Yerington

Other Names:
District: North American Cordillera
Commodities :   Copper, Molybdenum

Yerington is a porphyry-skarn copper district in western Nevada, USA. It is ~80 km to the south-east of the city of Reno in western Nevada.

Skarn mineralisation is developed within Triassic age limestones adjacent to a Jurassic batholith, which is dominatly a granodiorite, but contains quartz-monzonite stocks and dyke swarms that host major porphyry copper mineralisation. 

Production from the porphyry ores from 1953 to 1978 had been 160 Mt @ 0.55% Cu (Einaudi, 1982) with 95 Mt @ 0.55% Cu remaining (Skillings, 1978). 

The un-developed Ann Mason porphyry copper deposit at Yerington has reserves of 495 Mt @ 0.4% Cu. 

7 associated skarn deposits produced 3.8 Mt @ 1.5 to 6% Cu between. 1906 and 30 (Einaudi, 1982). 

In 1994, Yerinton was operated by Arimetco Inc. as an open pit leaching operation, with a remaining mine life of 13 years (Am. Mines H’book, 1995). 

Geology 

The Yerington mine is situated some 80 km to the east of the Sierra Nevada Batholith in the western Great Basin in Nevada, within a belt of Jurassic intrusives. One of these intrusives, the Jurassic Yerington batholith, intrudes strongly folded and faulted Triassic volcanic and sedimentary rocks which have a total thickness of about 3000 m. These volcanics and sedimentary rocks form an east-west trending septum 8 km long and 3 km wide between the Yerington Batholith and the next batholith to the south. The lower half of this section is occupied by metamorphosed andesite and rhyolite flows, breccias and sediments, while the upper parts largely comprise massive limestone, thin bedded black calcareous shale and silicic volcani-clastics. Limestone beds constitute the host rocks for numerous, small copper-bearing skarn deposits located on an outer fringe of a hornfels-skarnoid aureole extending 600 to 1800 m from the Yerington Batholith. The Yerington Batholith consists of medium grained, equigranular granodiorite intruded by quartz monzonite and, later, by quartz-monzonite (adamellite) porphyry dyke swarms. Known porphyry copper mineralisation is restricted to the core of the batholith and is associated with porphyry dyke swarms (Einaudi, 1982).
  
Volcanic rocks of Tertiary age are represented by more than a 1000 m of flows and pyroclastics, mainly of quartz latite and rhyolitic composition, with local overlying andesites and basalt. These volcanics are underlain by a conglomerate of variable thickness. Basin and range normal faulting resulted in up to a 60° westward tilt of all pre-Miocene rocks; the surface therefore represents aseries of fault bounded pre-tilt cross-sections of the original geology, with their upper portions being to the west (Einaudi, 1982).
  
The principal host rocks to porphyry copper mineralisation are quartz-monzonite (adamellite), which displays marked textural and compositional variations in contrast to the granodiorite of the batholith, ranging from coarse grained and porphyritic to fine grained and equigranular. The finer grained varieties appear to be related to the loci of mineralisation. Typically the quartz-monzonite (adamellite) is composed of phenocrysts of orthoclase, perthite and quartz in a consistently aplitic groundmass of quartz and oligoclase. The ratio of phenocrysts to matrix is of the order of 1:1. Ferro-magnesian minerals are biotite and hornblende. The youngest intrusives are the porphyry dykes, which are locally referred to as tonalite and are abundant as dykes and apophyses. An elongate mass of quartz-monzonite porphyry defines the locus of the orebody, recognised over a distance of 1800 m, with a width of 300 to 650 m, and a 300° trend. Mineralisation also occurs within the adjacent granodiorite on the margins of the quartz-monzonite (Wilson, 1963). 

Mineralisation and Alteration 

Porphyry dykes with a strike of 290° and dip of 40° to 70° north were intruded in three stages, i). early barren, ii). mineralised, and iii). late barren. Alteration and mineralisation patterns are zoned around the mineralised porphyries. In the eastern end of the pit, pervasive albitic alteration occurs with chalcopyrite. The core, or centre of the deposit is characterised by intense quartz veining, K-silicate alteration and chalcopyrite-bornite-magnetite assemblages. The western end of the deposit is dominated by sericitic alteration and pyrite-chalcopyrite (Howard, 1976). Due to the post-ore tilting described above, this zoning from east to west represents a transition from depth to shallow levels in the original porphyry system.
  
Significant Cu mineralisation is developed over a length of 1650 m with a width varying from a maximum of 500 m to 180 m on the western margin. In the main section it occurs as a flat lying lens around 150 m thick, although to the west where it thins it extends deeper to around 240 m where it has a 'V' shape in cross section, reflecting the shape of the quartz-monzonite host (Wilson, 1963).
  
A high grade hypogene core is developed in the centre of the orebody, decreasing outwards gradually, but not uniformly to grade boundaries. Primary sulphide minerals are essentially pyrite and chalcopyrite, with a pyrite:chalcopyrite ratio substantially <1:1. Minor bornite and covellite have been observed and trace primary chalcocite has been detected microscopically. Molybdenite is rare. Sulphides typically occur as minute, discrete grains in the groundmass of the porphyry and as narrow, randomly oriented, discontinuous veins. Sulphide grains are not uncommonly enclosed within feldspar and quartz phenocrysts. Aligned sulphide seams and veinlets attain their maximum development in the central, high grade core, accompanied by quartz dyking and pervasive quartz flooding of the porphyry (Wilson, 1963).
  
Important oxide accumulations were developed, with a sharp, slightly undulating lower limit, with minor fault off-set, but generally conforming to the pre-gravel surface. The maximum vertical extent of oxidised ore is confined to the eastern half of the orebody, where essentially the full vertical column of mineralisation has been converted to oxide. Re-deposition of oxidised Cu products is considered to have occurred for the most part in-situ, with some migration and re-deposition of exotic Cu evidenced by restricted concentrations within the oxide zone and by shallow secondary sulphide enrichment of the primary zone. The principal oxidation product is chrysocolla, which is irregularly dispersed throughout the rock and as narrow seams along fractures. Massive concentrations of this silicate are limited to discontinuous vein-like occurrences along fissures and interstitial filling of breccia zones. Cuprite, tenorite and melaconite all have a wide distribution, but are of limited importance in the main oxide zone. Malachite and azurite are not abundant. In certain areas ore grade zones have no recognisable Cu minerals, with the metal occurring as amorphous, hydrated iron-copper oxide of variable Cu content (Wilson, 1963).
  
Lying between the primary sulphide and chrysocolla mineralisation is a transition zone in which chalcocite, cuprite, melaconite, native copper and chrysocolla occur, super-imposed upon primary mineralisation. Limits of this zone are irregular, but seldom exceeds 6 m in thickness. Immediately underlying this there is a prominent layer of chalcocite replacement of chalcopyrite and to a lesser degree pyrite, has developed through an average vertical range of 9 to 12 m. Secondary chalcocite has been detected as deep as 30 m below the top of the sulphide zone, but in such instances is confined to narrow widths along recognised post mineral structures (Wilson, 1963).
  
Skarn mineralisation developed in the surrounding country rock is associated with alteration of the lower Mesozoic volcano-sedimentary section some 3 to 4 km from the porphyry copper mineralisation. Alteration included:
• An early hornfels-skarnoid phase producing garnet-pyroxene hornfels near the batholith contact, possibly an early metamorphic event synchronous with the intrusion of the batholith.
• The main skarn development which followed brecciation of the early hornfels-skarnoid and the formation of chalcopyrite-pyrite bearing skarns. Iron rich skarns formed near the batholith, and andradite-salite skarns formed on the fringes of the skarnoid aureole in dolomitised marbles. Six deposits are located in this zone, two near the contact in the early garnet-pyroxene skarnoid (with low sulphides, low pyrite:chalcopyrite ratios, absence of magnetite and pyrite, a gangue dominated by andradite, strong brecciation and no zoning of calc-silicates and sulphides) and four further removed, at 1 to 2 km from the batholith. The latter four have 10 to 25% sulphides, high pyrite:chalcopyrite ratios of generally >1, trace magnetite, talc and tremolite, a gangue of andradite-salite, no evidence of brecciation, but zoning of calc-silicates and sulphides. The general outward zoning is andradite ? andradite-salite-chalcopyrite-pyrite ? salite-actinolite-pyrite±magnetite ? chalcopyrite-tremolite-magnetite±pyrite ? talc-calcite± magnetite ? dolomite-calcite.
• Silica-pyrite occurred within a fault breccia at one locality between skarnoid developments and limestone and was associated with an intensely silicified and quartz veined porphyry dyke possibly related to the porphyry copper mineralising event (Einaudi, 1982). 

(Source: Porter GeoConsultancy, www.portergeo.com.au, 1995)

DM Sample Photographs