科研之友
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摘要
A compilation of economically viable gold concentrations containing >= 10 Moz in the North and South American Cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits, more formed over the last 150 m.y. The gold concentrations are attributed to eight widely recognised deposit types, of which porphyry, sediment-hosted, and high-sulfidation epithermal are economically the most important. Individual gold belts are typically several tens to hundreds of kilometers long, dominated by single deposit types, and metallogenically active for relatively brief periods (<5-20 m.y.).
Many of the gold belts and major isolated deposits were generated under extensional of transtensional tectonic conditions in either are or back-arc settings. Nevertheless, the two main high-sulfidation epithermal gold belts were generated in thickening or already-thickened crust during low-angle subduction. Either other gold belts or districts also accompanied compression or transpression, with two of them, the main orogenic gold belts, occupying fore-arc sites. There is a strong suggestion that the preeminent Cordilleran gold concentrations formed either during or immediately following prolonged contraction. The major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes, but almost all are of postaccretionary timing. Many of the gold belts and isolated deposits were localized by crustal-scale faults or lineaments, which may be either parallel or transverse to the Cordilleran margin.
The gold concentrations accumulated during active subduction, commonly ill close spatial and temporal association with intermediate to felsic, medium- to high-K calc-alkaline igneous rocks. By contrast, the low-sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc-alkaline, tholeiitic, or alkaline affinity. However, a gold-alkaline rock association is uncommon. A genetic link between gold mineralization coeval magmatism is widely accepted for most of the deposit types, the exceptions being the sediment and hosted and orogenic gold deposits.
Notwithstanding the small cumulative extent of the gold concentrations relative to the entire Cordilleran margin, there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted are (including fore- and back-arc) segments. In the case of the western United States, for example, six belts and four isolated major deposits make up a particularly prominent cluster. If fortuity is discounted, this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization. An analogous situation is evident for other metals, particularly copper and tin. The reason for the recurrent generation of major deposits and belts dominated by one or more metals remain uncertain, although heterogeneously distributed metal preconcentrations, favourable redox conditions, or other parameters somewhere above the subducted slab, between the mantle wedge and upper crust, are widely contemplated possibilities. Elucidation of the reason(s) for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs.
- 出版日期2008-7
