Summary of assemblages of alteration minerals, commonly used terminology, and the environment of formation.�(Source : Atlas of Alteration; A Field and Petrographic guide to Hydrothermal Alteration minerals. Editors : A.J.B. Thompson& J.F.H. Thompson, MDD Series editor: K.P.E. Dunne, 1996).
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Mineral Assemblage �(Key minerals are in bold) |
Standard Terminology |
Environment of Formation |
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Intrusion-related - High sulphidation Epithermal |
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| quartz, rutile, alunite, native sulphur, barite, hematite, pyrite, jarosite | vuggy silica, vuggy quartz | Typically occurs in structural zones or as replacement bodies in permeable lithologies, usully in the core of zones of advanced argillic alteration. This extreme form of leaching can occur in the upper parts of porphyry systems (telescoped) but is more common at higher (epithermal) levels. |
| quartz, chalcedony, alunite, barite, pyrite, hematite | silicic | Represents the addition of silica to the rock, resulting in replacement or, more commonly, the fill to vugs created during intense leaching. Silicification is common in high-sulphidation systems at porphyry to epithermal depths. It is sometimes confused with intense quartz stockwork veining at the top of some porphyry deposits. |
| quartz, kaolinite/dickite, alunite, diaspore, pyrophyllite, rutile, zunyite, alumino phosphate-sulphates, native sulphur, pyrite, hematite | advanced argillic-acid sulphite | Forms widespreadd zones in the upper parts of some porphyry systems (lithocap); also as more restricted alteration halos around high-sulphidation epithermal deposits. |
| kaolinite/dickite, montmorillonite, illite-smectite, quartz, pyrite | aegillic, intermediate argillic | May be present as a zone of lateration between advanced argillic and propylitic alterations, particularly in the high-sulphidation epithermal setting. |
| calcite, chlorite, epidote, albite, sericite, clay, pyrite | propylitic | May occur as an outer regionally extensive alteration zone in systems at moderate depths (>500m) |
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Low- sulphidation Epithermal - Geothermal |
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| quartz chacedony, opal, pyrite, hematite | silicic | Pervasive replacement of the rock by silica minerals. Occurs in some epithermal and geothermal systems as wallrock alteration around fractures and vein or within permeable zones, usually at relatively shallow levels. Also forms blanket-like zones of replacement at the water table below steam-heated advanced argillic alteration. Stratification may be mistaken for sinter. |
| orthoclase ("adularia"), quartz, sericite-illite, pyrite | "adularia" | Varies from wallrock alteration around veins, fractures and permeable zones to selective replacement of plagioclase in alteration envelopes. Common at shallow to intermediate depths in epithermal or geothermal systems; may be associated with boiling. Prevasive replacement by "adularia" is difficult to distinguish from silicification. |
| sericite (muscovite), illite-smectite, montmorillonite, kaolinite, quartz, calcite, dolomite, pyrite | sericite, argillic | Occurs as wallrock alteration around veins and replacement zones in permeable lithologies. May exhibit progression from sericite to mixed layer clays with increasing distance from mineralized (upflow) zones. Blanket-like carbonate-bearing alteration zones in the upper part of some geothermal/epithermal systems may reflect the condensation of gases (CO2) from deeper boiling zones. Carbonate may also be important in some deeper base metal-rich systems. |
| kaolinite, alunite, cristobalite (opal, chalcedony), native sulphur, jarosite, pyrite | advanced argillic-acid-sulphate | Forms extensive areas of alteration above the water (paleowater) table related to the condensation and oxidation of gases (H2S). Associated with mud pools, fumaroles and deposits of native sulphur. |
| quartz, calcite | silica-carbonate | Replacement of ultramafic rocks inthe shallow parts (low temperature) of geothermal systems. |
| calcite, epidote, wairakite, chlorite, albite, illlite-smectite, montmorillonite, pyrite | propylitic, zeolitic alteration | Regional extensive alteration around epithermal and geothermal systems. Mineralogical changes from zeolit-rich to propylitic assemblages reflect increasing depth and temperature. The concentration of CO2 also influences the stability of zeolites and the relative importance of calcite versus epidote. |