review of the previous geological cross-section interpretations constructed at north 20° east, approximately perpendicular to the strike of the mineralized body and spaced 50 m apart. These sections were used as the baseline geology for a re-interpretation of the drilling data on north-south and east-west cross-sections spaced 25 m apart. This work was advanced in conjunction with the interpretation of the geological bench plans on 50 m spacing during the first phase of modelling. The section and bench plans form a rectified geological model and were entered into a three dimensional model in GEMS software. The bench plans were refined on 10 m spacing within the zone of drilling and these bench plans were tied together to make a solid model. The solid model was modified bench by bench and section by section in order to adjust the hard boundaries to include most of the mineralized intervals and exclude weakly mineralized intervals where possible, to define the best estimation of the mineralized skarn body for the solid interpretation at this time.
The distribution of drilling data allows for the modelling of a mineralized skarn envelope which includes low grade internal waste that results in a larger volume of mineralized material at a lower overall grade. The skarn is interpreted to form at or near the intrusive contact, which has been confirmed in both the open pit and underground exposures within the mine area. Additional areas of mineralization occur in skarn type deposits at the skarn-marble contact with this zone only minimally tested at the Aranzazu project. These spatial associations are used to interpret the possible location of mineralization in areas with minimal or no drilling. The mineralized skarn envelope forms a relatively sharp contact with the intrusive rocks and marble or unmineralized skarn or hornfels. The mineralized solid volume defines the “ore” rock code and forms the hard boundary used for estimation of the block models. The block models were reviewed to insure agreement with the geologic model and any necessary adjustments were made to the block or geological model. The geological model is a constantly evolving process, updated with new drilling information and
68
interpretation. Aura should conduct its own drilling program to confirm the geological model and expand the skarn bodies.
The Arroyos Azules North resource evaluation area is approximately 1,600 metres east-west by 800 metres north-south and includes the BW body, the Arroyos Azules open pit, the Mexicana underground mine, the Glory Hole zone, and the Cabrestante underground mine areas on the northern and eastern intrusive body contact. The southern contact zone of the Arroyos Azules stock is unexplored and is a high priority exploration target. The Arroyos Azules project area and drill hole locations are shown in Figure 17.1.
Figure 17.1
Project Area and Drill Hole Location
Figure reproduced from the 2007 Zacoro Technical Report.
17.1.2.1 Assay and Composite Evaluation
The current digital dataset consists of 447 drill holes containing 11,594 assay intervals. None of the drill holes have had down-hole surveys conducted on them. The vast majority of the drill holes completed were diamond drill holes, but conventional rotary (6 estimated) and reverse circulation holes (2 estimated) are included in the database. The minimum and maximum values of assay interval lengths and assay values are shown in Table 17.2.
Previous drilling used irregular assay lengths varying from 0.05 m to 22.4 m with an average of 1.78 m (Table 17.3). Copper assays have a range from 0% to a maximum value of
35.83%. Gold assays range from a minimum of 0 g/t to a maximum of 28.8 g/t, silver has a range from 0 to 800 g/t, and zinc ranges from 0% to 47.5%.
Table 17.2
Assay File - Minimum and Maximum Assay Intervals and Assay Values
Item Description Minimum Maximum -AI- Assay length (m) 0.05 22.40 TCU % Total copper 0.00 35.83
ZN % Zinc 0.00 47.50
AU Gold g/t 0.00 28.80
AG Silver g/t 0.00 800.00
Table reproduced from the 2007 Zacoro Technical Report.
Table 17.3 illustrates the continuity of data available for each item. The table shows that while copper is assayed for 99.2% of all assay intervals, silver has been analyzed for only 45.6% of the intervals, zinc for 19.1% of the intervals, and gold for only 17.2% of the intervals. The average metal content and standard deviation of the sample populations show the variability of analyses in the raw data.
Table 17.3
Assay File Data Inclusion and Statistics Assay
Table reproduced from the 2007 Zacoro Technical Report.
The composite file is constructed using 5 m fixed length intervals. Composite items include total copper, gold and silver. The maximum values of each metal are lower in the composite samples than in the original assay intervals, providing a smoothed data set for evaluation.
The minimum and maximum values for each item in the composite file are shown in Table 17.4. Table 17.5 shows that while copper is assayed in 100% of all composite intervals, silver has been analyzed in 47.8% of the intervals and gold has been analyzed in only 19% of the intervals. A distribution of the composite intervals is very similar to the overall assay distribution. A 2 m and 3 m copper composite was calculated for comparison with the 5 m composite (Table 17.6.). The average and variance is similar to the 5 m composite. The 5 m composite was used for all block modelling as it was more compatible with the 5m by 5m by 12.5 m blocks and provided more smoothing to the data set by averaging out the short, high grade assay intervals. The five metre composites also more accurately reflect a bulk mineable volume.
70
Table 17.4
Composite File – Item Description
Table reproduced from the 2007 Zacoro Technical Report.
Table 17.5
Five Metre Composite File Data Inclusion and Statistics
5 m Composite
Table reproduced from the 2007 Zacoro Technical Report.
Table 17.6
Two and Three Metre Composite File Data Inclusion and Statistics
Composite Assay
Table reproduced from the 2007 Zacoro Technical Report.
The histograms in Figures 17.2 and 17.3 compare the copper assays and composites for all assayed intervals, the great majority of which are in the mineralized skarn. The mean and standard deviation of the two populations demonstrate the smoothing effect of the compositing, removing the effect of short, high grade copper assay intervals. This smoothing is considered to be more representative of the grades for a bulk-mineable skarn body.
Histograms were also plotted for gold (Figures. 17.4 and 17.5) and silver (Figures 17.6 and 17.7). These histograms show the same effect as the copper composites, a smoothing of data removing high grade spikes from short assay sample intervals.
Item Description Minimum Maximum
Length Composite length (m) 0.00 5.00 TCU % Total copper 0.00 14.94
AU Gold g/t 0.00 10.24
AG Silver g/t 0.00 303
Figure 17.2 Copper Assay Histogram
10083
883
Average 0.67%
Std Dev. 1.25 Variance 1.56
Figure reproduced from the 2007 Zacoro Technical Report.
Figure 17.3
Copper Composite Histogram
Figure reproduced from the 2007 Zacoro Technical Report.
Average 0.605%
Std Dev. 0.86 Variance 0.74
3067
925
72
Figure 17.4 Gold Assay Histogram
1762
Average 0.52 ppm Std Dev. 1.39 Variance 1.93
Figure reproduced from the 2007 Zacoro Technical Report.
Figure 17.5 Gold Composite Histogram
Average 0.47 Std Dev. 1.00 Variance 0.70
621
Figure reproduced from the 2007 Zacoro Technical Report.
Figure 17.6 Silver Assay Histogram
4858
Average 15.72 Std Dev. 31.02 Variance 962
Figure reproduced from the 2007 Zacoro Technical Report.
Figure 17.7
Silver Composite Histogram
Average 13.98 Std Dev. 18.78 Variance 350
946
1075
Figure reproduced from the 2007 Zacoro Technical Report.
74
Figure 17.8 is an oblique view of the Arroyos Azules North skarn area looking to the northwest. The view shows the extent of skarn from the BW body at the top of the figure to the Cabrestante shaft, which is 1.6 km. The mineralized zone outcrops or occurs in subcrop over nearly the entire length of the zone.
Figure 17.8
Oblique View Arroys Azules North Skarn from the BW Body to the Cabrestante Shaft
Figure reproduced from the 2007 Zacoro Technical Report.
17.1.2.2 Block Modelling
The block modelling process first created a lithology block model from the geology solid, which identified two lithological rock types: mineralized skarn (rock code 1700) and other (rock code 0). Lithology codes have not been entered for the individual drill holes and thus back coding from the geology solid model was used. The mineralized skarn blocks were
compared to the solid bodies created from the sectional and bench interpretations and these agreed well with the geologic model.
Block model interpolation was performed using ordinary kriging respecting lithology codes.
Abrupt ore-waste contacts do occur at lithological breaks, hence interpolation does not allow composites of a certain lithology code to interpolate model blocks of a different lithology.
The significant copper mineralization occurs in the mineralized skarn coded areas. The block model is constructed with a row and a column of blocks and levels of blocks according to elevation. The individual blocks are 5 m in dimension along the column and level directions and 12.5 m along the rows (Table 17.7). The area of interest for the Arroyos Azules North zone is approximately 1.6 km long (east to west) which contains 128, 12.5 m long blocks per row. The width (north-south) distance is approximately 800 m which contains 160, 5 m blocks per column. There are 20,400 blocks in the rows and columns per level. The 110, 5 metre levels which contain the mineralized zone have 2,252,800 blocks in the entire block model. The blocks within this volume which are coded as “ore” (rock code 1700) number 250,000.
Table 17.7
Aranzazu Project Block Model Dimensions
Minimum Maximum Block Size
Number of Blocks Easting 253300 254700 12.5 128 Northing 2723700 2724500 5 160
Elevation 1650 2200 5 110
Table reproduced from the 2007 Zacoro Technical Report.
17.1.2.3 Variography
Three-dimensional semi-variograms for copper, gold and silver were completed in the mineralized skarn lithology. Four variograms were used to asses the variance of grade and assign the interpolation parameters for copper in skarn. The variograms for gold and silver did not produce useable results probably due to the irregular data distribution and the small amount of assay data available for the drill holes
Field observations of the mineralized skarn indicate that the general strike of the mineral deposit and the protolith bedding is approximately 110° and the intrusive body contact dip is approximately 80°. The primary mineralization control is parallel to the intrusive contact and the protolith bedding is a strong secondary control. Variation in the grade of mineralization is larger perpendicular to the intrusive contact. As these controls have been established for the continuity of mineralization the following parameters were considered in calculating semi-variogram conditions (Table 17.8 and Table 17.9). Conceptually the range with the least variance should be parallel to the intrusive contact or along the 110o strike of the mineralization and down dip, parallel to the contact zone (Models 1 and 3). The shortest range should be perpendicular to the intrusive contact (Model 2). Model 4 is an omni-directional search and suggests a omni-directional control to the variance of composite grades.
76
Table 17.8
Variogram Model Parameters
Variogram Model Strike Dip Search Cone
1 120 0 15
2 30 0 15
3 30 80 15
4 0 0 180
Table reproduced from the 2007 Zacoro Technical Report.
Table 17.9
Variogram Model Parameters
Copper Composite Model 1 Model 2 Model 3 Model 4
Nugget effect 0.4 0.45 0.26 0.20
Sill (not inc. nugget) 0.34 0.29 0.48 0.54
Range (m) 113 61 129 31
Table reproduced from the 2007 Zacoro Technical Report.
Figures 17.9, 17.10, 17.11 and 17.12 present the copper variograms for Models 1 through 4.
Figure 17.9
Copper Variogram for Model 1
Figure reproduced from the 2007 Zacoro Technical Report.
Figure 17.10
Copper Variogram for Model 2
Figure reproduced from the 2007 Zacoro Technical Report.
Figure 17.11
Copper Variogram for Model 3
Figure reproduced from the 2007 Zacoro Technical Report.
78
Figure 17.12
Copper Variogram for Model 4
Figure reproduced from the 2007 Zacoro Technical Report.
Figures 17.13 and 17.14 present the gold variograms for Model 1 and Model 4.
Figure 17.13
Gold Variogram for Model 1
Figure reproduced from the 2007 Zacoro Technical Report.
Figure 17.14
Gold Variogram for Model 4
Figure reproduced from the 2007 Zacoro Technical Report.
Figures 17.15 and 17.16 present the silver variograms for Model 1 and Model 4.
Figure 17.15
Silver Variogram for Model 1
Figure reproduced from the 2007 Zacoro Technical Report.
80
Figure 17.16
Silver Variogram for Model 4
Figure reproduced from the 2007 Zacoro Technical Report.
The limited population of gold and silver assays limits the estimation of block model values for these elements. The correlation coefficients for 1,831 samples with copper, gold and silver analyses (Table 17.10), were compared to assess the distribution of the precious metals in relation to the copper distribution. In general, a moderate correlation exists where elevated copper concentrations contain elevated gold and silver values. Silver and gold were compared in 1,951 samples and a moderate correlation exists in the precious metal values.
The precious metal distribution varies within the skarn and metal zoning does exist. Gold and silver values are lower in the west end of the deposit in the BW area. Gold values are elevated in the central portion of the system near the east end of the open pit and glory hole areas, and elevated silver and gold occur with high copper in the eastern end of the system.
Zinc assays are limited but show elevated values on the west and east ends of the system with lower values in the central portion.
Table 17.10 Correlation Coefficients
Metals Correlation Coefficients Copper- Gold 0.587
Copper - Silver 0.693 Gold - Silver 0.522
Table reproduced from the 2007 Zacoro Technical Report.
17.1.2.4 Topography
The topography is based on contours interpreted from satellite imagery after the suspension of operations on January 7, 2002. A 3-dimensional surface was created in GEMS from the topography contours. This surface was then gridded into a surface model and subsequently into the block model to eliminate blocks above topography. The blocks above topography and in previously mined areas were included for the estimation of the grade in the surrounding blocks, but the associated tonnage and grades related to the above topography and previously mined areas were deducted from the total resource.
17.1.2.5 Specific Gravity
Specific gravity has been assigned to the blocks in the model assuming most of the rock included in the mineralized bodies is a compact garnet skarn rock type. Initial density determinations completed by PRA on four metallurgical samples of mineralized rock are shown in Table 17.11. A density of 3 was used for tonnage estimations in the mineralization for the block model. The mineralized samples measured ranged from 2.74 to 3.57. The material currently being mined and stockpiled is sample Met-3, which has a density of 3.57.
A density value of 3 is considered conservative in sulphide bearing garnet skarn. Further density measurements will be included in all of the next phases of drilling.
Table 17.11
Specific Gravity Measurements from PRA’s Testwork
Sample Density
Met-1 2.74 Met-2 2.74 Met-3 3.57 Met-4 3.50
Table reproduced from the 2007 Zacoro Technical Report.
17.2 GENERAL RESOURCE ESTIMATE DISCUSSION