Presentasjon om: "Hydrotermale mineralforekomster"— Utskrift av presentasjonen:
1Hydrotermale mineralforekomster The hydrothermal feeder systems are preserved as dikes and veins that are mostly composed of quartz and secondary ore-minerals such as for example gold.Here we can study the chemical properties of the hydrothermal solutions that actually formed our hydrothermal ore-deposit
2Ca Si K Li Na Ti Al Ta B O Fe P However, getting lost in the bizarre world of pegmatite minerals, let us have a look at the processes that characterises most granitic and pegmatitic melts.First we have the introduction of a silicate liquid at a temnperature of 700 to 900 oC. Being a granitic melt it typically comprise ~70 wt% SiO2 and the other major elements seen on this diagram. Most of the elements are quite common, but we also have important cocentrations of rare expensive metals, here exemplified by Ta (tantalum).BOFeP
3Au Li Cu Ta Ti B Mo Al Si Fe Bi Qz Kfld As the temperature is falling, common minerals such as biotite, quartz and K-feldspar will crystallise and wil remove various elements from the melt.However, the cocentration of our rare metals is to low to make its own mineral. Here, I have included some more rare metals that remain in the melt because they have nowhere else to goFeQzKfld
4Au Mo Au Li Li Cu B B Cu Ta Mo Ta Finally, the concentration of the exotic metals, Ta, Li and B becomes so high that they begin to form their own minerals. Here, Li goes into Li-mica, forexample the one called lepidolite, Ta makes the mineral tantalite and B makes tourmaline where B is a major component.However, we still have some elements left that have not obtained high enough concentrations to form their own mineral
5CuAuMoAgain, as with the chalcophile elements in mafic magma chambers we have to play a little trick. However, this time we are not experiencing liquid immiscibility under the formation of sulphide melts.In the granitic melts, the concentration of H2O is so high, that it is forced to exsolve from the silicate melt and make its own phase. Accordingly, all of a sudden we have a myriad of water droplets forming all over the place in the silicate melt that remain.It happens so that many of the elements that are left in the melt are attracted to the water-rich phase. We say that they fractgionate in favour of water and obtain high concentrations here.However, H2O is very light, so what happens next is that H2O, will accummulate in the top of the magmachamber
6However, H2O is very light, so what happens next is that H2O, will accummulate in the top of the magmachamber.From here, it may either escape along fractures into the coutry rock or by time, it may crystallise under the roof of the magma chamber under the formation metal rich deposits.Given that the mineral deposits are forming from varm water-rich solutions that were derived from the magmatic melts, we call them magmatic hydrothermal deposits and they are the topic of the next double lecture.
7Fluid inclusion trails in quartzThe reasons for this is that our hydrothermal solutions are preserved in microscopic cavities in the quartz as fluid inclusions. In otherwords small reservoirs filled with H2O, Salts and the economis elements that formed the deposit.GlassyMilky
8Different types of fluid inclusions Trains of secondary inclusionsalong healed micro-fractureThis is what the inclusions look like at high magnification. They are les than o.1 mm across and comprise a gass bubble, liquid and sometimes solid minerals that precipitated from the fluids after the fluid inclusion formed.Primary inclusion withdaughter mineralPrimary inclusions with sylvite (KCl)
9HYDROTERMALE PROSESSER MINERALFOREKOMSTERDANNET VEDHYDROTERMALE PROSESSERViktige forekomster av metaller: Cu, Mo, Sn, W, Fe,Au, Ag, Zn, PbViktige forekomster av industrimineraler: Zeolitt, asbest,talk, kaolin, pyrofylitt, flusspatEndogene, hypogene, epigenetiske og (stratabundete)Avsetninger som fyller alle typer av sprekker, hulrom og rom etter utlutete mineraler
10Skorpedyp: Episonale forekomster: 0-5 km dyp, kaolin, Zn-Pb, flusspat, Au-AgMesosonale: 5-10 km, Au, Sn-W, Cu-Mo, talk, asbestHyposonale: km, AuMineralene fyller:Forskastninger/store sprekker, gangmineraliseringerNettverk av sprekker, stokkverk- el. porfyr mineral.Tektoniske, kollaps og hydrauliske breksjerHulrom og porer i sandsteiner, karst-rom i karbonaterog fortrenger primære bergartsmineraler(hydrotermal omvandling)
11Drivkraft: Termiske gradienter og trykkgradienter Væsketyper: Metamorft vann (dehydrering av vann-holdige mineraler),magmatisk vann (salt-rikt vann avblandet fra granittiske smelter)og overflate vann/grunnvann.Gull-kvarts-turmalin gang
12We are now definitively leaving the magmatic systems that we were concerned about in chapters 1 and 2.The mineral deposit forming systems on which we are now focusing comprises aqueous solutions derived from nonmagmatic sources.The vast majority of mineraldeposits that can not be explained with a direct magmatic origin are formed by hydrothermal solutions derived from non-magmatic sources.Even in the famous gold deposits in the Witswaterrand basin in South Africa that previously were thought to be placer deposits that formed by rivers, hydrothermal processes were an important ore-forming player.Depending on their origin, hydrothermal fluids are divided in magmatic, we talked about them already, seawater, meteoric, connate and metamorphic fluids.Sea waterSea water comprises by far the largest water reservoir in the surficial environment of the Earth.Sea water is saline and the most important electrolytes dissolved in sewater are Na+, K+, Ca2+, Mg2+, Cl-, HCO3(-) and SO4(2-)Sea water is a relatively homogenous mixture with 3.5 wt% solvents and the composition is buffered by the decomposition of continental and oceanic material.Sea water is particularly important in the formation of oceanic mineral deposits, particularly associated with black smokers (Zn, Cu, Au) but also concretions of Mn and or Fe and a host of economic metals (Mn-Fe Nofules).Meteoric waterMetoric water comprises the second largest reservoir on Earth and is essentially a product of the hydrological cycle.In other words water that interacted with the atmosphere.Meoric water is infiltrating the soil profile and the upper part of the crust.It is involved in the formation of low T mineral deposits, for example the most important variety of U-deposits, known as roll front U-deposits.Connate WaterConnate water is also known as formation water and may comprise both seawater and meteoric components .Essentially it is water that is arrested in the pores of sedimentary formations.It has been removed from the meteoric cyclle and is isolated from the sea, so this property characterises connate water.Metamorphic waterMetamorphic water is forming when a mineral assemblage is heated and subsequently decompose under the formation of a less water rich mineral assemblage.We briefly talked about some of these important reactions in the formation of skarn deposits.Metamorphic reactions controls the water budget of metamorphic water.Essentially, minerals with progressively smaller proportions of OH or H2O is forming as the temperature is increasing to a point when only anhydrous minerals are stable.Particularly at high temperatures, CO2 becomes an important component together with water.
13So far we have mostly been concerned with the formation and chemical properties of ore-forming hydrothermal solutions.However, having formed an ore-forming solution, the next step is to mobilise the fluid phase.As mentioned earlier, the driving force is a pressure gradient that initiate a migration from high pressures to low pressures and temperatures.Contrary to magmatic hydrothermal solutions, the fluids we have talked about so far are not ore-forming before they have depleted economic elements from the formations through which they flow.Therefore, it is important that the hydrothermal fluids interact with as large a rock volume as possible.Subsequently the fluids must be focused through a narrow channel way, a fault for example, where the economic metals meet a chemical barrier that provokes the precipitation of economic minerals.In the first two scenarios shown on the figure, we are handling pervasive fluid flow. Here, the porosity and the permeability of unconsolidated sediments allows the hydrothermal fluids to move through entire formations as a massive front. In other words, fluid migration is not focused in narrow path-ways.The process is only relevant for the upper 1-2 km of the sediments where the absence of widespread diagenesis and the unconsolidated nature of the sediments allow pervasive flow.The driving force may be a pressure gradient maintained by the topographic height difference of the grounwater threshold in uplifted regions compared to the valleys and the plains.Alternatively, the pressure gradient is provided by orogenic uplift during regional compression.In the first scenario, water is passively flowing towards lower presures, whereas in the second case, water is actively squezed out of the rocks like water being squezed out of a sponge.The ore-forming fluids in both cases maintain relatively low temperatures and, mostly, will have a low concentration of economic elements.Focusing of the ore-forming solutions can be enhaced by impermeable topographic barriers or faults planes and narrow lithological units with higher than average permeability (e.g. Comglomerates).
14In the next two scenarios, the driving force is a strong temperature gradient, somewhat comparable to the situation around a hot magmatic body.The upper figure shows the wellknown scenario in the oceanic crust or a thinned continental crust where the proximity to the asthenospheric mantle provides the temperature gradient.Hot hydrothermal solutions in the lower parts of the profiles bouyantly flow toward the upper and colder regions.In the ocenaic crust, the fluid flow is strongly controlled by microfractures and fault planes.In Intracratonic basins with small sedimentary thicknesses, the fluid flow may be more pervasive and the fluids may establish convection cells.However, in thicker more compacted sedimentary rocks, the fluid flow will also be chanellised along faults and fractures.In both cases, the hydothermal solutions are hot and may have high concentrations of economic elements.
15At greater depth where the rocks are compacted and and diganesis or metamorphic reactions has sealed the rocks, all fluid migration is focussed along faults.Fluid migration along faults has experienced intensive research over the past years, not leats in conjunction with the massive efforts to understand the migration of hydro-carbons from the the source rock to the oil reservoirs.However, particularly in the migration of ore-forming hydrothermal solutions, a concept known as seismic pumping has emerged.The idea is that earthquakes along major fault zones forcefully drives the fluids towards more shallow regions.The flow rate may be more than 10 m/y which actually is a huge flow rate.
221-3 g/t Au Kaolinitt Kvartsårer med Ag +Au Champagne Pool, New Zealand AgS
23Ore deposits related to granitic plutons are formed by the interaction of twomajor processes:Exsolution of metalliferous magmatic waterduring crystallization of the pluton.Heat source for the convection of meteoric and/ormetamorphic water which may leach metals fromthe rocks along their pathways.
24Ores consisting of a stockwork of thin polygeneration veinlets of quartz, ore-sulphides and pyrite. Sulphides also as dissemination, i.e. along micro-fractures
25Stockwork of quartz-sulphide veins enveloped by quartz-pyrite-sericite alteration (phyllic; brown)in granodiorite
26Sprekkebundet talk omvandling Serpentinitt (venstre) and peridotite HYDROTERMAL OMVANDLINGHowever, as the hot hydrothermal fluids migrate through the country rock they actually change the composition of the rocks that are in contact with the hydrothermal feeder systems. We call this “hydrothermal alteration”.During this process they may actually form another type of mineral deposits.One type of deposits that are formed this way are talc deposits, magnecite and soap stone deposits (Kleberstien). Accordingly, the hydrothermal alteration of a mafic country rock composed of olivine and pyroxene form an entirely knew mineral assemblage of talc and magnesite (together fdorming soap stone).Sprekkebundet talk omvandlingSerpentinitt (venstre) and peridotite(høyre)
27Talc Linnajavri Very large talc resource Raudfjellet AltermarkMine in operationby NorwegianTalc (Omya)RaudfjelletLarge talc and magnesite resourceCaledonian mantle fragments Alteration TalcTalc and soapstone deposits are known from many localities in Norway and we have or have had several mining operations where they either quarry soap stone as a dimension stone, or talc as an industrial mineral.
28METAL ACCUMULATION DURING RESIDUAL FOREKOMSTERAlAuNiA1A2CDOBauxiteSaprolite goldNickel lateriteMETAL ACCUMULATION DURINGWEATHERINGHydrothermal solution also play an important role in forming so called residual ore deposit.In this type of deposit the ore forming solutions comes from above and due to a high content of organic material (=organic acids), they dissolve Au and NI and transport it deeper in to the soil profile. Here Au an Ni may be reprecipitated in a narrow zone forming an ore deposit.This proces can only take place in Tropical humid climates where we also have the formation of Bauxite deposits i.e. Al deposits. The oreforming proces is the same except that Al is not dissolved in the hydrothermal fluids. Rather it comprises the only element that is entirely insoluble, hence is left behind whereas all the rest is removed.
31Ni laterite deposits NEW CALEDONIA Garnierite, Ni-serpentine (Ni, Mg)3Si2O5(OH)4
32Utluting fra øvre del av en forekomst Utfelling lengre ned Gull Sølv Sekundær anrikningOverflatevannUtluting fra øvre del av en forekomstUtfelling lengre nedGullSølvResiduale forekomsterUtluting av løselige mineral fører til en oppkonsentrering av de ikke-løseligeGarnieritt (vannholdig silikat av nikkel og magnesium)Lateritt (jern og/eller aluminium)Bauxitt (lateritt med nesten bare aluminium)Lateritt består av et materiale rikt på oksider og hydroksider av treverdig jern og/eller aluminium. Med mye aluminium har man overgang til bauxitt. Det organiske materiale er fraktet bort.Eroderte, transporterte og resedimentert bauxitter.Finnes i nedbørrike tropiske eller skogkledte, varme og tempererte strøk.
33Precambrian Banded Iron Formations, Australia Sedimentære mineralforkomsterPrecambrian Banded Iron Formations, Australia(BIF, viktigste kilde til jern)