2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 1
Presentation Time: 8:15 AM


HOFSTRA, Albert H., U.S. Geol Survey, MS 973, Box 25046, Denver, CO 80225, ahofstra@usgs.gov

Sedimentary rock-hosted disseminated Au (SRHDG) deposits are often called Carlin-type even though they form in different tectonic settings and physical environments and have different characteristics and endowments of Au. Current information shows that SRHDG deposits form in sedimentary basins, orogenic belts, magmatic arcs, and continental rifts. They form in sedex, epizonal orogenic Au, distal portions and later stages of oxidized porphyry Cu and reduced intrusion-related Au, Carlin-type, and low-sulfidation systems from basin brines, metamorphic, magmatic, meteoric, and hybrid fluids. Many are insufficiently studied to be classified and a few don't fit these categories. Their mineralogy (gold, py, orp, real, stib, cinn, bar) and geochemistry (Au, As, Sb, Hg, Tl, Ba) indicate they all formed from H2S-rich fluids. They all occur in reduced, carbonaceous, pyritic, +/-baritic carbonate and/or shale sequences that lack significant pyrrhotite, magnetite, hematite, or chlorite to buffer fluids to low H2S activities. Cooling and reaction of fluids of diverse origins with these rocks yield H2S-rich fluids capable of transporting Au, As, Sb, Hg, Tl, and Ba, but little Ag or base metals (convergent evolution). As these fluids cool, mix with other fluids, or react with wall rocks, some or all of the above mentioned minerals precipitate. In Nevada, major Au belts are underlain by crustal structures produced by Late Proterozoic rifting that influenced subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. In the Carlin trend, Dev. Sedex, Jur. & Cret. intrusion-related, Eocene Carlin-type, and Miocene low-sulfidation systems deposited Au and are locally superimposed. Thus, structural and geochemical inheritance contributed to formation of this world class district. Apparently, any large hydrothermal system developed along major fault zones in reduced sedimentary rocks has the potential to form SRHDG deposits. The multi-event history of the Carlin trend suggests that other SRHDG districts and mineral belts may contain more than one type of Au deposit that remain to be detected and explored. It also is important to recognize and distinguish the deposit type(s) present in a given district or belt because they have different exploration models and different endowments of Au.