Abstract:
In section A a short description is given of the minerals encountered in 129 polished section3 of conglomerate from the Dominion Reefs Mine. These include: (a) The radio-active ninerals: Thorian uraninite, thorite, monazite, betafite, euxenite, lcucozene and zircon; (b) The oxide minerals: Cassi terite, chromite, columbite and ilmenite; (c) The sulphides: Pyrite, marcasite, balena, chalcopyrite, covellite, sphalerite, cobaltite, linnaeite and arsenopyrite; (d) Gold and some silicate minerals. In section B a study is made of the literature on various methods used for roundness, form and size studies, and a method is proposed which is suitable for a quantitative study of these properties in polished sections. The results obtained when this method was applied to samples from the Dominion Reefs Mine indicate that the roundness of uraninite the is in accordance with that of monazite and zircon. Uraninite in this conglomerate can therefore best be regarded as a detrital mineral concentrated together with other heavy minerals by sedimentary processes. However, some detrital minerals, such as cassiterite and garnet 5 have such small mean rounder values that they could not have shared the sedimentary history of the other detrital minerals. They were probably introduced into the sediments using another cycle of sedimentation. The same conclusions a1one borne out by the size distribution data. of hydrothermal activity and an increase of temperature caused sulphidization and of some minerals, the limited redistribution of gold, and corrosion and alteration of many detrital mineral grains.
