Where is Porphyry found?
Porphyritic igneous rock
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Open the appPorphyry occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Porphyry
Andean Volcanic Belt
Extensive porphyry copper deposits are found along the Andes Mountains in South America (e.g., Chile, Peru), associated with subduction-related magmatism.
Cordilleran Orogen
Significant porphyry occurrences and associated ore deposits are found throughout the North American Cordillera (e.g., British Columbia, Arizona, Utah).
Pacific Ring of Fire
Volcanic arcs and associated intrusive bodies around the Pacific Ocean are common settings for porphyritic rocks and related mineralization.
Ancient Shield Areas
Some older porphyritic rocks, often metamorphosed, can be found in Precambrian shield regions globally.
Collecting tips
- Look for Two Crystal Sizes. The most defining characteristic is the presence of large, distinct crystals (phenocrysts) embedded within a much finer-grained or glassy matrix. This bimodal texture is key.
- Examine Mineralogy. Identify the minerals forming the phenocrysts (e.g., quartz, feldspar, hornblende, biotite) and try to infer the composition of the groundmass to classify the specific type of porphyry (e.g., porphyritic granite, porphyritic andesite).
- Consider Geological Context. Porphyries are common in areas of past or present volcanic and shallow intrusive activity, such as volcanic arcs, subduction zones, and areas with extensive dikes and sills.
- Check for Ore Mineralization. In some regions, porphyritic rocks are associated with significant ore deposits (e.g., copper, gold). Look for signs of alteration or disseminated sulfide minerals.
Quick Check
- Color:
- Variable (light to dark), depending on mineralogy.
- Luster:
- Variable, often dull to vitreous.
- Streak:
- White or colorless (for felsic groundmass), or dark grey to black (for mafic groundmass), depending on the dominant minerals and their alteration.
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