Ore deposits represent work that nature does for us. For instance, Earth’s crust contains an average of about 55 ppm(parts per million) of copper, whereas copper ore deposits must contain about 5,000 ppm (0.5%) copper before we can mine them. Thus, geologic processes need to concentrate the average copper content of the crust by about 100 times to make a copper ore deposit that we can use. We then use industrial processes to convert copper ore into pure copper metal, an increase of about 200 times. By a fortunate coincidence, elements and materials that we use in large amounts need less natural concentration than those that we use in small amounts. Thus, we are likely to have larger deposits of mineral commodities that we use in large amounts. As long as energy costs remain high, the relation between work that we can afford to do and work that we expect nature to do will control the lower limit of natural concentrations that we can exploit, and this puts very real limits on our global mineral resources.

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I don't understand what the last part of this article means. Please explain in detail.

1) What does it mean in the context that energy costs remain high?

2) What we can do and what nature is expected to do regulate the lower limits of natural enrichment that we use. I don't know what this means.

3) What do you mean by putting real limits on our global mineral resources?

Question

Ore deposits represent work that nature does for us. For instance, Earth’s crust contains an average of about 55 ppm(parts per million) of copper, whereas copper ore deposits must contain about 5,000 ppm (0.5%) copper before we can mine them. Thus, geologic processes need to concentrate the average copper content of the crust by about 100 times to make a copper ore deposit that we can use. We then use industrial processes to convert copper ore into pure copper metal, an increase of about 200 times. By a fortunate coincidence, elements and materials that we use in large amounts need less natural concentration than those that we use in small amounts. Thus, we are likely to have larger deposits of mineral commodities that we use in large amounts. As long as energy costs remain high, the relation between work that we can afford to do and work that we expect nature to do will control the lower limit of natural concentrations that we can exploit, and this puts very real limits on our global mineral resources.

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I don't understand what the last part of this article means. Please explain in detail.

1) What does it mean in the context that energy costs remain high?

2) What we can do and what nature is expected to do regulate the lower limits of natural enrichment that we use. I don't know what this means.

3) What do you mean by putting real limits on our global mineral resources?

Answer

That is a hell of a complicated article with which to be learning English! 
1. "Energy costs remain high" - this sentence means that because of high energy costs we can only mine copper ore deposits (or other ores) at relatively high concentrations. We have to use energy (industrial processes) to get copper out of copper ore. If the concentration of copper in the copper ore is too low we'll have to use more energy, and if energy costs a lot we won't be able to afford it. 
2. "What we can do and what nature is expected to do ..." - nature (through geological processes) concentrates copper in copper ore. The more this happens through natural processes the less energy humans have to expend extracting copper from copper ore. If, for example, copper ore deposits only contain 1000ppm copper then energy costs would be too high to make it worthwhile extracting the copper from this ore. 
3. "Put real limits on our global mineral resources" - this means that the lower the concentration of a pure metal in its ore, the more energy we need to extract it. Because energy is expensive we can only mine good quality ore (with a high ppm - parts per million) otherwise it isn't worthwhile. That's the limit - in this example the limit for copper is 5000pmm copper in the copper ore. Any lower concentration of copper and it isn't worth using the ore. 
Good luck!!