Art. V.—On the Rôle of Metasomatism in the Formation of certain Ore-deposits.
[Read before the Otago Institute, 13th September, 1904.]
Until lately it was the common belief that ore-deposits merely filled pre-existing fissures and cavities in the country rock. In recent years, writers on ore-formation have become convinced, as the result of microscopic examination, that many ore-bodies were merely metasomatic replacements of country rock that followed certain well-defined crush-zones of zones of metamorphism. According to this, it is surmised that in many cases no previous cavities existed, but that the waters altered and removed certain tracks or zones of rock which they partially or completely replaced with orematter and gangue.
This process of replacement is known to petrologists to have taken place among the constituents of many rock-masses, no matter how dense, including all metamorphic rocks, and all older igneous and eruptive masses. It is known as “metasomatism” (meaning, change of body), and is due to internal chemical reactions which seem to take place as readily in rocks as do the equally obscure metabolic changes in living organisms.
In many cases minerals are replaced molecule by molecule, giving rise to what is termed “mineral pseudomorphism.” But in the processes which affect changes in rock-masses, reactions may be set up between the different constituent minerals, thereby forming new minerals capable of segregating themselves into large masses; or the rock may be altered, and some or all of the constituents removed and replaced by new
substances. Thus, while pseudomorphism and metasomatism are closely related processes, it is found that they differ widely in the scope of their operation. Gneiss and mica-schist are familiar examples of the work of segregation and molecular rearrangement of the dominant constituents of sedimehtary rocks.
The internal changes that affect eruptives are known to every petrologist. Besides these changes, which are chiefly molecular, rock-masses, and especially eruptive rocks, may be so altered by the action of circulating waters as to bear no resemblance to the original rock. Thus, in many cases andesites have been changed to propylite by the removal of certain essential constituents and the substitution of others.
Metasomatic replacement, as defined by Van Hise* and Emmons,† does not necessarily imply a mere substitution of matter, molecule for molecule, as happens in the process of pseudomorphism, which involves the preservation of the original form of the substance replaced, but an interchange of substance, the dissolved rock being replaced by grains or crystalline aggregates of one or more minerals. That substitution did, however, take place in some kinds of deposits is well known. In the tin impregnations found in granite in New South Wales, pseudomorphs of tin in the form of orthoclase are not uncommon; and many other examples could be quoted having reference principally to the replacement of isolated crystals in crystalline and eruptive rocks.
Slow replacement of substance by a progressive movement of the solutions in a definite direction must be assumed to have taken place in the formation of ore-deposits composed of massive aggregates of ore and quartzose matrix. In most cases the direction of movement would be determined by a rockfracture, fault-line, or crush-zone. In the case of deposits formed by deep-circulating solutions it is manifest that circulation could not be rapid, as the face or breast where metasomatic processes were active would form a blind end or cul-de-sac. Whatever circulation existed would be mainly due to convection currents, which in deep-seated cavities would of necessity be feeble.
This raises the question as to the transference and supply of dissolved matter to the continually advancing faces of metasomatic action.
The energy which caused, or, at any rate, accelerated, this transference was probably osmotic pressure, which is a force
[Footnote] * Van Hise, Sixteenth Annual Report U.S. Geol. Surv., part i, p.689.
[Footnote] † S. F. Emmons, U.S. Geol. Surv. Monograph xii, p.565.
of great intensity. It has been proved that when a portion of dissolved substance is deposited from a solution at any point the osmotic balance is disturbed, and immediately more dissolved matter travels to that point, in accordance with the well-established laws of osmotic diffusion, thereby providing new matter to augment the growing mass of ore. Osmotic pressure is the chemical principle which compels solutions to maintain an equal state of concentration throughout their whole mass; and since it is always called into being when precipitation commences, its operation as an agency in veinfilling must not be overlooked.
Metasomatism is a process of lode-formation, and does not concern itself with the source or origin of the dissolved matter contained in the solutions. It is almost certain that metasomatic processes, to a greater or less degree, were active agencies in the formation and filling of the majority of pyritic ore-bodies.
Veins in which the mineral contents are arranged in symmetrical bands or crustifications can only be satisfactorily explained by supposing that the vein-matter was deposited in open channels, beginning with a crust on each wall, followed by subsequent crusts until the channel became closed or the solution exhausted. It is not assumed that the vein fissure remained open its full width during the whole period of deposition of the vein-matter. It is more reasonable to suppose that the fissure gradually opened as the process of deposition proceeded, the newly formed matter affording the necessary support to the walls. The forces which initiated the fracture, if still in existence, would doubtless tend to reopen and widen the fissure from time to time.
Waldemar Lindgren's classic paper on “Metasomatic Processes in Fissure-veins”* represents a great advance in the scientific investigation of vein-formation. The author has followed Stelzner's methods of microscopic chemical research with conspicuous success, in a field hitherto much neglected. His work further shows that a clear understanding of the genesis of a vein can only be obtained by a minute study of the rocks contiguous to the ore-body. The metasomatism he describes is clearly not correlative with the metasomatic replacement defined by Emmons, but merely mineral pseudomorphism on a large scale. He defines his standpoint by repeating and adopting Becker's statement† that “the theory of the substitution of ore for rock is to be accepted only when there is definite evidence of pseudomorphic molecular replacement.” He mentions that quartz is found replacing
[Footnote] * Lindgren, Trans. Amer. Inst. Min. Eng., vol. xxx, 1900, p. 578.
[Footnote] † Becker, Discussion, “Genesis of Ore-deposits,” 1901, p. 204.
calcite or even orthoclase, and that rutile and anatase are common as secondary products after ilmenite, titanite, titaniferous magnetite, biotite, &c. Substitution of this kind is pseudomorphic rather than metasomatic. Upon these and other mineralogical replacements which he enumerates he implies that the formation of vein-filling was the result of replacement molecule by molecule.
Lindgren thinks this genetic theory may be fully sufficient for many veins, but admits that for many others, perhaps the majority of fissure-veins, there seems to be something lacking.
Vogt* classifies the metasomatic alterations caused by the circulation of ore-solutions as follows:—
Alterations forming greisen, cassiterite rock, &c.
Carbonatisation (with dolomitisation).
Intense contact metamorphism.
The tin-bearing rocks at Mount Bischoff, in Tasmania, are eurite and felsite wholly or partly replaced by massive topaz. To the list of metasomatic alteration of rock-masses must therefore be added topazisation.
Vogt agrees with Emmons, Becker, Lindgren, and others that metasomatic replacement plays an important part in the formation of mineral veins and ore-bodies.
[Footnote] * Prof. Vogt, “Problems in the Geology of Ore-deposits,” loc. cit, p. 660.