Ore Rocks and Minerals Name:
In this lab, we're going to learn about ore minerals and the geologic environments that result in their formation and concentration into economically-profitable ore bodies.
1.
First off, what is the difference between a mineral, rock, and an ore?
Part 1: Mineral Identification
For each mineral, record its luster, hardness, color, streak, transparency, crystal habit, cleavage, and density . A guide for how to describe these properties is provided.
Start with luster and hardness, and then look up the options in the mineral books to determine the mineral. Below is a list of major ore minerals; the samples that you must identify in part A are in RED , the samples in part B are in BLUE .
Iron Minerals
Pyrite FeS
2
Pyrrhotite Fe
0.83-1
S
Magnetite Fe
3
O
4
Hematite Fe
2
O
3
Limonite FeO(OH)●nH
2
O
Siderite FeCO
3
Copper Minerals
Copper Cu
Chalcopyrite CuFeS
2
Bornite Cu
5
FeS
4
Tetrahedrite Cu
12
Sb
4
S
13
Malachite Cu
2
(CO
3
)(OH)
2
Azurite Cu
3
(CO
3
)
2
(OH)
2
Manganese Minerals
Rhodocrosite MnCO
3
Pyrolusite MnO
2
Psilomelane Ba(Mn
+2
)(Mn
+4
)
8
O
16
(OH)
4
Lead Minerals
Galena PbS
Cerrusite PbCO
3
Arsenic Minerals
Realgar As
4
S
4
Orpiment AsS
Arsenopyrite FeAsS
Antimony Minerals
Stibnite Sb
2
S
3
Aluminum
Gibbsite Al(OH)
3
Zinc Minerals
Franklinite ZnFe
2
O
4
Sphalerite ZnS
Zincite (Zn,Mn)O
Titanium Minerals
Ilmenite FeTiO
3
Rutile TiO
2
Titanite CaTiSiO
5
Other Metals
Barite BaSO
4
Cinnabar HgS
Chromite (Fe,Mg)Cr
2
O
4
Associated Minerals
Feldspar (albite) NaAlSi
3
O
8
Quartz SiO
2
Calcite CaCO
3
Pyroxene (Ca, Fe, Mg)
2
Si
2
O
6
Non-metallic ore minerals
Garnet (Ca, Mg, Fe)
3
(Al,Fe,Cr)
2
(SiO
4
)
3
Borax Na
2
B
4
O
7
·10H
2
O
Beryl Be
3
Al
2
(SiO
3
)
6
Sulfur S

PART A: Write down three observations for each sample among the following: luster, hardness, density, crystal habit, cleavage, transparency, streak and color. Underline the characteristics that are diagnostic of that mineral, and write down the identity of each sample.
Sample 1:
Sample 2:
Sample 3:
Sample 4:
Sample 5:
Sample 6:
Sample 7:
Sample 8 (contains two minerals):
Sample 9:
Sample 10:
Sample 11:

Sample 12:
Sample 13:
Sample 14:
Sample 15:
Sample 16:
PART B: These samples are at the front of the room. (Same instructions as part 1.)
Sample 17:
Sample 18:
Sample 19:
Sample 20:
Sample 21:
Sample 22:

Uses of minerals
Think back to our many discussions of the use of different elements for energy-related technologies. Identify three of these technologies and their related ore mineral.
Ore formation
So now that you can identify ore minerals, I'm sure you want to know what kinds of geologic environments they form in (so you can go out in the field and prospect for ores, of course!). Following is an outline of some of the major ore-forming environments.
Ore minerals must be concentrated at several times their normal abundance in the crust in order to form an economically viable ore accumulation. This happens in four general ways:
1.
Preferential crystallization of magmas,
2.
Dissolution and redeposition by fluids,
3.
Erosion and gravity sorting,
4.
Leaching / weathering of surface deposits.
Chemical precipitation – banded iron formation
It is generally agreed that banded iron formations formed during the advent of oxygenproducing photosynthetic cyanobacteria. As these bacteria produced oxygen, the oxygen combined with the iron in the ocean, forming an insoluble precipitate that accumulated on the sea floor. As the oxygen levels were minor and probably variable, periods of iron oxide formation alternated with periods of anoxic deposition of silt/mud layers, leading to the banded appearance of these deposits. Many of the highest grade BIFs are further enriched by meteoric waters flushing through the deposit and leaching out silicates, leaving enriched iron.
Laterites
Laterites form when intense, long-lived weathering in tropical environments mobilizes the mobile elements (K, Na, Ca, et al) and leaches them out of the soil, leaving only the least mobile elements (Fe and Al) behind. This results in a rich red soil that is so poor in most essential nutrients that it is difficult for plants to grow. In regions where weathering is most extreme, even the iron is leached out, resulting in the formation of bauxite ore , which contains the primary ore mineral gibbsite (other aluminum hydroxides, limonite, and clays are also present). Laterites often have a pisolithic texture, consisting of spherical concretions.

Copper porphyry / Hydrothermal / metasomatic environments
Most copper deposits form from hydrothermal fluids associated with granitic intrusions, which mobilize copper from the granitic body and deposit it in susceptible surrounding rocks (usually in limestones). The heat and circulating hydrothermal fluids from the intruding body metamorphose the limestone into scarn , which is rich in copper and iron sulfides, copper carbonates, and other associated minerals. Zinc and lead are also common in hydrotherthermally altered regions. Copper is a chalcophile element, meaning that it will preferentially form sulfide minerals. However, in the absence of sulfur, it may form elemental copper, such as found extensively in the Keweenaw Mining
District in Michigan.
Supergene enrichment
This is a common process in which meteoric waters mobilize ore minerals, carry them downwards, redeposit them at the base of their penetration, leading to a highly enriched layer of ore minerals all at the same depth. This is one of the primary means of concentrating a low grade ore such as a copper porphyry into a high-grade deposit.
Layered mafic intrusives
Copper, Chrome, Nickel, and Platinum group metals are often mined from layered mafic intrusives, which are mafic or ultramafic volcanic bodies. As the magma incrementally cools, the most mafic minerals form first, settling out of the melt to the bottom of the magma chamber to form cumulate rocks, which are well-formed crystals of one or more mineral type forming a “grain-supported” rock texture, with another mineral filling the interstitial space. As the melt becomes progressively less mafic, different minerals become the dominant phase to crystallize out of the melt, which often leads to a progression of rock types from dunites to norites, pyroxenites, gabbros, diorites, granites, and then granophyres. This process is called fractional crystallization . Chromite often forms in monomineralic layers near the base of the sequence. Copper, nickel, and platinum group metals often form in association with iron and copper sulfides in particular layers within the lower portion of the sequence.
Pegmatites
As the last portions of a magma body crystallize, the elements that are least favorable to fit into the crystal structures of normal rock-forming minerals are concentrated in the melt. The final fraction of the melt usually intrudes into fractures to form veins, where the melt cools and crystallizes very slowly, therefore forming very large minerals with anomalously high concentrations elements rarely found in common minerals (for example, beryllium in beryl crystals). Sulfides of several metals are common in pegmatites (pyrite, chalcopyrite, et al.). Elemental, or native, gold and silver are also found in pegmatite veins called lodes .
Deposition in fault zones
Often, if circulating fluids are involved in the mineralization, ore-bearing minerals will preferentially deposit into the brecciated rocks in fault zones. For example, in metasomatism, mineralization is often most strongly concentrated in fault zones.
Another example of this is in the central Appalachians, where weathering of the iron-rich

mafic elements in the exposed granite led to the mobilization of iron in waters passing through the region, which then deposited the iron in the frontal main fault of the mountain range. This iron-cemented breccia was the primary source of iron ore throughout the 19 th
century in the US, until the exploitation of banded iron formations in the Great Lakes region.
Placer deposits
If any of the above formations are exposed to the surface and weathered, the heaviest and most resistant minerals (particularly gold) settle when the energy drops in rivers, such as in point bars on the inside of river bends, where they are then concentrated. Miners
“panning for gold” in the streams of the American West or Alaska are looking for such placer deposits.
Heavy mineral sands
Magnetite and other heavy metal minerals are found in low concentrations in many igneous and metamorphic rocks. As these rocks are eroded, the heavy metals are carried by streams to shores, where wave actions serves to sort the grains by size, and carry away less dense grains, leaving high concentrations of heavy metals in beach sands (which sometimes are lithified and found as sandstones). Sands of this type can create economic accumulations of magnetite (Fe), zircon (Zr), ilmenite (Fe-Ti), rutile (Ti), and occasionally diamonds.
Evaporites
When bodies of water have restricted circulation, and the evaporation rate exceeds the input of water into the lake, water will evaporate, causing dissolved salts to become oversaturated, and to precipitate out of solution in the reverse order of their solubility.
This results in the deposition of carbonates, sulfates, halides, nitrates, and borates.
Halides are obtained from evaporite deposits, as are potash (K) and nitrates (N) for fertilizers. Particularly in nonmarine evaporite environments (internally drained basins in arid environments), economic accumulations of borates (B) are mined for use in soaps.
Mobilization and precipitation in changing aqueous conditions
Some elements are easily dissolved in water under certain conditions. In their dissolved state, they can be carried by the waters through the pore spaces of a rock until different pH conditions or oxidation states are encountered, causing the elements to precipitate out into the pore space of the surrounding material. This is the case in uranium roll-type deposits, iron breccia deposits in the Blue Ridge Mountains, and bog iron (the most common source of pre-Industrial age iron).

Part 2: Rock description and Identification
Now that you are familiar with the major ore-forming processes, you will identify a series of rocks that represent some of the processes described above. Answer the following questions about each sample, and write the name of the rock type for each.
2a. Sample 2a:
Describe this sample:
In what setting does it form?
What is the major ore mineral in this rock?
2b. Sample 2b:
Identify the main mineral in this rock:
Is this rock sedimentary, igneous or metamorphic?
Describe the texture of this rock:
Under what geological conditions did this rock form?
2c. Sample 2c:
Describe this sample:
How does this kind of mineral deposit form?
2d. Sample 2d:
Identify at least two minerals present in the sample:

What environment did this rock form in?
Ores of what elements might be found in this sample or in associated rocks? Will they be associated with oxides, sulfides, hydroxides, or carbonates?
2e. Sample 2e:
Identify at least 3 minerals in this sample:
Based on the texture seen in this rock, how was it formed? In what direction did the crystals grow from?
What metallic elements are associated with this kind of ore formation?
2f. Sample 2f:
What are two minerals in this sample?
Is this rock igneous, sedimentary, or metamorphic?
This sample come from somewhere on the map on the next page. Identify a possible location for where the rock could have come from on the map based on how you think it formed. The pre-Laramide rocks consist of a sequence of Cambrian sandstone interbedded with limestone, early Paleozoic limestones and dolomites, and late Paleozoic interbedded shales and carbonates. The rest of the rocks are igneous intrusive rocks of varying silica content, where the orthoclase gabbros are the most silica-poor (mafic) and the quartz-monzodiorite is the most silica-rich (felsic). Justify your choice.

What other minerals from the first section might you find in this kind of geologic setting?
What was the likely composition of the original rock type based on what you know about this kind of ore forming process?
2g. Sample 2g:
What are three minerals in this sample?
What type of environment did this sample form in? How do you know?
2h. Sample 2h:
What are three minerals in this sample?

What element is this sample an ore of?
2i. Sample 2i:
Look at this sample with a hand lens. What are the tan colored grains made of?
How are they sorted? What does this imply for the pore space?
What element’s ore is deposited in the pore space?
What causes this mineral to be deposited in this way?
2j. Sample 2j:
What are the grains of this rock composed of?
Are they well or poorly sorted?
Based on the above, at environment was this rock deposted in?
Detection methods
We can broadly identify potential targets for mineral exploration by looking for regions where the geologic circumstances for ore formation are present. For example, the following is a cross-section of an ore body in southern Montana.

The rock units consist of the following: quartz monzonite (feldspars, amphibole, and mica), hornfels (contact metamorphic rocks), bronzitites (primarily pyroxene), peridotites
(olivine and pyroxene), the banded series (alternating layers of feldspar-rich and pyroxene-rich rocks), and sedimentary rocks that are primarily limestones.
(from McCallum, 1996)
3a. What kind of ore-forming geologic setting is this? How do you know?
3b. Which rock sample that you identified in part 2 is most likely to have came from this mine? What elements’ ores are most likely to be found here?
Geophysical methods
4a. What are four properties that distinguish metallic ore minerals from other minerals?
Which of these can we use to detect where these minerals occur in the subsurface?

The following is a gravity survey (labeled E), electromagnetic response (C), and apparent resistivity (F) of a suspected ore region near Voisey's Bay, Labrador, Canada (from the
Canadian geologic survey).
4a. Why might a metallic ore body have an electromagnetic signal associated with it?
4b. What is the sign of the associated gravity anomaly? Why?
4c. What is the associated resistivity signal? Why?