EVIDENCE FOR OPEN-SYSTEM BEHAVIOR IN IMMISCIBLE Fe S O LIQUIDS IN SILICATE MAGMAS: IMPLICATIONS FOR CONTRIBUTIONS OF METALS AND SULFUR TO ORE-FORMING FLUIDS

Larocque, Adrienne C. L.; Stimac, James A.; Keith, Jeffrey D.; Huminicki, M. A. E.

doi:10.2113/gscanmin.38.5.1233

Cited by 106 publications

(69 citation statements)

References 49 publications

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“…3a). They are broadly similar to the oxide replacement assemblages described by Larocque et al (2000) seen in a variety of igneous environments. These replacement assemblages are most common in GN-III, AN-II, and GN-II but are also observed in N-II, BZ, and PZ.…”

Section: Petrographysupporting

confidence: 72%

A study of the trace sulfide mineral assemblages in the Stillwater Complex, Montana, USA

et al. 2016

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The sulfide assemblages of the Stillwater Complex away from the well-studied ore zones are composed mainly of variable proportions of pyrrhotite, chalcopyrite, pentlandite, and ±pyrite. Excluding vein assemblages and those affected by greenschist and lower temperature alteration, the majority can be classified into two broad assemblages, defined here as pristine (multiphase, often globular in shape) or volatilebearing (multiphase, high-temperature, volatile-rich minerals such as biotite, hornblende, or an unmixed calcite-dolomite assemblage). The volatile-bearing assemblages are mainly found within and below the J-M reef, where native copper and sphalerite are also locally present. Pristine sulfides are found throughout the stratigraphy. Both groups can be affected by apparent S loss in the form of pyrite being converted to magnetite and chalcopyrite to a Cu-Fe-oxide (delafossite), with little to no silicate alteration. An upward trend from pentlandite-rich to pyrrhotite-rich to pyrite-rich assemblages is observed in the footwall rocks in upper GN-I, and the same trend repeats from just below the reef and continues into the overlying N-II and GN-II. Modeling suggests that the sulfide Ni in the Peridotite Zone is largely controlled by silicate Ni. When taken together, observations are most readily explained by the remobilization of selected elements by a hightemperature fluid with the apparent loss of S > Cu > Ni. This could concentrate ore metals by vapor refining, eventually producing a platinum group element-enriched sulfide ore zone, such as the J-M reef.

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Section: Petrographysupporting

confidence: 72%

A study of the trace sulfide mineral assemblages in the Stillwater Complex, Montana, USA

et al. 2016

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“…Although sulfide saturation could be viewed as destroying ore potential, the concentration of ore metals by saturation of a sulfide melt may in fact be a highly effective mechanism for converting a magmatic system with a dispersed, low concentration of metals into one with pockets of highly enriched sulfide melt. As long as the sulfide melt, or its crystallized products, are accessible to -and soluble in -fluids that are subsequently exsolved from the magma 72,76,88 , then the potential exists to produce anomalously metal-rich hydrothermal fluids 47 . Evidence for the operation of this process has been described from the active Merapi Volcano in Indonesia where it was inferred that the injection of sulfide-saturated mafic magma into a felsic chamber triggered volatile exsolution, dissolution of sulfide into the volatile phase and caused explosive eruptions 75 .…”

Section: Box 2 | Trigger 2 -Magmatic Sulfide Saturationmentioning

confidence: 99%

Triggers for the formation of porphyry ore deposits in magmatic arcs

2013

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Porphyry ore deposits source much of the copper, molybdenum, gold and silver utilized by humankind. They typically form in magmatic arcs above subduction zones via a series of linked processes, beginning with magma generation in the mantle and ending with the precipitation of metals from hydrous fluids in the shallow crust. A hierarchy of four key "triggers" involved in the formation of porphyry deposits is outlined. Trigger 1 (10 2 -10 3 km scale) is a process of cyclic refertilization of magmas in the deep crust. Trigger 2 (10 1 -10 2 km scale) is the process of sulfide saturation in magmas that can both enhance and destroy ore-forming potential. Trigger 3 (10 0 -10 1 km scale) relates to the efficient transfer of metals into hydrothermal fluids exsolving from porphyry magmas. Trigger 4 (~10 0 km scale) identifies processes that lead to the final precipitation of ore minerals. Although all processes are required to a greater or lesser degree, it is argued that trigger 3, as an over-riding mechanism, can best explain the restriction of large deposits to specific arc segments and time periods. Consequently, recognition of the fingerprint of sulfide saturation in igneous rocks may help mineral exploration companies to identify parts of magmatic arcs particularly predisposed to ore formation.Ore deposits are scarce. Their discovery consumes considerable time and resources and only about one in every one thousand prospects explored by companies is eventually developed into a mine. Deposits often occur in clusters and formed within specific time intervals. This non-uniform pattern provides keys to understanding how and why metals accumulate in some places and not in others and its explanation is fundamental for mineral exploration.The most important metalliferous ore deposits are hydrothermal deposits, formed from hot waters circulating in Earth's crust. Such deposits represent a highly efficient trap where fluids were focused into a limited volume of rock, became supersaturated and precipitated ore minerals. In theory, this does not require unusual fluid compositions 1,2 as long as fluid focusing, sufficient fluid flux and efficient precipitation conditions can be maintained. Recently, this paradigm has been challenged by the recognition that ore fluids in sediment-hosted 3-5 , epithermal 6,7 and porphyryrelated [8][9][10] hydrothermal deposits can carry orders of magnitude more metal than previously considered probable, or measured in modern fluid samples. This suggests that our understanding of metal extraction and transport processes is incomplete.Here, this theme is explored by consideration of porphyry ore deposits [11][12][13][14][15] , remarkable geochemical anomalies that can contain up to 1 Gt of sulphur 16 , 200 Mt copper, 2.5 Mt molybdenum and 2600 t gold 17 . The most copper-rich examples include the 4-5 million-year old El Teniente and Rio Blanco-Los Bronces deposits in Central Chile, and the most gold-rich is the 3 million-year old Grasberg deposit in Irian Jaya, Papua New Guinea 17 .Porphyry deposits...

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“…Regardless of the exact origin of the barite inclusions, they may indicate that the magma was more sulfur-rich than the final consolidated rock that has less than about 400 ppm sulfur. It is not unreasonable that over 90% of the magmatic sulfur was lost by degassing upon eruption or in subvolcanic magma chambers (Larocque et al 2000;Hattori and Keith 2001).…”

Section: Sulfur-and Water-rich Magmasmentioning

confidence: 99%

Contributions from mafic alkaline magmas to the Bingham porphyry Cu-Au-Mo deposit, Utah, USA

Maughan

Keith

Christiansen

et al. 2002

Mineralium Deposita

116

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The Bingham porphyry Cu-Au-Mo deposit, Utah, may only be world-class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc-alkaline system. Volcanic mafic alkaline rocks in the district are enriched in Cr, Ni, and Ba as well as Cu, Au, platinum group elements (PGE), and S. The bulk of the volcanic section that is co-magmatic with ore-related porphyries is dacitic to trachytic in composition, but has inherited the geochemical signature of high Cr, Ni, and Ba from magma mixing with the mafic alkaline rocks. The volcanic section that most closely correlates in time with ore-related porphyries is very heterogeneous containing clasts of scoriaceous latite, latitic, and minette, and flows of melanephelinite, shoshonite, and olivine latite in addition to volumetrically dominant dacite/trachyte. Bingham ore-related porphyries show ample evidence of prior mixing with mafic alkaline magmas. Intrusive porphyries that have not been previously well-studied have several chemical and mineralogical indications of magma mixing. These ''mixed'' lithologies include the hybrid quartz monzonite porphyry, biotite porphyry, and minette dikes. Even some of the more silicic latite and monzonite porphyries retain high Cr and Ba contents indicative of mixing and contain trace amounts of sapphire (<1 mm). The heterogeneous block and ash flow deposits also contain sapphire and are permissively correlated with the intrusions based on chemical, mineralogical, and isotopic data. Magma mixing calculations suggest about 10% of the monzonitic/latitic orerelated magma may have been derived from mafic alkaline magma similar to the melanephelinite. If the original S content of the mafic magma was about 2,000-4,000 ppm, comparable with similar magmas, then the mafic magma may have been responsible for contributing more than half of the S and a significant portion of the Cu, Au, and PGE in the Bingham deposit.

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EVIDENCE FOR OPEN-SYSTEM BEHAVIOR IN IMMISCIBLE Fe S O LIQUIDS IN SILICATE MAGMAS: IMPLICATIONS FOR CONTRIBUTIONS OF METALS AND SULFUR TO ORE-FORMING FLUIDS

Cited by 106 publications

References 49 publications

A study of the trace sulfide mineral assemblages in the Stillwater Complex, Montana, USA

A study of the trace sulfide mineral assemblages in the Stillwater Complex, Montana, USA

Triggers for the formation of porphyry ore deposits in magmatic arcs

Contributions from mafic alkaline magmas to the Bingham porphyry Cu-Au-Mo deposit, Utah, USA

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