Erosion

Erosion is the break down of materials forming the earth’s crust, otherwise known as weathering.  The two principle types of weathering include chemical and mechanical weathering.  Chemical weathering involves decomposition of rocks and their constiuent minerals by subjecting them to generally acidic waters, either in the form of acid rain or acidic groundwater.  Soluble minerals, such as calcite, are particularly effected, but many other minerals, including silicates and sulfide minerals can also be dramatically effected.  Chemical reactions (including solution, oxidation, carbonation and hydration reactions) separate the constituents of the minerals themselves.  In contrast, mechanical weathering involves physical forces which disintegrate rocks into constituent minerals, but do not dissociate the minerals into elemental their components.  Ultimately this process results in the creation of soil.  These forces include the movement of water, the freezing and thawing action of ice, the prying action of plant roots, or wind action.

Mechanical erosion loosens and wears away materials and transports these materials to a new location.  The most pervasive type of mechanical erosion in temperate climates is that caused by running water.   Rainfall begins the process.  The water then flows down slope or soaks into the ground.  The washing action of the water on hillsides carries rocks, and mineral grains formed by rock decomposition, down the slope where they eventually become stream sediments.  The turbulence of the water, especially in steeper areas where the current is faster, continues to carry the sediments in suspension downstream.  These suspended sediments are called the “stream load”.  Larger rocks which skip along the bottom of the stream bed due to sheer mass, are called the “stream bed load”. 

Placer Deposits 

Placer deposits, or simply “placers”, are accumulations of valuable minerals concentrated in overburden, in stream sediments or in beach materials by natural processes.  The minerals are freed from solid rock by mechanical and chemical weathering, and then transported usually by water or wind action to the final resting place.  Most of the placer deposits being mined today are Cenezoic or younger and occur in unconsolidated materials.  However, some ancient placers, or “paleo-placers”, are found in sedimentary rocks as old as Precambrian in age.  In fact, some paleo-placers which are eroded become the source of present day placer deposits.

Placer Minerals

Two types of minerals form placers:  1) minerals which are more resistant to chemical and mechanical erosion (called “resistate” minerals), and 2) minerals which have high specific gravities (called “heavy minerals”).  There are three categories of resistate minerals, including those which are relatively inert (non-reactive), those which are maleable (tend to bend rather than break), and those which have greater hardness :

Maleable Minerals:

Examples of heavy minerals include native metals, sulfide minerals (including pyrite and galena), magnetite and scheelite.  The high density of these minerals enables them to be concentrated because they are less easily mobilized by water currents.  As a result, less dense mineral grains surrounding them are “winnowed” (washed away) with ease leaving the heavy minerals to lag behind.  Winnowing also occurs as a result of wave action on beaches, and even as a result of wind action.  The minerals most likely to accumulate in placers are those which are both resistates and heavy.  This is the reason gold and magnetite (often called “black sand”) are the most common minerals to accumulate in placers. 

Other factors influence the ability of a mineral to become concentrated in a placer, such as the settling rate.  The settling rate is a function of the grain size, grains shape, specific gravity, surface roughness and electrostatic charge.  Larger grains, because they are heavier, settle faster than small ones.  Thin, flat grains (such as gold “flakes”) tend to catch currents and be whisked away more easily than rounded grains (such as gold “nuggets”).  Surface roughness causes greater friction, inhibiting ease of movement.  Some mineral grains are known to carry electrostatic charges which cause them to stick to other grains the way a balloon rubbed against your shirt will stick to your hand. 

Placer gold occurs in many shapes and sizes.  Larger pieces (generally > 10 mesh) are called nuggets, and smaller, flat pieces are called flakes.  “Colors” are the tiny pieces (generally < 0.001 oz) which are found by panning or sluicing.  Placer gold is not pure, but instead is a mixture of gold and other native metals (usually silver, copper or bismuth).  The purity of placer gold is referred to as the “fineness”, which is essentially the volume percent of gold stated in parts gold per 1000 millileters.  As gold particles travel further downstream, the metal impurities are leached out, causing an increase in gold fineness downstream.  The texture of gold is also an indication of distance of downstream transport.  Rough, angular texture is generally considered to be an indication of close proximity to the source. 

Hydrologic Environments

A general understanding of present day (or past) hydrologic environments is necessary in order to predict where placer minerals are deposited.  These environments include the action of water currents in streams and the action of waves on beaches.  Current velocity is faster in steeper areas.  Faster currents cause greater the turbulence, which in turn affects the ability of the stream to carry heavy minerals in the suspended load or bed load.  For this reason, where the stream gradient suddenly decreases is generally a good area to look for placer deposits.  

The ideal environments for heavy minerals to drop out of suspension are so called “velocity shadows”, where the current suddenly looses velocity due to a stream obstruction (Figure 11 – 1).  One example is where the current meets a an island or large boulder in the middle of the stream.  The current splits and goes around each side and then merges back together on the downstream side. Suction eddies occurring directly behind the boulder cause a velocity shadow where heavy minerals drop to the stream bed.  Similar eddies form both upstream and downstream from obstructions on the sides of streams, such as rock outcrops or bluffs.

 

FIG 11-1 pending permission

Figure 11 – 1.   Hypothetical stream showing hydrologic environments (above) and associated heavy mineral accumulations (from Boyle, 1979).

Placer minerals also accumulate in alluvial fan deposits, which are fan-shaped areas of unconsolidated, unsorted stream sediments at the mouths of major stream drainages.  This is due to the sudden decrease in the stream gradient and consequent decrease in stream velocity and turbulence.  The maximum amount of winnowing occurs in the middle portion of the fan, called the “mid-fan facies”, hence this is where the largest accumulations of placer minerals occur in the fans.  Ancient fan deposits which have been buried and lithified are the source of some very rich placer gold deposits in an area of South Africa called the Witwatersrand District.

Most of the concentration of heavy minerals occurs during flooding, when current velocity and winnowing are at a maximum.  Each time a flood occurs, heavy minerals which were once randomly scattered within the sediments end up resting on the new stream bed created by the scouring action of the flood (Figure 11 – 2).  Periodically, large scale floods scour the stream bed completely down to bedrock, resulting in deposition and accumulation of the heavy minerals on bedrock.  This is why the richest pay streaks of placer gold and other heavy minerals is usually found on or very near bedrock. 

Figure 11 – 2.  Profile of a stream showing how placer deposits form by the action of floods, and how major floods cause most placer deposits to accumulate on bedrock (after Faulkner, 1986).

Coarse-grained gold and other heavy minerals are usually found associated with coarse sediments, such as pebbles, cobbles, boulders and coarse sands.  The coarser sediments, due to size and weight, drop out of suspension in the same hydrologic environments that deposit the heavy minerals.  Similarly, fine-grained heavy minerals are usually found associated with deposits of sand or silt. 

Wave action on beaches winnows away light minerals and leaves heavy minerals to lag behind, similar to the manner in which water currents operate in the stream environment.  As each wave retreats it washes light minerals back towards the the ocean or lake.  As a result, heavy minerals, along with larger pebbles and cobbles, lag behind.  These “lag deposits” form a linear band which is generally parallel to the shoreline.  Regional subsidence can result in these deposits becoming submerged beneath the water, forming offshore placer deposits.  Regional uplift can result in these deposits occurring on high benches further inland, which represent ancient shorelines. 

Bedrock Traps

The shape of the stream bed, in particular the shape of the bedrock floor of the stream bed, is also a major factor determining how and where placer minerals accumulate.  Depressions or any type, such as plunge pools at the base of waterfalls, act as natural traps because the current lacks the force to move the heavy minerals out of the depression (Figure 11 – 3).  Joints or fractures of any type also tend to trap the heavy minerals.  The ideal bedrock floor contains natural “riffles”, where a series of resistant layers of rock occur at right angles to the axis of the stream.  Heavy minerals may accumulate on either the upstream or downstream side of these obstructions. 

 

FIG 11-3 Pending permission

Figure 11 – 3.  Hypothetical stream profile showing different types of traps in bedrock. A = joints or fractures, B = recent faults, C = outcrops of resistant dikes, D = outcrops of resistant beds, E = plunge pool (from Guilbert & Park, 1986).