Re: Rats!,,,a round the lake
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Message: Rats!
..whats interesting here, is that San knows Vanson ( 2.4 kil ) sits North on the same mine structure as their shaft and Luana sits 4 kil West from Vanson following the Wanipagow fault line. South of Luana running paralel with the Vanson structure the luana structure boxes in the West side of Round Lake with Vanson on the East. Sans Hang Wall running for 6000 ft West of the Rice Mine Headfram boxes in the south West side. So we"er does this leave the Ross Pluton Eruption flow to go, or settle and freeze from the ice age...PS" OLD HOUND, what "s under the Lake ...............................................................................
Class 8: Hydrothermal Veins and Alteration - Notes
Rock alteration simply means changing the mineralogy of the rock. The old minerals grow are replaced by new ones because there has been a change in the conditions. These changes could be changes in temperature, pressure, or chemical conditions or any combination of these. Hydrothermal alteration is a change in the mineralogy as a result of interaction of the rock with hot water fluids, called “hydrothermal fluids”. The fluids carry metals in solution, either from a nearby igneous source, or from leaching out of some nearby rocks. Hydrothermal alteration is a common phenomena in a wide variety of geologic environments, including fault zones and explosive volcanic features.
Hydrothermal fluids cause hydrothermal alteration of rocks by passing hot water fluids through the rocks and changing their composition by adding or removing or redistributing components. Temperatures can range from weakly elevated to boiling. Fluid composition is extremely variable. They may contain various types of gases, salts (briney fluids), water, and metals. The metals are carried as different complexes, thought to involve sulfur and chlorine.
Sources of hydrothermal fluids are not well understood, however, there are three main possibilities that exist. One source can be the magmatic rocks themselves, which exsolve water (called “juvenile” water) during the final stages of cooling. In metamorphic terranes a potential source of the fluids is dehydration reactions which take place during the metamorphic event. With increasing temperature of metamorphism, early, low temperature, hydrous minerals recrystallize into new, higher temperature, anhydrous minerals. The excess water circulates through the surrounding rocks and may scavenge and transport metals to sites where they can be precipitated as ore minerals. Near surface groundwater is another source of water (called “meteoric” water). Evidence from some ore deposits suggests meteoric waters may mix with juvenile or metamorphic waters during late stages of mineralization.
Hydrothermal fluids in plutonic settings are thought to circulate along a large scale convective path. It would be analogous to a pot of boiling water: hottest water rises fastest directly above the heat source, and at the surface changes flow direction to horizontal, and finally downwards along the sides of the pot. In a similar manner, hydrothermal fluids circulate upward and outward from an igneous intrusion at depth. Porous and permeable host rocks (those containing lots of interconnected pore spaces) allow this to happen more readily, for example, in a coarse-grained sandstone. Some types of rocks, like shale or slate, are extremely impermeable. A layer of shale can cause damming or ponding of the hydrothermal fluids, which can lead to a concentration of mineralization behind the impermeable barrier. Fluid migration can be also facilitated by the presence of lots of thin layers .
Hydrothermal fluids also circulate along fractures and faults. A which has a well-developed fracture system may serve as an excellent host rock. Veins form where the fluids flow through larger, open space fractures and precipitate mineralization along the walls of the fracture, eventually filling it completely. Fault zones are excellent places for fluids to circulate and precipitate mineralization. Faulting may develop breccia and gouge, which is often a good candidate for replacement style mineralization. The form of mineralization and alteration associated with faults is highly variable, and may include massive to fine-grained, networks of veinlets, and occasionally vuggy textures in some breccias.
Alteration Zoning
Although mineral zoning patterns are not uncommonly developed around ore deposits, they are not always present or obvious. The patterns can be caused by changes in temperature, fluid chemistry or gas content. The change in parameters over time, such as decreasing temperature of the fluids, can cause overprinting of lower temperature minerals by higher temperature minerals. Structural deformation, such as when a rock shattering or faulting event affects the host rocks, can cause more complexity.
Alteration zoning can occur in many different geometric forms, ranging from concentric shells, to linear forms, to irregular and complex. Porphyry copper deposits are characterized by concentric shell-shaped zones of alteration, which overlap to some extent Figure 8 – 1 A. The core area contains “potassic” alteration in the form of potassium feldspar and biotite. Further outward is a zone of “phyllic” alteration consisting of the assemblage quartz-sericite-pyrite. The outermost zone, called “propylitic”, is characterized by the assemblage quartz-chlorite-carbonate and locally containing epidote, albite or adularia. Epithermal deposits associated with major structures (faults or fractures) have linear zones which parallel the structure. The mineralogy is highly variable, as is the geometry. One example of alteration zoning associated with a volcanic vent is shown in Figure 8 – 1 B. This example indicates an inner zone of silicification forms within a central breccia formation, and an outer zone of propylitic alteration lies adjacent. Sericite is a common alteration mineral formed in zones along fault structures or fault zones in low to moderate temperature settings.
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Figure 8 – 1. A. Concentric shell-shaped alteration zones and associated mineralization pattern (after Lowell and Guilbert model). B. Alteration zones associated with epithermal mineralization hosted in alkalic volcanic rocks.
Alteration can be mapped graphically using patterns or colors in much the same way that lithologic units are mapped. The primary characteristics to note are the alteration mineralogy, style and intensity. The mineral assemblages can be coded using patterns or colors. The style of alteration refers to the form, which could be disseminated or massive or anything in between. Another form of alteration is “veinlet-controlled”, which indicates that alteration is restricted to narrow zones adjacent to veinlets. The intensity of alteration refers to how well-developed the alteration is. It could be incipient mineral growth due to weak development, or it may be pervasive throughout the rock, indicating strong development.
Mapping alteration can be used to predict mineralization. In theory, this is done by comparing the results of alteration mapping with known alteration zoning patterns for known mineral deposits. In practice however, the process is seldom so simple because every mineral deposit has some uniqueness to its alteration zoning.
Alteration Types
There are as many alteration types as there minerals. The following types are the most commonly described types:
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Propylitic: (Chlorite, Epidote, Actinolite) Propylitic alteration turns rocks green, because the new minerals formed are green. These minerals include chlorite, actinolite and epidote. They usually form from the decomposition of Fe-Mg-bearing minerals, such as biotite, amphibole or pyroxene, although they can also replace feldspar. Propylitic alteration occurs at relatively low temperatures. Propylitic alteration will generally form in a distal setting relative to other alteration types.
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Sericitic: (Sericite) Sericitic alteration alters the rock to the mineral sericite, which is a very fine-grained white mica. It typically forms by the decomposition of feldspars, so it replaces feldspar. In the field, its presence in a rock can be detected by the softness of the rock, as it is easily scratchable. It also has a rather greasy feel (when present in abundance), and its color is white, yellowish, golden brown or greenish. Sericitic alteration implies low pH (acidic) conditions.
Alteration consisting of sericite + quartz is called “phyllic” alteration. Phyllic alteration associated with porphyry copper deposits may contain appreciable quantities of fine-grained, disseminated pyrite which is directly associated with the alteration event. -
Potassic: (Biotite, K-feldspar, Adularia) Potassic alteration is a relatively high temperature type of alteration which results from potassium enrichment. This style of alteration can form before complete crystallization of a magma, as evidenced by the typically sinuous, and rather discontinuous vein patterns. Potassic alteration can occur in deeper plutonic environments, where orthoclase will be formed, or in shallow, volcanic environments where adularia is formed.
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Albitic: (Albite) Albitic alteration forms albite, or sodic plagioclase. Its presence is usually an indication of Na enrichment. This type of alteration is also a relatively high temperature type of alteration. The white mica paragonite (Na-rich) is also formed sometimes.
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Silicification: (Quartz) Silicification is the addition of secondary silica (SiO2). Silicification is one of the most common types of alteration, and it occurs in many different styles. One of the most common styles is called “silica flooding”, which results form replacement of the rock with microcrystalline quartz (chalcedony). Greater porosity of a rock will facilitate this process. Another common style of silicification is the formation of close-spaced fractures in a network, or “stockworks”, which are filled with quartz. Silica flooding and/or stockworks are sometimes present in the wallrock along the margins of quartz veins. Silicification can occur over a wide range of temperatures.
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Silication: (Silicate Minerals +/- Quartz) Silication is a general term for the addition of silica by forming any type of silicate mineral. These are commonly formed in association with quartz. Examples include the formation of biotite or garnet or tourmaline. Silication can occur over a wide range of temperatures. The classic example is the replacement of limestone (calcium carbonate) by silicate minerals forming a “skarn”, which usually form at the contact of igneous intrusions.
A special subset of silication is a style of alteration called “greisenization”. This is the formation of a type of rock called “greisen”, which is a rock containing parallel veins of quartz + muscovite + other minerals (often tourmaline). The parallel veins are formed in the roof zone of a pluton and/or in the adjacent country rocks (if fractures are open). With intense veining, some wallrocks can become completely replaced by new minerals similar to the ones forming the veins. -
Carbonatization: (Carbonate Minerals) Carbonitization is a general term for the addition of any type of carbonate mineral. The most common are calcite, ankerite, and dolomite. Carbonatization is also usually associated with the addition of other minerals, some of which include talc, chlorite, sericite and albite. Carbonate alteration can form zonal patterns around ore deposits with more iron-rich types occurring proximal to the deposit.
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Alunitic: (Alunite) Alunitic alteration is closely associated with certain hot springs environments. Alunite is a potassium aluminum sulfate mineral which tends to form massive ledges in some areas. The presence of alunite suggests high SO4 gas contents were present, which is thought to result from the oxidation of sulfide minerals.
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Argillic: (Clay Minerals) Argillic alteration is that which introduces any one of a wide variety of clay minerals, including kaolinite, smectite and illite. Argillic alteration is generally a low temperature event, and some may occur in atmospheric conditions. The earliest signs of argillic alteration includes the bleaching out of feldspars.
A special subcategory of argillic alteration is “advanced argillic”. This consists of kaolinite + quartz + hematite + limonite. feldspars leached and altered to sericite. The presence of this assemblage suggests low pH (highly acidic) conditions. At higher temperatures, the mineral pyrophyllite (white mica) forms in place of kaolinite. -
Zeolitic: (Zeolite Minerals) Zeolitic alteration is often associated with volcanic environments, but it can occur at considerable distances from these. In volcanic environments, the zeolite minerals replace the glass matrix. Zeolite minerals are low temperature minerals, so they are generally formed during the waning stages of volcanic activity, in near-surface environments.
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Serpentinization and Talc Alteration: (Serpentine, Talc) Serpentinization forms serpentine, which recognized softness, waxy, greenish appearance, and often massive habit. This type of alteration is only common when the host rocks are mafic to ultramafic in composition. These types of rocks have relatively higher iron and magnesium contents. Serpentine is a relatively low temperature mineral. Talc is very similar to the mineral serpentine, but its appearance is slightly different (pale to white). Talc alteration indicates a higher concentration of magnesium was available during crystallization.
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Oxidation: (Oxide Minerals) Oxidation is simply the formation of any type of oxide mineral. The most common ones to form are hematite and limonite (iron oxides), but many different types can form, depending on the metals which are present. Sulfide minerals often weather easily because they are susceptible to oxidation and replacement by iron oxides. Oxides form most easily in the surface or near surface environment, where oxygen from the atmosphere is more readily available. The temperature range for oxidation is variable. It can occur at surface or atmospheric conditions, or it can occur as a result of having low to moderate fluid temperatures.
Zoned vein deposits are deposits which form along fractures and faults as open-space fillings or replacements. They are generally polymetallic. Many have been mined for copper, lead and zinc, although substantial gold and silver credits occur locally. These deposits generally fall in the category of low tonnage, high grade types of deposits. There are two broad categories: 1) vein deposits associated with porphyry base metal deposits, and 2) vein deposits not associated with porphyry base metal deposits.
Zoned vein deposits which are associated with porphyry base metal deposits appear to form at lower temperatures during a later mineralization event. These veins are characterized by a strong sense of zoning from high temperature minerals in proximal (closer to the pluton) portions of the veins, to low temperature minerals in distal (far away) portions of the veins. Proximal portions of the veins are copper-rich and contain sulfide minerals with high metal:sulfur ratios. Distal portions of the same veins are lead-zinc-rich and contain sulfide minerals with lower metal:sulfur ratios. At Butte, Montana, alteration halos adjacent to the veins change dramatically along the length of the vein and with increasing distance from the central porphyry copper-molybdenum deposit (Figure 8 – 2). Proximal portions of the veins are characterized by advanced argillic alteration adjacent to the vein which is superceded outwards by sericitic alteration. Distal portions of the veins are characterized by propylitic alteration adjacent to the vein which gives rise to fresh unaltered rock further away from the vein. Zoned vein deposits which are not associated with porphyry base metal deposits are characterized by having moderate, more uniform temperatures over a larger area. Zoning in these types of vein deposits is usually a function changes in the fugacity of sulfur along the length of the vein.
Figure 8 – 2. Example of proximal and distal zoning of base metal vein deposit of the type associated with porphyry copper/molybdenum deposits.
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