Gangue and alteration minerals

The generalized sequence of alteration minerals from shallower to deeper portions and/or from lower to higher temperatures in active geothermal systems, which is constructed mainly based on the work by Henley and Ellis (1983), is given in Fig. 2.25. It is shown in Fig. 2.25 that the change in alteration and gangue minerals largely depends on temperature as well as on the other physicochemical parameters such as /s2, /o2 /c02 and pH. 

It has been pointed out by Giggenbach (1981) on the basis of thermochemical calculations that epidote occurs at higher temperatures of at least more than 240°C, and K-feldspar occurs at restricted temperatures, i.e. below ca. 250°C, in active geothermal systems. These theoretical results seem to be consistent with those observed in epithermal vein-type deposits in Japan. 

Epidote tends to occur in high-temperature Cu deposits and adularia occurs abundantly in precious deposits whose average homogenization temperature is ca. 240°C or slightly less. 

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Geological, mineralogical and geochemical features of these deposit types (distribution, age, associated volcanism, host and country rocks, fluid inclusions, opaque, gangue and hydrothermal alteration minerals, chemical features of ore fluids (temperature, salinity, pH, chemical composition, gaseous fugacity, isotopic compositions (O, D, S, Sr/ Sr, Pb), rare earth elements)) were summarized. 

The Special Issue of Resource Geology on the Hishikari deposits (Shikazono et al., 1993) includes various aspects of the Hishikari deposits (oxygen isotopes of gangue minerals, hydrothermal alteration, precipitation sequence, fluid inclusions, vertical electric profiling and electric sounding surveys, structural geological analysis, opaque minerals. 

The changes that occur in minerals as a result of thermal pretreatments are wide and varied. Some examples are given in Table 5.5. The resources listed, in their untreated conditions, are not amenable to dissolution by common reagents. The treatments mentioned have converted them to altered forms, which dissolve more readily. In these examples only the valuable components of the different resources have been altered. There are however, examples where the gangue is converted into its insoluble or less-soluble forms. In such situations the leaching reagent loss is cut down. As an example, reference may be drawn to... 

Micro-XRF transects evidenced a significant mineralogical control on the mobility of several elements released during sulfide and gangue minerals alteration (Fig. 3) in particular goethite-rich layers are enriched in Ni and Zn, whereas hematite-rich layers selectively concentrate As, Se, Mo, and Cu. 

Condition the solids to alter the wettability of the mineral and the gangue. The fundamental surface wettability for sulfide ores is different from oxides, silicates, and salt-type minerals. pH is a critical variable. Typical conditioning chemical additions include collector about 0.01 to 0.1 kg/Mg solids frother about 0.01 to 0.5 kg/Mg solids activator about 1 to 4 kg/Mg solids depressant about 0.02 to 2 kg/Mg solids. Allow 6-min contact for conditioning. Bubble size about 1000 pm. Flotation rate constant is 0.2 to 1 min sink rate constant is 0.005 min-. Flotation cells mechanical cell for fast float, sequential separation, and relatively coarse particle diameter 1.6 to 2.4 kW/m cell volnme. Pneumatic cell for relatively dilute feed concentrations and smaller particle diameters. Air blower 0.5 kW/m cell volume. Typical solids throughput 0.4 to 0.8 kg/s m feed concentration 10 to 40% w/w. Air escape velocity 0.02 m/s. Float times 6 to 20 min. Feed concentration to rongher or scavenger 30% w/w to cleaner 10% w/w. 

A mineral particle will attach itself to an air bubble and float when its surface is hydrophobic, or water-repellent and conversely, a particle with a hydrophilic, or water-attractive, surface will not become attached to an air bubble. Practically all minerals as they occur are hydrophilic. It is the function of one class of flotation agents, called promoters or collectors, to convert the surface of the mineral to be floated from a hydrophilic to a hydrophobic state. At the same time, the collector must not alter the hydrophilic surface of the gangue, which is to stay behind. One theory as to how this is done is that the collector is selectively adsorbed and oriented on the mineral surface to form a hydrophobic film. 

Condition the solids to alter the wettability of the mineral and the gangue. The fundamental surface wettability for sulfide ores is different from oxides, silicates and salt-type minerals. pFJ is a critical variable. 

Uranium occurs as concentrations of pitchblende with some coffinite in veinlets along faults, in brecciated zones and in satellite structures associated with the major structure. These minerals may occur also as fine disseminations in selected horizons in the host rocks. Secondary uranium minerals are locally abundant, especially in near-surface portions of deposits. Small amounts of quartz and carbonates are the gangue minerals. Associated minerals vary in type and abundance and determine whether a deposit is classified as monometallic or polymetallic. The latter may contain a wide variety of associated minerals, including sulphides. Hematitization and chloritization are the most common forms of alteration... 

Pitchblende is the dominant uranium mineral in all these vein-like deposits in sedimentary rocks. It occurs as small veinlets along fractures within and surrounding the structures and as finely disseminated crystals in porous breccia fragments within the structures. Pitchblende concentrations may be distributed zonally within the ore, and in some deposits may be concentrated in the upper levels of the structure. Associated minerals may include sulphides and sulpharsenides. Calcite and quartz are the most common gangue. Types of alteration include bleaching of red sediments, silicification, carbonatization and argillization. 

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