Narrow veins are major sources of many commodities, including gold, silver, tin, tungsten, coal, uranium and sometimes copper, lead and zinc. Mining this narrow style of mineralization can be considered as relatively high risk, not only due to the high cost of exploration and mining per unit of metal, but also to the frequently limited size of the resource. Estimation of the Mineral Resources and their location can be difficult. Underground development is generally required to confirm the continuity of mineralization.
Most vein modelling and Mineral Resource estimation methods for narrow vein deposits uses three dimensional (3D) block grade estimation, based on kriging and inverse distance weighting. In addition, most modelling packages require the assumption of a constant dip and dip direction for the whole vein. The main problems with these methods are the change of the sample support due to the variability of the vein width, the use of unrealistically small blocks, changes in vein dip and dip direction that are common with precious metal deposits, and the large variations in grade. AMC suggests the use of the two-dimensional (2D) accumulation estimation method for most narrow vein deposits, in conjunction with a variable-trend capability such as the Dynamic Anisotropy module developed by DatamineTM to accurately model the vein. Based on the vein boundary wireframe, it more accurately defines the vein dip and dip direction, when calculating the vein true thickness and true northing, easting or vertical thickness1.
A number of papers (example O Bertoli 2003) have described the method of 2D block grade estimation, using accumulation (grade x thickness) and thickness estimates. This method has proven to be successful, especially when the samples are given a constant easting, northing or RL for the variogram analysis and block grade estimation. AMC includes the estimation of the vein thickness based on the accumulation variogram (to back-calculate the true grade) and a second vein thickness based on its own variogram. Depending on the situation, it may also improve the representation of the vein geometry, which is initially based on the wireframe outline but can be also be based on the estimated true thickness using the drillholes and, where available, underground samples. Methods for efficient evaluation of changing scenarios for Mineral Reserves estimation have also been developed by AMC, allowing for easy assessment of possible dilution, previously mined areas, mining method and stope areas.
The data required to prepare the model includes:
- Drill hole and other sample data (in drill hole format)
- Wire-frames outlining the vein
- Wire-frames of existing workings (to exclude mined material from the Mineral Resources)
- Wire-frames of any major lithology changes within the vein
The result is a three-dimensional block model containing:
- Single block covering the full vein thickness
- Variable block size in the vein width direction (either east north or vertical) accurately defining the vein width in that direction
- True vein horizontal or vertical thickness
- Commodity grade percentage estimates
- Option to increase the block size to a defined minimum based on mining method
- Options to change block size in all directions to be consistent with mining requirements
- Location and grade of previously mined areas
The model is suitable for2 :
- Mineral Resource estimation
- Estimation of tonnes previously mined
- Estimation of planned dilution
- Assessment of possible unplanned dilution
- Locating areas of possible geotechnical concern based on the vein’s location compared to faulting or low strength rock
- Identifying the preferred mining method
- Estimation and scheduling of Mineral Reserves
- Rapid evaluation of the impact of changes of mining scenarios or costings
- Reconciliation with the results of past mining
The method can be used for veins with any dip or dip direction modelled in any plane. The final model can contain the results for a number of veins added together to form a single model.
The figure above is a plan view, from a real life example, comparing the results of the estimated true vein thickness using a constant vein dip and dip direction, with the results using dynamic anisotropy. Areas where the vein changes direction compared to the constant dip and dip direction values originally used, result in changes in the true vein thickness. This figure also shows the change in vein thickness compared to the wireframe outline provided.
1 Yellow areas are the block true north thickness using a constant dip and dip direction.
2. Blue outlines show the change in block north thickness based on dynamic anisotropy
3. Blue dotted line is the initial wire-frame of the vein outline
Fowler A, Davis C, 2011, “Quantifying Uncertainty in a Narrow Vein Deposit – An Example from the Augusta Au-Sb Mine in Central Victoria, Australia”, Proceedings of the Eighth International Mining Geology Conference, 2011. Bertoli O et al, 2003, “Two dimensional Geostatistical methods – Theory, Practice and a Case Study from 1A Shoot Nickel Deposit, Leinster, Western Australia”, Fifth International Mining Geology Conference.\
1. A paper by Dr A Fowler and C Davis discusses the use of conditional simulation to estimate the vein location and width, as well as the block grades. Further work is required to confirm the results of using this method
2. Some of these are not unique to the 2D method