Some genetic relations of tin deposits

Joseph Theophilus Singewald
1912 Economic Geology and The Bulletin of the Society of Economic Geologists  
Since the days of Werner, the different kinds of veins have been divided into groups according to their most prominent and characteristic features. To these groups, Werner applied the term vein formation; other authors have subsequently also used the term vein type. Even {n Werner's time, it was difficult to fit all known veins into sharply defined groups. With ,the rapid increase {n our knowledge .of ore deposits {n recent year•, as more and more new mining districts have been discovered and
more » ... en discovered and described, the dividing lines between 'these groups have become less and less sharp, and {n many cases .complete ,transition exists from one group to another. Among these groups, tin veins have stood out more distinct than probably any other type. They are almost always closely associated with granitic rocks; and are character{zeal by an abundance of fluor{he and boron, and to a less ext•nt lithium and phosphorus minerals, and the pneumatolytic metamorph{sm of the country rock. To one familiar only wi,th the more common vein types, a more different group could hardly be pictured. Yet, notwithstanding the unusual character{st{cs of this group of veins, there are many transitional types •vh{ch closely relate {t to other groups. These transitions have been brought about by minerals, which {n the type are acces. sory con-st{tuents of the vein filling, becoming the most prominent; and by the entrance into the vein filling of ne•v minerals, and the disappearance of some of the characteristic features of the type. No less distinct than the type itself, has the genesis of this group been considered. Moreover, our understanding of the genesis of this group dates back further and {s considered more certain than {n the case of any other. As early as I84I,' 'Daubr6e: ,/Inhales des Mines, 3d Ser., Vol. XX., •84•, pp. 65-•I2. 263 264 JOSEPH T. SINGEkVALD, JR. Daubrde called attention to .the fact that all tin veins are associated with granitic rocks, and that the tin and other characteristic elements of these veins were derived directly from the granitic magma. H•is synthetic experiments enabled him Vco arrive at conclusions as to the chemical processes involved. As the magma .cooled, metalliferous gases and vapors were given off; and these, reaching the already solidified peripheral portions and the adjoining country rock, deposited their metallic conteats, and at the same time, reacting chemically on those rocks, produced the profound metamorphism so constantly associ'ated with these veins. Daubrde's theory was substantiated by the observations of his contemporary Elie de Beaumont • and is today accepted as the expl'a0ation of the origin of this type of veins. So radically different was this theory from the views held as to the genesis of veins in general, that the older French school drew a sharp line between tin veins and sulphide veins, and some went so far as to divide all veins into two main groups--Filons stannif3res and Filons sulphurds dites plombif3res; the former fumerolic products of granitic magmas, the latter hydrothermal deposits. Such a genetic isolation of this group is no longer tenable. In the following paragraphs, I shall discuss briefly some transition'al occurrences of tin deposits and point out their genetic significance. ?EGMATITIC DEPOSITS. There are no magmatic segregati.ons of cassiterite of economic importance; yet cassi. terite is frequently found as a primary constituent of granite in tin districts. In pegmatite dikes, however, there are occurrences of sufficient richness to make a workable ore. Most closely related to a normal granite is the rock in which occurs the tin deposit at Etta Knob in the Black Hills of South Dakota. The acid intrusive there is in the form of a stock with a nearly circular outcrop, measuring from 3 ¸ to 60 m. in diameter. This rock is an unusually coarse grained pegmatite, x Elie de Beaumont: Bull. de la $oc. Gdol. de France, 2d Ser., Vol. IV., x847, PP. x249-I333. JOSEPH T. SINGEW.4LD, JR.
doi:10.2113/gsecongeo.7.3.263 fatcat:l2nxhyv5s5gc5c6umv54keyckm