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。

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Records show that during December and January, the two months with the fewest hours of daylight, attendance rates fall, average daily class participation drops, and grades decline.

记录表明在日光最少的十二月和一月，出席率下降，每日平均课程参加量减少，分数降低。

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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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The firm said it is also conducting a study to see about increasing the mill throughput by about 25% to continue with steady gold production as grades are expected to decline slightly in the future.

FORBES: Production At Agnico-Eagle's Meadowbank Mine Drives Performance

The Tory AM also highlighted the decline in the number of pupils achieving top grades in Wales.

BBC: Conservatives' debate

Several experts in the field outlined several issues that they said point to a decline in the mining industry, from poor ore grades to declines in drilling activity to misinformation on the investment side.

FORBES: PDAC: Many Factors Putting Mining Industry At Risk