Abstract

The tungsten province of northwestern Mexico is the southernmost of the Cordilleran tungsten provinces of North America and the westernmost of a parallel series of Mexican metal provinces which contain Cu, Au-Ag, Pb-Zn-Ag, F, Sn, Mn, U, Fe, and other metals. On its western side, the tungsten province primarily contains skarns associated with Paleozoic limestones. The eastern side primarily contains pegmatites and quartz veins in Laramide plutons. The eastern margin of the tungsten province overlaps the porphyry copper province where a series of breccia pipes contain copper and lesser tungsten.Isotopic dating, using K-Ar, 40 Ar/ 39 Ar, and Rb-Sr methods, indicates that tungsten mineralization is Laramide with ages ranging from 47 to 65 m.y. Hornblende and biotite K-Ar and 40 Ar/ 39 Ar ages are discordant by an average of 11 m.y., and biotite and muscovite associated with mineralization record ages averaging 14 m.y. younger than related intrusive rocks. These large discordances are inferred to be the result of slow cooling in a relatively deep batholithic environment.Sr isotope initial ratios for intrusive rocks associated with tungsten deposits indicate that mineralization occurred independently of basement rock type. Deposits within terranes underlain by cratonic basement are associated with intrusive rocks having higher initial ratios than deposits within terranes underlain by accreted oceanic basement. Initial ratios range from 0.70901 at San Antonio (eratonic) to 0.70570 at San Alberto (accreted).The age of the tungsten province is similar to that of the porphyry copper province and is approximately 20 m.y. older than Ag-Au and Pb-Zn-Ag mineralization in the Sierra Madre Occidental and the central plateau. Tungsten mineralization is approximately 10 m.y. older than rhyolite-hosted U mineralization and approximately 25 m.y. older than rhyolite-hosted F, Sn, and Fe mineralization. The northwestern Mexico tungsten province is slightly younger than other Cordilleran tungsten provinces which are pre-Cenozoic in age.

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