Thermal Action in Relation to Vein-Formation.
The occurrence of metallic sulphides in the sinters at Sulphur Bank, Steamboat Springs, and Ohaeawai hot springs; the mushroom-capped lodes at Waihi and Great Barrier Island; and the tree-stems replaced by sulphides found in veins at great depths below the present surface, afford conclusive evidence of the filling of veins by hot ascending waters and gases in areas occupied by later eruptive rocks. It is a notorious circumstance that ore-deposits are most numerous in the neighbourhood of extended zones of eruptive rocks, as in Hungary, Transylvania, Nevada, Colorado, and New Zealand, where the vein-bearing rocks are principally andesite, phonolite, and trachyte. In other rocks veins are fewer and more scattered.
For veins in these altered later eruptives Lindgren suggests the name “propylite veins,” but it is doubtful whether the genetic difference between propylite veins and true fissureveins is sufficiently marked to justify the distĩnction. Moreover, the roots of propylite veins will be difficult to distinguish from fissure-veins connected with a plutonic intrusion.
Professor Suess,* speaking of the importance of the rôle played by the waning phases of volcanic phenomena in the formation of mineral veins, says, “Hot springs may be taken as the latest phase of a whole series which led up to the present deposits of ore.”
In Nevada the sulphur-bearing rock occurs in beds lying between limestone and magnesian rocks. In Utah the sulphur occurs associated with gypsum near an old crater.
At Tikitere, in New Zealand, there are extensive deposits of sulphur in an old crater. A large proportion of the sulphur is the black amorphous variety. The heat of the fumaroles and hot springs is too great to permit the excavation of the sulphur to a greater depth than 6 ft. or 8 ft.
[Footnote] * Professor Edward Suess, Lectures, Royal Geographical Journal vol. xx, Nov. 1902, p. 520.
At White Island, in the Bay of Plenty, the deposits of sulphur occur in and around the crater-lake, mixed with gypsum. The crater-water is hot, and highly charged with free hydrochloric and sulphuric acids. The gypsum is deposited in crystalline incrustations on the sides and floor of the crater-lake. The source of the lime has not yet been determined; but the supply must be constant, as gypsum is being deposited continuously. The sulphur is deposited in the water from gas-springs which are seen bubbling everywhere in the floor of the lake; and also from fumaroles around the margin of the crater.