Diffusion in vein genesis at Cobalt [Ontario]
Economic Geology and The Bulletin of the Society of Economic Geologists
It is no less a convenience in geological as well as other kinds of work, to have standard tools and materials. It is therefore a great temptation for geologists to accept certain standard theories as legitimate working tools, which they need no longer hold in question, but may henceforth take for granted. And so it has come to pass that process,es of ore-formation have been standardized and listed, .to the exclusion of what may yet be recognized as one of the most potent genetic processes. In
... etic processes. In the silver deposits at Cobalt, the writer ,believes he has found an example of the operation .of this unrecognized proces_s, worthy of being described as a type case. Long ago the idea of "lateral secretion' as advocated by F. Sand•berger was condemned in geologic opinion. The verdict was probably just ir• respect to the letter, but unjust in respect to the nucleal idea of his conception. In Geologische Diffusionen, 2 R. E. Liesegang has indicated the ground upon which the decision against lateral secretion will probably be reversed. Posepny a quotes Sand'berger as saying: The theory of lateral secretion was conceived in this sense only, that the material for the filling of veins is derived from the country rock through gradual leaching by seepage-water, which brings the dissolved substances from both sides of the fissure, where it is then converted by chemical decompositions into insoluble gangue minerals and ores, and so deposited. The error in this theory lies in the assignmen•t of the task of transportation t.o circulating water; and the reversal of the decision against it will be based upon a recognition of the potency • Companion paper to that by W. L. Whitehead in this number. 2 Reviewed by Adolph Knopf in Ecoa. GEOL., Vol. 8, No. 8, Dec., I9T3. a,' The Genesis of Ore Deposits," Ed. 2, •9o2, p. 57. DIFFUSION IN VEIN-GENESIS AT COBALT. •37 of diffusion to perform tha't task without the aid of movements of water. In all likelihood, it will yet be adequately demonstrated that much more often than is now supposed, ore deposits originate through the migration of ore materials from country rock in or adjacent to which the deposits lie. The efficacy of diffusion to produce many geological phenomena has already been recognized. It has been assigned an important r61e in metalnorphism, in banded wea. thering, and in certain ore-form!ng processes; but either by silence or direct imputation it has been denied recognition in its true r61e as a prime agent in the formation of all metasomatic deposits. 'Before presenting the small array of evid.ence which has a direct bearing upon the problem, it will, perhaps, be best briefly to set forth the .main thesis, which is as follows: •. The cobalt-nickel-silver ores of the Cobalt District were derived by diffusion and metasomatic fixation from the sill of Nipissing dia•ase with which they are associated. 2. They were deposited essentially in their present form below the groundwater level, no't by circulating solutions, but by migrant ions traversing virtually stagnant or slowly circulating aqueous solutions, from all parts of the adjacent dia'base, to such favorable seats of deposition as were accessible in the neighborhood of its margins. 3-Mineral-laden water filled all the pores, joints, and fractures o.f the diabase and its neighboring rocks; and through it moved the ions of vein lnaterial, at a temperature considerably below the melting point of the dia•base, and at a pressure of several thousand feet of hydrostatic head. These ions were not capable of combination and precipitation without the help of an external agency; this consisted in catalytic influences or obscure metasomatic reactions of a kind which could take place only in rock relartively free from stress in one direction, as that must have been, which lined the sides of vertical tension joints lying parallel with the chief lateral stresses of the locality. •The effect of these reactions of replacement, although virtually grain-for-grain as is understood by the term metasomatic, was to send into solution I38 ALFRED R. WHITMAN. the aluminous and ferrous. silicates composing the wall rocks, and to fill the vacated space grain-f. or-grain as it developed, with the ore minerals. The first rock to be thus replaced was that under least stres's on the immediate walls of the indicated joints.