Abstract

Gold-bismuth-tellurium mineralization at the Lucky Draw mine is associated with pods, lenses, and veins of Fe-Mg-Al silicates, adjacent to the late Carboniferous Burraga Granodiorite. Alteration and mineralization is enclosed by micaecous quartzite and quartz-mica schist of the Triangle Group, immediately below ultramafic shoshonitic volcanic rocks of the Rockley Volcanics. Geologic relationships imply that mineralization and alteration are probably synchronous with later stages of contact metamorphism and, hence, postdate upper greenschist facies regional metamorphism. Barren stage I assemblages consisting of combinations of gedrite, biotite, staurolite, cordierite, quartz, albite, ilmenite, and hercynite are overprinted by stage II assemblages of chlorite, almandine, and biotite. Stage III alteration, consisting of green-brown biotite, is developed where the granodiorite intrudes stage II alteration. Stage I assemblages are poorly constrained to T nearly equal 600 degrees C at 2 to 3 kbars, whereas garnet-chlorite thermometry and phase equilibria indicate T nearly equal 550 degrees C for stage II assemblages. Native Bi, bismuthinite, native Au, maldonite, and various Bi tellurides were deposited interstitial to chlorite grains and along chlorite cleavage planes, during stage II alteration. Ore minerals are characterized by low melting points (< or =370 degrees C) but are in apparent equilibrium with stage II assemblages; this can be reconciled if the ore minerals collected as melts at higher temperatures. Laminae of heavy minerals, which are present in stage I and II alteration assemblages, are identical to those marking bedding and crossbedding in unaltered metasedimentary rocks enclosing the mineralization. Whole-rock immobile element concentrations- of the alteration assemblages are very similar to unaltered metasedimentary rocks of the local Triangle Group. However, stage I assemblages and mineral compositions have some similarities with local occurrences of cordierite-orthoamphibole rocks derived from altered mafic volcanics. Therefore, silicate assemblages at Lucky Draw may be the product of contact metamorphism and metasomatic alteration of quartz-rich metasedimentary and interbedded, mafic volcanic rocks.

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