JURASSIC FAULTS ALONG THE MARGIN OF LAURASIA: PRINCIPAL CONDUITS FOR GOLD-BEARING FLUIDS

Correlative, sinistral, Jurassic, faults and coeval transtensional and transpressional segments that have been identified in western North America (Mexico-Alaska megashear, MAM), Siberia (Aducha-Taryn and Yana-Indigirka), China (Tan-Lu) commonly are coincident with gold mineralization. Transtensional (e.g. Foothills belt, Klamath Mtns., Britannia and Eskay rifts, Sanandaj-Sirjan zone) and transpressional (e.g. Blue Mtns., Chersky collisional belt, Mian Lue suture) linkages also are mineralized. In western North America, the coincidence between the distribution of mines, prospects, and placers and regional structures such as the Juneau gold belt (Fanshaw and Sumdum faults), Bridge River (Bralorne and Pioneer faults), Sierra Foothills gold province (Melones fault), and the Caborca orogenic gold Belt (MAM) is clear. Analysis of the inferred ages and settings of these faults, among the collage of terranes that characterizes the margins of the western North America and eastern Asia, leads to the inference that the structures belong to the same Middle Jurassic fault set, which encircles the Laurasia plate and accommodated the last stage of the break-up of Pangea. The process zone along the principal strand of the MAM may be tens to hundreds of kilometers wide; however, most of the large mines are close to the main fault or adjacent strands. Ages of mineralization are diverse and commonly younger than the principal, movement between 169 and 148 Ma. As previously noted, reactivation of crustal scale faults commonly correlates with fluid flow, and formation of orogenic gold deposits. The erosional surface along the fault trace lies within with the level of rich mineralization.