As mines age, ore grades decline, since the best stuff is usually dug up first.
随着矿山年龄的增长，铁矿石的等级下滑，因为最好的往往被最先发掘。
As older mines get deeper, ore grades decline and extraction becomes more expensive.
随着旧矿越挖越深，矿石的品质下降使提取成本增加。
As older mines get deeper, ore grades decline and extraction becomes more expensive.
随着旧矿越挖越深，矿石的品质下降使提取成本增加。
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