Continental Arc Tectonics and Sulfide Deposits
The question describes a specific tectonic environment: a convergent plate boundary where oceanic crust subducts beneath continental crust. This process leads to melting and the formation of magma, which rises to create a continental arc, characterized by volcanic mountain ranges.
Magmatism in Continental Arcs
The intense magmatic activity associated with continental arcs is crucial for the formation of certain types of mineral deposits. Molten magma contains dissolved metals and volatiles. As this magma intrudes into the crust or erupts at the surface, changes in pressure, temperature, and chemistry cause these metals to precipitate and form ore deposits.
Identifying Deposit Type
The described setting, particularly the magmatism linked to subduction and continental arc formation, is the classic environment for the genesis of porphyry copper deposits. These deposits are characterized by:
- Disseminated sulfide minerals (primarily chalcopyrite and bornite, sources of copper) within intrusive igneous rocks (porphyries).
- Associated metals often include molybdenum, gold, and silver.
- They form at relatively shallow depths within the continental crust.
Excluding Other Options
- Mississippi Valley-type (MVT) deposits: Form in passive continental margins or stable cratons, typically within shallow marine carbonate rocks, not subduction zones.
- Besshi-type deposits: Found in metamorphosed sequences, often associated with accretionary prisms or mélanges closer to the trench, rather than the main magmatic arc.
- Kuroko-type deposits: Associated with submarine volcanism, typically in rifted island arc or back-arc basin settings, often related to bimodal volcanism, not the specific continental arc magmatism described.
Therefore, the specific tectonic setting detailed points directly to the formation of porphyry copper deposits.
Important Questions from Igneous Petrology (Classification, Forms, Textures, Genesis)
- Match the rock types in Group-I with their most likely corresponding lithospheric / tectonic settings of formation in Group-II
Group-I Group-II P. Boninite 1. Continental anorogenic Q. Lamproite 2. Island-arc R. Phonolite 3. Continental collision S. Leucogranite 4. Intraplate oceanic - The mole percentages of SiO$_2$, Al$_2$O$_3$ and K$_2$O in a granitic rock are 84.21, 7.89 and 7.89, respectively. The molar proportion (in %) of K-feldspar in the rock is ________ [ round off to one decimal place]
- Which one of the following is a mafic volcanic rock?
Match the following igneous textures in Group-I with their definitions in Group-II.
Group-I
Group-II
P Vitrophyre 1 Alkali feldspar rimmed by plagioclase Q Rapakivi 2 Aggregate of radially arrayed, needle-like crystals of plagioclase with or without clinopyroxene R Ocelli 3 Sub-parallel skeletal, platy olivine and/or pyroxene S Spinifex 4 Large phenocrysts within a glassy matrix - Which one of the following statements about igneous rocks is CORRECT?
