Lesson 7: Mineral Resources

Hi! I hope your having a great day. We are on our 7^th post here on our blog and I’m glad that you guys are still reading it. In today’s post, we’ll be talking about the formation of mineral resources. This is a continuation of the previous topic from our last blog posts. Stick around and I hope that you’ll learn something from this lesson.

At the end of the lesson, the learners will be able to:

• Understand the importance of the different minerals to the society; and
• Understand the different ways on how the mineral deposits are formed.

Mineral Deposits

A beautiful spray of highly lustrous, metallic, silvery Stibnite crystals With calcite from Xikuangahan Sb Deposit, Hunan Province, China. Image from https://scontent-cdg2-1.xx.fbcdn.net/v/t1.0-9/38282997_1848997021860295_435156908085608448_n.jpg?_nc_cat=0&oh=a8eeb7a960183aac1ea9e8d0eb783fe6&oe=5BFCD34B

There are a few terms to familiarize with when it comes to mineral deposits, namely:

Mineral Occurrence

• concentration of a mineral that is of scientific or technical interest Mineral

Deposit

• mineral occurrence of sufficient size and grade or concentration to enable extraction under the most favorable conditions

Ore Deposit

• mineral deposit that has been tested and known to be economically profitable to

Aggregate

• rock or mineral material used as filler in cement, asphalt, plaster, etc.; generally    used to describe nonmetallic deposits

 Ore

• naturally-occurring material from which a mineral or minerals of economic value can be

Most rocks of the Earth’s crust contain metals and other elements but at very low concentrations.  For example, the average concentration of Gold in rocks of the Earth’s crust is about 0.005 ppm (parts per million) which is roughly 5 grams of gold for every 1000 tons of rock.  Although valuable, extracting Gold at this concentration is not economic (the cost of mining will be too high for the expected profit).    Fortunately, there are naturally occurring processes (geologic processes) that can concentrate minerals and elements in rocks of a particular area.

Mineral resources have two types: metallic and nonmetallic

• Metallic mineral deposits: gold, silver, copper, platinum, iron

A sample of a gold nugget, image retrieved from: https://hobbylark.com/metal-detecting/Panning-for-Gold-In-Your-Backyard

• Non-metallic resources: talc, fluorite, sulfur, sand, gravel

Sulfur crystals, a non-metallic mineral resource, on Aragonite matrix. From Agrigento, Agrigento Province, Sicily, Italy. Source: http://www.exceptionalminerals.com/exceptionalroom5.htm

One has to know that minerals don’t just come out of scratch. There are  geologic processes involved in the rock cycle and theses processes play a major role in the accumulation and concentration of valuable elements/ minerals.

A diagram of the collision of two plates. Image from: http://pubs.usgs.gov/gip/dynamic/understanding.html

The concept of plate tectonics plays a role in the creation of mineral resources. Plate tectonics: the Earth’s crust is broken into a dozen or more plates of different sizes that move relative to one another (lithosphere).  These plates are moving slowly on top of a hot and more mobile material called the asthenosphere.

Different Types of Ore Deposits

Mineral resources can be classified according to the mechanism responsible for concentrating the valuable substance.

1. Magmatic Ore Deposits

The magma inside Yellowstone Volcano is responsible for creating magmatic ore deposits. Retrieved from: http://pubs.usgs.gov/gip/dynamic/understanding.html

• Valuable substances are concentrated within an igneous body through magmatic processes such as crystal fractionation, partial melting and crystal settling.
• Magmatic processes can concentrate the ore minerals that contain valuable substances after accumulating elements that were once widely dispersed and in low concentrations within the magma.

Examples:

• Crystal settling: as magma cools down, heavier minerals tend to crystallize early and settle at the lower portion of the magma chamber
• From a basaltic magma, chromite, magnetite and platinum can be concentrated through crystal settling

A chunk of chromite. Photo from http://hyperphysics.phy-astr.gsu.edu/hbase/minerals/chromite.html#c1

• Fractional crystallization: the residual melt contains high percentage of water and volatile substances that are favorable for the formation of pegmatites. Pegmatites are enriched in Lithium, Gold, Boron, rare elements and some other heavy metals
• Fractional crystallization of granitic magmas can concentrate rare earth elements (such as cesium and uranium) and heavy metals. This can also form pegmatites (large crystals of quartz, feldspars and muscovite) which may contain semi-precious gems such as beryl, topaz, and tourmaline

2. Hydrothermal Ore Deposits

Smoking Ocean Vents Found in Surprisingly Shallow Water. Source: https://news.nationalgeographic.com/2018/06/hydrothermal-vents-discovered-azores-science-environment/

• Concentration of valuable substances by hot aqueous (water-rich) fluids flowing through fractures and pore spaces in rocks – hydrothermal solutions
• Are hot, residual watery fluids derived during the later stages of magma crystallization and may contain large amount of dissolved metals. These can also originate from the ground water circulating at depth that is being heated up by a cooling and solidifying igneous body or along depths with known geothermal gradient.
• Such hot water can dissolve valuable substances (at low concentrations) from rocks. As the metal enriched hot waters move into cooler areas in the crust, the dissolved substances may start to precipitate
• There are numerous hydrothermal mineral deposits as compared to the different types of deposits

Examples:

Vein type deposits

http://www.freeportresources.com/s/SpanishMountain.asp?ReportID=290578&_Title=Sediment-Hosted-Vein-Deposits

• A fairly well defined zone of mineralization, usually inclined and discordant and typically narrow. Most vein deposits occur in fault or fissure openings or in shear zones within the country rock. Sometimes referred to as (metalliferous) lode deposits, many of the most productive deposits of gold, silver, copper, lead, zinc, and mercury occur as hydrothermal vein deposits

Disseminated deposits

• Deposits in which the ore minerals are distributed as minute masses (very low concentration) through large volumes of rocks. This occurrence is common for porphyry copper deposits

The disseminated deposits in this illustration are the red dots. The illustration is from https://www.researchgate.net/figure/Geologic-model-for-the-setting-of-disseminated-stockwork-and-crustiform-vein-deposits-in_fig2_284574136

Massive sulfide deposit (at oceanic spreading centers)

• Precipitation of metals as sulfide minerals such as sphalerite and chalcopyrite occurs when hot fluids that circulated above magma chambers at oceanic ridges that may contain sulfur, copper and zinc come in contact with cold groundwater or seawater as it migrate towards the seafloor.

Block model schematic diagram of a mound-like volcanogenic massive sulfide system from https://www.sciencedirect.com/science/article/pii/S0169136815000050

Stratabound ore deposits (in lake or oceanic sediment)

• This deposit is formed when the dissolved minerals in a hydrothermal fluid precipitate in the pore spaces of unconsolidated sediments on the bottom of a lake or ocean. Such minerals may contain economic concentrations of lead, zinc and copper, usually in sulfide form like galena, sphalerite and chalcopyrite.

An illustration of a stratabound ore deposit from http://www.freeportresources.com/s/SpanishMountain.asp?ReportID=290578&_Title=Sediment-Hosted-Vein-Deposits

3. Sedimentary Ore Deposits

• Some valuable substances are concentrated by chemical precipitation coming from lakes or seawater

Lake Moraine, Banff National Park, Alberta, Canada, a good place to find sedimentary ore deposits. Taken from: http://www.gettyimages.com/license/145174022

Examples:

• Evaporite Deposits: This type of deposit typically occurs in a closed marine environment where evaporation is greater than water inflow. As most of the water evaporates, the dissolved substances become more concentrated in the residual water and would eventually precipitate. Halite, gypsum, borax (used in soap) and sylvite ( from which potassium is extracted for fertilizers) are examples of minerals deposited through this process.
• Iron Formation: These deposits are made up of repetitive thin layers of iron-rich chert and several other iron bearing minerals such as hematite and magnetite. Iron formations appear to be of evaporite type deposits and are mostly formed in basins within continental crust during the Proterozoic (2 billion years or older).

4. Placer Ore Deposits

• Deposits formed by the concentration of valuable substances through gravity separation during sedimentary processes.
• Usually aided by flowing surface waters either in streams or along coastlines. – Concentration would be according to the specific gravity of substances, wherein the heavy minerals are mechanically concentrated by water currents and the less-dense particles remain suspended and are carried further downstream.

A raging river: a body of water that is responsible for creating placer ore deposits. From: http://hundred-acresss.tumblr.com/post/63451210015/

• Usually involves heavy minerals that are resistant to transportation and weathering. – Common deposits are gold and other heavy minerals such as platinum, diamonds and tin;
• The source rock for a placer deposit may become an important ore body if located.

5. Residual Ore Deposits

• A type of deposit that results from the accumulation of valuable materials through chemical weathering processes.
• During the process, the volume of the original rock is greatly reduced by leaching.

A representation of a residual ore deposit from https://slideplayer.com/slide/5822770/

• Important factors for the formation of residual deposit include parent rock composition, climate (tropical and sub-tropical: must be favorable for chemical decay) and relief (must not be high to allow accumulation) – Common deposits are bauxites and nickeliferous laterites.
• Bauxite, the principal ore of aluminum, is derived when aluminum-rich source rocks undergo intense chemical weathering brought by prolonged rains in the tropics, leaching the common elements that include silicon, sodium and calcium through leaching.
• Nickeliferous laterites or nickel laterites are residual ore deposits derived from the laterization of olivine-rich ultramafic rocks such as dunite and peridotite. Like in the formation of bauxite, the leaching of nickel-rich ultramafic rocks dissolves common elements, leaving the insoluble nickel, magnesium and iron oxide mixed in the soil.
• Secondary Enrichment Deposits are derived when a certain mineral deposit becomes enriched due to weathering.

There are a few unfamiliar terms that were mentioned, so I’ll list them down below for a quick definition.

• Magma chamber – a reservoir or pool of magma within the upper mantle or lower crust

An illustration of two volcanoes with two different magma chambers by Chuck Carter. Image from https://s-media-cache-ak0.pinimg.com/736x/44/f9/d8/44f9d8ce80b755bf5f5efaf576d861a0.jpg

• Pegmatite – an igneous rock formed during the latter stages of magma’s crystallization that has exceptionally large crystals (several centimeters or even a few meters in length). Most pegmatites are granitic.

Pegmatite granite with garnet. Source: https://geology.com/rocks/pegmatite.shtml

• Feldspar – used in production of ceramics

Spodumene on Feldspar. Taken from: https://www.bonhams.com/auctions/13327/lot/8221/

• Muscovite – used for electrical insulation and glitter

Fluorapatite with Arsenopyrite and Muscovite. Image from https://i.pinimg.com/originals/f6/2e/b5/f62eb51647334847269cfe717ae13088.jpg

• Discordant – cuts across existing structures.

• Porphyritic texture: formed through two stages of crystallization where in magma partly cooled below the surface of the earth providing time for the large crystals to grow (phenocrysts) before it is extruded to the surface forming the fine-grained matrix (groundmass).

Close-up views of porphyritic texture of the felsite. Retrieved from http://blogs.agu.org/mountainbeltway/2015/04/16/eocene-dike-and-sill-in-ordovician-limestone/

• Aphanitic texture: fine-grained texture; minerals not visible to the naked eye; relatively fast rates of cooling/ solidification prevent the formation of large crystals.

Aphanitic Volcanic Bomb. A volcanic bomb found in the Mojave Desert National Preserve. The smooth texture of this basaltic volcanic bomb is aphanitic. From: https://i.pinimg.com/originals/28/22/3f/28223f4be41c2a5b5bca764b80cd4e6c.jpg

• Special textures would include: vesicular, glassy and porphyry
• Vesicular texture: voids created by rapid cooling which causes air bubbles to be trapped inside.

Pumice has a vesicular texture. Image from: https://geology.com/rocks/igneous-rocks.shtml

• Leaching – the removal of soluble materials in rocks or ore body through the percolation of water
• Relief – the configuration of an area that pertains to the elevation and slope variations and the irregularities of the land surface
• Laterization – conditions of weathering which leads to the removal of alkalis and silica, resulting in a soil or rock with high concentrations of iron and aluminum oxides.

Thanks for poring over the whole post! I hope you learned something amidst this brief passage of information. Please  keep on supporting our blog!

Sources:

• https://mining.cat.com/cda/files/2786351/7/GroundRulesMineralsEverydayLife-15-18.pdf Accessed 25 Sep 2018
• http://www.tulane.edu/~sanelson/eens1110/minresources.htm. Accessed 25 Sep 2018
• http://encyclopedia2.thefreedictionary.com/hydrothermal+solution. Accessed 25 Sep 2018
• http://www.britannica.com/science/hydrothermal-solution. Accessed 25 Sep 2018
• http://earthsci.org/mineral/mindep/depfile/vei_dep.htm. Accessed 25 Sep 2018
• http://geology.com/rocks/pegmatite.shtml. Accessed 25 Sep 2018
• Frank, D., Galloway, J., Assmus, K., The Life Cycle of a Mineral Deposit – A Teacher’s Guide for Hands-On Mineral Education Activities, USGS General Information Product 17, 2005
• Carlson, D. H., Plummer, C. C., Hammersley L., Physical Geology Earth Revealed 9thed, 2011, pp 560-563
• Marshak, S., Essentials of Geology, 4th ed., 2013, pp 375-379
• Tarbuck, E. J., Lutgens, F. K., Tasa, D., Earth An Introduction to Physical Geology, 11thed, 2014, pp 802-806

ABOUT THE AUTHOR

 

The writer of this blog post was written by Josias Abrielle Meneses. Like the rest of the staff of this website, he is also a student from Aurora National Science High School. He likes to play basketball and he also listens to Korean music (Kpop) on a daily basis. Amidst all this, he still excels at his studies, proving once again that ANSciestas are not only limited to being intellectuals.