Pegmatite Zones Research Articles

Crystallization Sequence of Minerals Leading to Formation of Ore Deposits in Quartz Monzonitic Rocks in the Northwestern Part of the Boulder Batholith, Montana

The crystallization succession of minerals in quartz monzonitic rocks in the northwestern part of the Boulder batholith from the magmatic to deuteric to hydrothermal stages is described. Ore minerals began crystallizing in stable-feldspar or deuteric-stage assemblages irrespective of structures and in hydrothermal veins with associated sericitization and argillization of the wall rocks. Tourmaline, which began crystallization in the deuteric stage and continued crystallizing into the hydrothermal stage, provides an important link in demonstrating the continuity of crystallization of the ore minerals from the deuteric into the hydrothermal stage. Pyrite, tourmaline, molybdenite, and rare chalcopyrite occur in stable-feldspar or deuteric-stage associations in pegmatitic zones in aplite, in locally derived segregation pods, and in primary joints which postdate aplite. The same minerals with quartz occur in veins where the feldspars are altered to sericite and clay minerals. Sphalerite, galena, silver, and gold crystallized at relatively cooler temperatures and somewhat farther from the point of derivation; this accounts for the localization of base metals in open spaces in veins near the top of the batholith and in replacement bodies in limestone adjacent to the batholith. The writer proposes that the sulfur, iron, copper, zinc, and lead were derived from magma in the normal course of magmatic crystallization.

Basic Dike Injections in Magmatic Vein Sequences

Zonal Theory of Ore Deposition In 1907 2 I presented the “Zonal Theory” of ore deposition in a paper, the conclusion of which was as follows: “Summing up, this theory proposes that metalliferous fluids, from which most ore deposits are precipitated, are extreme differentiation phases of rock magmas; that most ore deposits and ‘mineral veins,’ as a class, represent one or the other of the extreme products of magmatic differentiation; and that the most striking chemical differences between ore deposits is due (in the more important class representing the siliceous extreme) to successive precipitation in theoretically vertical zones, as the fluid migrates toward the surface, and with diminishing heat achieves more and more mature crystallization.” I defined the principal zones as: 1, the pegmatite zone, containing tin, tungsten, molybdenum, etcetera; 2, the gold quartz-auriferous pyrite zone; 3, the copper zone; 4, the lead-zinc zone; 5, the silver zone with much gold; . . .