Field mapping and economic geology

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Field mapping and economic geology

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1. Field mapping and economic geology Lecture 10 - Hydrothermal Deposit Types Ch 17 Evans, 1997. An introduction to Economic Geology and its Environmental Impact.

2. Links

4. The Vein Association In geology, a vein is a finite volume within a rock, having a distinct shape, filled with crystals of one or more minerals, which were precipitated from an (aqueous) fluid. Veins are formed by fluids carrying mineral constituents into a rock mass as a consequence of some form of hydraulic flow within the rock. Usually this is the result of hydrothermal circulation. So how do veins form? Veins are classically thought of as being the result of growth of crystals on the walls of planar fractures in rocks, with the crystal growth occurring normal to the walls of the cavity, and the crystal protruding into open space. This certainly is the method for the formation of some veins. However, it is rare in geology for significant open space to remain open in large volumes of rock, especially several kilometres below the surface. There are two main mechanisms considered likely for the formation of veins: open-space filling and crack-seal growth. Kinds of Veins Hydrothermal solutions ppt metals in environments including, near magmatic high temperature, high pressure, near surface low-T low-P conditions Gangue minerals dominant constituents, commonly quartz, calcite depending on the composition of the host rock indicating derivation from the surrounding host rocks Sulfides are the most important ore bearing minerals but in the case of tin and U oxides are predominant

5. Open space filling Open space filling is the hallmark of epithermal vein systems, such as a stockwork, in greisens or in certain skarn environments. For open space filling to take effect, the confining pressure is generally considered to be below 0.5 GPa, or less than 3-5 kilometres. Veins formed in this way may exhibit a colloform, agate-like habit, of sequential selvedges of minerals which radiate out from nucleation points on the vein walls and appear to fill up the available open space. Often evidence of fluid boiling is present. Vugs, cavities and geodes are all examples of open-space filling phenomenon in hydrothermal systems. Alternatively, hydraulic fracturing may create a breccia which is filled with vein material. Such breccia vein systems may be quite extensive, and can form the shape of tabular dipping sheets, diatremes or laterally extensive mantles controlled by boundaries such as thrust faults, competent sedimentary layers, or cap rocks.

6. Crack-seal veins When the confining pressure is too great, or when brittle-ductile rheological conditions predominate, vein formation occurs via crack-seal mechanisms. Crack-seal veins are thought to form quite quickly during deformation by precipitation of minerals within incipient fractures. This happens swiftly by geologic standards, because pressures and deformation mean that large open spaces cannot be maintained; generally the space is in the order of millimetres or micrometres. Veins grow in thickness by reopening of the vein fracture and progressive deposition of minerals on the growth surface.

7. Tectonic implications Veins generally need either hydraulic pressure in excess of hydrostatic pressure (to form hydraulic fractures or hydrofracture breccias) or they need open spaces or fractures, which requires a plane of extension within the rock mass. In all cases except brecciation, therefore, a vein measures the plane of extension within the rock mass, give or take a sizeable bit of error. Measurement of enough veins will statistically form a plane of principal extension. In ductilely deforming compressional regimes, this can in turn give information on the stresses active at the time of vein formation. In extensionally deforming regimes, the veins occur roughly normal to the axis of extension.

8. Hydraulic Fracturing

9. Mineralisation and veining Veins are of prime importance to mineral deposits, because they are the source of mineralisation either in or proximal to the veins. Typical examples include gold lodes, as well as skarn mineralisation. Hydrofracture breccias are classic targets for ore exploration as there is plenty of fluid flow and open space to deposit ore minerals. Ores related to hydrothermal mineralisation which are associated with vein material may be composed of vein material and/or the rock in which the vein is hosted. Gold-bearing veins In many of the gold mines exploited during the gold rushes of the 19th century, vein material alone was typically sought as ore material. In most modern mines, ore material is primarily composed of the veins and some component of the wall rocks which surrounds the veins. The difference between 19th century and modern mining techniques and the type of ore sought is based on the grade of material being mined and the methods of mining which are used. Historically, hand-mining of gold ores permitted the miners to pick out the lode quartz or reef quartz, allowing the highest-grade portions of the lodes to be worked, without dilution from the unmineralised wall rocks. Modern mining using larger machinery and equipment forces the miners to take low-grade waste rock in with the ore material, resulting in dilution of the grade. However, modern mining and assaying allows the delineation of lower-grade bulk tonnage mineralisation, within which the gold is invisible to the naked eye. In these cases, veining is the subordinate host to mineralisation and may only be an indicator of the presence of metasomatism of the wall-rocks which contains the low-grade mineralisation. For this reason, veins within hydrothermal gold deposits are no longer the exclusive target of mining, and in some cases gold mineralization is restricted entirely to the altered wall rocks within which entirely barren quartz veins are hosted.

10. Important Vein Deposit Types Archaean Greenstone Belts Epithermal Deposits in Volcanic terranes Carlin-type Deposits

11. Archaean Vein Gold Deposits Yilgarn Block WA contains thousands of individual deposits the majority of which produce < 1t Au. However they can contain several giant deposits eg Kalgoorlie golden mile Contained in greenschist facies metm rocks in structures of brittle-ductile transition regime Dominant host rock is tholeiitic basalt and komatiites. The physical properties of these rocks favour hydraulic fracturing and fluid access and their composition controls gold deposition within the veins. Some of the gold bearing solutions are metamorphic in origin Wall rock alteration involves addition of SiO2, K2O, CO2, H2O and Au

12. Yilgarn Craton

13. Gold in the Yilgarn The Yilgarn Craton gold endowment is considered to be a process of a prolonged period of cratonic development during a series of orogenic episodes beginning at about ~2.9Ga and culminating in ~2.67Ga. These events saw the assembly of the Yilgarn Craton from several 'proto-cratons' or unconsolidated terranes of perhaps older earlier-formed granite-gneiss, probably of similar nature to the Narryer Gneiss Terrane. These have been mostly destroyed by the voluminous tonalite-trondhjemite-granodiorite (TTG) magmatism of c. 2.75-2.85Ga, which saw vast quantities of essentially uniform igneous-derived granitoids intruded into the existing greenstone belts, thus forming the cratonising event. These granites now form pillow-like flatly-dipping to steeply dipping sheath-like margins to the greenstone terranes, and may have contributed to the gold mineralisation either during the metamorphic decarbonation-dehydration reactions or as heat engines to drive thermal convection and hydrothermal fluid flow. The greenstone-granite terranes of the Yilgarn Craton have subsequently been affected by several later metamorphic events and deformations, which have now overprinted the craton with zones of steeply-dipping foliation and vertically thrust-offset fault blocks. These later events tend not to cause mineralisation, instead causing structural disruption of the gold lodes. Gold mineralisation in the Yilgarn Craton usually occurs at the contact between the veins and wallrocks. Formation of these deposits is linked to mid-crustal level processes during regional metamorphism.

15. Metamorphic Origin of Hydrothermal Fluids

16. Hydrothermal Systems, Fluid Inclusions & Mineral Exploration Fluid inclusions are droplets of fluid trapped in crystals at the time of their growth or subsequently introduced along microcracks and cleavages They represent samples of hydrothermal fluids and range in size from a single water molecule up to several mm and avg about 0.01 mm Practical uses of fluid inclusions include; information on the T, P, density and composition of the mineralising fluids

17. Types of Fluid Inclusions There are 3 types of fluid inclusions: Primary – those that became trapped during the growth of the host mineral & are therefore assoc with crystallisation features such as growth zones Secondary – those that form after the growth of the host mineral is completed. They cut across growth zones and crystal boundaries and may represent infilling of microcracks by late fluids Psuedo-secondary – these form during the 2 stages above and are characterised by their alignment with microcracks that end against a growth zone

18. Types of Fluid Inclusions

19. Fluid Inclusion Classification Monophase – entirely filled with liquid (L) Two-phase – filled with L phase and a small vapour bubble (L+V) Two-phase – in which vapour phase is dominant & occupies more than 50% volume (V+L) Monophase vapour inclusion (V) – generally mixtures of H2O, CH4, CO2 Multiphase inclusions containing solids (S+L+/-V) – contain solid crystalline phases known as daughter minerals. Commonly halite NaCl & sylvite KCl & sometimes sulfides Immiscible liquid inclusions – 2 liquids, usually one H2O-rich and the other CO2-rich (L1+L2+/-V)

20. Fluid Inclusion Types

21. Fluid Inclusion Interpretations The coexistence of L+V and V+L phases may indicate that the fluid was boiling during entrapment In the case of boiling; In a one component system the gas bubble is the vapour phase of the host liquid In a heterogeneous system the gas phase exsolves by effervescence However, gas bubbles may also indicate immiscibility eg CO2, when present, will separate on cooling The presence of daughter minerals indicates that solids nucleated from an oversaturated liquid sol’n In these hypersaline fluids Na+, Cl-, Mg2+, Ca2+ are the most common dissolved ions

22. Fluid Inclusion Measurements Measurements on fluid inclusions are carried out by means of heating and freezing thick-sections on specially designed microscope stages Homogenisation of the liquid and gas phase will be seen to occur at a given T whilst gradually heating the inclusion This T is a lower limit, having been obtained at atmospheric pressure, & pressure correction for the original depth is required Salinity of inclusion is determined by first freezing the inclusion & then raising the T & observing the first & final melt T. The first melt T indicates the type of salt (eg NaCl or MgCl) while the last melt indicates the degree of salinity, usually measured in equivalent NaCl

23. Temperature-salinity fields

24. Fluid Inclusion Compositions The liquid of the inclusion is normally an aqueous sol’n with dissolved ions of Na+, Cl-, Ca2+, Mg2+, SO42-, HCO32-, CO32- The concentration of the salts in the sol’n ranges from <1 wt. % to >50 wt. % Different styles of mineral deposits have vastly different homogenisation T’s indicating a large range of hydrothermal fluids can be responsible for mineralization eg epithermal Au-Ag generally has low conc of NaCl indicating low T of formation (200-300 C) whereas porphyry Cu have high conc. indicating high T

25. Fluid Composition and Metal Partitioning Analysis of typical hydrothermal fluids indicates that Na, K, Cl & Ca are almost always the major components Minor components include Sr, Fe, Zn, Mg, Mn, CO2, SO2, H2S and NH3 The most striking feature is that concentrations of ore-forming metals is generally low. Therefore it can be deduced that metal conc’s in hydroth fluids need not be high to form an ore deposit The critical factors for ore dep’n must be time & deposition rate

26. Composition of Hydrothermal Fluids

27. Metals in Hydrothermal Fluids Hydrothermal fluids acquire their dissolved constituents by one of 2 processes; Constituents are released to a fluid by a crystallising magma eg Cu in porphyries Constituents are derived from the rock thru which the hot fluid is circulating A rock need not be enriched in certain elements initially to serve as the final source of these elements Experiments show that elements such as Fe, Zn, Cd, Cu & Mn are strongly partitioned into chloride-rich hydroth fluids rather than staying within the crystal lattice of rock forming minerals, referred to as leaching or partitioning

28. Metal Transport Complex Ions & Ligands Important in the dissolution of Ni, Cu, Zn, Pt, Au, Co, Cr, Mo, W Important ligands include NH3, H2O, Cl-, OH-, HS- Eg Pt(NH3)42+ + 2Cl = Pt(NH3)4Cl2 (complex cation) Pt(NH3)Cl3- + K = Pt(NH3)Cl3K (complex anion) Au + H2S HS- = Au(HS)2- + 1/2H2 (Au thio-complexing)

29. Metal Deposition Ppt’n of dissolved constituents in hydrothermal fluid occurs as a result of either, T variations, P changes and boiling, rea’ns between wallrock and fluid, mixing of different fluids (black smokers) Boiling is the most important of these as it results in almost instantaneous removal of volatile phases and a sudden increase of metal concentrations in the remaining sol’n which may not be able to keep these metals dissolved. This is an important mechanism of Au, Ag ppt’n in geothermal systems by removing ligands from sol’n HCO3- + H+ = CO2 + H2O HS- + H+ = H2S (g)

30. Boiling

31. Black Smokers

32. Black smoker metal precipitation

34. Carlin-type Deposits

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