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The Round Mountain Mine, Nevada
Transcript of The Round Mountain Mine, Nevada
This project is possible because of University of Idaho's Special Topics course on Ore Deposits offered by professor Dennis Geist. If you have any questions, please feel free to email me.
Thanks and enjoy!
firstname.lastname@example.org Round Mountain is an epithermal gold mine in the state of Nevada, USA. It is a 50/50 venture between Barrick Gold Company and Kinross Gold Corporation, with the latter operating the mine [Hanson, 2006]. Round Mountain Winnemucca Las Vegas Reno Nevada Round Mountain is located approximately 250 miles north of Las Vegas and east of Reno in Nye County, Nevada [Hanson, 2006] http://mapsof.net/uploads/static-maps/nevada_road_map.png The Round Mountain Gold deposit is a very large, epithermal, low-sulfidation, volcanic-hosted, hot-springs type, precious metal deposit, interpreted to be located along the margin of a buried volcanic caldera. The Round Mountain Gold Mine currently operates as a conventional open pit that is approximately 8,200 feet long in the north-west, south-east direction and 5,000 feet wide in the north-east to south-west direction [Hanson, 2006]. The basement rocks in the area of the Round Mountain Mine are highly deformed Paleozoic quartzite, carbonaceous argillite, schist, and impure limestones [Sander and Einaudi, 1990]. Deformation and volcanism related to subduction on the western margin of North America played a large role in the creation of the ore deposit at Round Mountain. From Paleozoic to Late Mesozoic time, low-angle subduction along this western margin caused thrusting inland, the product of which are the three thrust belts found below Round Mountain [Hanson, 2006]. These juxtaposed Cambrian and Ordovician units above Permian units and are correlated with the Devonian Roberts Mountain Allochton in northern Nevada [Hanson, 2006]. Several igneous plutons then intruded these at 95 Ma [Shawe, et al., 1990]. maps.google.com Increase in the angle of subduction during the Eocene initiated extension and extensive volcanism, dubbed the "ignimbrite flareup" [Schmandt and Humphreys, 2011]. The west-northwest trending belt of Oligocene and early Miocene volcanic rocks that extend from south-central Utah to east-central California are the expression of this [Boden, 1986]. Maximum extension from 30-16 Ma coincides with the volcanism in Nevada [Hanson, 2006]. Basin and range extension has chopped up these volcanics so that a lot of the rocks have been downthrown and covered by basin fill in the last 15 million years. Round Mountain sits on the edge of Big Smoky Valley in the Toquima Range, a remnant caldera complex that was active from 32-15 Ma [Shawe, 1986]. The Round Mountain Tuff, the main ore-containing formation, has been dated at 26.1 +/- 0.8 Ma and mineralization occurred shortly after at 25.2 +/- 0.8 Ma [Shawe, 1986]. From Hanson, 2006 The primary host rocks for gold mineralization are the volcanic rocks: poorly to strongly welded tuffs and silicified breccias (Types 1, 2, 9). A minor amount of ore occurs in the Paleozoic rocks along the caldera margin (Type 4) associated with narrow northwest-trending structures [Hanson, 2006]. Intracaldera collapse features and similar faulting in the metasedimentary rocks provided the major structural conduits for gold-bearing hydrothermal fluids [Hanson, 2006] Later, basin and range faulting (which is not associated with mineralization) lowered the western part of the caldera which has since been covered by basin fill of Big Smoky Valley [Henry, 1997]. Shear zone fractures, veins and disseminates within the more permeable units host the mineralization [Hanson, 2006] In volcanic units, ascending hydrothermal fluids deposited gold along a broad west-northwest trend.
1. Strongly welded tuff is largely fracture controlled
2. Poorly welded tuff is finely disseminated gold
9. Moderately welded tuff is both fracture-controlled and disseminated
[Hanson, 2006] Gold Mineralization within the Round Mountain deposit occurs as electrum (alloy of gold with at least 20% silver) in association with quartz, adularia, pyrite and iron oxides [Hanson, 2006] Primary sulfide mineralization consists of electrum associated with or internal to pyrite grains. In oxidized zones, gold occurs as electrum associated with iron oxides, or as disseminations along fractures [Hanson, 2006] Northwest-striking steep veins consists of quartz, adularia, limonite (oxidized from pyrite), manganese oxide, and associated free gold. Minor alunite, fluorite, and realgar have been reported [Shawe, 1986] From Sander, 1990 There are also flat veins similar in character and mineral content, although the wall rocks of the flat veins tend to be more brecciated than the steep faults. The flat veins also proved to be richer in gold than the steep veins; the richest ore shoots are found along intersections of the flat faults with the steep faults [Shawe, 1986] Henry, 1997 This is the distribution of hydrothermal alteration of the upper member of the tuff of Round Mountain. Propylitic, potassic, and high-level silicic alteration occurred in that order with the transition from propylitic to potassic being the main ore-forming stage of alteration [Henry, 1997 and Sander, 1990]. Fluids were channeled along northwest-striking faults and joints, which are marked by linear bands of 100% replacement [Henry, 1997] Quartz+adularia+albite+chlorite+calcite+pyrite+rutile +/- epidote.
Adularia selectively replaced primary sanadine of the host tuff, preserved primary phenocrystic textures.
Adularia is key indicator of propylitic alteration and a neutral chlorite, low-sulfidation system. Quartz+adularia+calcite+white_mica+pyrite+rutile
Replacement of plagioclase phenocrysts or older albite by nearly pure adularia
Strongest potassic alteration occurs only in lower poorly welded tuff below the open pit, where its distribution coincides with that of ore grade rocks. Clay minerals and remnant albite replaced by quartz+chalcedony+opal
Disseminated pyrite occurs in silicified and intermediate argillic rocks
Not associated with ore Fluid inclusion work by Sander and Eunuadi (1990) found that adularia inclusions had a homogenization temperature of 265˚C. This is the highest temperature found in the system and they concluded no boiling occurred. With a melting point of 0.0˚C, the inclusion is not a brine. Most gold deposition occurred at the transition from propylitic to potassic alteration. Inclusions in adularia show a drop from 250˚C to 150˚C at constant pH [Sander, 1990]. This coincides with the overgrowth veins in the upper tuff and pervasive alteration in the lower, poorly-welded tuff. This change is believed to be from influx of cold groundwater into the system at high levels (the upper tuff). In the post-ore, late silicic and intermediate argillic alteration, quartz crystals show a homogenization temperature of 180˚C and only appear at high levels of the Round Mountain system [Henry, 1997]. The idea of a short-lived, caldera-related epithermal system has been put together by Henry, et al. (1997), from K-Ar dating of adularia, which was crystallized at all alteration states. They suggest that gold mineralization occurred over the course of only 50,000 years due to a shallow silicic intrusion along the ring fracture of the caldera [Henry, 1997]. The mixing of cold groundwater is key to the deposition of gold in the highly porous volcanic rocks in high enough concentrations to be economical [Sander and Einuadi, 1990]. This series of cartoons shows the idealized orogenesis at Round Mountain [from Sander and Einuadi, 1990]. The implications for exploration would be that a hot, propylitic system can have enough gold but will be largely disseminated without the concentration of the ore from a quick change in temperature, which groundwater provides [Sander and Einuadi, 1990] flickr user: Uncle Kick-Kick The alteration history of propylitic superimposed by potassic alteration can lead to the discovery of other large gold deposits such as Round Mountain [Sander and Einuadi, 1990] Thank you for watching my presentation and check these references for more information! Boden, David R. "Eruptive history and structural development of the Toquima caldera complex, central Nevada." Geological Society of America Bulletin 97.1 (1986): 61-74.
Hanson, W. "Round Mountain Mine Technical Report." Kinross Gold Corporation (2006) http://kinross.com/pdf/operations/Technical-Report-Round-Mountain.pdf
Henry, Christopher D., et al. "Brief duration of hydrothermal activity at Round Mountain, Nevada, determined from Ar 40/Ar 39 geochronology." Economic Geology 92.7-8 (1997): 807-826.
Sander, Mark V., and Marco T. Einaudi. "Epithermal deposition of gold during transition from propylitic to potassic alteration at Round Mountain, Nevada." Economic Geology 85.2 (1990): 285-311.
Schmandt, Brandon, and Eugene Humphreys. "Complex subduction and small-scale convection revealed by body-wave tomography of the western United States upper mantle." Earth and Planetary Science Letters 297.3 (2010): 435-445.
Shawe, D. R., et al. "Ages of igneous and hydrothermal events in the Round Mountain and Manhattan gold districts, Nye County, Nevada." Economic Geology 81.2 (1986): 388-407.
Aerial photos by Google Maps
"another view of Round Mountain gold mine" by flickr user Uncle Kick-Kick
http://www.flickr.com/photos/28016468@N06/3933537102/ From Henry, 1997