Finding the Mother Lode
Rachel Hughes based on a lesson described by Peter Modeski, USGS
Time:
1 class period
Preparation
Time:
Materials: Laminated maps, white-out
Abstract
In this lesson students engage in a paper treasure hunt to find a lode deposit of gold using the
placer deposit concentrations of metals found in stream sediments. Students use topographic
maps and metal concentrations in stream sediments to determine the location of the lode deposit
much like early prospectors did and modern geochemical prospecting does today. This lesson
provides an opportunity to discuss the distribution of minerals throughout the bedrock and also in
surrounding surface and ground water. This lesson, along with subsequent lessons, addresses
not only the differential distribution of gold, but also of arsenic in both bedrock and drinking water
supplies. Arsenic in drinking water is a substantial public health issue in countries across the
globe.
Objectives
Students will be able to:
1. Establish where a lode deposit is located using a topographic map, stream sediment data
and deductive reasoning.
National Science Education Standards:
Content Area D: Earth and Space Science
Geochemical Cycles
The earth is a system containing essentially a fixed amount of each stable chemical atom or
element. Each element can exist in several different chemical reservoirs. Each element on
earth moves among reservoirs in the solid earth, oceans, atmosphere, and organisms as part
of geochemical cycles.
Teacher Background
In addressing a public health issue like arsenic in drinking water, students should ask not only
“What are the affects of arsenic in drinking water on health?”, but also “Why is the arsenic in the
drinking water?” and “Are there ways to remove the arsenic from the drinking water?” This
learning cycle uses the distribution of gold throughout the bedrock, something students are more
familiar with, as a starting point for understanding why arsenic has variable distribution throughout
the bedrock and so then in drinking water.
Gold, like arsenic, is deposited and concentrated in the bedrock by hydrothermal activity. The
superheated hydrothermal water fluids and the pressure generated by a pluton result in mineral
alteration in the surrounding bedrock. The pressure of the pluton uplifting the crust results in
vertical faulting along which the hydrothermal fluid flows. The hydrothermal fluids carry metals
from the magma source and also change the bedrock mineral composition, leaching out some
minerals into the hydrothermal fluid to be deposited elsewhere.
This distribution results in deposits known as lode deposits. One famous lode deposit during the
Californian Gold Rush of 18** was named the Mother Lode and while I’m not sure of the etiology
of the phrase “Finding the mother lode” is a term that students may be familiar with.
Related and Resource Websites
Activity
Before class prepare enough maps for pairs of students.
On the board write the question, “What does the phrase ‘To find the mother lode’ mean?” As
students walk in ask them to respond to the question on the board in their notebooks.
1. After giving the students time to respond to the question in their notebooks ask them to
discuss it for a few minutes. Even if this isn’t a phrase that they use they can attempt to
answer.
2. Tell students that today they will be prospecting for gold, looking for the mother lode. Ask
students how they think early prospectors looked for gold. If they suggest panning for gold,
ask them “How did the early prospectors, the successful ones, know where to pan?” Lead
students to recognize that understanding geology is central to knowing where to mine for
whatever mineral you are looking for. Ask students if they know areas that are famous for
specific minerals, for example, areas of Arizona are important in copper mining, Colorado,
South Dakota have gold mines. Like early prospectors, the group that gets to the mother lode
first is the one who gets to lay the claim on it.
3. On each table place the laminated topographic map. Point out the circled sampling sites.
Explain to students that at each site are the metal values underneath the white-out. The
challenge is to follow the metal values to find which drainage has the ore deposits and the
goldmine close by. The challenge is to do this in as few moves as possible. In other words
they are to do this without scratching out all the values. They must use their knowledge of
topographic maps to follow the deposits upstream. Provide students with the following hint:
Gold is heavy. It settles where there is slow water such as on the inside of a bend or behind a
large boulder. As students scratch off the white-out they must calculate the concentration of
metal in their sample.
4. There are multiple mines on the map. The multiple mines reflect a vein across the map.
Before the students stake their claim they must make sure that they are able to explain their
choice of sites. What does this tell them about where the gold is situated? How did the gold
get in the sediments? Why in the area represented by the map do some drainages have gold
and others don’t? Students should respond to these questions before being able to stake
their claim.
5. As students map the gold mines mark them off on an overhead copy of the map.
6. Once all the mines are claimed ask students what this information tells them. They should
respond to this in their notebooks.
Embedded Assessment
Students’ participation in the class discussions and their “T” charts will be evaluated.
Homework
None