Stocks of major Australian mining firms plummeted to a record low since the global financial crisis, fueling speculations on the longevity of the resources boom.

Shares of BHP Billiton (ASX: BHP) declined 2 per cent on Wednesday to $30.18, which was its lowest close since March 2009. The fall in share prices occurred despite a report on the same day that BHP logged a 20 per cent boost in its iron ore production in the year to June to 179 million tonnes.

Rio Tinto (ASX: RIO) shares also declined 3 per cent, Fortescue Metals (ASX: FMG) stocks dipped 3.5 per cent and Newcrest (ASX: NCM) dropped 4 per cent.

Market observers attributed the decline in mining share prices to the assessment by the U.S. Federal Reserve of a weak outlook for the United States economy and its domino effect on China and Australia.

Another area of concern for the mining sector is the continuous weak commodity prices which are 30 per cent lower compared to 2011 prices. Other issues cited by the analysts to explain the dropping share prices are the rising costs of miners, volatility in Europe, uncertainty over China and the U.S. growth prospects and the strong Canadian currency.

Rio Tinto Chief Executive Officer Tom Albanese said the firm is keeping a close eye on the pace of the American economy's recovery, the ongoing eurozone crisis and the impact of efforts to stimulate the Chinese economy, which is a major market for Rio.

Rio reported a production of 133,500 tonnes of copper, up 5 per cent for a year ago. It is 45 per cent lower due to lower grades at Rio's Kennecott Utah Copper Unit. However, Rio reaffirmed it would produce 580,000 tonnes of copper for 2012.

Rio also suffered a 12 per cent decline in its aluminium output to 841,000 tonnes due to the labour problem at its Alma unit but reported a 13 per cent increase in hard coking coal production to 2 million tonnes due to gains in output at its Kestrel mine.

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