GOLDEN HIGHS AND SOGGY BOTTOMS: THE LINK BETWEEN EOCENE PALOEGEOGRAPHY AND GOLD DEPOSITION, NORTHERN CARLIN TREND, NEVADA

A link between gold deposits in the northern Carlin trend (NE Nevada) and the Eocene paleogeography is provided by the Eocene Elko Formation, which filled the Elko Basin. This basin was bounded by topographic highs, one of which was the northern Carlin trend. The non-marine clastic rocks of the Elko Formation filled a broad extensional basin by 46 m.y.-ago. Sedimentary facies indicate at least 2 large sub-basins. Shallow lakes filled the sub-basins and coalesced to form a large lacustrine system ringed with swamps. Restricted boulder conglomerate deposited as talus aprons shed from normal fault scarps mark the eastern basin margin. Paleocurrent data from massive, lensoidal beds of chert-pebble sandstone, and 100’s of metres of lacustrine oil shale and siltstone indicates that the depocentre developed in the eastern Elko Basin, west of the modern Ruby Mountains-East Humboldt Range. The western Elko Basin is filled predominantly by ash-flow tuff erupted from the Tuscarora Volcanic Field. Clastic sedimentary rocks beneath the tuff in the west are less than 200 m of cobble conglomerate and cherty limestone, which represent an alluvial braidplain with many shallow ponds. Differences in basin fill lithology and thickness suggests that a paleohigh in the area of the modern Adobe Range subdivided the Elko Basin. Paleocurrent data from outcrops of conglomerate and sandstone in the western sub-basin further demonstrate that the flow was to the north and west away from the northern Carlin trend. The Elko Formation is absent in the area of the gold deposits and apatite-fission track geochronology of Paleozoic strata along the northern Carlin trend suggest limited (<1 km) erosion since the Eocene. Closely-spaced, rotational extensional faulting, progressing from west to east, broke the Elko Formation into shallowly SE-dipping structural panels between 40.5 and 37.5 Ma in the Eocene, a time which overlaps with gold deposition. The evidence suggests that gold deposition was localized beneath a topographic high, and was broadly synchronous with normal faulting, and with lakes being progressively concentrated in the eastern Elko Basin. The lakes potentially provided a source of water for circulation through the upper crust, and for thermally driven circulation toward paleotopographic highs where gold was deposited between 800 and 2 km beneath the extant surface.