Market Cap, NAV and finite resources

The futures market will determine the value of their copper, which may still be in the ground or has been mined and stockpiled. It isn't uncommon in the mining industry for companies to hold a reserve of what they've mined back in order to be able to take advantage of sudden short-term price spikes, in which case there is a future yet-to-be-determined valuation that can be placed on it. One must also consider that while they know how much of a resource they have still in the ground, it may not be feasibly minable (is that a word?) at current prices, or they're waiting for some technology that will help them recover enough from the ore to make it worth mining. There's still a valuation attached to it (usually based on something approaching current market prices), but until it's produced and deliverable, that value will always be in doubt. The copper mining industry went through that back in the 1990's, when copper prices plunged and the cost of production didn't merit trying to extract anything but the most high-grade and readily accessible ore. Some companies went bankrupt after borrowing against the value of their reserves and then prices fell off the cliff. Oil companies face this all the time. There's technically a "today value" to all of the oil in the ground that they have rights to produce, but until it is produced and sold (or a futures contract based on it is sold), the value will fluctuate all over the place.

A mining company could have a market cap beyond the value of its resource holdings because it perhaps is viewed as a strong player in its particular field with a better chance of surviving the peaks and valleys of the commodities markets and would thus be in a position to acquire assets of less-fortunate rivals at prices below market when they fail. The company may hold a near-monopoly in its field. Another reason could be that it, unlike some or any of its competitors, already has long-term delivery contracts in place with major buyers that guarantees its revenue stream well into the future, enabling it to better forecast its cash flows, borrow at better rates, and be more successful at efficiently planning and executing its operational plans.

To answer your question with another question, why would Apple's market cap be several multiples of the value of all the products it sells in any particular year? And for that matter (and as a rhetorical question), why would ANY company have a market cap that exceeds the value of the goods and services it produces, if the sum value of those goods and services should serve as the basis for that valuation?

Many times, the additional market cap reflects investor faith in the long term prospects of the company relative to other players in its sector, thus making it worth more to them than anyone else with less certain prospects.