Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 73

Chapter 4
Hydrothermal Deposits

4.1 Introduction

This important class of ore deposits is the source of most of the world's metals.
Hydrothermal deposits provide almost 100% of our Pb, Zn, Mo, and Ag, 60-90% of
our Cu, Au and U, as well as gemstones and industrial materials such as clay
minerals and quartz. Hydrothermal deposits are diverse, being present in a wide
range of geological settings and tectonic environments: some are closely associated
with granitic intrusions, others form on the ocean floor and still others are in
sedimentary basins. What all the deposits have in common is their origin via the
precipitation of metals or ore minerals from hot aqueous fluids.

4.2 Key Factors in the Formation of a Hydrothermal
Ore Deposit

To form a hydrothermal deposit requires: (1) a source of fluid, (2) a mechanism
by which the metals or minerals are dissolved in the fluid, (3) a trigger of
circulation of the fluid, (4) a mechanism that precipitates the metals or minerals
(Fig. 4.1 ). We will now discuss each of these factors in turn before illustrating,
through the description of five selected types of deposit, how they are related to
ore formation.

4.2.1 Source of Metals

Some types of hydrothermal deposits are directly linked to plutonic rocks, usually
granitoids, and in such cases it is evident that the ore metals are derived from the
magmas themselves. The best examples are the so-called “porphyry-copper