Abstract
Students will explain how
solutions precipitate out in different conditions and
compare this to the
formation of ore deposits in hydrothermal systems.
Objectives
Students
will:- Understand how mineral deposits are formed and why
they are not evenly dispersed.
- Create and describe
three different precipitates from four solutions simulating
mineral ore deposit formation
in sedimentary rock.
National
Science Education Standards
ECONTENT STANDARD B Physical Science
CHEMICAL REACTIONS
Chemical reactions can take place in time periods ranging
from the few femtoseconds (10-15 seconds) required
for an atom to move a fraction of a chemical bond distance
to geologic time scales of billions of years. Reaction
rates depend on how often the reacting atoms and molecules
encounter one another, on the temperature, and on the
properties--including shape--of the reacting species.
Teacher
Background
The
three main types of
rock are:
Sedimentary
rock is formed in three main ways—by
the accumulation of other rocks pieces (known as
'clastic' sedimentary rocks), by the accumulation
of biogenic activity (fossils), and by precipitation
from solution. As sediment deposits build up, the
overburden (or lithostatic) pressure squeezes the
sediment into layered solids in a process known as
lithification ("rock formation").
Sedimentary rocks contain important information about
the history of the earth. They contain fossils, the
preserved remains of ancient plants and animals.
The composition of sediments can also provide clues
to the original igneous rock. Differences among successive
layers of sedimentary rock indicate changes to the
environment which have occurred over time. Sedimentary
rocks can contain fossils because, unlike most igneous
and metamorphic rocks, they form at temperatures
and pressures that do not destroy fossil remnants.
Sedimentary rocks include common types such as chalk,
limestone, sandstone, clay and shale. Sedimentary
rocks cover 75% of the earth's surface, but the total
contribution of sedimentary rocks is estimated to
be only 5% of the total. As such, the sedimentary
sequences we see represent only a thin veneer over
a crust consisting mainly of igneous and metamorphic
rocks.
The process of precipitation of solids as two solutions
meet is critical to the creation of mineral deposits.
Gold deposits are formed by a very wide variety of
geological processes. The majority of primary gold
deposits fall into two main categories: lode gold
deposits or intrusion-related deposits.
Lode gold deposits are generally high-grade, thin,
vein- and fault-hosted. They are comprised primarily
of quartz veins also known as lodes or reefs. Lode-gold
deposits are intimately associated with tectonic
plate collision events within geologic history.
Most lode gold deposits are sourced from metamorphic
rocks because it is thought that the majority are
formed by dehydration of basalt during metamorphism.
The gold is transported up faults by hydrothermal
waters and deposited when the water cools too much
to retain gold in solution.
Intrusive related gold is generally hosted in granites,
porphyry or rarely, dikes. Intrusive related gold
usually also contains copper, and is often associated
with tin and tungsten, and rarely, molybdenum,
antimony and uranium. Intrusive-related gold deposits
rely
on gold existing in the fluids associated with
the magma, and the inevitable discharge of these
hydrothermal
fluids into the wall-rocks.
Related
and Resource Websites
Rocks
Rock Hounds http://www.fi.edu/fellows/fellow1/oct98/index2.html
Rocks For Kids http://rocksforkids.com/
Rocks on Wikipedia http://en.wikipedia.org/wiki/Rocks
Gold
Mineral Information Institute on Gold
http://www.mii.org/Minerals/photogold.php
World Gold Council http://www.gold.org/index.html
Gold
on Wikipedia http://en.wikipedia.org/wiki/Gold
Gold Mining on Wikipedia http://en.wikipedia.org/wiki/Gold_mining
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