Art. III.—Contact Metamorphism in its Relation to the Genesis of certain Ore-deposits.
[Read before the Otago Institute, 13th September, 1904.]
A Molten magma tends to effect changes in the rocks with which it comes in contact. In the case of overflow magmas the thermal changes are generally trifling, and in many cases hardly appreciable. Even magmas that have cooled in rents in sedimentaries at shallow depths have not always caused great changes in the enclosing rock.
The greatest alteration will naturally take place in the case of magmas that do not reach the surface, but cool slowly under great pressure. The greater the mass of the intrusive magma, the slower will be the rate of cooling; and the slower the rate of cooling, the longer will the adjacent rocks be heated. The rate of cooling will be mainly dependent upon the mass of the intrusion, the distance from the surface, and the relative thermal conductivity of the adjacent rocks.
The changes effected in the country rock by the intrusion of an igneous magma will be mechanical and hydrothermal. The intruded sedimentaries will be compressed, bent, and more or less shattered and fissured along the line of intrusion. The magma will part with its heat by slow radiation into the adjacent rocks. The magmatic steam and gases, together with the gases generated in the sedimentaries,* will pass into and permeate the latter, and cause a molecular rearrangement of the constituent minerals, resulting in what is termed contact metamorphism. As the igneous magma and the heated sedimentaries cool they will contract in area, and when the temperature normal to the depth has been reached
[Footnote] * Professor Joseph Barrell has shown that the heat of an igneous mass acting upon sedimentaries librates enormous volumes of steam and gases, attended by a shrinkage of volume of the rocks and the formation of vein fissures: “The Physical Effects of Contact Metamorphism,” Am. Jour. Sci., vol. xiii, April, 1902, p. 279.
the contraction will tend to cause the two rocks to shrink from each other, resulting in the formation of cavities along the line of contact.
Above a temperature of 365° C. and a pressure of 200 atmospheres, water and all more or less volatile compounds will exist as gas. Aqueous vapours above the critical temperature and under great pressure will act as strongly upon the cooling magma as upon the adjacent rocks. They will possess a solvent power which will be greatest at the depth where the highest temperature and pressure are reached. The pressure will cause the heated steam and gaseous emanations carrying the heavy metals to permeate the bedding-planes of the sedimentaries, and fill all accessible cracks and fissures. In this way bed-impregnation may be effected, and even ore-bodies formed at points some distances from the genetic eruptive magma. A decrease in the temperature and pressure will cause the least soluble substances to be deposited; and as the temperature and pressure continue to diminish, the dissolved substances will be thrown out of solution in the inverse order of their solubility. It is manifest that the later phases of the eruptive after-actions will represent in a modified form the waning effects of solfataric action. The deep-seated conditions will also favour the action of metasomatic processes in the zone of metamorphism, and veins will be formed, some of which may rise to the surface. It is probable that the circulation of the heated mineralised solutions in the later phases will tend to effect a redistribution of the ores and minerals deposited in the earlier stages. In some cases the ascending waters and gases may reach the zone of surface circulation and mix with the meteoric waters, which will then reappear as hot springs, forming ore-bodies and veins not directly in contact with the eruptive magma.
Weed and some other writers have made an attempt to subdivide contact-metamorphic deposits into groups depending mainly upon the mode of occurrence. But the form and mode of distribution may be due to accidents of density or porosity, composition and hydrous condition of the rocks affected, rather than differences in genetic formation. Moreover, the mass of the magma, the weight of superincumbent rocks, the amount of heat and subsequent contraction, and phase of the after-action are all doubtless contributing factors in connection with the form and distribution of the heavy metals. Masses of ore occurring as contact deposits, fissure-veins, and bed-impregnations in the zone of metamorphism may all be traced to the same genetic causes.
Professor L. de Launay, of Paris, supports the views of the school of De Beaumont and Daubrée in respect to the primary influence of volatile mineralisers emanating from eruptive
magmas. The emanations, he contends, must have prepared the way by introducing into the enclosing rocks, or simply by depositing in the vein fissures, elements such as sulphides, fluorides, chlorides, &c., which, subsequently dissolved anew by the circulation of superficial waters, have rendered the latter essential aid in the processes of alteration.*
The extent of contact metamorphism effected by the granite intrusions of Albany, in New Hampshire, was fully investigated by Hawes.† His analyses showed a progressive series of changes in the schists as they approached the granite. The rocks are dehydrated, boric and silicic acids have been added to them, and there appears to have been an infusion of alkali on the line of contact. He regarded the schists as having been impregnated by hot vapours and solutions emanating from the granite.
Contact deposits frequently lie at the boundary between the eruptive and the country rock; also at variable distances from the eruptive, but never outside the zone of metamorphism. More particularly, contact ores occur in limestones, marly and clay slates, and are accompanied by the usual contact minerals, garnet, vesuvianite, scapolite, wollastonite, augite, mica, hornblende, &c., and in clay-slate by chiastolite, &c. contact ores are principally magnetite and specular iron, but sulphides of copper, lead, and zinc often occur. Pyritic contact deposits are typically represented by those of Vegsnas, in Norway; Rio Tinto, Tharsis, and San domingo, in Spain.
The pyritic ore-mass in Mount Lyell Mine, in Tasmania, is generally described as a contact deposit, although its geologic occurrence does not strictly conform to the common definition of such a body. It is a boat-shaped body lying between talcose schists and conglomerates.‡ The mine-workings have shown that it gradually tapers downwards from the outcrop, being cut off below by a great thrust-plane. There are no eruptives in actual contact with the ore-body, but dykes of diabase and other igneous rocks occur in the district at no great distance. The existence of these dykes and of bands of schist impregnated with sulphides forming fahlkands would lead to the belief that there at one time existed channels of communication leading from the eruptive rocks to the vein cavities. It seems probable that the ore-bodies in the Mount Lyell field were formed in the later or solfataric stages of eruptive after actions.
[Footnote] * L. de Launay, “The Genesis of Ore-deposits,” 1901, Discussion, p. 616.
[Footnote] † G. W. Hawes, Amer. Jour. Sci., vol, xxi, 1881, p. 21.
[Footnote] ‡ Prof. J. W. Gregory, “The Mount Lyell Mining Field,” Trans. Aust. Inst. Min. Eng., vol. i, part iv, July 1904, p. 281.
Among ore-deposits genetically connected with eruptive after-actions Vogt* includes cassiterite and apatite veins and “ore-deposits of contact-metamorphic zone.” Cassiterite deposits are everywhere connected with and eruptives, principally granite, and occasionally quartz-porphyry and rhyolite. Partly for this reason, and partly because of the characteristic paragenesis of fluoride, borate, and phosphate minerals, he supports the common view that tin-deposits are genetically connected with granitic eruptions, and that various volatile fluorides took part in their formation. Cassiterite veins were formed, he thinks, by pneumatolytic processes†—that is, by the action of gases and water at high temperature and pressure. He further urges that they were formed immediately after the eruption, and before the complete cooling of the granite, one proof of which is the occurrence of tin-vein minerals in veins of pegmatite in the granite.
Cassiterite veins are admittedly independent of the immediately adjacent country rock, and for this reason seem to be more nearly related to deposits of magmatic segregation than to contact-metamorphic deposits.
It is probable that the magmatic segregation of chromite in peridotite was in some cases effected by pneumatolytic agencies before the complete cooling of the magma. It is not uncommon to find chromite in vein-like masses that have the appearance of having been segregated in cavities of contraction in the pasty magma. As the agency of underground water cannot have been active in this class of ore-deposit, the aggregation must have been effected by metal-bearing steam and gases occluded in the igneous magma.
Pegmatite veins, while genetically connected with granitic eruptions, seem to be of later formation than the cassiterite veins. They often pass into quartz, and frequently possess sharp well-defined walls, which suggest their formation in shrinkage-cracks by pneumato-hydatogenetic agencies in the waning phases of the after-actions developed by the progressive cooling of the eruptive magma. The different phases of after-action must necessarily merge into each other, and hence we may expect to find, as we do, tin-vein minerals and even cassiterite in veins of pegmatite.
Among ore-deposits of contact-metamorphic origin Vogt includes the ore-bodies which occur within the metamorphosed contact zone of deep eruptives, especially granite. He distinguishes several types of contact deposit. The Chris-
[Footnote] * J. H. L. Vogt, “The Genesis of Ore-deposits,” New York, 1901, p. 636.
[Footnote] † “Pneumatolyis” is a term first used by Bunsen to desoribe the combined action of gases and water.
tiania type includes iron-ore deposits that appear to have been formed before the solidification of the granitic magma. These ores are never found in the granite, but always in the adjcent rocks. If they had been introduced after the cooling of thé magma they would also have been deposited in the granite. The eruptive magma is believed to be the source of the metal, which is expelled in the heated steam into the surrounding rocks.
The synthetic experiments of Daubrée seem to justify the views of Vogt, Beck, and other observers that cassiterite and pegmatite veins are formed by gaseous and aqueous emanations, and not by direct segregation.