Post by
brasil500 on Nov 18, 2009 11:27am
past 400 million lbs now
more great work by BeerBaron over at the hot copper baord
Resource Estimation Rossing South 18 Nov 09
Summary
All results estimated by the author unless otherwise noted.
Zone 1 – 169 Mlb
Zone 2 & 3 – 214 Mlb (Note 1)
Salem – 5 Mlb
Ida Dome – 25 Mlb (JORC)
Total – 413 Mlb
Estimatesare now rounded to the nearest million pounds. The estimationtechniques are not accurate enough to justify more precision.
Note1: The southern end of Zone 2 is vague and there is no clear indicationof where new zones might start. Consequently I have included all thedata down to 7500000N as “Zone 2 & 3”, and I have assumed that theuranium mineralisation is contiguous down to this line. It is likelythat this uncertainty will continue until it is positively establishedwhere the geological splits in the resource are.
Previous Estimates by the Author
Zone 1
- based on drill data to 31 Mar 09 - 107.6 Mlb
- based on drill data to 27 Apr 09 - 117.9 Mlb
- based on drill data to 28 May 09 - 127.8 Mlb
- based on drill data to 31 Aug 09 - 143.6 Mlb
- based on drill data to 9 Oct 09 – 155.7 Mlb
- based on drill data to 18 Nov 09 – 169 Mlb
(latest release by EXT 2 Jul 09 - 145 Mlb)
Zones 2 & 3
- based on drill data to 28 May 09 - 103.6 Mlb
- based on drill data to 31 Aug 09 – 178.0 Mlb
- based on drill data to 9 Oct 09 – 181.2 Mlb
- based on drill data to 18 Nov 09 – 214 Mlb
(latest release by EXT 22 Jul 09 - 122 Mlb)
Salem
- based on Fig 1 App 1 of EXT report 31 Aug 09 - 6.1 to 10.6 Mlb
- based on drill data to 18 Nov 09 – 5 Mlb
Ida Dome – 25 Mlb (JORC)
Total (including Ida Dome and Salem)
- 31 Mar 09 – 133.0 Mlb
- 27 Apr 09 – 143.3 Mlb
- 28 May 09 – 256.8 Mlb
- 31 Aug 09 – 353.1 to 357.6 Mlb
- 9 Oct 09 - 368.4 to 372.9 Mlb
- 18 Nov 09 - 413 Mlb
Resource Estimation
Theresource estimate for Zone 1 using data up to 18 Nov 09 came in at 169Mlb, a typical increase in Zone 1. This shows deeper and higher graderesults as more drilling is carried out.
The Zone 2 estimatebased on drill data to 18 Nov 09 is 214 Mlb, a significant increase onthe 181.2 Mlb on 31 Aug 09. This is partly due to the inclusion ofdrill reports down to 7500000N, which have greatly expanded the area ofZone 2 by 800 metres in a north-south direction, and between 500 to1,750 metres in an east-west direction. It is not yet clear that thisarea is entirely contiguous, there may geological breaks as the zoneappears to bifurcate. If it is not contiguous there may be a slightoverestimate of the southern 400 metres or so, but as the overallestimation technique is fairly conservative this may have little effecton the total.
The northern part of Zone 2, where the drillresults are spaced at approximately 100m, is estimated using ageostatistical program called ILWIS. The southern part, where the drillresults are spaced at about 400m, is estimated using a simpleraveraging method, as the spacing is too great to use ILWIS.
TheSalem estimate was calculated using both ILWIS and the simpleraveraging method used in the southern half of Zone 2. The ILWIS programdid not give convincing results, and changes to pixel size (whichshould have had negligible effect) resulted in large changes to theestimate. It is possible that the number of data points (14) is toosmall for ILWIS to be properly used. The simpler averaging method gavegood correlation with earlier estimates using the median of all data inRossing South (excluding Ida Dome), and appeared to be “about right” tothe author based upon examination of the drill results and previousexperience gained in using estimation methods for Rossing South.
Detailed Description of Method - Zones 1 & 2
Resourceestimation was carried out using a program called ILWIS (v 3.6). Drilldata was interpolated using a moving average operation. The movingaverage operation is a point interpolation which requires a point mapas input and returns a raster map as output. The values for the outputpixels are the weighted averages of input point values. Weightedaveraging is the calculation of the sum of the products of weights andpoint values, divided by the sum of weights.
The weight factorsfor the input points are calculated by a user-specified weightfunction. There are two methods: inverse distance and linear decrease.Both methods ensure that points which are close to an output pixelobtain large weights and that points which are farther away from anoutput pixel obtain small weights. Values of points which are close toan output pixel are thus of greater importance to output pixel valuethan the values of points which are farther away.
By specifyinga limiting distance, you can determine the distance from any outputpixel that points will be taken into account for the calculation avalue for that output pixel; for each output pixel, only the values ofthe points which fall within the limiting distance to this output pixelwill be used. Values of points that are farther away from an outputpixel than the specified limiting distance, obtain weight zero by theweight calculation, and these values will thus not be used in theoutput pixel value calculation. This speeds up the calculation andprevents artefacts.
Several methods were trialled using the 27Mar 09 data set for Zone 1 (see next section for the reason why thisdata set was chosen). This included both weight factor methods (inverseand linear decrease), different weight factors (0.5, 1 and 2),different limiting distances (up to 300m), and different pixel sizes(1x1 m and 5 x 5 m). Other interpolation methods were trialled,including Moving Surface (Plane), Moving Surface (2nd degree linear)and an attempt at Ordinary Kriging.
As a result of all thesetrials, the best method was determined to be Moving Average with alinear decrease (factor 1) and a limiting distance of 150 m, with apixel size of 5 x 5 m (smaller pixel sizes made little difference andgreatly increased processing time).
Ordinary Kriging (OK) is amore sophisticated form of averaging often used for resource estimationby professional geologists. Instead of just using a linear or a inversedecrease for the weight function, more complex weight functions can beused. These functions are determined by statistical analysis of thedata set. This analysis requires rather sophisticated interpretation ofthe data set, and relies on the (geological) experience of the user.The simpler nature of my techniques did not seem to justify the use ofOK, and I am not nearly qualified enough to make appropriateinterpretations of the data set when determining the weight functions.
Iconsidered it safer to use a simpler interpolation method that did notrely on interpretation. This also was better suited to the areas of thezones where less data was available.
After interpolation wascarried out, the resulting raster map was converted to a point map.Pixels containing very small amounts (less than 1 lb of uranium persquare metre) were discarded. These were mostly pixels on the outeredge of the resource and were probably an effect of the interpolationsoftware than an actual picture of the resource. The pixels were thentotalled to arrive at a resource estimate.
Data Set
All drill data released by EXTwas used in the resource estimate. Corrections were made to radiometricdata in accordance with the formula given in the NI 43-101 reportparagraph 17.3.1. The specific gravity (SG) is the same as that assumedby the NI 43-101 report. Corrections were made to account for drillangle (usually 60 degrees for most data).
An attempt was made torecreate the data set used when the initial resource was released inthe NI 43-101 report. If I had such a data set, I could compare my ownmethod of estimation with the known value of the JORC estimate of 108.3Mlb. The NI 43-101 report made an estimate based upon 11 diamond and174 RC drill holes (paragraph 17). Using drill data to 27 Mar 09, 154drill holes are found of which 12 are diamond. It's not quite right butis the closest match.
Using the map from the NI report (page 33)showing all the drill holes used to determine the resource, the dataset to 27 Mar 09 was overlaid on top of this. There is a closecorrelation between the 27 Mar 09 data set and the NI report. Usingthis data set, a result of 107.6 Mlb was calculated for Zone 1,remarkably close to the NI 43-101 calculation of 108.3 Mlb. Latercalculations showed reasonable correspondence with the official reportsissued by Extract.
Extract uses radiometric data whencalculating resource estimates. This is available to the companyquickly and is not usually released separately. Later, chemicalanalysis of drilled holes is released once it has been completed -usually two months after the radiometric data. As a result, it isimpossible to exactly correlate the data used by Extract for resourceestimation with the data released.
To ensure a firm edge to thedata, a series of false data points with zero uranium is spaced 50 maround the edge of the resource. This forces the program to sharplydelineate the resource near the edges of the current drilling.
Detailed Description of Method – Zones 2 (southern end) & 3, and Salem
Thesouthern part of Zones 2 & 3, where the drill results are spaced atabout 400m, is estimated using a simpler averaging method, as thespacing is too great to use ILWIS. Salem also uses this estimationmethod as the number of data points (14) seem to be too few to getconsistent results from ILWIS.
The averaging method looks ateach line of data points running in an east-west direction. the amountof uranium for each data point is averaged and an amount per squaremetre is calculated. Between each pair of east-west lines, the uraniumper square metre is averaged again, and the width east-west is averagedbetween the two lines. The area is usually a trapezium (UK English) ora trapezoid (US English).
A buffer of 50 m is allowed for in aneast-west direction to match the 50 m edge used by the ILWIS program.No buffer is used in a north-south direction.
The result isusually conservative because the east-west lines are only to a mediumdepth as a guide to further exploration. Infill drilling is usually togreater depths and discovered areas of greater mineralisation areusually targeted by the drilling program. This results in infilldrilling data points being higher in uranium content and, once theILWIS program can be used, result in a greater total for the resourceestimate.
Disclaimer
It is still possible thatthere are errors in my calculations, which will lead to errors inestimation of the resource size. However, the results obtained so farhave been reasonably close and it is assumed that the differences arecaused by the use of different data that is released later by Extract.The estimates given here are intended to be a general, quick look atwhat might be possible for Rossing South, with no claims as to theiraccuracy.