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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Mesothermal |
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| calcite, ankerite, dolomite, quartz, muscovite (Cr-/V-rich), chlorite, pyrite, pyrhotite | carbonate | Wallrock alteration in and around veins or shear zones, and extensive replacement of ultramafic to mafic rocks. CArbonate-rich alteration may be regionally extensive and is not always related to mineralization. |
| chlorite, muscovite, quartz, actinolite, pyrite, pyrhotite | chlorite | Wallrock alteration in and around veins and shear zones, particularly in mafic volcanic and volcaniclastic sedimentary rocks. |
| biotite, chlorite, quartz, pyrite, pyrhotite | biotitic | WAllrock alteration in and around veins and shear zones, particularly in sedimentary rocks. |
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Sediment-hosted gold |
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| quartz, pyrite, hematite | jasperoid | Complete replacement of limestone, and occasionally other rock types, by fine-granied quartz; often associated with brecciation. Jasperoid can form as regionally extensive zones, as small bodies related to sediment-hosted Au deposits (Carlin-type), and as the upper or outer alteration zones associated with intrusion-related skarn/sulphide replacement bodies. Depth of formation is probably moderate (>2km-'mesothermal'), although shallower zones of jasperoid may form; fluids may be metamorphogenis (classic mesothermal), connate, or magmatic. |
| Volcanogenic massive sulphide | ||
| sericite, quartz, pyrite, chlorite, andalusite, chloritoid | sericite | Pervasive replacement of rocks in the footwall below massive sulphide lenses; concentrated in stockwork feeder zones but may be laterally extensive both deeper in the footwall and extendeing into the hanging wall in some deposits. Most common in intermediate to felsic volcanic rocks but may also replace the more mafic units in lower temperature systems. Andalusite and chloritoid occur in metamorphosed alteration zones. |
| chlorite, quartz, sericite, pyrite, cordierite, biotite | chlorite | Pervasive replacement of rocks in the footwall below massive sulphide deposits. Fe-rich chlorite occurs in the core of stockwork feeder zones in mafic footwall sequences (e.g. Archean deposits) whereas Mg-rich chlorite has a more erratic distribution, generally around the periphery or upper parts of stockwork zones. Cordierite +/- biotite is common in metamorphosed Mg-Fe-rich alteration zones. |
| quartz, pyrite, sericite, K-feldspar | silicic | Pervasive replacement of rocks in the footwall below massive sulphide deposits; particularly common in permeable siliceous ash-rich beds, where the silicified rock may be mistaken for cherts (chemical sediments). Also occurs as wallrock alteration in some quartz vein stockwork zones. |
| dolomite, siderite, ankerite, calcite, quartz, sericite, chlorite, pyrite | carbonate | Usually occurs as disseminated alteration in footwall sequences, commonly over extensive lateral and stratigraphic intervals. The composition of carbonates may change with distance from ore zones. |
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