Park. — Bifilribution of Ores m Horizontal Zones. 91 



At the Broken Hill mines in New South Wales the general 

 distribution of ores in vertical depth has been as follows : — 



(a.) Oxidized ores of lead and silver. 

 (b.) Galena with blende, 

 (c.) Blende with galena. 



Weed, in his paper on '' Ore-deposition and Veiji-enrich- 

 ment by A.scending Hot Waters,"* appears to support the 

 hypothesis which assumes that the distribution of ores in hori- 

 zontal zones is the result of primary concentration by ascending 

 hot solutions. 



The eruption of igneous magmas is often succeeded by in- 

 tense solfataric action, of which notable examples are found in 

 the Yellowstone Park in the United States, and in the volcanic 

 region of the North Island of New Zealand. The ascending 

 waters slowly circulating in contact with the heated rocks below 

 become superheated, and in their upward course dissolve various 

 substances, which they carry with them along the line of least 

 resistance — that is, towards the hot-spring pipe or vent. Many 

 substances insoluble in normal conditions are rendered easily 

 soluble in the presence of heat and pressure. The underground 

 water will therefore possess its greatest solvent power where 

 the greatest heat and pressure are attained, which will naturally 

 be at the greatest depth. With loss of heat and pressure the 

 less soluble substances held in solution will be precipitated — 

 that is, those substances whose dissolution was effected under 

 extreme heat and pressure. As the waters ascend they will 

 continue to lose heat and be relieved of pressure, with the result 

 that the dissolved minerals will be precipitated in the inverse 

 order of their solubility. When the hot waters reach the sur- 

 face the only substances in solution, in most cases, will be the 

 extremely soluble alkaline sulphates, carbonates, and silicates. 

 An obvious result of this process of vein-filling will be an im- 

 poverishment of the veins at great depths, due to the migration 

 of the valuable minerals from below to the zones of precipitation 

 above. It is a notorious fact that hot springs seldom deposit 

 metallic sulphides at the surface. The great mushroom-capped 

 veins of the Hauraki region and Great Barrier Island, in New 

 Zealand, are composed of siliceous sinters, chalcedonic and 

 crystalline quartz, manifestly the result of long-continued 

 solfataric activity. The overhanging mushrooms of Cjuartz 

 are almost devoid of gold and metallic sulphides : but the 

 necks in all cases contain gold, and sulphides of silver and iron. 



* W. H. Weed, Aiuer. Inst. Mining Eng., Vol. xxxiii, l'.t(_l3. 



