Chalcopyrite and Arsenopyrite - Indicators for Gold and Copper Mineralized Zones.
Well to start things off the pictures aren't the best quality but hey it's a $50.00 digital microscope and we get to see things up closer than we do with a loupe. These pictures are of the same rock just different sections and were assayed to have gold content in them. The gold was only in the 500 ppb range but you can expect that the gold was probably trapped inside the matrix of the chalcopyrite and the Arsenopyrite. The presence of these minerals don't guarantee the presence of gold however they are a really god starting point. If you are out prospecting and find yourself in areas with pyritic mineralization then take a closer look around you. The chalcopyrite is above which has more of a gold hue to it where as the Arsenopyrite is below with more of a silvery tone to it.
One of the things to look for when checking these rocks out in the field is the minerals surrounding them. Pyrite on it's own in any form is relatively common however when you get pyrite in strongly sheared rocks accompanied by Quartz veins the chances increase because these rocks have been reformed from their original state. During these processes heat and pressure can cause accumulations of minerals or re-mobilisation / concentration of minerals. Now to give you an idea of what this might look like refer to the picture below. This is a strongly sheared sedimentary rock which has no quartz veining but has pyrite inclusions. This rock was also assayed and contained around 400 parts per million copper along with about 18 parts per billion gold. Now in some rocks the 18 PPB gold would say HEY, might be worth looking into this further. However it's a sedimentary rock with no quartz veining so chances are the sediments which comprised this rock had a low concentration of gold to begin with.
In summary though each of the times I seen this mixture of pyrite/quartz and metamorphic rock, it set off alarm bells for me to look into it further. Gold is often where you find it and some of the biggest gold discoveries in our province have been in relatively barren looking material. Devils Pike Brook had a very plain looking quartz which assayed over 500 grams per ton.
As you progress with prospecting you will learn from your assays what to look for and what clusters of colorful minerals can help you find what you are looking for.
Until Next time - NB PROSPECTING
