Great Northern Minerals (ASX:GNM) started a three-week fieldwork program at its Catalyst Ridge project in California, according to a Thursday filing with the Australian bourse.

Exploration will focus on sediment stream and rock chip sampling, accompanied by reconnaissance mapping, the filing said.

The collected samples are to be dispatched to American Analytical Services in Idaho, with assay results expected within a two-week timeframe following submission, according to the filing.

Shares fell nearly 5% in midday trade Thursday.