Correlation study between reverse
circulation and diamond drilling in
iron ore deposits
Diniz Ribeiro*, Cid Monteiro*, Evandro Cunha*, Mariella Catarino*, Vagney Augusto* and Márcia Gomes*
*Ferrous Planning and Development Department – Vale Brazil
INTRODUCTION
Sampling iron ore deposits in Brazil is traditionally carried out using diamond drill holes (DDH). The reason to adopt core drill sampling as a standard is explained by grain size partition requirement, and also grain size chemical estimates in block model. Grain size information is important to estimate benefit function and geotechnical parameters, used in mine planning studies.
Application of RC drilling in iron ore can improve production rate, reduce evaluation time of new projects. The choice for a destructive method of drilling implies a restriction to collect geological details such as contact, bands attitude, fractures frequency, etc. The impact of those restrictions can be reduced if the RC drill hole could be logged by indirect methods (geophysical logging, hole´s imaging, etc).
OBJECTIVE
The purpose of this study is to compare Reverse Circulation and Diamond Drilling methods using twin holes in four different iron ore deposits and to evaluate global bias, different support effects and chemical grades (co) regionalization.
Despite the good response of models evaluated by DDH, Vale aims to investigate the feasibility of using reverse circulation drilling (RC) combined with DDH method in iron ore deposits evaluation.
LOCATION The data came from João Pereira, Baú, Apolo and Capão Xavier mines, in Quadrilátero Ferrífero, Minas Gerais State, southeast Brazil, where 1,027 samples from 85 Reverse Circulation (RC) drillholes were collected. These samples sum 5,883 drilling metres.
DRILLING SAMPLESSamples were regularized at each 10 m length, considering lithological intervals of DDH drilling. A total of 567 regularized samples were generated, distributed in JoãoPereira (309), Baú (106), Apolo (75) and Capão Xavier (77) iron mines.
Rock type/Deposit CPX APL BAU JPE Total
hematite 29 22 9 1 61
friable itabirite 12 7 53 287 359
contaminated itabirite 33 28 27 1 89
compact itabirite 0 12 5 1 18
quartzite 0 0 5 9 14
canga duricrust 3 6 3 0 12
others 0 0 4 10 14
Total 77 75 106 309 567
METHODOLOGY�The sampling points were selected near old shallow vertical diamond drill holes (DDH), at a maximum horizontal distance of five meters from original DDH;
�Geological description and RC sampling were done taking into account DDH intervals. All RC samples were analysed in four different grain size screens for Fe, P, Al2O3, SiO2, Mn e LOI (loss on ignition) in Vale’s laboratories. The screening tests can vary in each deposit, but in general the four grain size fractions analysed were: G1 (> 8 mm), G2 (< 8 mm > 1 mm), G3 (< 1 mm > 0.15 mm) and G4 (< 0.15 mm);
�For data analysis, statistical and geostatistical tools were used, such as: mean and variance comparison, spatial correlation graphs, distribution comparisons and regression analysis between RC and DDH. Besides the spatial correlation graphs, simple and cross variograms were calculated for DDH and RC iron samples.
SAMPLE SUPPORTThe mass of each sample collected using RC drilling method is approximately six times higher than half side core of DDH drilling samples. The impact of sample support heterogeneity for resource estimation could be strong if the change of variance is so important that modifies tonnage/grade curves. In theory, data support should be homogeneous, but in practice, it is important to evaluate the real impact on quality estimation.
10
m
10
m
Ha
lfo
fd
rill
core
12.06cm
7.1cm
Sample Vol: 19.80 liters
DDH Sample RC Sample
~6X Greater
Sample Vol: 114.22 liters
GEOLOGY
The homogeneous geological domains are defined by the crossed action of different phenomena that modifies the texture, mineralogy and chemistry of the rock. The Banded Iron Formation – BIF – that occurs in Quadrilátero Ferrífero –Brazil, is a result of superimposed diagenetic, metamorphic, deformation and weathering processes that affected these rocks from the Paleo Proterozoic until the Cenozoic ages. The sampling of these rocks is affected by the intrinsic variability of the matrix rock. The friability, for example, can affect the drilling recovery.
JPE CPX APL BAUMain Lithology itabirite friable hematite + contamineted itabirite contaminated itabirite itabirite friableFe% 38.98 61.97 56.3 53.88Mean Grain Size% (-0.15mm) 42.2 55.7 27.9 24.8
GLOBAL BIAS
To simplify the comparison between pairs of RC and DDH samples closer to the horizontal distance between them (<5m) and the difference in sample support was not taken into account in the statistical analysis.
The bias were evaluated using the methodology proposed by Sinclair and Blackwell (2002), based on paired t-test. If no bias exists between the duplicate samples, the mean difference (m) ZDDH – ZZRC , where Z is the variable, should be close to zero. According to the authors the mean (m) and the standard deviation (s) of paired samples (n) can be used to test whether or not mean difference (m) is significantly different from zero (paired t-test).
RC x DDH CORRELATIONS
In that case the correlation between the same variables, ZRC against ZDDH, is a spatial correlation spaced by a vector h. In this data base, the vector h has horizontal component lower than 5 meters and its vertical component is not significant.
The correlation coefficients indicate good correspondence among RC and DDH for most part of paired variables in each deposit. Low values of correlation are associated with variables that have erratic distribution (Mn, P, for example).
RESULTS - Global Bias and RC X DDH correlation
Variable (ZRC + ZDDH)/2 (ZRC - ZDDH) error(%) bias t-test σ(ZRC , ZDDH) Variable (ZRC + ZDDH)/2 (ZRC - ZDDH) error(%) bias t-test σ(ZRC , ZDDH)
Fe 53.886 -0.003 -0.0001 no 0.962 Fe 61.561 0.809 0.013 no 0.527SiO2 18.895 0.867 0.0459 yes 0.954 SiO2 6.154 -0.272 -0.044 no 0.520
Al2O3 0.764 0.005 0.0065 no 0.776 Al2O3 2.068 -0.312 -0.151 yes 0.805
P 0.051 0.003 0.0594 yes 0.863 P 0.053 -0.012 -0.229 yes 0.815Mn 0.284 0.031 0.1093 no 0.855 Mn 0.233 -0.118 -0.508 yes 0.632LOI 1.869 0.049 0.0262 no 0.947 LOI 3.013 -0.416 -0.138 yes 0.820
Variable (ZRC + ZDDH)/2 (ZRC - ZDDH) error(%) bias t-test σ(ZRC , ZDDH) Variable (ZRC + ZDDH)/2 (ZRC - ZDDH) error(%) bias t-test σ(ZRC , ZDDH)
Fe 56.156 0.284 0.005 no 0.901 Fe 39.585 -1.214 -0.031 yes 0.905SiO2 11.192 -0.185 -0.017 no 0.926 SiO2 40.651 2.041 0.050 yes 0.914
Al2O3 2.459 -0.130 -0.053 no 0.932 Al2O3 0.954 -0.108 -0.113 yes 0.808
P 0.087 0.007 0.081 no 0.756 P 0.047 -0.001 -0.022 no 0.825Mn 0.309 -0.073 -0.236 no 0.717 Mn 0.141 0.002 0.014 no 0.708LOI 5.277 0.054 0.010 no 0.946 LOI 1.425 -0.079 -0.055 yes 0.886
BAU
APL
CPX
JPE
JPE
APL
30
30
40
40
50
50
60
60
70
70
30 30
40 40
50 50
60 60
70 70
FeDDH
rho=0.905
40
40
50
50
60
60
70
70
FeRC
FeRC
40 40
50 50
60 60
70 70
FeDDH
rho=0.901
JPE
APL
30
30
40
40
50
50
60
60
70
70
30 30
40 40
50 50
60 60
70 70
FeDDH
rho=0.905
40
40
50
50
60
60
70
70
FeRC
FeRC
40 40
50 50
60 60
70 70
FeDDH
rho=0.901
CPX
0
0
5
5
10
10
LOIRC
0 0
5 5
10 10
LOIDDH
rho=0.820
0
0
1
1
2
2
3
3
4
4
5
5
6
6
LOIRC
0 0
1 1
2 2
3 3
4 4
5 5
6 6
LOIDDH
rho=0.947
BAU
RESULTS - Global Bias and RC X DDH correlation
In terms of grain size partition, the results indicate that the DDH borehole core and RC sampling procedures show a granulometric bias with no correlation. Fragmentation of rock matrix is higher in RC samples, increasing the proportion of fine material (G4, fraction below 0.15 mm) and decreasing in coarse material (G1, fraction above 8mm). This does not implicate in important chemical biases.
DepthDepth
% G
4
% G
4
DDH RC
RESULTS - RC X DDH correlation - grain size partition
N0
6941261
518
361 632
398
D-90
8
77
45
10063
0 250 500 750 1000 1250
Distance (m)
0
10
20
30
40
50
60
70 Variogram: FeRC
N0694
1261
518361 632
D-90
8
77
4510063
0 250 500 750 1000 1250
Distance (m)
-50
0
50
Variogram: FeDDHx FeRC
N0
694
1261
518361 632
398
D-90
8
77
45
100
63
0 250 500 750 1000 1250
Distance (m)
0
10
20
30
40
50
60
70
Variogram: FeDDH
Var. Covar. MatrixFeDDH 67.27 60.97FeRC 60.97 66.54
nugget matrixFeDDH 8.90 4.96FeRC 4.96 6.68
nugget/sillFeDDH 13% 8%FeRC 8% 10%
N0
6941261
518
361 632
398
D-90
8
77
45
10063
0 250 500 750 1000 1250
Distance (m)
0
10
20
30
40
50
60
70 Variogram: FeRC
N0694
1261
518361 632
D-90
8
77
4510063
0 250 500 750 1000 1250
Distance (m)
-50
0
50
Variogram: FeDDHx FeRC
N0
694
1261
518361 632
398
D-90
8
77
45
100
63
0 250 500 750 1000 1250
Distance (m)
0
10
20
30
40
50
60
70
Variogram: FeDDH
Var. Covar. MatrixFeDDH 67.27 60.97FeRC 60.97 66.54
nugget matrixFeDDH 8.90 4.96FeRC 4.96 6.68
nugget/sillFeDDH 13% 8%FeRC 8% 10%
�The direct and cross variograms of iron for friable itabirite (IF) from JPE mine was modelled. This iron co-regionalization model shows similar sills and nugget effects for direct variograms (gRC, gDDH) and cross variogram (gDDHXRC);
RESULTS - RC X DDH - Regionalization
�The RC proportion nugget/sill is lower than the respectively proportion of DDH variogram. The reduction of sill and nugget effect in RC variograms, compared to DDH variograms, is due to difference of sample support;
CONCLUSIONS
�The results indicate a small global bias for some chemical variables;
�The bias associated with contaminants (silica, alumina, phosphorous, manganese and LOI) are, in general, inversely proportional to iron bias;
�Due to difference in volumetric sample support between RC and DDH drilling methods, variance of RC samples are smaller than DDH samples, generating a conditional bias;
�Spatial correlation coefficients among chemical variables of samples (RC / DDH) are in general greater than 0.7;
�Direct and cross variograms of chemical variable Fe are similar for RC and DDH, varying only sill contribution;
�The low ratio nugget effect and sill can be probably related to mixture of fragments of adjacent intervals, mainly in RC samples;
CONCLUSIONS
�Grain size variables do not show correlation comparing both drilling methods, and RC samples show finer particles with the increase of depth;
�The use of both methods is indicated in global chemical evaluation of iron ore. Deeper RC samples (> 100 m) and samples below water level should be better investigated in future studies. The combined use of both methods is also possible, global bias is acceptable for the majority of variables.
