A considerable group of minerals, including the clay minerals, zeolites, pyrophyllite, chlorite, alunite, vermiculite, certain carbonates, sulfates, and other minerals under special circumstances, may originate through the interaction of thermal fluids and rock masses. The fluids consisting of heated waters, water vapor under pressure, acids, and frequently metallic ions originate well within the earth’s crust as an aftermath of vulcanism. The reaction halos, aureoles, disseminations, and bleached zones in which the products of hydrothermal alteration are found represent the locale of reaction. In the interpretation of the origin of mineral deposits alteration areas are frequently as significant as associated intrusives.
Alteration minerals form a group with unusual stability which exhibits a remarkable tendency toward convergence.
The effects of weathering and the minerals produced are often similar to the products of hydrothermal alteration insofar as the hydrous silicates are concerned. Temperature and chemical environment are equally important as controlling factors in supergene processes as under hypogene conditions.
The alteration effects may represent a single progressive epoch in a single rock type or may be due to different epochs embracing several types of wall rock. Alteration of younger igneous rocks may have a high argillic content, as in the Tertiary sequences of the western United States. On the other hand, alteration of old rock masses where regional metamorphism has been prevalent may yield a large content of chlorite of different types, hydromica, pyrophyllite, or serpentine. Both are more or less widespread hydrothermal effects in contrast to more local alteration such as zeolitization.
The fluids responsible for aggregates of hydrothermal minerals have long since vanished. However, through a gradual accumulation of evidence on mineral synthesis, microscopic study of sequence, observations on thermal springs and fumaroles, a much improved picture of the conditions prevalent during hydrothermal alteration is emerging.
The accumulation of field relationships becoming known through the studies of various observers places emphasis upon the relative role of wall rock in determining the nature and magnitude of alteration halos. Such factors as permeability, porosity, and conduits for fluid penetration assume an importance parallel to rock type. Essentially similar alteration aggregates may result from rocks as dissimilar as diorite and alaskite, while tremendous differences in the magnitude of the alteration halo may be noted between a porous tuff and a compact quartz monzonite even in the same district.
Halos of hydrothermal alteration are frequently a prelude to metallic mineralization. This has been particularly noted in connection with certain tungsten-bearing veins, disseminated porphyry copper deposits, certain lead-zinc deposits, some gold-bearing veins, and some uranium-bearing veins. In such deposits the metallic epoch is ordinarily closely associated with the culmination of hydrothermal activity.
The source solutions with H2S, HC1, HF, steam, and other reagents may be neutralized or even rendered slightly alkaline by reaction with wall rock and release of alkali or alkaline earth elements. The temperatures under which the alteration takes place as interpreted from the minerals formed appear to lie in the range 100°–400° C with emphasis on the middle temperature zone.
Through the utilization of improved techniques in identification the fine mineral aggregates of alteration halos are becoming better understood. The evidence of such study is contributing to a better understanding of the origin of associated metallic minerals.
