Geochemical Sampling

Geochemical Sampling

kacimo samy   8:07 ص

What is geochemical sampling?

Geochemical sampling is taking a small portion of Earth's material for finding its mineralogy, composition and grade such that it represent the whole area. Geochemical sampling is the basic technique used for the exploration of  minerals and their ores. So for this exploration multiple sampling techniques can be used to determine the place of ores. There are several methods of sampling

• Stream sampling
• Vegetation sampling
• Hydrogeochemistry
• Soil sampling
• Gas sampling
• Rock sampling

Stream sampling

Stream sampling is the samples taken from stream running through area and providing water and sediments from catchment area. These water when flow in stream it provides picture of the area from where water flows out. The stream source of sediments are by erosion of soil and rocks. It also gets water from inflow ground water which gives the subsurface mineralogy. So taking samples from the stream requires choosing area which provides picture of whole stream, sediments should be 80 mesh size acquired by sieving. Gold, magnetite concentrates in the stream sediments will settle down on the sieves as they are heavy minerals and finer sediments will be sieved. 

For base metals and geochemical mapping 0.5 kg sample should be taken but for Gold 10 kg sample should be analysed. For an active stream sampling should be at interval of 20-30 meters or 50-100 meters. Sampling should be from a depth of 10-15 centimetres to avoid excessive Iron and Manganese oxides. Panning should be done for Gold, Zinc, Garnet, Magnetite and Diamond.

Vegetation sampling

Vegetation sampling should be done where soil and ground water chemistry knowledge is required. It is useful in areas where soil samples are not available.

Geobotanical survey

Geobotanical survey is looking for a specific specie presence or absence that can indicate mineralization or specific rocks type. Certain plants especially flowering plants act as a exploratory guides or indicator to certain elements. Other cases are discoloration or stunted growth of plants. Example Mauve Cu flower or Beccium Humblei of Zambian Cu belt requires 50-1600 ppm (parts per million) to thrive.

Biogeochemistry

Biogeochemistry is the most used sampling technique in countries like Siberia and Canada where soil sampling is not easy so plant sampling is acquired to understand the subsurface geochemistry. Plants need most trace elements which are transmitted from ground through roots. Sampling is done through one plant specie where its first or second year leaves and twigs and samples. Sample 0.5 kg is then burned to ash which will be analysed through XRF, XRD and AAS.

Hydrogeochemistry

Water is used as medium of sampling because ores and minerals have effect on composition of water through dissolution. This technique have restriction as not all elements have equal dissolution rates, Some are insoluble, trace element concentration is very low to detect, Concentration is highly depending on climate and weathering, easily contaminated and ineffective in dry conditions.

However it is useful in areas where network of interconnected streams are present. Effective for fluorine and associated minerals, dissolved uranium and radon can help for uranium ores exploration and areas where sediments are not available. Lakes and bogs water are analysed for underlying mineralizations.

Soil sampling

Soil are of two types and accordingly can be used.

• Residual soil
• Exotic soil

Residual soil

Residual soil are those which are derived from the underlying rocks and gives clear picture of the subsurface. It is effected in areas where rock outcrop is not available so for this purpose selection of horizon is important. Soil horizons are OABC where B horizon is sampled because it is zone of accumulation and provides overall soil mineralogy. Where B horizon is not present so any horizon can be samples or all of the soil can be samples. Usually 100-200 grams is samples for lead, zinc, copper and 0.5-2 kg for gold.

Exotic soil

Exotic soil are those which is derived from other places so these can be used in glacial environments and is sandy desserts where finer particles are blown away so coarser are sampled for geochemical surveys.

Gas sampling

Gas sample are used as they can diffuse through over burden. A number of gases are used in finding different ores. Mercury is used for mercury deposits, uranium deposits and sulphide deposits. Radon and helium and gases used as an indicator for uranium deposits. Sulphide di oxide is used for oxidising sulphide mineralization. 

Rock sampling

Rock sampling is the most reliable and flexible method where large outcrops are available because rocks cannot be contaminated easily. Rock samples can be directly used for rock types, structure, mineralization and alteration. Rock sample can be stored for later examination and testing. Limitation in rock sampling are: outcrop not always available, scope is narrow and it shows only specific site not entire area. 0.5-1 kg samples is used for base metals and 10 kg sample for gold. Rock sample testing can only be done in laboratory and it consumes time.

Depositional Environments

Depositional Environments

kacimo samy   4:09 ص

There are many environments and every one has it's own characteristics. Every single depositional system have many environments where sediments deposits. On a broader sense there are three main types of depositional environments.

1. Continental
2. Terrestial
3. Marine

Continental

Continental of-course the name simplifies it in understanding, it is the all environments on the continent. Continental environments include 

• Glacial 
• Lacustrine
• Fluvial
• Dessert

Glacial

Glacial environments are those related to snow and ice. When it snows on the continent or mountain ranges after certain time snow becomes more rigid and become ice. The ice melting water carry sediments with itself and these deposits are then glacial deposits. Snow and ice creeps along the slopes where it carries sediments and boulders along itself and deposits where it velocity decreases. As ice can carry sediments to boulders therefore it's deposits are not sorted (fining upward), it's deposit can be distinguished by unsorting called tillites. 

Lacustrine

Lacustrine are the deposition in lakes. The sediments coarser to finer according to the streams power to carry sediments are deposited in the lakes. As to the gravity action the coarser sediments deposits first and then the finer particles therefore it gives a fining upward succession. Coarser are at the base and fine upward.

Fluvial

Fluvial deposits are the water channels deposition. These can be in the streams and rivers. Water manages to carry load from boulder to fine sediments according to the velocity of the water. Boulders are usually carried to a certain distance in a mountain ranges sometimes during flooding the capacity increases where water possess huge velocity. Water dumps it's load where the velocity breaks  As velocity decrease will first deposit coarser sediment and then finer, it also have fining upward succession.

Dessert 

Desert are the places where average rainfall is less than 25mm/year. So any deposition in such areas are considered as dessert deposits. In dessert mostly deposition is aeolian (wind). Wind takes finer sediments with itself and deposits elsewhere when wind speed breaks. The sediments are rounded as the corners are round by friction.

Terrestial 

These are environments that are present in the junction of marine and continent. These includes

• Delta
• Lagoon
• Beach

Delta

Delta deposits are those of the rivers mouth. Rivers carry continental sediments and are deposited where rivers open into sea/ocean. The coarser sediments are deposited at the early opening of river mouth where water velocity breaks and finer sediments are carried along water to the sea/ocean. Water channels are present like birds feet as water moves along different direction. 

Lagoon

Lagoon environments are those when sea/ocean water moves landward and it separates from main water body. These separated water makes environment where sediments deposits on continent.

Beach

Beach deposits are the terrestial environments on the onshore. Here deposits are mainly sand particles and some finer sediments are also present.

Marine

Marine environment as name indicates it is the deposition in the sea/ocean, open water body. These includes

• Continental shelf
• Continental slope
• Open marine

Continental shelf

Continental shelf is the shallow marine environment where landmass from continent is submerged by ocean water. These environments comes in marine as landmass in submerged by water. Continental shelf has major deposits of sand, clay  and corals. Coral reefs live in colonial form and are mostly on the continental shelf, shallow marine environment.

Continental slope

Continental slope is environment where continental landmass slopes into sea/ocean. Deposition on this slope are referred as continental slope deposits which are finer sediments. These sediments can be reworked by turbiditic currents to open marine environments. 

Open marine

Open marine environment is the overall ocean deposits where it starts after continental slope. This environment deposits are from finer to coarser, organism remains, basalts etc in deposit. All animal living in free water are deposited in open marine along with sediments (Continental and terrestial). These sediments supply are deposited in ocean.

Igneous rocks

Igneous rocks

kacimo samy   6:45 ص

Igneous rocks are those formed by the direct cooling of magma or lava.
Magma or lava is the molten form of rocks where intense heat turns them into hot magma. This is semi fluid and is in the subsurface. This magma can come out and produce lava and this also intrude in the joints or into the parent rock which is called dykes or sills. Igneous rocks are of two types according to the place where they forms i.e. intrusive and extrusive.

Intrusive

Intrusive rocks are those which are formed in the subsurface by direct cooling of the magma with overburden pressure. These rocks cools down gradually thereby giving time for minerals forming larger crystals. These are also called plutonic rocks. Intrusive rocks possess greater hardness than that of the volcanic rocks, this is because of the gradual cooling example are Granite, Gabbro and Peridotite. 

Extrusive

Extrusive or volcanic rocks are formed by the lava when it erupts from the subsurface. As in the open air and release of overburden pressure they cools rapidly and produces smaller crystals. The examples of volcanic rocks are Rhyolite, volcanic glass and many more.

Metamorphic rocks

Metamorphic rocks

kacimo samy   2:38 ص

Metamorphic rocks are formed by the heating of pre-existing rocks. The heat provide to a rock changes it mineralogical and physical changes which are called metamorphic rocks.

These rocks forms mostly where magma chamber is available to heat enough for mineralogical changes occurrence. These rocks have multiple features in distinguishing like schistosic, gneissic and slaty texures.

Texture is the physical character or a pattern.

Slaty texture

Slate are formed by the metamorphism of shale. Sheets are formed in the slates where it can broke  into sheets. This help in determining the slates.

Schistose

Schistose is formed after metamorphism of slate where it rearrange in forming irregular sheets like character which is schistosic texture.

Gneissic

Gneiss is the high grade metamorphic rock of shale which is distinguished by regular interval of dark and light bands present in it. These are called gneissic bands which is the recognizable character.

Metamorphism and its changes are following

Shale- Slate- Schist - Gneiss

Shale is sedimentary rock when metamorphose produces slate and slate to schist and schist to gneiss with increasing metamorphic grades. In the same way marble is produced by the metamorphism of limestone.

Sedimentary rocks

Sedimentary rocks

kacimo samy   12:54 م

Sedimentary rocks

Sedimentary rocks are those which are made up of pre-existing small particles called sediments. There are two types of sedimentary rocks

• clastic
• non clastic

Clastic 

Clastic rocks are those which are made up of sediments that are transported from other places. The sediment supply to the deposition are can be by the help of wind, gravity, water or ice. These rocks have many examples including sandstone and shale. The sandstone are made up of grains clustered together by the means of cements. Cement is the mineral that hold rock together, not splitting them in finer grains. Cement in the sandstone are silica or calcite. The sand particles are carried to the depositional area with the help of wind, an example of desert or the rivers deposits with the help of water. The grains are suspended either in the water or wind and when both medium speed breaks it can not carry the weight so are then deposited. It takes million years in order to the deposited sediments to form a rock as it needs pressure which can be overburden. It forms a rock which is called clastic rock.

Non clastic

Non clastic rocks are those which deposits at the area of origin. These sediments are not reworked by trasporting agents but are thus deposited where it have been formed or at lesser distance travel. These are also formed by the weathering process but not the erosional process. The best example of non clastic rocks are coral reefs. Corals habitat is colonial so when they die, deposits at its origin place. Limestone can also be formed clastically when the shells are reworked by turbiditic currents and drops calcium carbonate shells from shallow marine to deeper portion but eventually that won't be non clastic.

Weathering

Weathering

kacimo samy   12:21 م

Weathering

Weathering is the dissemination, destruction or breaking of the rock to finer particles. Weathering occur by many means, it can be either chemical, organic or mechanical.

Chemical weatheing

Chemical weathering occurs by the ion exchange method, dissolution of some particles which breaks the bond and the rock is thus weathered. The rock is dissolved when it is acid rain and the rock weathers. the best example in understanding chemical weathering is water dissolving limestone producing Karst topography. This help in the building of sedimentary rocks by erosion. Erosional means are wind, gravity, ice and water. When sediments are eroded it deposits at some other place, builds sedimentary rocks

Organic weathering

Organic as the name indicates is done by organisms. Plant roots break rock when they move into the rock and so the weathering process occurs.Rock is first weakened by penetrating forces of roots and thus split into smaller and smaller pieces. These are then eroded.

Mechanical weathering

Mechanical weathering is the physical breaking of a rock by means of force. The best example of the mechanical weathering is the permafrost. The water enters the pores and joints of a rock and when it freezes at low temperature areas. As the property of water at freezing is expansion so the water in rocks expands which widens the spaces and after some time disintegrated.

The deposition of these sediments occur when wind, water or ice slows in speed and have no capacity to blow away the particles, it deposits.

Types of rocks

Types of rocks

kacimo samy   4:52 ص

There are three main types of rocks

Sedimentary

Metamorphic

Igneous

Sedimentary rocks

Sedimentary rocks are those that are produced by the deposition of sediments (small particles). The sediments cluster and are cemented that produces a rock. Sedimentary rocks are also called soft rocks as they are not produced by heat factor but are produced by the weathering of the rocks that are exposed on the Earth surface. 

Metamorphic rocks

Metamorphic rocks are those which minerals are changed by heat not directly magma but heat which does not melt it but only changes the mineralization. 

Igneous rocks

Igneous rocks are those which are produced by the direct melting rocks (magma). The magma cooling in the subsurface are called plutonic or intrusive rocks and those lava cooling on the surface are the volcanic or extrusive rocks.

The rock cycle changes the rocks from one form into another.