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Are there any valuable deposits (gold ore, iron ore, silver, copper, gemstones) which are likely to be found in or around excavation sites for building materials like limestone, sandstone, slate or clay?

Is it likely for one to encounter the former while extricating the latter?

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  • $\begingroup$ Do you consider granite a building material? $\endgroup$ – Shalvenay Jul 18 '15 at 17:18
  • $\begingroup$ Yes - although I was initially thinking only of resources used for fairly large-scale building projects, granite would work roughly as well for my purposes. $\endgroup$ – esckelbröd Jul 18 '15 at 20:15
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In general,

The building materials you mentioned are all sedimentary (rocks either eroded from volcanic rock and then reformed by action of water) or metamorphic rocks (sedimentary rocks subsequently modified by exposure to subsequent high temperature and/or pressure).

Meanwhile the minerals you mentioned (especially gold, diamonds, platinum, etc.) are typically (but not exclusively) found in certain kinds of igneous rocks.

So the general case is:

No

However, gold especially has occasionally been found in large quantities within sedimentary rocks (mostly sandstones) and/or sandbars of rivers draining mountainous regions. These rivers contain detritus from the erosion of quartz veins. Gold solidifies around the same temperature and pressure as these quartz veins and, therefore, is often found embedded in quartz veins. It would tend to collect in sandbars of the rivers containing these quartz particles. This is what happened at Sutter's Mill in California.

Since gold (and other noble/precious metals) can be found in sand bars, it isn't a very big stretch to imagine we could possibly find large quantities of gold in certain sandstone deposits. These placer deposits can contain nuggets of platinum group metals and gemstones too.

The concentrations will not be very high compared to what we normally think of as high concentrations but they will be high in comparison to the normal concentration of such materials on the surface of the Earth.

So, in special circumstances, the answer can be

Yes

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    $\begingroup$ And don't forget areas of extreme folding, with much older, igneous material side-by-side with recent sedimentaries. $\endgroup$ – WhatRoughBeast Jul 19 '15 at 20:10
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Yes

While the building materials mentioned may be primarily sedimentary (as noted in Jim2B's answer), there's nothing to stop you, as the author, from inventing some geological event that will overlay all those sedimentary layers with some igneous or metamorphic rock. You can put as many types of rocks close to each other as you need as long as you have a reasonable explanation for how those rocks got to be close together. If the civilization is primitive enough, there isn't any explanation needed beyond "The Gods have made this a bountiful land".

And you have a lot of wiggle room

Unless you're writing for a geologists conference, most people aren't going to know enough about geology to know that a particular configuration of rock is unrealistic. It may seem a little unrealistic to happen to have every single precious metal/gem be found within a 10 mile radius, so just be careful.

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