- Narrow Vein Mining
- Media & Presentations
Nature of the Veins
The Co-O Mine narrow quartz veins can be thought of as sheets of white paper generally 1-2 metres thick suspended in green space. The white sheets or veins are essentially filling spaces in the green volcanic wall rocks which form when the rocks crack or are faulted. These spaces are not regular hence the vein which fills the crack will take the shape of the crack and form sheets which change in width and attitude according to the shape of the original crack. Some cracks are long and continuous, others are shorter and less continuous. Veins can have complex shapes, with branches or splits, and many other variations.
The volcanic wall rocks are green hence the white veins are visually distinctive underground and are easily followed by the miners. The veins can develop at any depth so the tops of different veins can be at different depths and the bottoms as well at different depths. Not all veins are the same size in terms of lateral extent or vertical extent.
In summary, the geometry of narrow veins can be very complex, and cannot always be simplistically interpreted from drilling results.
Narrow Vein Mines
As a generalisation, many narrow vein mines do not have long life reserves, but in general have significant resource bases with potential to increase. This applies not only to narrow vein gold mines, but also to silver and tin narrow vein mines, diamond fissure mines, base metal narrow vein mines, and others.
Narrow veins are challenging to evaluate because of their geologically variable nature. They are generally less than 3 metres wide, discordant, of variable dip, variable width and commonly displaced by faults.
Hence many operators of narrow vein mines use drilling to initially locate the veins and to estimate Inferred Resources, but use infill drilling and underground development along the veins to raise the category of the resource to Indicated (and maybe Measured) as the level of confidence in the resource interpretations increases.
A well known Australian example of long lived narrow vein gold mine is the Norseman Mine in Western Australia which has been in almost continuous operation for some 70 years but is understood to have rarely had more than 2 years reserve life. A well known United Kingdom example is the South Crofty Tin Mine in Cornwall.
Resources and Reserves Policy
The Company has adopted the policy of aiming to replace the reserve ounces it mines each year, both as resources and reserves within the area of the Co-O Mine resource model by underground drilling and development. Using extensive and expensive deep drill drilling from surface to increase resources at depth in the mine, in areas which will not be mined for many years, is viewed as not being an efficient deployment of capital, hence this capital is now deployed into more effective underground drilling.
Additional resources may be added to the resource inventory through extensions of known veins at depth, the discovery of new vein systems either immediately adjacent to or as extensions to the Co-O Mine resource model, or new discoveries within the general area of the Co-O Mine.
Resources and Reserves
Minerals Resources and Ore reserves are defined under the guidelines of the recently adopted JORC Code 2012. Prior to 01 December 2013, the Company had been reporting Mineral Resources and Ore Reserves under the guidelines of JORC Code 2004. As a consequence of the recently adopted guidelines, resources and reserves previously reported can only be reported now providing that there has been no material changes to the resource or reserve model as a results of such factors as new drilling results, mining, change of commodity prices,etc.
Inferred Resources at the Co-O Mine are drill indicated, ie, the drill hole intersections are used to develop the 3-dimensional computer model for the shapes of the veins. 3-dimensional computer modelling involves fixing the shape of both walls of the vein in space so that the volume and grade of the material between the walls can be estimated.
Inferred resources are the category of resource with the lowest level of confidence.
Indicated Resources are Inferred Resources that have been converted by infill drilling and underground development along the vein, ie, tunnelling along the vein so that the veins can be joined continuously laterally and vertically. This development is conducted on each level which are spaced every 50 metres vertically.
Development along veins in the Inferred Resources category is required as part of the conversion to Indicated Resources , hence a proportion of ore production from Inferred Resources contributes to the mill feed.
Indicated Resources are the category of resources with the middle level of confidence.
Probable Reserves are estimated at a point in time annually and are estimated by applying specified mining and financial parameters to the Indicated Resources which are located in areas of vein where development (in the Inferred Resources) has been undertaken in the year subsequent to the previous reserve estimate, and will also include reserves still not mined from areas included in the previous year’s estimate. Probable Reserve blocks generally have development at least top and bottom (on the level drives) and generally, some vertical development. Some reserve blocks may be partially mined at the time of the reserve estimate.
The reserve estimate takes into account the need to leave pillars, ie, areas of unmined rock below each level (floor pillar) and above each level (roof pillar) and other areas that need to remain to hold the walls so the vein material can be safely removed from between the walls. Hence not all the Indicated Resources become Probable Reserves, but an indicative conversion factor at Co-O is approximately 80% for shrink stoper.
Figure 1. Composite level plan of Co-O Level 8 as at 30 June 2015.
Figure 2. Co-O Mine composite longitudinal projection showing the locations of reported significant drill intercepts (since 2010)
The challenge in mining narrow veins is to guide the operations so as to produce ore economically with as little dilution as possible, at the least cost commensurate with needs for safety and the environment.
Stoping is the process of extracting the vein material from above each development level and below the one above, a vertical distance of approximately 50 metres.
Different stoping methods are used for different orebodies in different mines based on factors such as wall rock competence, vein widths, dips and grades and volumes of mineralised material.
Two mining methods are currently utilised at the Co-O Mine:
(i) Shrink stope mining
This method (Fig 1) is predominantly used on vertical to sub-vertical veins where dilution can usually be reasonably well controlled. Shrink stopes have a minimum mining width of 1.25 metres; and
(ii) Room and Pillar mining
This method is used on the numerous low-angle veins (described above) where it is difficult to control the dilution from the hanging wall or roof. The minimum mining width for low angle veins is 1.5 metres, hence the higher dilution in low-angle stopes is partly responsible for the overall lower than average grade achieved from the mine.
The productivity of room and pillar stopes is not as high as shrink stopes, and hence they also incur slightly higher costs. Figure 2 shows a room and pillar stope where the ore is physically scraped from the blasted face to the ore pass.
Approximately 20% to 30% of the development ore at Co-O is by definition (in narrow vein mines) taken from Inferred Resources as this development is required as part of the conversion process from Inferred to Indicated Resources (as described previously).
Figure 1. Diagrammatic representation of shrink stoping.
Figure 2. Room and Pillar Stope Development
Level and vertical development will continue at approximately 1,500 metres per month to maintain the stope development cycle. Development ore will provide approximately 30% of the mill feed.