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Epidote and chlorite mineral chemistry from the Yerington porphyry copper district, USA : genetic and exploration implications


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Ahmed, AD ORCID: 0000-0001-5713-4331 2019 , 'Epidote and chlorite mineral chemistry from the Yerington porphyry copper district, USA : genetic and exploration implications', PhD thesis, University of Tasmania.

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The Yerington district, Nevada, USA, hosts a range of Mesozoic ore deposits, including porphyry Cu (Mo-Au), skarn Cu and Fe-oxide Cu. Mesozoic rocks in the district were tilted up to 90° west by post-mineralisation Basin and Range faults, resulting in a series of fault blocks that expose extensive vertical sections within and around the deposits. The Ann Mason fault block provides the most laterally and vertically extensive exposures of a magmatic hydrothermal system in the district, from its roots through to its roof. It includes the giant Ann Mason porphyry Cu (Mo-Au) deposit (1,400 Mt @ 0.32% Cu, 0.006% Mo, 0.03 g/t Au), the smaller Casting Copper Cu skarn deposit, and other skarn deposits. It also contains a mineralised and an unmineralised batholith, providing the opportunity for direct comparison between productive and unproductive magmatic-hydrothermal systems.
Copper mineralisation in the Yerington district is spatially and genetically associated with the composite Yerington batholith. High-K, calc-alkaline arc-related magmas were emplaced in response to east-facing subduction beneath the western margin of North America in the Late Jurassic. New CA-TIMS and LA-ICP-MS U−Pb zircon dates constrain emplacement of the earliest intrusive phase, the McLeod Hill quartz monzodiorite, to 167.60 ± 0.22 Ma. It was followed closely by the Bear quartz monzonite (167.1 ± 1.1 Ma), Luhr Hill granite (167.47 ± 0.30 Ma) and the Luhr Hill granite porphyry dykes (166.66 ± 0.42 Ma). New Re-Os dates of molybdenite from quartz − chalcopyrite − muscovite veins in the Luhr Hill granite porphyry dykes indicate that mineralisation was co-genetic with host rocks (between 167.0 ± 0.9 Ma and 166.1 ± 0.8 Ma). The Shamrock monzonite is unrelated to mineralisation and was emplaced a minimum of 0.44 m.y. after the Luhr Hill granite porphyry dykes at 165.53 ± 0.27 Ma.
Intrusive phases of the Yerington batholith define a fractionation trend of early plagioclase suppression and progressive hornblende fractionation with increasing silica contents over time. These processes caused increasing (Ce/Nd)/Y, Eu\(_N\)/Eu\(_N\)* and Dy/Yb values in zircon and increasing whole-rock values of Sr/Y, Sr/MnO, and La/Yb\(_N\) with each successive phase of the batholith. Compared to the Yerington batholith, the Shamrock batholith has: (1) overall lower Na\(_2\)O and Sr values; (2) overall higher Cr, Mn, Sc, Nb, Y, MREE, and HREE values; (3) larger amplitude whole-rock and zircon negative-Eu anomalies; (4) lower whole-rock Sr/Y, Sr/MnO, and La/Yb\(_N\) values, and (5) lower (Ce/Nd)/Y and (10000*Eu\(_N\)/ Eu\(_N\)*)/Y in zircon values. These contrasting compositions suggest different mid-crustal or deep-crustal reservoirs for the Yerington and Shamrock batholiths. The Shamrock monzonite data reflects a lower initial magmatic water content, which prevented the accumulation of large volumes of volatiles during emplacement.
Within the Ann Mason fault block, epidote − albite ± chlorite ± magnetite ± actinolite ± titanite alteration extends from the centre of Ann Mason to the most distal margins of the Yerington batholith and out to adjacent Triassic − Jurassic volcanic and sedimentary rocks. The concentrations of Na, Mg, P, Ca, Cu, As, Sr, Mo, Sb, and Pb in altered rock samples vary laterally and vertically from the centre of Ann Mason, and are correlated with the intensity of epidote alteration. The addition of Cu and Mo in the deposit centre (> 10,000% absolute mass change) is broadly coincident with Ca, Sr, As, and Sb removal (up to 99% depletion). These patterns are inferred to be the result of destruction of calcic-plagioclase and hornblende during potassic alteration. Addition of Ca (up to 1,570%) and Sr (up to 315%) define a whole-rock geochemical footprint 4 km laterally from the centre of Ann Mason.
In the shallow porphyry environment (<3 km paleodepth) low Li, Mn, Zn, Sr, and Pb, and elevated Zr, Tl, and Au in chlorite define geochemical footprints 1.5 – 2 km laterally outward from Ann Mason. Similarly, low concentrations of Mn, Ti, Cu, Co, Zn, Zr and Na in epidote define a 3 km radius footprint. The corresponding whole-rock geochemistry footprint extends laterally 2.8 km out from the deposit centre, based on univariate trace element data. Lateral changes in mineral chemistry are attributed to decreasing temperatures and increasing water-rock interaction with distance from Ann Mason. Significant inter- and intragranular compositional variability exist in epidote, as recorded by LA-ICP-MS raster maps (4 μm pixel size). Concentrations of Sr, Mn, As, Sb, Ce, Th and U can vary up to three orders of magnitude within individual grains. This variability is controlled primarily by sector and lamellar zoning, and twinning. Compositional variability is also manifested in different generations of epidote. Early-formed epidotes are typically enriched in trace elements (As, Sb, Ce, La) relative to later overgrowths. Variability in the concentrations of trace elements such as Cu and Zn in epidote, as measured by standard deviation of LA-ICPMS spot data, increases with distance from Ann Mason. Samples of epidote from the deposit centre have low standard deviations (<0.5 ppm) whereas samples 3 km from Ann Mason have higher standard deviations (6 − 7 ppm).
Epidote fertility can be gauged by plotting mineral chemistry data on conventional whole rock Cu prospectivity diagrams of Sr/Y vs. Y and La/Yb\(_N\) vs. Yb\(_N\). Epidote from the productive Yerington batholith has higher Sr/Y (>300) and higher La/Yb\(_N\) (>25) values than the unproductive Shamrock batholith. The concentrations of these elements in epidote are inferred to reflect primary hornblende and plagioclase compositions in host rocks.
The compositions of chlorite and epidote from an 8 km vertical transect through Ann Mason, along the axis of the Ann Mason dyke swarm, change systematically with depth. Ratios of Fe/Mg, Cu/V, Fe/V, Cu/Ag, and Pb/Ag in chlorite increase up to four orders of magnitude over the 8 km vertical profile. A series of bathymeter equations using these ratios provides a precise vectoring tool that can be used to predict the distance to the top of the Luhr Hill granite cupola. Concentrations of Pb, Mn, Ti, Zr, and Yb in epidote increase consistently with depth, but these systematic changes are limited to above 6 km paleodepth.
\(^{87}\)Sr/\(^{86}\)Sr\(_{initial}\) values of representative samples from epidote-bearing mineral assemblages at Yerington range from 0.7066 to 0.7048. The component of seawater-derived Sr in epidote from intrusive rocks was calculated to range from 21 to 83%. This suggests that epidote formed as a result of: (1) fluid mixing between non-magmatic hypersaline brine and magmatic-hydrothermal fluids; (2) interaction between magmatic-hydrothermal fluids and sedimentary wall-rocks; and/or (3) interaction between non-magmatic hypersaline brines and igneous wall rocks. Alteration in the core of Ann Mason is inferred to have been dominated by magmatic-hydrothermal fluids, whereas the flanks and deeper regions were dominated by basinal hypersaline brines. Intense Na and Ca metasomatism in the deep parts of Ann Mason are unlikely to be common features of porphyry deposits. They are instead likely to be characteristic of porphyries emplaced into sedimentary basins that contain evaporites.

Item Type: Thesis - PhD
Authors/Creators:Ahmed, AD
Keywords: Mineral chemistry, porphyry copper deposits, epidote, chlorite
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