GREENWOOD VILLAGE, Colo. (AP) â¿¿ Shares of Molycorp Inc. tumbled to a new all-time low Thursday after the mining company said it needed more time to file its annual report to account for a hefty impairment charge. The Greenwood Village, Colo.-based company said it estimates that the noncash goodwill impairment charge will be "substantial." The charge will be recorded for the fourth quarter and will be reflected in the company's full-year 2012 results. Molycorp was supposed to file its annual report no later than March 1. It now expects to file the report by March 15. The delay is another blow for the company, which said in January that its revenue and cash flow would be significantly lower than expected in the first half of 2013 and that it would sell a combination of common stock and notes to cover an expected cash shortfall this year of about $250 million. The company has long been working to expand its Mountain Pass open-pit mine in San Bernardino County, Calif., which became operational earlier in January. That process has been more expensive and taken longer than it expected, crimping production volumes. Meanwhile, prices for the rare earth minerals it produces have dropped significantly, hurting sales. Rare earth minerals are used in consumer electronics and other products. Molycorp shares fell 36 cents, or 6 percent, to $5.88 in morning trading, after tumbling to $5.60 shortly after the market opened and passing their previous all-time low of $5.75. Over the past year, Molycorp shares have steadily fallen, losing about 75 percent of the value.