Cryptic variation and the petrology of the Sudbury nickel irruptive

Abstract

The Sudbury Irruptive outcrops as an oval ring and has been emplaced at the base of the Whitewater group of sedimentary rocks. The Irruptive rocks of the north and east ranges of the ring resemble one another and are distinctly different to those of the south range. On the north and east ranges, 1,000 from 1,500 feet of "felsic noritc, " a plagioclase-hypersthene-orthocumulate, consisting of about 50% plagioclase, 15 % hypersthene, 7 % augitc and 25 % of a micrographic intergrowth of quartz, plagioclase and K-feldspar are overlain by about 700 feet of "oxide-rich gabbro," a plagioclase-augite-ulvospinel orthocumulate, consisting, on average, of 47 % plagio-clase, 22 % augitc, 8 % opaque oxide and 23 % micrographic intergrowth. The propor-tiqn of intergrowth increases rapidly to 60 percent over the upper third of the oxide-rich gabbro as the gabbro grades into the overlying "micropegmatite. " The "micropegma-tite " is a uniform rock, about 4,500 feet thick, with an average modal composition of 70 % micrographic intergrowth, 20 % plagioclase and 10 % augitc, biotite, hornblende, and secondary minerals.