LODE GOLD PROVINCES IN THE EVOLVING NORTHERN CORDILLERA

The fore-arc regions of Cordilleran orogens are inherently characterized by widespread orogenic gold deposits. With the onset of subduction-accretion and the deeper-later style of metamorphism, economically significant gold ores have formed within mainly greenschist facies rocks, but also in higher (Pogo) and lower (Donlin Creek) metamorphic grade sequences, of the northern Cordilleran orogen for the previous 170 m.y. The youngest such ores are the 50 Ma deposits on Chichagof Island, SE Alaska; however, younger deposits have likely continued to form at depth within active parts to the fore-arc of the orogen since the middle Eocene and are not yet exposed.

The most abundant lodes are those of a metamorphic fluid origin, typically of high grade(>=1-3 g/t Au) and are associated with terrane-bounding, deep-crustal fault systems. Where auriferous fluids are not well-focused into such conduits, world-class economic gold lodes are commonly absent, despite the presence of significant placer accumulations (Seward Peninsula, Klondike, Chugach-Kenai Mountains). Both exotic oceanic blocks (Juneau gold belt) and terranes of pericratonic affinity (Fairbanks district) are likely to host orogenic gold deposits.

The oldest gold lodes in the northern Cordillera are those of the Middle and Late Jurassic in the Canadian sector. In Alaska, both exposed gold deposits and arcs young seaward, from ca. 110-90 Ma in the north and interior, to ca. 60-50 Ma along the present active margin. Typically, ores occur in geologically and structurally favorable parts to the fore-arcs. However, where arcs are relatively diffuse, rather than being present as Andean-style batholiths, the deposits may be located in the evolving arc itself. Some melts within these diffuse arcs and of dominantly mantle affinity may be uniquely gold-rich and sites of low-grade (<1 g/t Au), but high-tonnage, economic intrusion-related gold deposits. In some cases, where deformation and subduction-related plutonism continue landward of the evolving continental margin arc, orogenic gold formation may occur inboard of the most voluminous magmatism (Bridge River, Polaris-Taku). Clearly, the thermal profile of a Cordilleran orogen, rather than simply geographic location in a growing margin, controls localization of gold orebodies seaward of a craton margin.