Volcanotectonic Setting of World-Class Alkalic Porphyry and Epithermal Au ± Cu Deposits of the Southwest Pacific
Anthony C. Harris, David R. Cooke, Jacqueline L. Blackwell, Nathan Fox, Evan A. Orovan, 2013. "Volcanotectonic Setting of World-Class Alkalic Porphyry and Epithermal Au ± Cu Deposits of the Southwest Pacific", Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings, M. Colpron, T. Bissig, B. G. Rusk, J. F. H. Thompson
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Some of the world’s largest and highest grade alkalic porphyry Au-Cu-(Mo) deposits and related epithermal Au deposits occur in the southwest Pacific. Alkalic deposits of this region share many geologic similarities in their environments of formation. Source magmas are highly oxidized and alkali rich, being derived from enriched mantle sources that were previously modified by subduction processes. The more Cu rich systems formed by high K calc-alkalic and alkalic magmatism are typically located along the main magmatic arc. These subduction-related fluids and mantle-sourced mafic magmas evolve in an environment associated with a thickened crust. In contrast, more Au rich systems appear to be associated with rifting of oceanic crust in back-arc settings. Here, primitive mantle-derived magmas evolve in upper crustal magma bodies to form Au- and PGE- rich alkalic porphyry and epithermal deposits.
The gold-rich alkalic porphyry and epithermal deposits formed in and along the margin of sedimentary basins that were intruded by alkalic dikes and stocks. In the largest example (Cadia East), deep mineralization is hosted by sheeted quartz-sulfide veins associated with potassic alteration, while near-surface mineralization is disseminated in both permeable clastic units and quartz-sulfide veins. Potassic alteration grades laterally into proximal, hematite-bearing propylitic alteration, and transitions upward from deep K-feldspar to shallow biotite-tourmaline. The shallow biotite alteration domain is overprinted by a complex, late-stage assemblage of pervasive K-feldsparalbite-sericite-pyrite, and structurally focused sericite-pyrite.
In the alkalic epithermal environment, near-surface K-feldspar-quartz-carbonate-anhydrite (± sericite) alteration associated with epithermal Au-Ag mineralization occurs in and around dikes, fault intersections, and along extensive low-angle faults. Catrastrophic failure of the overlying volcanic edifice has the potential to cause superposition of alkalic epithermal mineralization onto porphyry deposits.
Given their potential to form in a back-arc setting, alkalic porphyry deposits are considered more likely to be preserved in the ancient rock record than their calc-alkalic counterparts, due to burial in the sedimentary basins in which they form. Thus, areas of fragmented intraoceanic arc terranes within orogenic belts should be considered prospective for Au-rich alkalic porphyry deposits like those found in the southwest Pacific, particularly when they occur in regions overlain by postmineralization sedimentary and/or volcanic cover. Alkalic epithermal deposits offer more challenging exploration targets, as they are likely to be exhumed and eroded soon after their formation, unless a tectonic switch causes burial before any significant erosion occurs.
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Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings
The northern Pacific Rim—for the purposes of this contribution—comprises the Mesozoic and Cenozoic magmatic-arc and associated terranes of eastern China, Korea, Japan, the Russian Far East, Alaska, Yukon, British Columbia, the western United States, and Mexico. This ~1,800-km-long segment of the Pacific Rim is marked by a broad spectrum of metallogenic environments and mining jurisdictions, which combine to dictate where and how exploration is conducted and the overriding character of any resulting discoveries.
This summary report commences with a brief metallogenic overview of the northern Pacific Rim, with particular attention paid to the world-class Mesozoic and Cenozoic ore deposits that define the region’s premier metallogenic provinces. This is followed by a summary of the relative attractiveness of the region’s various mining jurisdictions, as recorded by recent exploration activity. The major discoveries made along the northern Pacific Rim, particularly during the past half century, are then placed in this metallogenic and regulatory context as a basis for determining the successful exploration methodologies employed. This discovery track record is then used to predict what the future of exploration in this vast and varied region may hold.
Much of the northern Pacific Rim, from eastern China and the Russian Far East in the northwest through Alaska to western parts of Canada, the United States, and Mexico in the southeast (Fig. 1), is characterized by a complex array of oceanic, accretionary prism, magmatic arc, and back-arc basin terranes and associated microcontinental blocks accreted to the North China, Siberian, Hyperborean, and North American cratons, mainly during Mesozoic times (Coney et al., 1980; Campa and Coney, 1983; Kojima, 1989; Nokleberg et al., 2005; Yakubchuk, 2009). The metallogeny of these tectonic collages is dictated by various combinations of pre-, syn-, and postaccretion ore-forming events, the last of which are generally preeminent, except in British Columbia (Nokleberg et al., 2005; Nelson and Colpron, 2007).
Although the Meso-Cenozoic metallogeny of the northwestern and northeastern Pacific quadrants displays some similarities, it is the contrasts that are most marked. The main contrasts stem from the preeminence of tin, tungsten, and antimony in eastern China, Korea, Japan, and the Russian Far East and of copper and silver in Western Canada, the conterminous United States, and Mexico. Nonetheless, both the northwestern and northeastern Pacific quadrants are exceptionally well endowed with gold and molybdenum deposits. The northeasternmost Russian Far East, Alaska, and Yukon Territory display elements of both northwestern and northeastern Pacific metallogeny (Fig. 1). These metallogenic contrasts between the northwestern and northeastern quadrants result in China being the world’s leading producer of tungsten, tin, bismuth, and antimony, mostly from its eastern Mesozoic metallogenic province.