Rattlesnake Hills greenstone belt
Encyclopedia
The Rattlesnake Hills greenstone belt represents a fragment of a partially exposed synformal
Archean
greenstone belt
within the Wyoming craton
that was intruded by Tertiary
alkalic volcanics. The supracrustal belt has been subjected to multiphase deformation during the Archean and later brittle deformation during the Laramide orogeny
. Ductile deformation during the Archean produced foliation, and at least three episodes of folding
.
. Mapping by Hausel indicated that the supracrustal belt consisted of a refolded, isoclinally folded, greenstone belt
fragment with a lower(?) metasedimentary unit of metapelite
, quartzite
, banded iron formation
and amphibolite
named the Barlow Springs Formation. The Barlow Springs Formation is overlain by pillowed metatholeiite
s of the McDougal Gulch Metavolcanics, which, in turn, is overlain by metagreywacke
with intercalated metachert
s of the UT Creek Formation. The northern limb of the Rattlesnake synform was not uplifted and presumably underlies the Phanerozoic
sediments north of the uplift.
mineralization
are recognized in the district. These include syngenetic stratabound
exhalitive
mineralization, epigenetic mineralization, and disseminated epithermal gold associated with the volcanic activity.
The presence of anomalous gold associated with the Tertiary alkalic volcanism suggests this region to represent a favorable exploration target for low-grade, large-tonnage, gold mineralization. The greenstone belt was considered to have high potential for gold mineralization by Hausel who investigated the greenstone belt initially in 1982 and made several significant discoveries. Since 1996, several companies, the most notable at present being EVG (Evolving Gold Corp), have explored the belt finding additional gold anomalies including at least one deposit with > 1 million ounces of gold.
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
greenstone belt
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
within the Wyoming craton
Wyoming craton
The Wyoming craton is a craton located in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah...
that was intruded by Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
alkalic volcanics. The supracrustal belt has been subjected to multiphase deformation during the Archean and later brittle deformation during the Laramide orogeny
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
. Ductile deformation during the Archean produced foliation, and at least three episodes of folding
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
.
Folding
The exposed core of the Rattlesnake Hills forms an Archean supracrustal belt of metasedimentary and metavolcanic rock that has been intruded by more than 40 Tertiary alkalic plugs and dikesDike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
. Mapping by Hausel indicated that the supracrustal belt consisted of a refolded, isoclinally folded, greenstone belt
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
fragment with a lower(?) metasedimentary unit of metapelite
Pelite
Pelite is old and currently not widely used field terminology for a clayey fine-grained clastic sediment or sedimentary rock, i.e. mud or mudstone. It is equivalent to the Latin-derived term lutite. More commonly, metamorphic geologists currently use pelite for a metamorphosed fine-grained...
, quartzite
Quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
, banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
and amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
named the Barlow Springs Formation. The Barlow Springs Formation is overlain by pillowed metatholeiite
Tholeiite
The tholeiitic magma series is one of two main magma series in igneous rocks, the other magma series being the calc–alkaline. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
s of the McDougal Gulch Metavolcanics, which, in turn, is overlain by metagreywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
with intercalated metachert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
s of the UT Creek Formation. The northern limb of the Rattlesnake synform was not uplifted and presumably underlies the Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
sediments north of the uplift.
Mineralization
At least three episodes of goldGold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
mineralization
Mineralization (geology)
In geology, mineralization is the hydrothermal deposition of economically important metals in the formation of ore bodies or "lodes".The first scientific studies of this process took place in Cornwall, United Kingdom by J.W.Henwood FRS and later by R.W...
are recognized in the district. These include syngenetic stratabound
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
exhalitive
Sedimentary exhalative deposits
Sedimentary exhalative deposits are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir , resulting in the precipitation of stratiform ore....
mineralization, epigenetic mineralization, and disseminated epithermal gold associated with the volcanic activity.
The presence of anomalous gold associated with the Tertiary alkalic volcanism suggests this region to represent a favorable exploration target for low-grade, large-tonnage, gold mineralization. The greenstone belt was considered to have high potential for gold mineralization by Hausel who investigated the greenstone belt initially in 1982 and made several significant discoveries. Since 1996, several companies, the most notable at present being EVG (Evolving Gold Corp), have explored the belt finding additional gold anomalies including at least one deposit with > 1 million ounces of gold.