Resources Estimation on further Exploration activities in PT. Trimegah Bangun Persada (Harita Group) Kawasi Village, South Halmahera District, North Maluku

PT. Trimegah Bangun Persada intends to do mining activity at the north part of IUP as the south and west parts have already been mined. This research aimed at producing natural resources of laterite nickel in the advanced exploration activity of Tangkuban Block. Besides, it also compared and determined the estimation methods having a good correlation with drilling results. Tangkuban Block carried out core drilling within total drill numbers of 286 spots and space distance 25 meters at the block area 22.16 Ha. This research began with determining the geological domain consisting of limonite, saprolite, and bedrock through a geo-statistical approach. After that, the researcher estimated nickel resources by three methods namely ordinary kriging, inverse distance weight, and nearest neighbor point. The result of estimation by ordinary kriging obtained a total volume of limonite layer 1,345,313 m3 with the content average of 1%, while the total volume of saprolite layer was 1,850,000 m3 1.64%.


Introduction
Laterite nickel is one of popular mining commodity used for electronic battery because of it lithium content. Nickel laterite can be extracted from earth used the open cast mining method. Before extracted, nickel laterite needs prospecting and sampling to determine its value and grade.
Prospecting is the activity for surveying and collecting data or information about the existence of mining commodities [1]. One of the prospecting methods is drilling exploration. Drilling is used to determining the geological domain and minerals grade [2]. Geological domain and minerals grade used for estimated distribution of minerals grade. For volume estimated used three methods: Inverse Distance Weight, Nearest Neighbour Point, and Ordinary Kriging.
Based on the above, a detailed prospecting activity was conducted to determine the amount of laterite nickel resources in the Tangkuban block. This study will compare three estimated methods that are Ordinary Kriging, Inverse Distance Weight, and Nearest Neighbor Point.

Methodology
The data used are data from further exploration of the Tangkuban block drilling activities as many as 286 drill points resulting in a sample of 4,967.

Data Analysis
The IDW method determines the sample weight (w) as a function of the sample distance to the estimated block [4]. The formula for this method is: The estimation of reserves using the nearest neighbor point (NNP) method is based on a block model framework. In general, the calculation method using the NNP method gives the same results as the manual section method, but the results of this assessment can be used directly in mine planning using a computer [3].
Things that need to be considered in kriging [3] : This is a very common problem and a graph that is presented as a representation of Y and X (in a cross-cartesian axis system) in statistics is called a scatter diagram [4], [5].

statistical analysis
In estimating nickel resources, the statistics of all data samples must be known in advance to find out whether the sample has one population or more than one population which is determined by the frequency distribution and linear regression of the drill data sample.

Figure 1. The Distribution of Drill Holes
Statistical analysis starting from the minimum data, the maximum to the sample variance, is more clearly seen in the table below. Based on the statistical analysis above, it can be seen that the data in the standard deviation table is quite large, this shows that the thickness sample distribution tends to be uneven while the CV value is> 0.5, which indicates that the sample is quite heterogeneous. Figure 1 shows a map of the distribution of drill holes at the research location.

Nickel Resources estimate
Estimation of laterite nickel resources uses three resource estimation methods, namely the Inverse Distance Weight, Nearest Neighbour Point, and Ordinary Kriging methods. The nickel resource estimation is carried out based on the geological domain, which means that the estimation of nickel resources separately in the limonite and saprolite layers. Meanwhile, the bedrock layer resource was not estimated because it is a layer containing a very low nickel content.

a. Inverse Distance Weight (IDW)
The estimation of laterite nickel resources using the inverse distance weight method on the limonite layer, the maximum distance estimation of the grade was limited to 51.27 meters based on the nickel variogram analysis. The total blocks formed were 8,647 blocks.
The resource estimation using the inverse distance weight method for the limonite layer obtained a total volume of 1,395,094 m3 with a minimum grade of 0.32%, a maximum of 2.17% with an average grade of 1%. The estimation results of limonite layer nickel resources using the IDW method are as follows. In the saprolite layer, the maximum distance assessment is limited to 67.78 meters based on nickel variogram analysis. The total blocks formed are 11,895 blocks.
The resource estimation using the inverse distance weight method for the saprolite layer obtained a total volume of 1,858,594 m3 with a minimum grade of 0.36%, a maximum of 2.91% with an average grade of 1.64%. The estimation results of saprolite layer nickel resources using the IDW method are as follows. Estimation of laterite nickel resources using the nearest neighbor point method on the limonite layer, the maximum distance of the estimated grade is limited to 51.27 meters based on nickel variogram analysis. The total blocks formed were 8,603 blocks.
The resource estimation using the nearest neighbor point method for the limonite layer obtained a total volume of 1,344,219 m3 with a minimum grade of 0.26%, a maximum of 2.21% with an average grade of 0.99%. The estimation results of the limonite layer nickel resource using the NNP method are as follows. In the saprolite layer, the maximum distance assessment is limited to 67.78 meters based on nickel variogram analysis. The total blocks formed are 11,895 blocks.
The resource estimation using the nearest neighbor point method for the saprolite layer obtained a total volume of 1,858,594 m3 with a minimum grade of 0.26%, a maximum of 3.44% with an average grade of 1.64%. The estimation results of saprolite layer nickel resources using the NNP method are as follows.

c. Ordinary Kriging (OK)
The estimation of laterite nickel resources using the ordinary kriging method in the limonite layer, the maximum distance of the estimated grade is limited to 51.27 meters based on nickel variogram analysis. The total blocks formed were 8,610 blocks.
The resource estimation using the ordinary kriging method for the limonite layer obtained a total volume of 1,345,313 m3 with a minimum grade of 0.32%, a maximum of 2.17% with an average grade of 1%. The estimation results of the limonite layer nickel resource using the OK method are as follows. In the saprolite layer, the maximum distance assessment is limited to 67.78 meters based on nickel variogram analysis. The total blocks formed are 11,840 blocks.
The resource estimation using the ordinary kriging method for the saprolite layer obtained a total volume of 1,850,000 m3 with a minimum grade of 0.36%, a maximum of 2.91% with an average grade of 1.64%. The estimation results of saprolite layer nickel resources using the OK method are as follows.

Estimation Validate
Validation of resource estimates is carried out to compare and determine the estimation results with what method will be used for reporting the nickel laterite resources of the Tangkuban block. All methods used will be compared with the results of the drilling which have been composited based on the distribution at X, Y, and Z coordinates.
To determine the method used as resource reporting, correlation regression is used to determine which method has a correlation coefficient that is closest to 1.
The following is the regression correlation of the estimation method with the composite drilling results which is calculated based on the distribution on the X, Y, and Z coordinates.     The regression correlation result of the OK method of laterite nickel estimation with the distribution of nickel content based on the drilling results in the saprolite layer which was calculated along the X, Y, and Z coordinates obtained a regression correlation at X coordinates 0.83, Y 0.99 and Z 0.98. If average, the regression correlation value is 0.933.

Discussion
The regression correlation results show that the estimation using the ordinary kriging method is the best correlation with the drilling results in the limonite and saprolite layers. Then the reporting of laterite nickel resource estimates in the Tangkuban block uses the ordinary kriging estimation method.

Conclusion
Tangkuban block is an advanced exploration activity of PT. Trimegah Bangun Persada has nickel resources of 1,345,313.0 m3 with a minimum grade of 0.32%, a maximum of 2.17%, and an average nickel grade of 1%. While the saprolite layer is 1,850,000.0 m3 with a minimum content of 0.36%, a maximum of 2.91%, and an average nickel grade of 1.64%.
Determination of the appropriate estimation method in the Tangkuban block is based on the regression correlation of the estimation results with the results of the composite drilling which is measured based on the distribution at the X, Y, and Z coordinates. It is then obtained that both the limonite and saprolite layers use the ordinary kriging estimation method because they better correlation than inverse distance weight and nearest neighbour point.