Skarn deposits in the Yerington District, Nevada; metasomatic skarn evolution near Ludwig

In the Yerington district of western Nevada, large bodies of skarn formed in a limestone unit of Triassic age adjacent to a Jurassic batholith. This batholith is dominantly granodiorite but contains quartz monzonite stocks and dike swarms that host major porphyry copper deposits. The units enclosing the limestone, an andesitc tuff and a silty limestone, were also extensively metasomatized, as were outlying intrusions of granodiorite.Skarn formation is divided into an early skarnoid stage and a late metasomatic skarn stage. The skarnoid stage formed massive fine-grained granditc garnet in the andesitc tuff, limestone, and silty limestone, with subordinate amounts of pyroxene. The iron content of garnets increases significantly away from the granodiorite, whereas pyroxenes maintain a relatively constant diopsidic composition.Granodiorite apophyses, which occur in the Triassic rocks up to 0.5 km from the main batholith contact, are extensively altered to endoskarn. The main alteration type consists of massive, anhedral granditc garnet with a significant titanium component. The distribution of endoskarn suggests that proximity to limestone was a key factor in endoskarn formation and that one hydrothermal fluid altered both sedimentary and igneous rocks simultaneously.The second stage of alteration most dramatically affected the limestone, forming coarse-grained skarn. Two skarn types are recognized: (1) a magnesium-rich type in dolomitized marble, dominated by pyroxene, and (2) an iron-rich type in calcite marble, dominated by andradite. Age relations between the two are ambiguous, but constraints inferred from T-X (sub CO 2 ) stability relations suggest that magnesium-rich skarn formed first and at high temperatures, followed by iron-rich skarn. During a late stage, intermediate granditc garnet veined both skarn types, and actinolite (+ or - salite) formed locally at the andradite-marble contact.In detail, magnesium-rich skarn nearest the granodiorite is zoned from diopside through serpentine (replacing diopside) + calcite, then from clinohumite + calcite + dolomite to calcite + dolomite marble. Additional phases include monticellite, spinel, magnetite, ludwigitc, and szaibelyite. Farther from the batholith, clinohumite is absent, tremolite separates serpentine from pyroxene, and periclase occurs locally.Pyroxene and serpentine are enriched in iron with distance from the granodiorite, the pyroxene changing from nearly pure diopside to intermediate smite a kilometer away. This compositional variation is consistent with an increase in mu Fe /mu Mg , with time and with distance from the batholith. The presence of magnetite pseudomorphs after hematite and the variation in magnetite composition indicate that the oxidation state decreased with time near the granodiorite.Iron-rich skarn consists of andradite garnet, which on the outer skarn contact directly replaced calcite. Inclusions of wollastonite noted in garnet cores from one sample suggest that initially a wollastonite zone was locally present at the skarn margin. Veins of actinolite and magnetite cut garnet skarn; late actinolite at the garnet-marble contact, locally a site of sulfide deposition, suggests a reversal in the trend toward increasing mu Fe /mu Mg . At the Douglas Hill mine, apatite, quartz, and sulfides replaced andradite.Mineral assemblages and fluid inclusion data indicate that both magnesium- and iron-rich skarn formed at low X (sub CO 2 ) , generally less than 0.1. Temperatures of formation of magnesium-rich skarn are inferred to have been in the range 650 degrees to below 400 degrees C. Temperatures of formation of iron-rich skarn are not constrained by observed assemblages. Fluid inclusion data show that late apatite replaced andradite at temperatures between 120 degrees and 200 degrees C.