(Adds quote on stimulus package and economic growth and context)
LIMA, June 10 (Reuters) - Peru is revising down its estimate of this year's economic expansion from 5.7 percent to an unspecified new figure because of weaker-than-expected mining activity, the finance minister said on Tuesday.
The minister, Luis Miguel Castilla, added that the government is preparing policies to help jump-start the economy, which grew 4.8 percent in the first quarter from the same period a year earlier.
Peru's economy slowed on lower prices and softer demand for its key mineral exports, to expand under 6 percent last year for the first time since 2004, with the exception of the 2009 financial crisis.
"The package of measures to reactivate the national economy is going to be discussed this week in the cabinet," said Castilla.
Castilla and the central bank president have said that economic growth in April was likely poor because of weak mining activity. The result for April will be released on Monday.
Peru posted an $861 million trade deficit in April on tumbling mineral exports, its widest negative gap on record.
"The numbers for the second quarter are not what we had anticipated because of ... a drop in mining and a major external shock" to mineral exports, Castilla said.
He said the economy is expected to grow more robustly in the second half of 2014 and that he will likely announce a revised forecast for the full year next month.
The central bank has said the economy will probably grow 5.5 percent this year.
Last year the economy grew 5.8 percent, according to the state statistics agency, Inei.
Inei initially reported a 5.2 percent expansion in 2013, but revised that upward to 5.6 percent and then to 5.8 percent after a new method for calculating gross domestic product was adopted.
The Andean country is a top exporter of copper, silver, gold and other minerals. (Reporting by Marco Aquino; Editing by James Dalgleish)