Deuteric alteration of some aplite-pegmatities of the Boulder Batholith, Montana, and its possible significance to ore deposition

Abstract

The Boulder batholith contains many texturally complex leucogranitic aplite-pegmatite intrusives. Both the aplite-pegmatites and the enclosing quartz monzonites were hydrothermally altered along steep zones of closely spaced fractures that are in part the hosts for vein deposits. Mineralogically and chemically similar alterations of different origin are common in some aplite-pegmatite intrusives near Boulder, Mont. These internal alterations are small, irregular, and vaguely outlined and are commonly related spatially to pegmatitic textures. The altered rocks were formed during crystallization of the aplite-pegmatites and are thus deuteric in nature. These deuteric alterations and other mineralogic changes related to textural variations imply the physical-chemical conditions of crystallization in the batholith. Because hydrothermal alterations in the quartz monzonite are closely similar to the deuteric ones, the deuteric alterations have some significance to the origin of ore deposits.The geometrically irregular textural variants and deuteric alterations in the aplite-pegmatites may be the result of increased vapor pressure and of concentration of mineralizers in local, hermetically sealed volumes of rock. This increase led to coarsened grain, recrystallization, to the occurrence of tourmaline and sulfides, and/or to major stability adjustments of mineral composition and crystal structure. Mineral assemblage is systemmatically related to textural type as shown by the progressive decrease of plagioclase and coarsening of grain until only separate bodies of quartz and of K-feldspar rock remain. Chemically deuteric alteration resulted in similar differentiation.The differentiation pattern of the aplite-pegmatites closely resembles Neuman9s hypothetical process of hydrothermal differentiation, leading to aplites and pegmatites and also to an aqueous product, possibly rich in ore metals. The similarity of deuteric alteration of aplite-pegmatites to hydrothermal alteration of quartz monzonite and aplite-pegmatites clearly indicates that residual magmatic fluids alone may cause all the observed features of hydrothermal alteration in the quartz monzonites. Theoretically, the migration of residual fluids into volumes of lessened pressure resulting from thermal contraction may cause the observed hydrothermal alterations of the batholith and may also transport ore-metals. These fluids also may gain additional metals released by alteration of the original rock minerals. If hydrothermal alteration releases ore metals from rock minerals, several hundred thousand tons of metal could have been made available to ore solutions from an estimated 5 cubic miles of altered batholithic rock.These three processes of ore deposition--hydrothermal differentiation, migration of fluids, and alteration release of metals--may be amenable to geochemical-geological testing. Systematic analyses of rocks of varying chemistry and structural positions are proposed as a basis for such tests.