Mining Area and Gold Origin
The sorting and composition of the gravels suggest a definite colluvial origin in that they are the product of slumping from the sides of the valley which are extremely steep. The rapid uplift and erosion in Papua and the nature of the slate and phyllite give rise to frequent slumping.
The gold is found only at depth in the gravels. According to the local miners, the highest grades are not on bedrock probably due to the disruption of the terraces by slumping. The gold is extremely coarse and consists of flattened elongate nuggets ranging from 50mm to several centimetres in size. Nuggets have been reported of over a kilo by the artisanal miners who have also reported no fine gold which in CSA Global's view provides for an extremely unusual situation.
Gold does not form like this in many hard rock deposits nor is it found in this form in most alluvial deposits. The nuggets show definite signs of fluvial transport. Some nuggets have are like folded wafers with strong suture lines where the folded edges have been joined. Other nuggets show the typical oval shapes formed by rolling in fluvial transport.
In Area 81 CSA Global's view that gold has been transported in a river bed and since covered by colluvial deposits. This suggests the gold has come from remnant terrace deposits included in, or just covered by, colluvium arising from massive slumping. These terraces would themselves be highly unusual, containing only coarse gold, which suggests they must be the result of very high energy and are close to source.
This is analogous to the "potato patch" in the Sierra Nevada of California where nugget?only deposits were derived from the "Mother Lode" formed high in the mountains with lower grade deposits of finer gold forming downstream and on the alluvial plains.
The nature of the gold also varies from area to area. The gold in Area 81 is soft, very yellow and has a fineness of ~96% whereas the gold in Area 99 is harder, less yellow and slightly less in fineness. The size of the gold found in Area 99 is also larger than that found in Area 81. Nuggets up to 1kg are reported from Area 99.
Gold Origin
Boulders and clasts in the gravel show strong mineralization with massive pyrite found in graphitic slate and phyllite. Quartz veining, vuggy quartz and silicification are well developed in the same rocks. There is widespread iron staining and oxidation seen in boulders and disseminated pyrite, chalcopyrite, sphalerite and galena occur in the slate with quartz veins.
It does not seem likely that the gold has been transported any great distance even if the clasts in the gravels are from remnant terraces of an earlier course of the Derewo. All the mining areas seem within 2/3km of each other and a fairly local gold source seems most likely. Normally, very large gold nuggets are unlikely to travel far from source although in a very rapidly eroding mountain environment, they could move many kilometres. The most likely source would seem to be quartz vein stockworks in the graphitic slate and phyllite, i.e. a mesothermal vein deposit.
Mesothermal gold deposits are commonly associated with regional structures where new terrain has been accreted onto continental margins or island arcs and where there has been significant horizontal shortening, uplift and late magmatism. This describes the Derewo fault zone.