Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label ore. Show all posts
Showing posts with label ore. Show all posts

Igneous Origin of Diamonds

Diamonds are a rare occurrence on the surface of the planet because it takes extremely hot and high pressure conditions to create them. Physical and chemical conditions where diamonds form only exist in the mantle, nearly 70 miles down or more. In that environment in the upper mantle, diamonds may be a common mineral! It takes incredible events, nothing that has ever been witnessed in historic times, to bring diamonds to the surface.
Kimberlite-Diamond-mantle-rock

Diamond deposits around the world (that have any economic significance) are associated with volcanic features called 
diatremes.  A diatreme is a long, vertical pipe formed when gas-filled magma forces its way through the crust to explosively erupt at the surface. Kimberlite a special kind of intrusive igneous rock associated with some diatremes that sometimes contain diamonds, typical coarse grained an bluish in color.    

Diamond-bearing kimberlite pipes are diatremes that originate in the mantle.
   
Diamonds are xenoliths carried up from deep sources in the mantle, and often occur in association with other gem minerals including garnet, spinel and diopside. Most "economically significant" diamond deposits occur in ancient rocks (Precambrian age), but have been discovered on all continents. Because diamonds are so hard, they survive torturously-long histories, recycled through sedimentary and metamorphic environments without being destroyed. As a result they have been found almost everywhere as very rare, isolated discoveries. Diamonds of microscopic size have been discovered in meteorites and asteroid impact sites, and some metamorphic rocks. They are most extensively mined from kimberlite pipes or from alluvial gravels derived downstream from diamond source areas. It should be noted that most diamonds are not of gem quality, but those that are not are used for industrial purposes.

Credits to Phil Stoffer at geologycafe.com
Text and figures are used with permission.

Metals and Ores

Metal and Ores

Metal and Its Discovery 

Metals are opaque, shiny, smooth solids that can conduct electricity and can be bent, drawn into wire, or hammered into thin sheets. In this regard, they look and behave quite differently from wood, plastic, meat, or rock. This is because, unlike in other substances, the atoms that make up metals are held together by metallic bonds, so electrons can flow from atom to atom fairly easily and atoms can, in effect, slide past each other without breaking apart. The first metals that people used copper, silver, and gold can occur in rock as native metals. Native metals consist only of metal atoms, and thus look and behave like metal.
Gold occurs as native metal within quartz veins. The quartz breaks up to form sand, leaving nuggets of gold.
Gold nuggets, for example, are chunks of native metal that have eroded free of bedrock (figure above). Over the ages, people have collected nuggets of native metal from stream beds and pounded them together with stone hammers to make arrowheads, scrapers, and later, coins and jewellery. But if we had to rely solely on native metals as our source of metal, we would have access to only a tiny fraction of our current metal supply. Most of the metal atoms we use today originated as ions bonded to non-metallic elements in a great variety of minerals that themselves look nothing like metal. Only because of the chance discovery by some prehistoric genius that certain rocks, when heated to high temperatures in fire (a process called smelting), decompose to yield metal plus a non-metallic residue called slag, do we now have the ability to produce sufficient metal for the needs of industrialized society. 

What Is an Ore? 

Examples of ore minerals.
The minerals from which metals can be extracted are called ore minerals, or economic minerals. These minerals contain metal in high concentrations and in a form that can be easily extracted. Galena (PbS), for example, is about 50% lead, so we consider it to be an ore mineral of lead (a in figure above). We obtain most of our iron from haematite and magnetite. Copper comes from a variety of minerals, none of which look like copper (b in figure above). Geologists have identified a great variety of ore minerals. Many ore minerals are sulphides, in which the metal occurs in combination with sulphur (S), or oxides, in which the metal occurs in combination with oxygen (O).
To obtain the metals needed for industrialized society, we mine ore, rock containing native metals or a concentrated accumulation of ore minerals. To be an ore, rock must not only contain ore minerals, it must also contain a sufficient amount to make the rock worth mining. Iron constitutes only about 6.2% of the continental crust's weight but makes up about 30% to 60% of iron ore. The concentration of a useful metal in an ore determines the grade of the ore the higher the concentration, the higher the grade. Whether or not an ore of a given grade is worth mining depends on the price of metal in the market. 

How Do Ore Deposits Form? 

Ore minerals do not occur uniformly through rocks of the crust. If they did, we would not be able to extract them economically. Fortunately for humanity, geologic processes concentrate these minerals into accumulations called ore deposits. Simply put, an ore deposit is an economically significant occurrence of ore. The various kinds of ore deposits differ from each other in terms of which ore minerals they contain and which geologic conditions led to their formation. Below, we introduce a few examples.
Various processes that form ore deposits.

Magmatic deposits 

When a magma cools, sulphide ore minerals crystallize early, then, because sulphides tend to be dense, hey sink to the bottom of the magma chamber, where they accumulate; this accumulation is a magmatic deposit. When the magma freezes solid, the resulting igneous body may contain a concentration of sulphide minerals at its base. Because of their composition, such concentrations are known as “massive- sulphide deposits” (a in figure above).

Hydrothermal deposits 

Hydrothermal activity involves the circulation of hot-water solutions through a magma or through the rocks surrounding an igneous intrusion. These fluids dissolve metal ions. When a solution enters a region of lower pressure, lower temperature, different acidity, and/ or different availability of oxygen, the metals come out of solution and form ore minerals that precipitate in fractures and pores, creating a hydrothermal deposit (figure above b). Such deposits may form within an igneous intrusion or in surrounding country rock. If the resulting ore minerals disperse through the intrusion, we can also call the deposit a disseminated deposit, but if they precipitate to fill cracks in pre-existing rock, we can call the deposit a vein deposit; veins are mineral-filled cracks.
In recent decades, geologists have discovered that hydrothermal activity at the submarine volcanoes along mid-ocean ridges leads to the eruption of hot water, containing high concentrations of dissolved metal and sulphur, from a vent. When this hot water comes in contact with cold seawater, the dissolved components instantly precipitate as tiny crystals of metal-sulphide minerals (c in figure above). The erupting water, therefore, looks like a black cloud, so the vents are called “black smokers”. The minerals in the cloud eventually sink and form a pile of ore minerals around the vent. Since the ore minerals typically are sulphides, the resulting hydrothermal deposits constitute another type of massive-sulphide deposit.

Secondary-enrichment deposits

Sometimes groundwater passes through ore-bearing rock long after the rock first formed. This groundwater dissolves some of the ore minerals and carries the dissolved ions away. When the water eventually flows into a different chemical environment (for instance, one with a different amount of oxygen or acid), it precipitates new ore minerals, commonly in concentrations exceeding that of the original deposit. A new ore deposit formed from metals that were dissolved and carried away from a pre-existing ore deposit is called a secondary-enrichment deposit. Some of these deposits contain spectacularly beautiful copper-bearing carbonate minerals, such as azurite and malachite. 

MVT ores 


Rain falling along one margin of a large sedimentary basin may sink into the subsurface and then flow as groundwater along a curving path that takes it first down to the bottom of the basin, and then eventually back up to the opposite margin of the basin, hundreds of kilometres away. At the bottom of the basin, temperatures become high enough that the water dissolves metals. As the water returns to the surface and enters cooler rock, these metals precipitate in ore minerals. Ore deposits formed in this way, containing lead- and zinc-bearing minerals, appear in dolomite beds of the Mississippi Valley region of the United States, and thus have come to be known as Mississippi Valley–type (MVT) ores. 

Sedimentary deposits of metals 

A Precambrian banded iron formation from northern Michigan.
Some ore minerals accumulate in sedimentary environments under special circumstances. For example, between 2.5 and 1.8 billion years ago, the atmosphere, which previously had contained very little oxygen, gained oxygen because of the evolution of abundant photo synthetic organisms. This change affected the chemistry of seawater so that large quantities of dissolved iron precipitated as iron oxide minerals that settled as sediment on the sea floor. The resulting iron-rich sedimentary deposits are known as a banded iron formation (BIF) (figure above), because after lithification they consist of alternating beds of Gray iron oxide (magnetite or haematite) and red beds of jasper (iron-rich chert).
The chemistry of seawater in some parts of the ocean today leads to the deposition of manganese-oxide minerals on the sea floor. These minerals grow into lumpy accumulations known as manganese nodules. Mining companies have begun to explore technologies for vacuuming up these nodules; geoscientists estimate that the worldwide supply of nodules contains 720 years’ worth of copper and 60,000 years’ worth of manganese, at current rates of consumption. 

Residual mineral deposits 

Recall from Interlude B that as rainwater sinks into the Earth, it leaches (dissolves) certain elements and leaves behind others, as part of the process of forming soil. In rainy, tropical environments, the residue left behind in soils after leaching includes concentrations of iron or aluminium. Locally, these metals become so concentrated that the soil itself becomes an ore deposit. We refer to such deposits as residual mineral deposits. Most of the aluminium ore mined today comes from bauxite, a residual mineral deposit created by the extreme leaching of rocks (such as granite) containing aluminium-bearing minerals.  The figure below shows the residual mineral deposits as well as placer deposits.

Placer deposits 

Placer deposits form where erosion produces clasts of native metals. Sorting by the stream concentrates the metals.
Ore deposits may develop when rocks containing native metals erode, producing a mixture of sand grains and metal flakes or nuggets (pebble-sized fragments). For example, gold accumulates in sand or gravel bars along the course of rivers, for the moving water carries away lighter mineral grains (quartz and feldspar) but can’t move the heavy metal grains (gold) so easily. Concentrations of metal grains in stream sediments are a type of placer deposit (figure above). Panning further concentrates gold flakes or nuggets swirling water in a pan causes the lighter sand grains to wash away, leaving the gold behind.

Where Are Ore Deposits Found? 

The Inca Empire of fifteenth-century Peru boasted elaborate cities and temples, decorated with fantastic masks, jewellery, and sculptures made of gold. Then, around 1532, Spanish ships arrived, led by conquistadors who quipped, “We Spaniards suffer from a disease that only gold can cure.” The Incas, already weakened by civil war, were no match for the armorclad Spaniards with their guns and horses. Within six years, the Inca Empire had vanished, and Spanish ships were transporting Inca treasure back to Spain. Why did the Incas possess so much gold? Or to ask the broader question, what geologic factors control the distribution of ore? Once again, we can find the answer by considering the consequences of plate tectonics. Several of the ore-deposit types mentioned above occur in association with igneous rocks. Igneous activity does not happen randomly around the Earth, but rather concentrates along convergent plate boundaries (specifically, in the overriding plate of a subduction zone), along divergent plate boundaries (along mid-ocean ridges), continental rifts, or hot spots. Thus, magmatic and hydrothermal deposits (and secondary-enrichment deposits derived from these) occur in these geologic settings. Placer deposits are typically found in the sediments eroded from such magmatic or hydrothermal deposits. The Inca gold formed in the Andes along a convergent plate boundary.

Credits: Stephen Marshak (Essentials of Geology)

Ore forming processes

Ore is a rock body that is enriched in one or more minerals.

Ore forming processes

Magmatic process

Magmatic is when liquid magna cools and solidifies to form igneous rock, it forms ore of metals, gems and other precious resources.

Layered plutons

Magma of the mafic nature solidifies in layers. Every layer has different mineralization. Some layers may have rich deposits. There are three type of processes which produces layered plutons
  • As magma solidifies (plutonic) its temperature gradually decreases. Minerals crystalize first in the liquid magma. Some crystals are denser than the magma which will settle down and is concentrated at the bottom which is also called crystal settling.
  • In the mafic magma crystallization usually starts from the bottom so early forming minerals are concentrated at the base.
  • When these processes occur and form minerals, at some time different magma can enter these mineraliztion which can change the early minerals formed.

Kimberlites

Kimberlites are igneous rocks that are produced in the mantle and are the main diamonds source.

Volcanic vent deposits

Volcanic vent deposits are from the gases escaping from the volcano where it precipitates such as sulphur. Sulphur usually deposits as a pure yellow deposition from the escaping gases and are used in sulphuric acid manufacturing.

Hydrothermal process

Hydrothermal process is the most common ore forming process. Water is heated by the magma chamber and this water dissolves metals. Metals are dissolved in the water and this solution seeps through cracks, fractures and through permeable rocks until they are precipitated and form a deposit. There are three types of water sources
  • Magmas of granitic composition when solidifies leaves a water-rich residual fluid which precipitates and form ore body.
  • In active volcanic activities when ground water seeps down through the crust, these magma chamber heats the water which provides the hydrothermal solution. This hydrothermal solution is at shallow depths and its precipitation makes an ore.
  • Sea water is heated when it seeps along cracks in the oceanic crust mostly at the mid oceanic ridges and submarine volcanoes. 
As of the fact that salty water increases the solubility of the water which is further enhanced by the heating. Hot salty water is a powerful agent in dissolving and transporting metals.Small amount of metal is present by an average in every crustal rocks so these hydrothermal solutions dissolves the tiny concentrates which percolating through permeable rock and cracks, ultimately in huge amount is precipitated to form an ore.
There are several types of hydrothermal mineralization


  • Hydrothermal vein deposits are formed when these hydrothermal solutions enter a country rock along cracks and fractures. It precipitates in the parent rock in a vein like structure.
  • The hydrothermal solution in vein deposits can also soak through the country rock which for disseminated ore deposits. This is less concentrated but as formed with the vein together they form economical deposit.
  • Disseminated copper depoits are associated with porphyry copper deposits. This depoist is associated with granitic to dioritic composition. 

Sedimentary process

Sedimentary process includes sedimentary sorting and precipitation.

Sedimentary sorting

Sedimentary sorting is the settling down of heavy minerals first and later of the less denser particles. example of such sorting is settling of gold in a stream when water slows down. As streams flows it carries clay, sand and gold particles. Gold settles down and conceive in the bedrock into coarser sediments. Gold concentrates in the bedrock and coarser gravel and is placer deposits.

Precipitation

Water seeping through the soil and bedrock dissolve ions which then flows into streams and oceans providing sodium and chloride ions which makes water salty. these can deposit only by the evaporation process which is not possible in the ocean however, lakes developed in the deserts evaporates and evaporites deposits are formed by precipitating crystals. 

Weathering process

Environments of high rainfall, water dissolves most soluble ions forms the soil and exposed surface rocks where insoluble ions are left as a residual. Iron and aluminium have less solubility so these ions are left behind which forms the bauxite. Bauxite is the principal aluminium providing residual but in come cases iron can also accumulate enough to be economical deposit. Weathering process can also provide metals to ores which are produced by other processes. Disseminated deposits contain metals but are not economical to mine so ground water and rainfall weathers these metals. In some cases metals react to weathering instead of removal which creates a supergene ore rich in metals above low grade mineralization.

Metamorphic process

Metamorphic rocks form by heat and pressure altering mineralogy and it can also expel water from the rocks. This water due to heat makes hydrothermal deposits so they are also associated with metamorphism