A fire which ripped though the top of the Gothic bell tower of this city's cathedral caused extensive damage but also revealed an important artistic discovery, the nature of the spire's original roof, officials said here. The fire broke out early Thursday morning hours when the 13th century tower was struck by lightning during a violent storm.

The lightning bolt apparently hit and travelled down the metal cross held by a statue on top of the tower, the 'Angiol d'Or' (Golden Angel), the thunder from which was heard throughout this northern Italian city. The force of the lightning bolt also stopped the tower's clock, at 2:29am, something which last occurred when the city was rocked by a powerful earthquake in 1971. Firefighters were not able to fully extinguish the blaze until late morning, also because they had to limit the amount of water they used to avoid causing further damage to the tower's upper structure and artwork inside. The fire destroyed much of the spire's copper roof and to everyone's surprise revealed that the cone-shaped pinnacle was originally covered with clay tiles.

''Last night's events brought to light something which we were all unaware of. Now what we need to do is go through the historic documents we have for more details,'' the city's councillor for art, Luciano Serchia, said. Underneath the copper covering, he added, were ''clay tiles of two different colors shaped like petals''. Repair of the spire will involve ''not only reinforcing the structure's support beams but also restoring its original roof so the people of this city can again appreciate the tower in its 13th century glory,'' Serchia said. City officials have already announced that they will pick up the tab for the tower's full restoration.

''We are ready to pay for all operations needed to repair and restore this structure, which we hope will not only make it more structurally sound but also improve it aesthetically,'' Parma Mayor Pietro Vignali said. Experts must now ascertain the full extent of the structural damage to the spire and in the meantime put up scaffolding to support the most damaged areas.

Church officials said the fire and restoration work will not interfere with the cathedral's normal functions because the tower is attached to the side of the church. The 'Angiol d'Or' on top of the tower is an early 20th century copy of the original statue which is currently housed in the diocese museum.