Class 9 Metamorphic Rocks And Metamorphic Deposits

Class 9 Metamorphic Rocks and Metamorphic Deposits - Notes • •

Metamorphic Rocks Metamorphic Deposits

Metamorphic Rocks Metamorphism Metamorphic rocks are the product of “metamorphism”, which is the partial or complete recrystallization of rocks. The original rock (called the “protolith”) is either an igneous or sedimentary rock. The protolith is subjected to a change, over time, in the physical and chemical conditions surrounding it, resulting in the growth of new minerals at the expense of the old ones. The physical conditions include the temperature, lithostatic pressure, hydrostatic pressure, directed pressure, vapor pressure, and other forces. They are “solid state” changes, which means that they do not occur as a result of magmatic crystallization, but instead occur much later. Changes in chemical conditions do occur, but are typically restricted to just those changes involving local components (minerals and pore fluids). Generally these changes are When large amounts hydrothermal fluids become involved, bringing in new components or metals and causing more or less wholesale replacement, the process is called “metasomatism”. A skarn (see Class 5 Notes) is an example of a metasomatic rock. Metasomatism usually causes nearcomplete destruction of the original fabric of the rock due to the growth of new minerals. In contrast, metamorphism usually retains the original fabric, called “relict”. The two biggest variables in metamorphism are the temperature and pressure. The general temperature and pressure conditions at a given point in time are called the “metamorphic facies”. Eskola (1915) created a scheme to describe these conditions (Figure 9 – 1). The scheme is based on “indicator minerals” - minerals which are characteristically associated with some specific temperature –pressure parameters. For example, at low pressures, the minerals albite and epidote are stable at low temperatures (300 – 450 deg. C), but give rise to the formation of hornblende at higher temperatures (500 – 600 deg. C). Facies conditions such as the zeolite, prehnite-pumpellyite, greenschist, albite-epidote and hornblende facies may be conducive to growth of hydrous minerals (those containing water in their crystal structure). Other higher temperature/pressure facies conditions cause dehydration reactions to occur, and therefore favor the growth of anhydrous minerals such as pyroxene and garnet.

Figure 9 –1. Metamorphic facies as a function of temperature and pressure. There are many ways that rocks can be subjected to new physical and chemical conditions. These ways mostly involve plate tectonic activity over long periods of geologic time. Movement along plate boundaries results in subduction, shearing or rifting. In the subduction zone environment, several different metamorphic facies conditions are present in different portions of the system (Figure 9 – 2). Zeolite and prehnite-pumplellyite facies occur at shallow crustal levels near the interface of the ocean and sediments, in a trench setting. At greater depths (higher pressures) and higher temperatures, regional metamorphism occurs (greenschist, amphibolite and granulite facies). Magmas formed by partial melting ascend towards the surface where they eventually form plutonic or volcanic rocks. The igneous rocks intrude portions of the crustal rocks, at lower lithostatic and hydrostatic pressures. As the plutons shoulder aside the adjacent rocks during ascent towards the surface, they generate thermal changes in these adjacent rocks, called “country rocks”. These high temperature, generally low pressure conditions of metamorphism are called “contact metamorphism”. Contact metamorphism occurs in a restricted aureole surrounding the igneous intrusion. The temperature gradient of the contact aureole decreases outward from the pluton in a series of concentric or irregular-shaped zones.

Figure 9 – 2. Subduction zone setting showing locations of metamorphic facies, including: 1) zeolite, 2) prehnite-pumpellyite, 3) glaucophane schist, 4) eclogite, 5) greenschist, 6) amphibolite, 7) granulite, 8) pyroxene hornfels, 9) hornblende hornfels, and 10) albite-epidote hornfels. Metamorphic Zones The heat source involved with a metamorphic event will typically set up a gradient in the country rocks which decreases outward away from the source of heat. The heat source can be can be an igneous pluton, a large-scale igneous batholith, a small-scale dike, or just the normal geothermal gradient of the earth which increases with depth. As a volume of the rock is subjected to the new temperature conditions, the minerals comprising the rock become unstable and react with adjacent minerals to create new minerals which are stable at the higher temperatures. Some of the reactions which are known to occur are summarized in the Figure 9-3. The mineral assemblage present in the metamorphic rock provides the most important evidence about the temperature and pressure conditions of the past. Careful analysis of the mineral composition of the rock, or petrologic evaluation, is required, often requiring the use of petrographic microscope or electron microprobe to determine the exact composition of the individual minerals. Key indicator minerals are used to identify the metamorphic zone. The metamorphic zone is the manifestation of a certain set of physical and chemical conditions. For example, look at the minerals which form by contact metamorphism of an impure limestone (mostly calcite, but with impurities of quartz and clay)(Figure 9 – 3). Calcite, quartz and clay minerals react to form talc at low temperature. With increasing temperatures, talc becomes unstable and reacts to form tremolite. Further temperature increase causes tremolite to become unstable and react to form diopside.

Figure 9 – 3. Chart of metamorphic facies showing mineral reactions and indicators for impure limestone, pelitic and mafic igneous protoliths (modified from Brownlow, 1979). Metamorphic Fabrics The mineralogy of the a metamorphic rock is not the only line of evidence to determine its history. The fabric of a metamorphic rock also tells of its history. Typically metamorphic rocks develop some kind of metamorphic fabric during the metamorphic event(s). The fabric can be either a planar fabric, called “foliation”, or a linear fabric, called “lineation”. figure will be added pending permission

Figure 9 – 4. Metamorphic rock fabrics. A. Schistosity formed by muscovite and biotite. B. Lineation formed by stretching of round clasts. C. Lineation formed by elongate mineral growth within schistosity plane (from ). The fabric develops because during recrystallization, the new minerals which grow have a preferred sense of orientation with respect to directional forces of pressure at work. The pressure may be static, as in the lithostatic pressure which results from the weight of the overlying rocks. Pressure may also be directed by tectonic forces, which can be tensional, compressional or shear forces. The new minerals can grow in a number of different orientations with respect to the lines of force, ranging from perpendicular to parallel. Planar fabrics, called “foliations”, develop in one of two ways: 1) by growth of planar minerals, or 2) by development of compositional layering. The most common planar minerals are the mica’s, but these are hydrous minerals and generally occur in lower grade metamorphic rocks. This type of fabric is called a schistosity, because it is characteristic of the metamorphic rock called “schist”. Mica’s grow perpendicular to the directions of force in most situations involving nondirected pressure, such as simple burial metamorphism. In this situation the micas grow horizontally, with the flat sides facing up, perpendicular to the pressure from the overlying column of rock. Micas and other minerals also grow in conditions of directed pressure, such as in environment in which there is compression and shearing. In these situations, the micas grow with their flat sides parallel to the direction of shearing. At very high grade metamorphic conditions, close to the melting temperature of the rock, there can be larger scale movement of chemical constituents, leading to the development of compositional layering or banding. The effect is to create alternating mafic-rich and quartz-feldspar-rich bands. Metamorphic Lithologies Metamorphic rocks are classified on the basis of their mineralogy and texture: special texture Example: porphyroblastic

mineralogy

rock name

garnet-mica-quartz

schist

It is known that at higher grades of metamorphism, rocks tend to recrystallize into coarser-grain sizes than at lower grades of metamorphism. Arbitrary grain size ranges used to classify metamorphic rocks are < 0.1 mm (fine-grained), 0.1 – 1.0 mm (medium-grained) and > 1.0 mm(coarse-grained). The grain size of the rock is specified in the rock name itself, for example phyllite is medium-grained, schist is coarse-grained, and gneiss is very coarse-grained (Figure 9 – 5). The most abundant mineral is listed last, so garnet is least abundant and quartz is most abundant. Porphyroblastic is a special textural name for metamorphic rocks that have larger crystals forming bumps or knots, which disrupt the primary foliation due to formation late in the metamorphic event.

Figure 9 – 5. Metamorphic rock classification based on grain size and protolith composition (from Mason, 1978). Metamorphic Deposits Metamorphic ore deposits are those which form as a result of metamorphic processes and are hosted in metamorphic rocks. This group does not include previously existing ore deposits which are later altered or deformed during a metamorphic event. It is not always clearcut whether a deposit is of the former or the later category. The distinguishing feature all of these deposits do have in common is the lack of a clear link to a causitive pluton, ie, a pluton which provided both the metals and the fluids which generated the deposit. Instead, the metals and hydrothermal fluids were derived from the metamorphic rocks during the metamorphic process. Metamorphic deposits are formed in different types of metamorphic conditions, ranging from low to high temperature and low to high pressure. The generation of fluids to transport and precipitate metals is critical. This means one might expect more deposits to form in metamorphic conditions which generate water as a by-product of dehydration reactions. In a regional metamorphic setting these conditions are most likely to be met in the greenschist facies of metamorphism. Further metamorphism into the amphibolite facies tends to drive all water from the system. This is why many metamorphic deposits form in greenschist facies rocks and relatively few form in high grade metamorphic rocks. Brittle, massive rocks often become good host rocks in areas affected by dynamic metamorphism because shearing causes the rocks to shatter, thereby developing fluid pathways and sites for mineralization. Mineralization may transcend across different rock lithologies, indicating that bulk composition of the host rock is a less important factor. Further evidence for syn-metamorphic ( formation is the fact that the ore and gangue minerals comprising the cross-cutting features are often compatible with the regional metamorphic grade, indicating they did not form during a post metamorphic event of higher or

lower temperature. Three general types of metamorphic deposits are known: 1) copper-rich, 2) gold-rich, and 3) lead-zinc-silver-rich. Copper-Rich Types: These metamorphic deposits are characteristically associated with very low grade to low grade metamorphism. Most often they form in terrains where mafic or ultramafic basement rocks are overlain upsection by organic-rich sedimentary rocks. Background copper values of the mafic rocks are low, nevertheless they are believed to provide the source of the copper by liberation during a leaching process caused by passing of low temperature hydrothermal fluids. The fluids move upwards above local hot spots, and especially along fractures and faults. Richest zones are usually in close proximity to a fault or other structure where fluids migrate. When the fluids encounter the rock layers containing the organic matter, precipitation results. Examples: Kennicott, Alaska: Ore = Chalcocite + Bornite. Average grade 6 % copper, 14 opt silver. High grade veins were the Jumbo and Bonanza-Mother Lode veins. Ore localized in the lower of the Chitistone Limestone formation, which has high organic content. Source of copper = Nikolai Greenstone = subaerial mafic volcanic flows. The flows have naturally high copper values. Doming of the rock layers caused fracture systems which localized the ore. Local flat faults. (Figure 9 – 6) White Pine, Michigan: Ore = chalcocite + bornite + chalcopyrite + minor sphalerite Proterozoic subaerial basalts overlain by sandstone-shale sequence. Shales contain high organic content and are host rocks for the ore. Native copper occurs in the basalts, which are altered to chlorite and zeolites.

Figure 9 – 6. Cross section of Kennicott copper deposit (source unknown). Gold-Rich Types:

Gold-rich metamorphic deposits are of two general types: 1) Archean iron formation types, and 2) quartz-carbonate veins. Archean Iron Formation Types: Occur mostly in Precambrian shield areas. Vein morphology but most veins apparently concentrated in peculiar iron-rich shales and sandstones which are upgraded by at least one and usually several metamorphic events. Iron-bearing minerals form a mineral zonation down dip from oxides (hematite, magnetite), to silicates (including Fe-rich clays & chlorite), to carbonates (siderite), to sulfides (pyrite, pyrrhotite). The zonation is thought to result from differences in water depth during diagenesis. Gold occurs in quartz veins in the silicate or sulfide facies host rocks. Examples: Precambrian of Wyoming Jardine, Montana Quartz-Carbonate Types: Often associated with greenstone belts in shield areas. Serious deformation of host rocks. Ore formed in structural zones/shear zones which are regional in scale. The districts usually contain large scale folding as well. Typically dismembered. Associated with greenschist facies rocks, namely greenstone. Also hosted in deformed metaigneous rocks, particularly where they intrude shaley rocks. Moderately high temperature fluids with significant CO2 content. Boiling (when CO2 exsolves from fluid) thought to be important mechanism in the precipitation of gold. Typical mineralogy is quartz + carbonate + sericite (or chlorite) + pyrite (or arsenopyrite) + native gold. Examples: Valdez Creek District, Alaska Conn Mine, Eastern Canada AJ Mine, southeast Alaska Lead-Silver-Rich Types: Lead-silver-rich types of metamorphic deposits typically contain galena, sphalerite, and locally tetrahedrite and chalcopyrite as ore minerals. The gangue is typically quartz and siderite (iron carbonate). Mineral zoning is from galena + sphalerite in the lower portion of the veins to galena + siderite in the upper portion of the veins. Example: Coer de Lane district, Idaho