Plat tectonic setting for porphyry copper deposits

دسته زمین شناسی اقتصادی واکتشاف
گروه سازمان زمین شناسی و اکتشافات معدنی کشور
مکان برگزاری بیست و پنجمین گردهمائی علوم زمین
نویسنده Yasushi Watanabe
تاريخ برگزاری ۰۲ اسفند ۱۳۸۵

P
late tectonic models for porphyry copper deposits originated in the early ۱۹۷۰s just after the emergence of plate tectonic theory. It became widely accepted that porphyry deposits from beneath andesitic and dacitic startovolcanoes at convergent plate margins as a result of magnetite-series calc-alkaline magma-tism. As porphyry deposits tend to cluster in long but narrow metallogenic zones, and as the ages of deposits seem to match periods of faster plate convergence, a causal Connection between shallow-dipping plate subduction and porphyry mineralization was suggested.This paper reviews the relationships between tectonics and porphyry deposits mainly in the western Americas, and presents a revised tectonic modal for porphyry mineralization.
 

 
 
Plat tectonic setting for porphyry copper deposits
Yasushi Watanabe
Institute for Geo-Resources and Environment, Geological Survey of Japan, AIST
 
 
P
late tectonic models for porphyry copper deposits originated in the early 1970s just after the emergence of plate tectonic theory. It became widely accepted that porphyry deposits from beneath andesitic and dacitic startovolcanoes at convergent plate margins as a result of magnetite-series calc-alkaline magma-tism. As porphyry deposits tend to cluster in long but narrow metallogenic zones, and as the ages of deposits seem to match periods of faster plate convergence, a causal Connection between shallow-dipping plate subduction and porphyry mineralization was suggested.This paper reviews the relationships between tectonics and porphyry deposits mainly in the western Americas, and presents a revised tectonic modal for porphyry mineralization.
 
 
 
Tectonic settings favorable for porphyry mineralization
Compilation of tectonic setting that produced porphyry deposits in the western Americas reveals that porphyry deposits are predominantly formed under the following tectonic regimes: (1) fast plate Convergence (more than 150 mm/y;e.g., Kluane belt in Alaska-Yukon, British Columbia, Arizona-Northern Mexico); (2) during subduction of an aseismic ridge or seamount chain (southern Mexico, Panama, cental Peru, centrl Chile,and Argentine); or (3) during subduction of thick, cold, oceanic or microcontinental terranes (Central and Eastern Alaska belts, Aleutian-Alaska Peninsula, Washington). These tectonic settings are characterized by diminished magmatic activity related to the backward shift of the volcanic arc, or by formation of a magmatic gap along the arc. Porphyry deosits tend to from in these perturbed zones and gaps. Porphyry formation is commonly accompanied or followed by shortening deformation, as is represented by the Laramide orogenic tract in the southwestern United States, the Eastern Chiapas Laramide fold belt in southern Mexico, the Cordillera de Talamanca in Panama, and the Zongo-San Gaban zone in southern Peru.
 
 
Role of tectonic stress on porphyry copper mineralization
Fast convergence rates between the subducting and overriding plates, subduction of aseismic ridges and seamount chains, and terrane subduction are factors that contribute towards shallow-slab subduction of oceanic plates. Porphyry copper deposits in the western Americas formed in zones of shallow-slab subduction, including transitional areas between shallow- and steep-slab regions. The slab angle associated with porphyry mineralization in northern Mexico, southern Mexico, and central Chile appears to be about 20° or less, if the dip of the subducting slab is averaged from the trench to the volcanic arc. No porphyry deposits of Pliocene or younger age have been discovered in southwestern Mexico (except for the area where the Tehuantepec ridge has been subducting), Nicaragua, northern Costa Rica, and northern Chile where the present angle of subduction is more than 30° . This suggests that porphyry mineralization is intrinsically related to the    angle of the down-going slab.
Shallow-slab subduction strengthens the coupling force between the slab and overriding plate, and reduces the extensional stress in the volcanic arc and backarc region. A less intense extensional stress field, lower than that associated with moderately dipping (>30°) slabe, can be conducive to the formation of porphyry deposits for the following reasons:
(1) Relatively large crustal magma chambers and subvolcanic porphyry stocks can from. The size of volcanoes and of subvolcanic magma chambers is related to the amount of differential horizontal stress in the lithosphere when the input rate of magma is constant. Stratovolcanoes with large underlying porphyry stocks and large parental magma chambers form in a regime of relatively low differential (i.e.,low extensional) stress. As a magma volume of 10-100 km3 is required to supply sufficient hydrothermal fluid to from a porphyry copper deposit, and as a shallow porphyry stocks (with maximum volume of several cubic kilometers) cannot itself exsolve a sufficient volume of hydrothermal fluid to from a porphyry copper deposit, and as a shallow porphyry stocks (with maximum volume of several cubic kilometers) cannot itself exsolve a sufficient volume of hydrothermal solutions,a porphyry stock and its contained metals represent products of volatile-rich magmatic discharge from a deeper and lager magma chamber. Low extensional stress due to shallow-slab subduction therefore favors formation of porphyry deposits because large porphyry stocks fed by large magma chambers can from.
(2) Compressive stress may prevent eruption of subvolcanic porphyry intrusions. Although porphyry deposits commonly form at shallow crustal level (2-3 km from surface) beneath stratovolcanoes, formation and preservation of a deposit require that hydrothermal solutions discharge through the apex of the stock without the porphyry magma erupting. Ambient compressive stress may favor deposit formation by inhibiting eruption of shallow, volatile-rich porphyry magma.
(3)Lithostatic pressure is maintained near the apex of a porphyry stock during fluid exsolution and mineralization. porphyry copper mineralization is formed by high-temperature (>350°C) hypersaline magmatic solutions.Phreatomagmatic or phreatic brecciation changes the pressure condition near the apex of a porphyry stock from lithostatic to hydrostatic, and allows meteoric water incrusion into the system. Introduction of meteoric water may change the style of mineralization form porphyry to epithermal (Sillitoe 1994). Ambient compressive stress may therefore help preserve lithostatic pressure near the apex of a porphyry stock during fluid exsolution and mineralization.
 
 
Conclusions
porphyry copper deposits formed during: (1) fast plate convergence (more than 150 mm/y); (2) subduction of an aseismic ridge or sea-mount chain; or (3) subduction of oceanic or microcontinental terranes. These conditions favor shallow-slab subduction (an average dip from the trench to the volcanic arc of c. 20° or less). Such shallow-slab subduction strengthens the coupling force between the downgoing slab and the overriding plate. The reduced extensional stress field is conducive to the generation of porphyry copper deposits because of (1) relatively large magma chambers and subvolcanic porphyry stocks can from, (2) compressive stress prevents eruption of subvolcanic porphyry intrusions, and (3) lithostatic pressure is maintained near the apex of the porphyry stock during fluid exsolution and mineralization.
 

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