I found this rock while hiking in a creek. It has some bits of white quartz, sheen and some brownish coloring. Does anyone know what it could be? Is there any possibility that it might be gold ore? It is heavy for its size and does not have any magnetic properties.

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  • $\begingroup$ Thanks for your reply. I’m no geologist either. However, It doesn’t smell like sulfur. I have posted more pictures if that helps $\endgroup$ – Philip Jan 7 at 8:01
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    $\begingroup$ Some more details like the approximate location you found this at, its size etc might be helpful. $\endgroup$ – Erik Jan 7 at 8:03
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    $\begingroup$ Gold ore? Probably not! The presence of quartz is irrelevant to the presence of gold. The brown stain may be due to iron oxides. $\endgroup$ – Fred Jan 7 at 8:21
  • $\begingroup$ Eric - I just added some dimensions . I found the rock while hiking in the mountains in California. $\endgroup$ – Philip Jan 7 at 8:49
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    $\begingroup$ @Philip California is a BIG state and has a lot of geologically distinct mountain ranges. Also, it's not clear that the rock has any quartz at all -- the white bits could be feldspar, in which case you probably have diorite. $\endgroup$ – Spencer Jan 7 at 11:52

Without any further information I would suggest that your rock is a type of Gneiss. Gneiss is a metamorphic rock that forms under moderate temperatures and pressures, where some re-crystallization occurs which obscures most of the original form of the rock. I believe it is Gneiss because of the following:

1) Distinct layering (seen in the last two photos)

2) Biotite Mica (The black crystals, this can be confirmed by looking for platy type crystals)

3) Phencrysts (Large crystals) of quartz or feldspar, see Spencer's comment

4) The rock looks like classic Gneiss

I doubt that the rock makes a significant gold ore. In the formation of Gneiss there is nothing that would tend to concentrate the gold within the rock. Most gold ores are found in hydro-thermal types of metamorphic rocks where the gold is dissolved out of the surrounding rock by heated water under pressure. As that water cools or depressurizes the gold precipitates out of solution generating the gold ore, see this wiki on Ore Genesis . Finally the staining on rock is likely simply staining from the soil or from iron.

Although this rock is not likely a gold ore, there could be gold areas nearby. Some areas in California have some very complex geology making it possible to have different types of formations located in close proximity to others.

Just to be clear I am not a geologist so you might get a better informed answer.


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