RIO DE JANEIRO, Oct 28 (Reuters) - Brazilian miner Vale SA (VALE3.SA) reported on Thursday a third-quarter net profit of $3.9 billion, significantly below analysts" forecasts of $6.2 billion, due to lower iron ore prices and an impairment at its coal business. While Vale"s net income fell by almost half from the previous quarter, it surged 34% when compared with a year-ago period. Vale, one of the world"s biggest iron ore miners, said its quarterly numbers were dented by iron ore prices that plunged 31% in the reported period, as well as a labor disruption at its Sudbury nickel mine in Canada that led to a hit in production. The miner said it had an impairment charge in its coal business that totaled almost $2 billion, but did not give further details. In the previous quarter, the company had disclosed a writedown on its coal assets resulting from a "lower long-term price assumption for metallurgical and thermal coal." Vale, one of the largest companies in Brazil, said on Thursday it would buy back 200 million shares, or about 4.1% of the firm"s outstanding shares. "The new program reflects the confidence of the company"s management in Vale"s potential to create and share value in a consistent way." The company reported adjusted earnings before interest, taxes, depreciation, and amortization (EBITDA) of $6.9 billion.