Chapter 18: Geology of Round Mountain, Nevada: A Giant Low-Sulfidation Epithermal Gold Deposit

2020 ◽  
pp. 375-397
Author(s):  
David A. Rhys ◽  
Nadia St. Jean ◽  
Rodolfo Lagos ◽  
David Emmons ◽  
George A. Schroer ◽  
...  
Keyword(s):  
Lacustrine Sediments ◽  
Ore Formation ◽  
Kinematic Indicators ◽  
Welded Tuff ◽  
Basement Rocks ◽  
Epithermal Gold Deposit ◽  
Volcaniclastic Rocks ◽  
Volcanic Facies ◽  
Low Sulfidation ◽  
Late Stages

Abstract The Round Mountain low-sulfidation epithermal Au deposit occurs within the rhyolitic tuff of Round Mountain (26.86 Ma) on the northeast side of an elliptical volcanic center that has morphology and volcanic facies suggesting it originated as a caldera. The hosting tuff comprises three pyroclastic flow and fall deposits (units T1 to T3). These are overlain successively by lacustrine sediments and volcaniclastic rocks. which may contain paleowater table levels formed at the time of ore formation and a 26.4 Ma postmineralization tuff unit. A linear vertical drop in the basement contact coincides with thick tuff fill and megabreccia, which is interpreted to follow the position of a WNW-trending ring fissure or vent wall that may have focused the locations of subsequent hydrothermal upflow zones. Orebodies are developed in strata-bound zones that are most extensive in poorly welded tuff, focused below overlying impermeable welded tuff in a WNW-trending, gently NW-plunging corridor above and mantling the SW-dipping paleoslope of basement rocks. Ore comprises disseminated pervasive adularia-quartz-pyrite ± illite alteration with electrum. The disseminated mineralization surrounds, and is most intensely developed in association with, a low-displacement extensional fault-vein network composed of conjugate NE- and SW-dipping faults and steeply dipping extensional veins. Vein orientations and kinematic indicators suggest ore formation occurred during localized NE-SW-directed extension that may have been related to late stages of volcanic subsidence, potentially in association with deep resurgent magmatism into ring fissures approximately 0.5 m.y. after deposition of the host tuff sequence.

Jurassic–Cretaceous arc magmatism along the Shyok–Bangong Suture from NW Himalaya: Formation of the peri-Gondwana basement to the Ladakh Arc

2021 ◽  
pp. jgs2021-035
Author(s):  
Wanchese M. Saktura ◽  
Solomon Buckman ◽  
Allen P. Nutman ◽  
Renjie Zhou
Keyword(s):  
Late Cretaceous ◽  
Nw Himalaya ◽  
Content Type ◽  
Magmatic Arc ◽  
Basement Rocks ◽  
Ladakh Himalaya ◽  
Accreted Terranes ◽  
Volcaniclastic Rocks ◽  
Supplementary Material ◽  
Depositional Age

The Jurassic–Cretaceous Tsoltak Formation from the eastern borderlands of Ladakh Himalaya consists of conglomerates, sandstones and shales, and is intruded by norite sills. It is the oldest sequence of continent-derived sedimentary rocks within the Shyok Suture. It also represents a rare outcrop of the basement rocks to the voluminous Late Cretaceous–Eocene Ladakh Batholith. The Shyok Formation is a younger sequence of volcaniclastic rocks that overlie the Tsoltak Formation and record the Late Cretaceous closure of the Mesotethys Ocean. The petrogenesis of these formations, ophiolite-related harzburgites and norite sill is investigated through petrography, whole-rock geochemistry and U–Pb zircon geochronology. The youngest detrital zircon grains from the Tsoltak Formation indicate Early Cretaceous maximum depositional age and distinctly Gondwanan, Lhasa microcontinent-related provenance with no Eurasian input. The Shyok Formation has Late Cretaceous maximum depositional age and displays a distinct change in provenance to igneous detritus characteristic of the Jurassic–Cretaceous magmatic arc along the southern margin of Eurasia. This is interpreted as a sign of collision of the Lhasa microcontinent and the Shyok ophiolite with Eurasia along the once continuous Shyok–Bangong Suture. The accreted terranes became the new southernmost margin of Eurasia and the basement to the Trans-Himalayan Batholith associated with the India-Eurasia convergence.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5633162

Woodcutters 55 Years Later: A New Look at an Old Discovery

SEG Discovery ◽  
2020 ◽  
pp. 15-21
Author(s):  
Dave Shatwell
Keyword(s):  
Chloride Content ◽  
Mississippi Valley ◽  
Deep Basin ◽  
Widespread Occurrence ◽  
Great Oxidation Event ◽  
Basement Rocks ◽  
Reducing Conditions ◽  
Volcaniclastic Rocks ◽  
Pine Creek ◽  
Shallow Marine Sediments

Abstract The Woodcutters Zn-Pb-Ag deposit in the Rum Jungle district of the Pine Creek orogen in northern Australia was discovered in 1964 and produced 4.6 Mt of ore grading 12.3% Zn, 5.6% Pb, and 83 g/t Ag between 1985 and 1999. Woodcutters, together with several other polymetallic, uranium, and phosphate deposits, is within a Paleoproterozoic sequence of fluviatile and shallow marine sediments deposited in a deepening basin between ~2100 and 2025 Ma around the margins of an Archean granitic and gneissic dome. These sediments were overlain by turbidites and volcaniclastic rocks until the basin was inverted and the sediments and mineral deposits were deformed and metamorphosed at 1860 Ma. Whereas the polymetallic and uranium bodies at Rum Jungle are considered to be syngenetic or syndiagenetic, sulfides in the Woodcutters orebody replace dolomitic horizons in an otherwise carbonaceous unit. This suggests that Woodcutters is similar to Mississippi Valley-type mineralization and rules out affinities with younger sedimentary exhalative-style deposits elsewhere in the Pine Creek orogen. A model is proposed whereby metals were eroded from Archean basement rocks into Paleoproterozoic sandstone aquifers following the Great Oxidation Event, which also liberated sulfur by oxidation of pyrite. Evaporative conditions, as suggested by the widespread occurrence of dolomite and magnesite, may have increased the chloride content of seawater and enhanced its capacity to transport metals. Subsequently, deeply circulating seawater leached metals from the aquifers and ascended up a deep, basin-penetrating fault until it intersected carbonaceous sediments. In this environment, Zn and Pb sulfides were deposited under reducing conditions, while sulfur may have been provided by H2S from organic material. The Woodcutters and other deposits at Rum Jungle show how metals formerly locked up in Archean cratons were delivered by erosion under an oxygenated atmosphere to Paleoproterozoic shorelines, where they were further mobilized and concentrated by a variety of processes.

Volcanic reconstruction of Paleoproterozoic arc volcanoes: the Hidden and Louis formations, Flin Flon, Manitoba, CanadaThis is a companion paper to DeWolfe, Y.M., Gibson, H.L., and Piercey, S.J. 2009. Petrogenesis of the 1.9 Ga mafic hanging wall sequence to the Flin Flon, Callinan, and Triple 7 massive sulphide deposits, Flin Flon, Manitoba, Canada. Canadian Journal of Earth Sciences, 46: this issue.

2009 ◽  
Vol 46 (7) ◽  
pp. 481-508 ◽  
Author(s):  
Y. M. DeWolfe ◽  
H. L. Gibson ◽  
B. Lafrance ◽  
A. H. Bailes
Keyword(s):  
Hanging Wall ◽  
Massive Sulphide ◽  
Gradual Change ◽  
Shield Volcano ◽  
Ore Formation ◽  
Massive Sulphide Deposits ◽  
Volcanogenic Massive Sulphide ◽  
Volcaniclastic Rocks ◽  
Flin Flon ◽  
Sulphide Deposits

The hanging wall to the Flin Flon, Callinan, and Triple 7 volcanogenic massive sulphide deposits of the Flin Flon district is composed of the Hidden and Louis formations. The contact between these formations is marked by mafic tuff that represents a hiatus in effusive volcanism. The formations form a composite volcanic edifice that was erupted and grew within a large, volcanic–tectonic subsidence structure (hosting the deposits) that developed within a rifted-arc environment. The formations are evidence of resurgent effusive volcanism and subsidence following a hiatus in volcanism marked by ore formation since they consist of dominantly basaltic flows, sills, and volcaniclastic rocks with subordinate basaltic andesite and rhyodacitic flows and volcaniclastic rocks. The Hidden formation is interpreted to represent a small shield volcano and the Louis formation a separate shield volcano that developed on its flank. Both the Hidden and Louis volcanic edifices were constructed by continuous, low-volume eruptions of pillow lava. A gradual change from a dominantly extensional environment during the formation of the footwall Flin Flon formation to a progressively more dominant convergent environment during the emplacement of the hanging wall suggests that the Hidden and Louis formations are unlikely to host significant volcanogenic massive sulphide-type mineralization. However, synvolcanic structures in the formations define structural corridors that project downwards into the footwall where they encompass massive sulphide mineralization, indicating their control on ore formation, longevity,and reactivation as magma and fluid pathways during the growth of the Hidden and Louis volcanoes.

U–Pb geochronology of minerals from the Midwest uranium deposit, northern Saskatchewan

1984 ◽  
Vol 21 (6) ◽  
pp. 642-648 ◽  
Author(s):  
H. Baadsgaard ◽  
G. L. Cummino ◽  
J. M. Worden
Keyword(s):  
First Generation ◽  
Second Generation ◽  
Uranium Deposit ◽  
Paragenetic Sequence ◽  
Secondary Material ◽  
Basement Rocks ◽  
Northern Saskatchewan ◽  
Host Rocks ◽  
Sampling Problems ◽  
Late Stages

Analyses of U/Pb ratios in 30 microsamples of pitchblende and coffinite from the Midwest uranium deposit in northern Saskatchewan, as well as two altered zircons from host rocks, indicate an age of mineralization for primary pitchblende of 1328 ± 17 Ma. The primary material was remobilized at 1110 ± 28 Ma, forming the second generation of pitchblende and coffinite. First-generation pitchblende appears to have lost Pb either by diffusion or by a series of episodic losses at 300–100 Ma, whereas the secondary material proved more susceptible to Pb loss in the recent past.There is evidence of migration of pitchblende downwards into the basement rocks under the deposit at least as recently as 700 Ma ago, but we have been unable to date with certainty any late stages in the paragenetic sequence because of sampling problems associated with the small size of material clearly identifiable as stage 3 or 4 of the paragenetic sequence of Wray et al.

scholarly journals Composition of spherules and lower mantle minerals, isotopic and geochemical characteristics of zircon from volcaniclastic facies of the Mriya lamproite pipe

2020 ◽  
Vol 242 ◽  
pp. 150
Author(s):  
Ivan YATSENKO ◽  
Sergey SKUBLOV ◽  
Ekaterina LEVASHOVA ◽  
Olga GALANKINA ◽  
Sergey BEKESHA
Keyword(s):  
Intermetallic Alloys ◽  
Primary Metal ◽  
Mantle Minerals ◽  
Basement Rocks ◽  
Reducing Conditions ◽  
Mantle Mineral ◽  
Geochemical Study ◽  
Metal Silicate ◽  
Volcaniclastic Rocks ◽  
Different Sources

The article presents the results of studying the rocks of the pyroclastic facies of the Mriya lamproite pipe, located on the Priazovsky block of the Ukrainian shield. In them the rock's mineral composition includes a complex of exotic mineral particles formed under extreme reduction mantle conditions: silicate spherules, particles of native metals and intermetallic alloys, oxygen-free minerals such as diamond, qusongite (WC), and osbornite (TiN). The aim of the research is to establish the genesis of volcaniclastic rocks and to develop ideas of the highly deoxidized mantle mineral association (HRMMA), as well as to conduct an isotopic and geochemical study of zircon. As a result, groups of minerals from different sources are identified in the heavy fraction: HRMMA can be attributed to the juvenile magmatic component of volcaniclastic rocks; a group of minerals and xenoliths that can be interpreted as xenogenic random material associated with mantle nodules destruction (hornblendite, olivinite and dunite xenoliths), intrusive lamproites (tremolite-hornblende) and crystalline basement rocks (zircon, hornblende, epidote, and granitic xenoliths). The studied volcaniclastic rocks can be defined as intrusive pyroclastic facies (tuffisites) formed after the lamproites intrusion. Obviously, the HRMMA components formed under extreme reducing conditions at high temperatures, which are characteristic of the transition core-mantle zone. Thus, we believe that the formation of primary metal-silicate HRMMA melts is associated with the transition zone D".

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