How to detect gold in loose alluvial ground has been already described. A similar method may be employed in the case of auriferous quartz after grinding it. If the concentrates obtained in either case contain sulphides, these are collected, roasted or treated by nitric acid and re-ground. The light particles of oxide of iron can now be separated from any gold that may be present by washing. “ Colour” may often be obtained thus when none could be seen after the first concentration. The washing is made easier by removing from the concentrates the magnetic oxides and iron from the grinding tools by a magnet. All the finest particles of gold are lost in the process of washing, and consequently many auriferous ores cannot be made to “ show colour.”
The following method devised by Wm. Skey, analyst to the Geological Survey of New Zealand, is said by him to give good results. The sample of ore is carefully roasted, then digested with an equal volume of an alcoholic solution of iodine for a length of time varying from twenty minutes to twelve hours, the longer time being allowed if the ore is poor. A piece of Swedish filter paper is then saturated with the clear supernatant liquid and afterwards burnt to an ash; if gold is present in the ore the ash is coloured purple, and the colouring matter can be quickly removed by bromine. This method is said to show the presence of as little as 2 dwts. gold per ton in certain ores, but is not uniformly successful. Bromine or chlorine may be substituted for iodine. A mixture of 5 to 10 parts of bromine with 100 of water may be used to 100 parts of ore. The ore must be fine enough to pass an 80-mesh sieve and should be re¬ground after roasting. After leaving the mixture to stand for some hours with occasional stirring, the liquid is filtered and the excess of bromine evaporated from the clear solution, which may then be tested by stannous chloride. Dr. Don found these methods defective, and was forced to use the ordinary crucible assay when examining material containing small amounts of gold, taking samples of 4.48 lbs. of ore. L. Wagoner determines the weight of small beads by fusing them and measuring their diameter under the microscope. The error in reading was about 0.001 mm. or 6.36 per cent, on a gold bead weighing 0.001 mg.