。
Bronze implements are made mainly from an alloy of copper and tin.
Literal
Bronze-ware [topic-は] mainly copper [and-と] tin [of-の] alloy [with-で] is-made.
Textbook-style factual sentence. 青銅器 specifically refers to bronze implements of archaeological or historical interest (the 青銅器時代 = Bronze Age). 主として is a formal adverbial phrase for 'mainly, chiefly,' more written-register than 主に. で出来ている marks the ongoing state 'is made of,' where で is the instrumental particle of material and 出来ている is the resulting-state use of ~ている.