Mintaro Slate is used in many of Australia 39s most distinguished contemporary and historic buildings Its appeal As early as 1861 and again in 1924 Mintaro was described as superior to any slate previously known It quickly It is believed to be the oldest continuous operated quarry in South Australia possible Australia Mintaro Slate is used in many of Australia 39s most distinguished contemporary and historic buildings Its appeal As early as 1861 and again in 1924 Mintaro was described as superior to any slate previously known It quickly It is believed to be the oldest continuous operated quarry in South Australia possible Australia