BRINE MIGRATION AND FORMATION OF HYDROTHERMAL ZEBRA DOLOMITE, SEDEX, AND PB-ZN DEPOSITS ALONG THE PALEOZOIC CARBONATE PLATFORM OF THE GREAT BASIN, NEVADA AND UTAH

Zebra-textured dolomite is characterized by alternating layers of fine-grained gray and coarse-grained white sparry dolomite. Worldwide, hydrothermal zebra dolomite (HZD) occurs in carbonate-platform margins in settings with elevated heat flow and extensional faulting. Compositions of fluid inclusions in sparry dolomite indicate that they formed from hot (Th > 70-150°C), saline (> 10 eq. wt. % NaCl) residual brine produced by the evaporation of seawater (from Na-Cl-Br ratios). HZD is frequently associated with MVT, Sedex Pb, Zn, Ag, and barite deposits.

In the Great Basin, episodes of brine migration are recorded by occurrences of HZD in the north-trending Paleozoic carbonate platform. HZD occurs in Neoproterozoic, Cambrian, Devonian, and Mississippian carbonates but has not been identified in upper Paleozoic and Mesozoic carbonates. At each locality, HZD exhibits similar structures and textures that suggest brine migration was channeled along extensional and strike-slip faults and permeable fabric elements in carbonate aquifers.

Spatially associated mineral deposits are either the same age (i.e. Sedex Barite-Zn-Au, MVT Pb-Zn-Ag) or younger (i.e., pluton–related polymetallic replacement, distal-disseminated Ag-Au, and Carlin-type gold deposits) than the HZD. The most Fe-rich HZD yet identified formed in the footwall feeder zone of a Devonian Sedex system in the Carlin trend. This Fe-rich HZD was an ideal chemical trap during formation of the Eocene Meikle Carlin-type gold deposit. In the Eureka and Carlin gold areas of northern Nevada, laser ablation identified only trace amounts of Zn in dolomite. Conversely, Zn-rich dolomite (£33 wt. % Zn within dolomite lattice structure) is associated with predominantly Pb-Zn-Ag ore deposits in the southern half of Nevada.

Carbon isotope values of HZD reflect the composition of the seawater when the host rocks formed. The δ¹⁸O(SMOW) HZD values from Neoproterozoic (19–24‰), Cambrian (20–25 ‰), and Mississippian (20–23‰) rocks are similar. However, the wide range (16–28‰) of the Devonian HZD values records multiple fluid sources or different brine evolution pathways. Mississippian data are comparable to MVT HZD in Belgium. However, other δ¹⁸O values do not correspond to Cambrian or Devonian HZD in Canada or to Carboniferous HZD in Spain.