Hi 2Luke,
A good question. These terms relate to different intensities of weathering (where the original rock has been subjected to a tropical climate and rain has partly-dissolved the rock turning it into clay and iron-rich minerals). This weathering could be ongoing today, or could have happened many millions of years ago. See below for a breakdown of each item:
Laterite - An iron-rich near-surface cap which comprises mainly of iron-oxides and aluminium-rich minerals. The composition can vary significantly depending on what the original rock-type was. You will hear this term in gold deposits, where often there is an enrichment of gold in the laterite due to the removal of other soluble minerals. This is considered mechanical enrichment, as gold is very insoluble.
Oxide - The layer beneath the laterite that is mainly comprised of clay minerals. This zone is much thicker than the laterite, and can be up to 200m thick in strongly weathered orebodies. The Oxide zone is usually more interesting for gold miners because it is easy to mine (doesn't require explosives) and treat (gold is usually liberated and "free", meaning it is easier to recover). Therefore, you will see that miners report mineral resources in the oxide zone at a lower cut-off due to the lower mining costs. Its a bit like the icing on the cake. If the weathering has been happening for a long time, sometimes you can see an enrichment of gold near the base of the Oxide zone. This is considered chemical enrichment, where gold has been leached from further up the profile, and redeposited near the base. THis is discussed further below.
Transition - Transition is the boundary between the Oxide Zone and the "fresh" unaltered rock beneath. This zone is problematic for miners, as it often requires blasting and blending with other ore sources to optimise the recovery of gold. In geology terms, it is usually thinner than the Oxide zone, and can sometimes contain higher grades. The higher grades are the result of gold that has been leached from the Oxide zone, and re-precipitated in the transition zone due to changes in chemistry. This creates a "blanket" of enriched "supergene" gold that has a much larger footprint than the actual orebody in the unaltered rock beneath. This supergene blanket is very useful for gold explorers. A similar process happens in copper deposits, where the supergene zone is higher grade and very useful for the economics of the deposit.
Fresh - Simply unaltered rock beneath the transition zone. Often sulphide-rich material (sulphide is not stable in the Laterite or Oxide zone), which requires a different treatment route.
These four terms are usually used to separate mineral resources and reserves, as they represent the four key rock types that are treated differently in the processing plant. As the mining and processing costs are different for each rock type, different cut-off grades will be applied during mining.
These are just generic descriptions for weathered gold deposits (Australia, West Africa, South America, southern regions of North America, etc.), and its worth noting that as each orebody is different.
Hope this helps.
Desertpunter.
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