Exam 2 Essay | eportfolio

Becca Hedges

Geo

Dr. Kaiser

6 Nov. 2016

Exam 2 Essay

The minerology of a hydrothermal deposit is a concentration of minerals formed by the precipitation of solids from hot mineral rich water. The mineral rich water is called a hydrothermal solution. The hydrothermal solution contains saturated minerals and oxygenated water. Through chemical processes these minerals are deposited in the openings of rocks. These deposits fill in empty cavities. They can also replace the rock themselves creating a replacement deposit. Four conditions are required for this process to occur.

First, the presence of hot water is needed in order to dissolve and transport minerals. The hotter the water, the more dissolved the solution will be. At certain points in the system, colder water will have more precipitation occurring. The speed of the fluid movement can affect if the deposit is a large mass or if it was spread over some distance. This water could come from rain or snow melt that seeps downward from the surface slowly heating due to the geothermal gradient, a nearby magma body creating partial melt, or seawater from an oceanic plate subducting.

Second, interconnected openings like fractures from a magmatic intrusion or a fault line in the country rock is needed to allow for the transport of the hot mineral rich water. These can be either sill or dyke orientations which match the stratigraphy of the location.

Third, and there must be empty areas or cavities available for the deposition to occur in. This could occur with any zones of weakness in the crust.

Fourth, there must be some type of chemical reaction occurring either by gas emissions and or the oxygenated water to create or alter the material being deposited.

It really does not matter what type of material the country rock is made of or the geologic environment these veins are located, this process will occur. The chemistry of the country rock will change from one location to another while being influenced by intrusions. Even the depths will vary. A pattern is formed from this process that is very important to the exploration of these deposit locations. The veins where the minerals of interest are more concentrated are typically surrounded by this system. On the surface the center is the location you would find a gossan mound. Surrounding the gossan area is the sericite, followed by the argillic and then the propylitic. Each of these areas contain different associated minerals.

The supergene process explains in a bit more detail of the hydrothermal process affects the surroundings due to depth, pressure, or other outside influences. The gossan is filled with materials of the supergene process that are relatively insoluble so, they will not dissolve in the water. As the oxygenated solution travels the system, oxides and carbonates are formed in the oxidized zone. This zone produces sulfuric acid as a by-product of the oxidation as it moves through the system. The original material present in this zone will determine if oxide or carbonate minerals are formed as well as how much space is available to deposit materials. If a softer less resistant material like a limestone is present in this zone, the easily dissolved limestone would create a larger cavity for material to be redeposited. If the oxidized zone contains any silicate minerals it will cause them through chemical reactions to break down in to clays. At the bottom of this leached zone is the water table. The depth at which this level is located will vary based on location. By this time all of the excess oxygen in the water has been depleted and what remains becomes diluted. This new zone is the reduced zone. This is where highly enriched concentrations of sulfides are deposited. Below that is the last zone of the system. This is the hypogene zone. This material is unaltered through this supergene process.

There are many different types of deposits that can be created during this process. Being largely dependent on the material present and the geologic environment which has the most control of the chemistry associated, there are different variations possible. The geologic history is one of the most important and complex factors of this. For example, the difference between continent-continent and ocean-continent zones will have produce different minerals. This process is however more common with mid-ocean ridges; a divergent setting and subduction zones; a convergent setting. As water is a main player hydrothermal deposits are less common along continent-continent convergence areas.

At divergent plate boundaries on the ocean floor, the boundary line pulling apart allows magma to rise. This is a low pressure, high temperature location. The sea water is heated from the magma and holds high concentrations of zinc, copper, lead. Gold, selenium, cadmium, bismuth, tin and silver are also in the dissolved solution. Through convection currents and black smokers this solution is transported to a different locations. Once this solution comes into contact with the colder water precipitation occurs. It will also mix with the sediments on the ocean floor. Mid-ocean ridges also contain manganese-rich polymetallic nodules. These nodules are most likely due to a slow accumulation of minerals from the sea water over a larger area. The major minerals associated with this area are pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena. In the outer layers adjacent to the clusters of the major deposits is called a fringe area. In this environment silver and gold are found in higher concentrations. Pyrrhotite and magnetite tent to occur on the boundary of the core and fringe zones. Minor minerals commonly associated with these areas are tetrahedrite, marcasite, bornite and barite. The barite and cherty silica are more commonly associated with gangue minerals. Gangue minerals can have similar compositions and or economic value as the major minerals but are not what is desired in that particular area. Pyrite is also fairly concentrated with fringes.

Areas where subduction of one plate underneath another is common around the globe. When an oceanic plate is the one subduction hydrothermal deposits are abundant. These areas have it all, seawater carried down with the plate, partial melting and intrusions from the magma, and meteoric water percolating through the system. This gives a large variety of deposits occurring. This would be a low pressure, low temperature system near the surface changing into a high pressure, high temperature system associated with depth. The oceanic plate being subducted carries hydrated silica rich material. This material interacts more and more with the crustal plate the deeper into the earth is goes. These areas also have many counter parts which aid in the development of different systems.

A vein deposit is a zone that has a high concentration of mineralization. Most veins occurs in zones of weakness in the crust. Igneous rocks and other hard materials break and fracture allowing for more free space unlike softer material which stay fairly close together. These deposits are often referred to as a metalliferous lode deposit. It contains valuable ore deposits like native or free gold, silver, copper, lead, zinc and other metal sulfides. Gangue minerals like quartz and calcite are associated. If there is more than one vein of the same contents in a particular location it is referred to as a vein system. Veins rarely show on the surface gold, or other ore minerals desired. Because of this geochemical pathfinders are required. The typical elements searched for are arsenic, antimony or mercury. These deposits can occur in three main varieties.

Mesodermal deposits occur in moderate temperatures and pressures. This area will produce major minerals of chalcopyrite, sphalerite, galena, tetrahedrite, bornite and chalcocite. Minor minerals found include Fluorite, tourmaline, apatite, fluorite, and gold. Gangue minerals commonly associated are quartz, carbonates (calcite, siderite, rhodochrosite), and pyrite. This usually involves some kind of alteration process in the oxidized zone. Associated with the altered zone is more quartz, sericite, calcite, dolomite, pyrite, orthoclase, chlorite and other clay minerals like weathered feldspars.

Epithermal zones are typically located within low temperatures low pressures. These are associated with volcanic activity. This hydrothermal system presents at the surface in hot springs or fumaroles. Major minerals here include cinnabar, selenides native silver, gold, and mercury with some arsenic and antimony. Minor minerals associated are galena, sphalerite and chalcopyrite. Gangue minerals here include quartz, amethyst, chalcedony, calcite, rhodochrosite with some fluorite, barite and hematite. Open spaces show characteristics of repeated mineralization of free solution creating drusy cavities and banding. Some younger deposits in areas have been removed most likely due to erosion. Most of the major deposits have be found in tertiary age. There are some in the Jurassic. Some alteration minerals include chlorite, calcite, pyrite, sericite, alunite, zeolites, and silica. These areas are often so diluted with excess water that the mineral content is low. These areas are more commonly associated with andesite filled country rock. Vein deposits associated with intermediate to felsic composition could havemajor minerals of pyrite, sphalerite, galena, chalcopyrite, and tetrahedrite. Minor minerals include bornite, chalcocite, gold, hematite, and pyrrhotite.

The alteration of these locations will vary dependent on the country rock. Sulphide minerals are easily oxidized to create sulphates. Most of which are soluble in water. This means that no sulphides will be found in gossans. The secondary or enriched zone is where the system will precipitate them out.

Some key features to keep in mind when exploring these zones and locating an area suitable to host veins. High concentrations of silica, clays, and pyrite can help determine if that is a good area to drill. Trace element geochemistry to fine pathfinders. A detailed map of alteration to find the direction of the mineralization. A knowledge of minerology associations.

Porphyry copper deposits can have so many variations one model would not give an accurate series of deposition. The most classification that occurs are orthomamatic which are driven by waters derived from molten rock and convective systems derived from meteoric waters. Orthomagmatic systems are dependent on an igneous intrusion, high salinity, long periods of consistent high temperatures with the system being closed and re-fractured over and over again. This would also cause the material to be altered and mineralized repeatedly. Metals and sulfur from the magma are also a must. Convective systems require the country rock to contain the water for the system, low salinity, boiling period must be short and localized. The temperatures will spike but will cool down rapidly and stay cool for extended periods of time. Metals and sulfur are acquired through the country rock from oxidation and alteration.

These deposits are typically located along orogenic belts. Within two settings along these belts island arcs and continental margins are more common. Different ages have produced different concentrations of deposits. The Paleozoic and pre Cambrian age are the most uncommon. This is due to the severity of metamorphism that has occurred. The Mesozoic are is the most abundant.

There are three major types noted. Plutonic, volcanic, and classic. Plutonic occur in batholithic areas. Mineralization occurs in one or more phases. Volcanic are directly involved with volcanos themselves. This happens in the volcanic rocks and any associated plutons. Classic occurs unrelated to the host rocks. Mineralization will occur in any part of the system. The earliest material mined of this type came from the Cenozoic age.

High alteration occurs with granitic rocks. The alkali metal to hydrogen ratio directly effects this system. If it is a low ration feldspars, micas and other silicates stability decreases and hydrolysis begins. This lets cations free and tries to reach equilibrium. Four alteration types that are common are propylitic, argillic, phyllic, and potassic. Propylitic involves weak hydrolysis. This type will produce plagioclase, chlorite, epidote and carbonate. Quarts and alkali feldspars are stable. Argillic has a higher hydrolysis and is associated with quartz, kaolinite and chlorite. Phyllic produce quartz, sericite, and could have pyrite. Potassic occurs in high temperatures altered by solution. Everything is unstable in this type. This could produce quartz, k-feldspars, biotite and intermediate plagioclase like labradorite. All four of these setting occur adjacent to intrusions and highly dependent on the country rock. Potassic and propylitic are the most common.

Major minerals found in these locations are pyrite, chalcopyrite, bornite, and molybdenite. These are usually mixed with quartz and sericite. Minor minerals include bornite chalcocite, magnetite, hematite, covellite, native copper and ilmenite. The more permeable material will host more deposits. In the field things to look for are dykes and granitic rocks with porphyritic textures. Breccia zones that contain sulfides in the crevasses, epidote, chlorite, quartz, sericite and secondary biotite alterations. Fractures with sulfide or quarts is a good indication. Trace copper in the soil or streams could also be a good sign.

Some other less common deposit types include: Mercury-sulfides. These are found in areas with low-temperature hydrothermal environments. Major deposits include cinnabar, pyrite, and marcasite Minor minerals are Native mercury, sphalerite, chalcedony, barite, carbonates and quartz. Tin-tungsten-bismuth deposits. These are found veins associated with granitic intrusions. Major minerals are arsenopyrite, pyrite, marcasite, and pyrrhotite. Lead-zine deposits in a carbonate host. These are located with replacement of dolostone. Typically low temperatures and not affiliated with any type of magma processes. Major minerals include Galena, sphalerite, fluorite, pyrite, marcasite, chalcopyrite, calcite, dolomite, quartz, and aragonite. Minor minerals are wurtzite, millerite, bornite, and covellite.

Skarn deposits can form thru any process of metamorphism but most are common in contact metamorphism of limestones. They can be found in most geologic setting but the difference between rocks and skarns is the minerology. They are associated with a large range of minerals but the most common are with garnets and pyroxene. The mineralogical classifications are Exoskarns which are developed in the sedimentary surrounding and the thermal source. Endoskarns are found within igneous intrusions. Skarns are typically found along magmatic arcs or beneath continental crusts associated with subduction. Seven main types of skarns are common. Iron skarns are the largest. They are desired for magnetite. Minor minerals containing elements nickel, copper, cobalt and gold. Gold skarns are common with mafic diorite. The desired material here is gold, iron, and copper. Tungsten skarns are associated with calc-alkali plutons or orogenic belts. These are found in high temperature, high pressure areas. Copper skarns are the most abundant. They are the most associated with orogenic areas relating to subduction. This is a very shallow low temperature, low pressure setting. They are also common with porphyry copper deposits. Zinc skarns are more common with subduction or rifting. They can be mined for lead and silver. Molybdenum skarns are common with leucocratic granites. This material does not have any ferromagnesian minerals. This type contains molybdenum, tungsten, and copper. Tin skarns are only associated with high silica granites created by partial melting. Skarn deposits are typically a high temperature system where silica rich material intrudes carbonate rocks. In general, major minerals desired are magnetite, molybdenite, sphalerite, galena, chalcopyrite, quartz, garnet, amphibole, pyroxene, epidote, and carbonates. Minor minerals include pyrrhotite, hematite, and gold.

All of the hydrothermal deposits representations we have discussed are related to each other. Some processes could happen as a single system or within a combined system. Mineral associations are very important though, they can be misleading. There is much overlap between major and minor minerals and the different processes in which they can be precipitated from the system. The geologic history and current environment is what help us determine individual mineral deposits. In closing, a list of common minerals discussed with their chemical formulas to help represent in which hydrothermal deposits they are most likely to be associated with.

Chalcocite Cu2S Gold Au

Galena PbS Silver Ag

Sphalerite ZnS Copper Cu

Pyrrhotite ? Magnetite

Cinnabar HgS Hematite

Covellite CuS Ilmenite

Pyrite FeS2 Garnet

Marcasite FeS2

Molybdenite MoS2

Chalcopyrite CuFeS2

Bornite Cu5FeS4

Tetrahedrite (Cu,Fe)12Sb4S13

Quartz SiO4

Calcite