The city has been offered a $750,000 donation from Partners HealthCare Systems Inc., a Boston-based nonprofit group that has plans to expand into Marlborough, toward the renovation or replacement of its senior center. Mayor Arthur Vigeant is seeking to build a new 22,000-square-foot senior center in Ward Park, at a cost estimated at $6 million to $7 million, although some neighbors have voiced objections to the plan. The City Council on Monday referred the Partners Healthcare offer to its Finance Committee for consideration. As a tax-exempt operation, Partners will not have to pay property taxes on a facility it is building on a 22-acre site on Forest Street. Mayoral aide Michael Berry said the company is offering the grant to be a “good neighbor” and provide a tangible benefit to the city.