Estimating and Reporting Grade at Bendigo
Key Points
- Most of the gold in Bendigo occurs as visible pieces of gold, greater than 1 mm in diameter (coarse gold), and is distributed very erratically within quartz reefs. This natural phenomenon is termed the nugget effect.
- The nugget effect means that no small sample, for example a single drill intersection, can accurately represent the grade of the entire reef, as small samples will tend to miss the coarse pieces of gold. To obtain a representative grade of the reef requires many small samples, perhaps 100s of drill intersections, or several big samples, say one to two 5,000 tonne parcels.
- The challenge at Bendigo is to utilise information available from drilling in determining grade; as drilling usually precedes development, it is a highly efficient sample collection method and often provides the only source of grade information.
- The Company uses a more fundamental grade estimation approach which is better suited and appropriate to local conditions. Company geologists now use their accumulated knowledge to make assessments of size and quality of mineralisation based on the visual examination of core.
- The approach taken by the Company is to use the physical characteristics of the reef intercept, in combination with the assays to estimate the grade range of each intercept. The basis of this approach is that the physical characteristics of the reef are a far more consistent indicator of gold grades than simply relying on chemical assays.
- The geological grade range estimate is based on a ranking of the key geological textures and minerals visible in the drill core: including quartz percent, quartz textures, presence of free gold and sulphides.
The key geological textures that are used in the visual grade estimation technique are:
- Quartz abundance - The quartz is the sole host of the gold in a Bendigo style reef system, hence the more quartz there is in an intercept, the greater the chance of there being gold in that intercept.
- Quartz texture - The deposition of the quartz in a reef, although occurring over a relatively short geological time frame, comprised many different pulses of gold-bearing silica rich fluid. The texture of the quartz in a reef intercept is the best guide to how many fluid pulses occurred, with multiple fluid events increasing the chance of gold mineralisation. Therefore a quartz intercept with a large number of fluid flow textures (called stylolites or laminations) will have a greater chance of containing gold bearing phases than a plain white featureless quartz vein.
- Gold abundance - The gold in Bendigo generally occurs as large (>1 mm) nuggets which are visible when intersected in drill core. Hence, the relative abundance and size of visible gold particles in an intercept will have a significant bearing on the estimated grade of the reef.
- Proxy sulphide abundance - When the gold was deposited in the quartz reefs, a number of other minerals were typically deposited at the same time. These minerals comprise three main sulphide species: arsenopyrite, sphalerite and galena. Arsenopyrite is by far the most abundant proxy sulphide species in the Bendigo Goldfield and where it occurs as large clasts or bands in the quartz, it is regarded as an indicator of gold mineralisation. Sphalerite and galena, although less abundant and more sporadic than the arsenopyrite are very closely associated with gold, hence intersecting these sulphides in drill core is also indicative of strong gold mineralisation.
- The physical characteristics of each intercept are logged in detail and verified by a senior geologist, with their relative abundance and importance ranked to derive a visual grade range for the intercept.
- The drill core samples are assayed via screen fire assay (the most robust analytical method for high nugget ores) with the results used to supplement the visually estimated grade range. The assays contribute up to one third of the overall grade range estimate.
