Método de cálculo de comprobación del IDAE 47

The modal mineralogy for several minerals can be calculated from the chemical assays that are routinely undertaken on-site. The number of minerals that can be calculated is dependent upon the elements that are routinely assayed for and the number of minerals present in the deposit that contain unique elements in their crystal lattice (i.e. no other minerals contain that element). This approach has been undertaken in this research for both Cadia East and Ernest Henry. The method for calculating mineral abundances from assay will be outlined in this section using a worked example from Cadia East (CE109: 442-444m).

3.7.1 Calculating mineral abundances from chemical assays: An example from Cadia East

The elements that are routinely assayed for at Cadia East are copper (Cu), Copper extracted from Cyanide leaching (CuCN), gold (Au), molybdenum (Mo), Lead (Pb), Zinc (Zn), Iron (Fe), and Sulphur (S). The minerals at Cadia East that can be attributed to these elements were determined by extensive SEM analysis and are outlined in Table 3.9.

Figurre 3.20. Unclassified (AA) and classified (BB) images of the same sample. The top image shows chalcopyrite grains with distinct grain boundaries, however when classified with a SEM-MLA system, the grain boundaries between adjacent chalcopyrite grains cannot be distinguished

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Table 3.9. Common and rare minerals that contain can be attributes to elements assayed for at Cadia East.

Element Common minerals Rare minerals

Cu Chalcopyrite, Bornite Digenite, Tetrahedrite, Tennanite

CuCN Bornite Digenite, Tetrahedrite, Tennanite

Mo Molybdenite

Pb Clausthalite Galena

Zn Sphalerite

S Chalcopyrite, Bornite, Molybdenite, Pyrite Digenite, Tetrahedrite, Tennanite, Galena, Sphalerite, Barite

Fe Magnetite, K-feldspar, Albite, Biotite, Chlorite, Epidote, Pyrite, Chalcopyrite, Bornite

Digenite, Tetrahedrite, Tennanite

The assay results for CE109: 442-444m are:

Cu 5380 ppm CCuCN 284 ppm

Mo 909 ppm SS 8450 ppm

Fe 31000 ppm ZZn 41 ppm

Pb 1 ppm

The first mineral abundances to be calculated are those that have one assayed element that does not overlap with another mineral. At Cadia East these are molybdenite, sphalerite and clausthalite (PbSe). Despite both clausthalite and galena occurring as minor inclusions in copper sulphides, SEM analysis indicates that galena is rare compared to clausthalite. For every mineral’s abundance that is calculated, the other elements in each equation that are also present in the assay analysis need be adjusted accordingly. For example, based on the molecular weights of both molybdenum (Mo) and sulphur (S), for each 1ppm of Mo attributed to molybdenite (MoS2), 0.67 ppm of S will be

deducted from the total S assay. The mineral abundances for molybdenite, sphalerite and clausthalite were calculated using the following formula:

For Cadia East where the majority of the copper mineralisation is contained in chalcopyrite and bornite, the CuCN concentration is used as an estimate for the bornite abundance. The CuCN leach will dissolve out a number of copper sulphide and copper oxide minerals, however, it does not leach out chalcopyrite. SEM and MLA analysis indicate that bornite mineralisation can occur in exsolution with digenite, however this is not common. For example, routine MLA-XMOD analysis of 23 samples that contained bornite, showed that digenite occurred in only four of these samples. Hence, the CuCN values can be used to determine bornite content.

To calculate the mineral abundance of chalcopyrite the total Cu assay value is adjusted by deducting the CuCN assay value. In cases where the CuCN value exceeds the total Cu assay, these results are considered inappropriate to use and were discounted. The estimated mineral abundances for CE109 442-444m are given in Table 3.10.

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Table 3.10. Estimated mineral abundances calculated from elemental assay data at Cadia East. CE109 442-444 m. Mineral abbreviations as in Appendix 1.1.

Mineral Assay E Element Assay p ppm Calculated Mineral elements ((ppm) Mineral A Abundance (%) Assay Balance Cu CuCN S Fe Mo Pb Zn Mo Mo 909 MMo 909 S S 607.5 0.15 5380 284 7842.5 31000 0 1 41 Sp Zn 41 ZZn 41 S S 20.10 0.0061 5380 284 7822.4 31000 0 1 0 Chl Pb 1 PPb 1 Se 0.38 0.00014 5380 284 7822.4 31000 0 0 0 Bn CuCN 284 CCu 284 Fe 49.92 S 114.69 0.045 5096 0 7707.7 30980 0 0 0 Ccp Cu 5096 CCu 5096 Fe 4479 S 5142 1.47 0 0 2566.0 26501 0 0 0 Py S 2566 SS 2566 Fe 2235 0.48 0 0 0 24266 0 0 0

The mineral abundances for chalcopyrite, bornite, pyrite, molybdenite, sphalerite and clausthalite have been calculated from assay data for CE082, CE098 and CE143 (Appendix 3.8). Any samples that were found to contain errors have been removed (CE082 298, 302 and 306). 3.7.2 Mineral abundances calculated from assay data: Ernest Henry

The mineralogy at Ernest Henry has been defined using detailed SEM (MLA) analysis. The dominant and rare mineral phases at Ernest Henry are displayed in Table 3.11. These will determine which minerals can be calculated from elemental assays at Ernest Henry. The elements that are assayed at Ernest Henry are Arsenic (As), Gold (Au), Cobalt (Co), Copper (Cu), Iron (Fe), Molybdenum (Mo), Nickel (Ni) and Sulphur (S). The mineral abundances that have been calculated are chalcopyrite, pentlandite, arsenopyrite and molybdenite (Appendix 3.9). If Barium (Ba) were also assayed then barite and pyrite abundances would also be able to be calculated.

Table 3.11. Dominant and Rare mineral phases for Ernest Henry

Element Common minerals Rare minerals

As Arsenopyrite, sulpharsenite, cobaltite

Au Electrum, gold

Co Sulpharsenite, cobaltite

Cu Chalcopyrite Bornite

Fe Magnetite, biotite, chlorite, pyrite, K-feldspar Arsenopyrite, allanite, serpentine, sulpharsenite

Mo Molybdenite

Ni Pentlandite

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3.8 Measuring for Mineral Processing attributes

The results of large and small scale physical tests undertaken at the Julius Krütschnitt Minerals Resource Centre (JKMRC) at the University of Queensland on the same drill-core are briefly described in Table 3.12 and Appendix 3.7, 3.8 and 3.9.

T

Table 3.12. Summary of the physical tests that have been used for comparisons with mineralogical and textural data within this research. Further detail regarding these methods can be found in Appendices 3.8, 3.9 and 3.10.

Physical Test Units Scale Description

EQUOtip Ls (Leeb hardness

unit)

Small The EQUOtip is a portable electronically controlled velocity rebound tester (Verwaal and Mulder, 1993). It is a non

destructive method for observing changes in rock hardness down hole (See Appendix 3.7).

JK Comminution Index (JKCI)

No units Small The JKCI uses an algorithm that involves examining breakage distributions size by size and examining the fines generated (See Appendix 3.8)

JK Rapid Breakage Tester (JKRBT)

kWh/t (as well as the a*b parameters used by JKMRC)

Small Developed at the JKMRC as part of the AMIRA P9N research the JKRBT tests particles across a wide range of sizes using precise energy inputs (See Appendix 3.9).

Point Load Test MPa and compressive strength

Small A geomechanical test used to measure rock fragmentation strength (Broch and Franklin, 1972). The PLT measures the Point Load Strength (Is) of the rock sample.

Drop Weight Test kWh/t Large Developed at JKMRC, the DWT is combined with a data reduction technique determines the relationship between the specific energy input and resultant product size (Napier-Munn et al., 1996).

3.9 Summary

Described in this chapter are the methods, technologies and associated protocols that have been used within this thesis. The typical procedures have been described for the meso-scale visual drill-core logging; the creation of meso-scale mineral maps using the GEOTEK Multi-Sensor Core-Logger (MSCL); the creation of mineral maps using images created using automated microscopy; and the classification and creation of mineral maps using the Mineral Liberation Analyser (MLA). Also listed in this section are the large and small scale physical tests that the results from this thesis will be compared to.