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

Abstract

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.