Midas, Ken Snyder

Nevada, USA

Main commodities: Au Ag
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The Midas (Ken Snyder) low sulphidation epithermal gold-silver deposit is located in Elko County of northern Nevada, USA., approximately 55 km north of Battle Mountain and 65 km NW of Carlin.

It is the largest known high-grade Au-Ag vein deposit within the Northern Nevada Rift and belongs to a suite of middle Miocene low-sulphidation epithermal systems associated with magmatism and faulting along the rift. The Northern Nevada Rift is a lineament that extends from east-central Nevadato southern Oregon and formed during Basin and Range extensio. It locally corresponds to an alignment of Middle Miocene eruptive centres, including mafic dyke swarms, intermediate to silici intrusives and vents, fossil geothermal systems and grabens.

The Midas deposit is hosted by a deeply eroded Miocene volcanic rocks on the eastern margin of the Northern Nevada Rift. The Midas hydrothermal system was developed following a change at about 15.6 Ma from mafic-dominated bimodal volcanism and basin formation to felsic volcanism, folding, and faulting. Between 15.6 to 15.2 Ma, sediments and tuffs were deposited on a relatively impermeable rhyolite flow. During this period, faulting and tilting of the volcanic edifice created pathways for hydrothermal fluids that flowed to the surface forming sinters and hydrothermal breecias.

Approximately 0.2 Myr after the change in volcano-tectonic regime, oblique-slip faulting occurred along zones of pre-existing weakness, creating dilational zones and additional channelways for hydrothermal fluids. At 15.4 Ma, high-grade veins formed in fault zones throughout the region, depositing more than 125 t of gold and 1250 t of silver, closely coinciding with the age of rhyolite intrusions whose source magma chamber likely provided the heat necessary to drive the hydrothermal system. The age of an unaltered tuff that unconformably overlies opalized sediments establishes that tilting of the units and the hydrothermal system had ceased by 15.2 Ma.

The gold-bearing quartz veins are associated with normal and oblique displacement faults with predominant orientations of N to 330°, 300 to 310° and east to ENE. The Colorado Grande vein formed in a long-lived, laterally and vertically persistent, north-south to 330° striking, steeply northeast dipping normal fault (growth fault), while subsidiary mineralised faults within the hanging wall display some reverse movement. The Gold Crown vein formed in a steeply northeast diping 300 to 310° striking fault that splays into the footwall of the Colorado Grande fault. Fault splays of similar orientation in the hanging wall host additional veins.

Alteration assemblages display a crude zonation centered on the main veins. Weak propylitic alteration occurs at distances of >400 m from the veins. Minor veining and partial replacement of phenocrysts and groundmass by chlorite, calcite, minor smectite (predominantly montmorillonite), and a trace of pyrite characterise this alteration. Regional weak propylitic alteration was developed prior to main-stage alteration. Propylitic alteration becomes more intense toward the veins with an increased density of fracturing and veining and more complete replacement of phenocrysts and groundmass and/or matrix. Calcite, chlorite, pyrite, and smectite (montmorillonite and nontronite) are abundant. Albitisation of plagioclase and adularia replacement of sanidine are common. Moderate propylitic alteration grades into intense propylitic alteration within 50 to 100 m of the main veins and at depth. In zones of intense propylitic alteration, epidote and prelmite are variably present and are accompanied by increased abundances of calcite, pyrite, quartz, and adularia. Intense propylitic alteration grades into potassic alteration within 30 m of the main veins, thelatter being characterised by nearly complete replacement of phenocrysts and groundmass and/or matrix, accompanied by an increased density of veining. Abundances of adularia, pyrite, marcasite, smectite (montmorillonite and illite), quartz, chalcopyrite, and sphalerite increase, and abundances of Fe rich chlorite decrease. More intense potassic alteration commonly occurs within 20 m of the veins where original textures are destroyed, and abundances of pyrite and/or marcasite increase. Illite ±sericite are slightly more abundant but overall present in trace amounts. Adjacent to the veins, wall rocks contain abundant silica and traces of selenide minerals, deposited during the earliest stages of the main orebearing veins.

Argillic alteration is most strongly developed in permeable volcaniclastic rhyolites, along faults, and at shallower levels of the deposit where it appears to overprint propylitic and potassic alteration. Phenocrysts and matrix are replaced by montmorillonite and minor kaolinite. Along the main faults, late deposition of montmorillonite, pyrite, calcite, and siderite in crosscutting veinlets followed formation of bonanza veins, possibly during collapse of the hydrothermal system.

The Colorado Grande and Gold Crown veins formed during multiple episodes of deposition and brecciation, with early silica flooding and brecciation of the wall rocks, followed by deposition of banded veins containing high-grade ore, several centimetres to several metres wide. Dark bands (up to 20 per vein) which are variably enriched in electrum, naumannite, chalcopyrite, pyrite, sphalerite with minor galena, aguilarite and marcasite alternate with quartz-, chalcedony, adularia-, and calcite-rich bands. Bladed calcite, adularia and mosaic quartz are more abundant in earlier bands, while pyrite and crystalline quartz are more abundant in later bands. Several episodes of brecciation disrupted veins and wall rocks during and following deposition of banded ore. Calcite and silica were deposited during and following these events.

Pre-mining reserves at Midas were approximately 94 tonnes of Au and 1100 t Ag at an 8.5 g/t Au cut-off.

Reserve and resource figures published by Newmont for the Midas operation at Dec. 2003 were:
    Proven + probable reserves - 3.4 Mt @ 17.75 g/t Au = 60 t Au
    Measured + indicated resources - 0.9 Mt @ 12.8 g/t Au = 11.5 t Au
    Inferred resources - Nil

Reserves and resources remaining, as published by Newmont for the Midas operation at Dec. 2013 were:
    Proven + probable reserves - 0.24 Mt @ 3.19 g/t Au
    Measured + indicated resources - 0.1 Mt @ 1.38 g/t Au
    Inferred resources - 0.4 Mt @ 1.29 g/t Au.

The most recent source geological information used to prepare this summary was dated: 2004.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Leavitt ED, Spell TL, Goldstrand PM and Arehart GB  2004 - Geochronology of the Midas Low-Sulfidation Epithermal Gold-Silver Deposit, Elko County, Nevada: in    Econ. Geol.   v99 pp 1665-1686

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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