Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 113

Chapter 5
Deposits Formed by Sedimentary and Surficial
Processes

5.1 Introduction

We have chosen in this chapter to group several types of ore bodies that have the
common characteristic of having formed at or very near the surface of the solid
earth. One group of deposits results from sedimentation; i.e., the accumulation of
detrital grains or chemical precipitates in rivers, lakes, coastal settings or in shallow
to deep oceans. The other group comprises deposits that form in zones of
weathering just below the surface, usually in humid tropical environments.
Table 5.1 summarizes these processes and gives examples of each type of deposit.
In each case the minerals that are mined in these deposits are stable at low
temperatures in the humid, usually oxidising environment at the surface of our
planet. Three broad types of ore-forming processes can be identified.

1. A placer ore body is a concentration of eroded particles of valuable minerals in
an alluvial or eluvial deposit of sand or gravel. The ore minerals in such deposits
crystallize initially within the crust, in magmas or metamorphic rocks, or from
hydrothermal fluids. In some cases their concentrations in these source rocks are
higher than in normal crustal rocks, as in a gold vein or diamond-bearing
kimberlite, but in other cases the minerals are accessory phases present in their
usual concentrations. These minerals are released from their host rocks by uplift,
weathering and erosion and are then concentrated in ore bodies by sedimentary
processes. Placer gold deposits in rivers gravels or aeolian sands, and deposits of
heavy Zr and Ti minerals in beach sands, are examples of this type of deposit.
2. Other ore minerals precipitate from lake water or seawater to form chemical
sedimentary rocks. The metals or other valuable minerals in such deposits are
soluble in surface waters but precipitate when they reach saturation levels or
when the composition or physical conditions of the water changes. Examples
include salt deposits that result from the evaporation of waters in lakes or
shallow seas, and Fe- or Mn-rich sediments that result from mixing of waters
with different compositions or redox states.