Fortunately there are techniques to deal with this. However, nature being capricious, we have to learn to live with it. A 10 gram nugget, while technically the same occupies a tiny area and requires a much larger amount of material to be found. So 10 grams of gold particles are more likely to be distributed throughout a given area. Fine gold particles while they can and will cluster, are more likely to be more distributed in throughout a deposit. The final twist is presence or absence of actual nuggets. However, as any 49’er could tell you, gold will be deposited in very specific points in the fluvial system with large areas of barren gravel in between. Unlike veins, these deposits have been sorted by water so nuggets of different sizes will be found in different areas, associated with different sizes of sediments. Another class of nuggety gold deposits are placer and paleo-placer deposits. The inevitable result is masses of gold nuggets in very specific locations and very sterile gangue everywhere else. In vein deposits this can be particularly pronounced when veins kink and bend, forming or removing spaces for gold to precipitate. Unlike a porphyry copper deposit, where vast volumes of fluid move through the rock and distribute mineralization widely in fracture sets and as replacement of rock forming minerals, gold in mineral deposits tends to be formed in where the exact conditions are right to precipitate gold from hydrothermal fluids. Moreover, many of the mechanisms of gold deposit formation are prone to irregular deposition in time and space. Gold, and PGE’s are particularly susceptible to this as they are not reactive and are not commonly found combined with ore forming minerals. Where metals are very tightly clustered (nuggety), then finding the nugget will give a wild overestimation of the amount of metal, inversely missing it will underestimate the amount of metal. The Nugget Effect simply is the product of the clustering of metals in a given deposit. There was a gold nugget, it was assayed the first time, and now it’s gone for the second assay. What happened? Did the assayer pocket that 5 grams of gold? Nope. What’s even more frustrating, from the authors own personal experience, is when core is first assayed by one technique and then re-assayed by another and the gold disappears. The core will assay 44 kilos of gold per tonne in 1 metre and 0.1 g/t per tonne the next. I’m not talking about the time the CEO of a junior exploration company picked up drill core freshly drilled from the corebox and called a zone of pyrite the best core he’d ever seen (it was if you like shiny things). One of the first things to recognize in the mining industry, is that all that glitters may not be gold.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |