Distribution of hydrothermal zebra dolomite and its relation to base and precious metal deposits in the Great Basin

Zebra dolomite is typically comprised of alternating layers (ca. 1–10 mm) of dark
replacement and light sparry dolomite. Worldwide, zebra dolomite is associated with
oil fields, Mississippi Valley-Type (MVT) Pb-Zn deposits, and Sedex Pb-Zn±Ba±Au
deposits that are produced by movement of hot basinal brines through permeable
strata and faults.
The initial goal of this study is to determine the geographic and host rock occurrences
of zebra dolomite in carbonate platform rocks of the Great Basin and their
relation to one another and to contemporaneous and subsequent ore deposits. The
stratigraphic, structural, and diagenetic controls on zebra dolomite and its spatial
and paragenetic relation to ore deposits are being documented to provide a framework
for interpretation of subsequent chemical and isotopic studies.
In the Great Basin, zebra dolomite occurs in platform margin to inner shelf facies
rocks of Cambrian, Devonian, and Mississippian age. Occurrences were sampled
along the carbonate platform, south at Las Vegas, north to the Snake Mountains, east
to the Confusion Range and west to Death Valley. In the southern part of the study
area zebra dolomite occurs in Neoproterozoic, Cambrian and Mississippian rocks,
and in the north, Devonian rocks. Some occurrences are stratabound (e.g. Death
Valley) and others discordant to bedding (e.g., Meikle mine, Carlin trend). Zebra
dolomite is genetically associated with Devonian Sedex gold and barite in the Carlin
trend and undated MVT Pb-Zn mineralization in the Goodsprings district, Nevada
(unconstrained at Lower Mississippian-Lower Triassic) and Death Valley region,
California (≤Cambrian). In the Eureka district, Nevada, zebra dolomite served as a
host rock for Cretaceous pluton-related polymetallic replacement deposits (e.g. Ruby
Hill) and Late Eocene Carlin-type gold (Windfall, Achilles). In the Carlin trend, it
was critical to the formation of the large high-grade Meikle gold deposit.
In zebra dolomite occurrences, horizontal layers of dark-gray dolomitized host
rock alternate with one or more generations of white coarse-grained dolomite voidfilling
cement. Sparry dolomite also cements fractures, vugs, and breccia. Dissolution
of carbonate host rock was an important factor in zebra dolomite formation. For
example, at outcrop-scale, high-angle normal faults exposed on pit walls (e.g., Windfall
mine) are interpreted as accommodation faults due to dissolution and removal of
carbonate host rock. In hand specimen (e.g., Rose and Diamond mines), high-angle faults with centimeter- to millimeter-scale offsets commonly form horst-and-graben
structures that terminate at stylolitic dissolution surfaces. Sparry dolomite can
exhibit complex zoning under cathodoluminescence, reflecting variations in Fe and
Mn content (e.g. Meikle mine) or dull diffuse zoning (e.g., Cedar Peak, Snake Mountains,
Nevada). The sulfide minerals, galena and sphalerite, commonly occur in latestage
carbonate-filled veins that cross cut the zebra texture, or as cement in
solution-collapse breccia. At Meikle, zebra dolomite was partially to completely dissolved
and its Fe component sulfidized, producing high-grade gold ore in resulting
dissolution cavities. Calcite commonly fills the remaining porosity or fractures that
crosscut zebra dolomite.
The extent of hydrothermal dolomite in the Great Basin requires brine movement
on a scale similar to that documented in western Canada where it is associated with
Devonian Sedex and MVT Pb-Zn deposits. Because of high permeability and Fe content,
zebra dolomite is an ideal host for younger polymetallic replacement and Carlintype
gold deposits. An improved understanding of zebra dolomite is, therefore,
relevant to development of genetic and exploration models for base and precious
metal deposits in the Great Basin.