Hydrotermale mineralforekomster

Presentasjon om: "Hydrotermale mineralforekomster"— Utskrift av presentasjonen:

1 Hydrotermale 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

2 Ca 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). B O Fe P

3 Au 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 go Fe Qz Kfld

4 Au 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

5 Cu Au Mo Again, 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

6 However, 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.

7 Fluid inclusion trails
in quartz The 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. Glassy Milky

8 Different types of fluid inclusions
Trains of secondary inclusions along healed micro-fracture This 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 with daughter mineral Primary inclusions with sylvite (KCl)

9 HYDROTERMALE PROSESSER
MINERALFOREKOMSTER DANNET VED HYDROTERMALE PROSESSER Viktige forekomster av metaller: Cu, Mo, Sn, W, Fe, Au, Ag, Zn, Pb Viktige forekomster av industrimineraler: Zeolitt, asbest, talk, kaolin, pyrofylitt, flusspat Endogene, hypogene, epigenetiske og (stratabundete) Avsetninger som fyller alle typer av sprekker, hulrom og rom etter utlutete mineraler

10 Skorpedyp: Episonale forekomster: 0-5 km dyp, kaolin, Zn-Pb,
flusspat, Au-Ag Mesosonale: 5-10 km, Au, Sn-W, Cu-Mo, talk, asbest Hyposonale: km, Au Mineralene fyller: Forskastninger/store sprekker, gangmineraliseringer Nettverk av sprekker, stokkverk- el. porfyr mineral. Tektoniske, kollaps og hydrauliske breksjer Hulrom og porer i sandsteiner, karst-rom i karbonater og fortrenger primære bergartsmineraler (hydrotermal omvandling)

11 Drivkraft: 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

12 We 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 water Sea 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 water Metoric 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 Water Connate 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 water Metamorphic 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.

13 So 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).

14 In 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.

15 At 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.

16 Volcanic hosted Cu, Zn, (Au)

17 Volcanic hosted Cu, Zn, (Au)

18 Volcanic hosted Cu, Zn, Pb (Au)

19 Sedimentary hosted Zn, Pb, Cu, Ag

20 Orogenic gold, shear zones (greenstone belts)

21 Carbonate hosted (MVT)
Pb – Zn - Ag

22 1-3 g/t Au Kaolinitt Kvartsårer med Ag +Au Champagne Pool, New Zealand
AgS

23 Ore deposits related to granitic plutons
are formed by the interaction of two major processes: Exsolution of metalliferous magmatic water during crystallization of the pluton. Heat source for the convection of meteoric and/or metamorphic water which may leach metals from the rocks along their pathways.

24 Ores consisting of a stockwork
of thin polygeneration veinlets of quartz, ore-sulphides and pyrite. Sulphides also as dissemination, i.e. along micro-fractures

25 Stockwork of quartz-sulphide veins
enveloped by quartz-pyrite-sericite alteration (phyllic; brown) in granodiorite

26 Sprekkebundet talk omvandling Serpentinitt (venstre) and peridotite
HYDROTERMAL OMVANDLING However, 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 omvandling Serpentinitt (venstre) and peridotite (høyre)

27 Talc Linnajavri Very large talc resource Raudfjellet
Altermark Mine in operation by Norwegian Talc (Omya) Raudfjellet Large talc and magnesite resource Caledonian mantle fragments Alteration  Talc Talc 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.

28 METAL ACCUMULATION DURING
RESIDUAL FOREKOMSTER Al Au Ni A1 A2 C D O Bauxite Saprolite gold Nickel laterite METAL ACCUMULATION DURING WEATHERING Hydrothermal 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.

29 Bauxite ores, Al(OH)3

30 Saprolite gold deposits

31 Ni laterite deposits NEW CALEDONIA Garnierite, Ni-serpentine
(Ni, Mg)3Si2O5(OH)4

32 Utluting fra øvre del av en forekomst Utfelling lengre ned Gull Sølv
Sekundær anrikning Overflatevann Utluting fra øvre del av en forekomst Utfelling lengre ned Gull Sølv Residuale forekomster Utluting av løselige mineral fører til en oppkonsentrering av de ikke-løselige Garnieritt (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.

33 Precambrian Banded Iron Formations, Australia
Sedimentære mineralforkomster Precambrian Banded Iron Formations, Australia (BIF, viktigste kilde til jern)

34 SEDIMENTÆRE FOREKOMSTER

35 Entrapment mechanisms for gold other heavy minerals in fluvial systems
and other heavy minerals in fluvial systems Physical barriers

36 Gold accumulation Old river bed Meandring stream Heavy-mineral sand Zone of suiteable velocity of flood water

37 Witwatersrand alluvial fans
Ventersdorp basalt VCR conglomerate

38 Ore deposit models for Witwatersrand gold

39 Placer mineralforkomster

40 Witwatersrand

41 Beach-placers formed by heavy-mineral
separation by action of waves, wind and current Normal situation Open stormy coast Storm waves Windy

42 Kennecott Open Pit Copper Mine, Utah
Reserver Kendte ressurser der kan utnyttes idag innen givne økonomiske og juridiske rammer Kennecott Open Pit Copper Mine, Utah

43 International terminology platetectonic elements
Destructive plate margin Contructive plate margin Destructive boundary Contructive boundary Marginal basin Spreading ridge International terminology for the different platetectonic elements Ensialic island arc Continental island arc Subduction trench Ensimatic island arc Back-arc basin Foreland basin Continent Oceanic crust (ophiolite) Continental crust Fore-arc prism Benioff zone (zone of earthquakes)

44 Present plate configurations
DV CV TF Intracontinental Intracratonic Present plate configurations DV = divergent plate margin CV = convergent plate margin TF = transform fault Continental drift

45