Activity variations attending tungsten skarn formation, Pine Creek, California

An integrated geochemical analysis of the well-exposed Pine Creek, California tungsten skarn deposit has been undertaken to evaluate changes in chemical gradients across various lithologies. Thermodynamic calculations using available experimental and thermodynamic data allow limits to be assigned to the activities of important chemical components in the metasomatic environment. Quantifiable changes in “non-volatile” component activites (CaO, MgO, Al 2 O 3 , Fe 2 O 3 , WO 3 ) and in fugacities (O 2 , F 2 ) have been traced across the system. The activities of Al 2 O 3 , Fe 2 O 3 and WO 3 generally increase from the marble (<10 2 , <10 −6 , <10 −5 respectively), through the outer skarn zone and into the massive garnet skarn (10 −1.7±0.3 , 10 −3.4±0.4 , 10 −4.8±0.1 ) While CaO and MgO activities decrease for the same traverse from 10 −5 and 10 −2.1±1 respectively, to <10 −5.7 and <10 −3 . Calculated oxygen fugacities are 10 −23.5+1.0 at T =800 K (527° C), about one log unit below QFM, and more reducing than that required by Mt-Py-Po. The high variance of the garnet-pyroxene-quartz assemblages adds sufficient uncertainty to the calculated activities for individual specimens that only the large-scale trends survive the small-scale scatter. None of the chemical variables emerge as major independent or controlling factors for the mineralogy or phase compositions. Changes in the activity of one component may be offset by compensatory changes in another resulting in an environment that, while different from Pine Creek, could still host scheelite mineralization. Mass balance calculations indicate that the exposed endoskarn cannot have supplied the necessary chemical components to convert the country rock to skarn.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47341/1/410_2004_Article_BF00381557.pd

Regional-scale Proterozoic IOCG-mineralized breccia systems:\ud examples from the Wernecke Mountains, Yukon, Canada

A large scale Proterozoic breccia system consisting of numerous individual breccia bodies, collectively known as Wernecke Breccia, occurs in north-central Yukon Territory, Canada. Breccias cut Early Proterozoic Wernecke Supergroup sedimentary rocks and occur throughout the approximately 13 km thick deformed and weakly metamorphosed sequence. Iron oxide–copper–gold ± uranium ± cobalt mineralization is associated with the breccia bodies and occurs as veins and disseminations within breccia and surrounding rocks and locally forms the breccia matrix. Extensive sodic and potassic metasomatic alteration occurs within and around breccia bodies and is overprinted by pervasive calcite and dolomite/ankerite, and locally siderite, alteration, respectively. Multiple phases of brecciation, alteration and mineralization are evident. Breccia bodies are spatially associated with regional-scale faults and breccia emplacement made use of pre-existing crustal weaknesses and permeable zones. New evidence indicates the presence of metaevaporitic rocks in lower WSG that may be intimately related to breccia formation. No evidence of breccia-age magmatism has been found to date

Magmatic-dominated fluid evolution in the Jurassic Nambija gold skarn deposits (southeastern Ecuador)

The Jurassic (approximately 145 Ma) Nambija oxidized gold skarns are hosted by the Triassic volcanosedimentary Piuntza unit in the sub-Andean zone of southeastern Ecuador. The skarns consist dominantly of granditic garnet (Ad(20-98)) with subordinate pyroxene (Di(46-92)Hd(17-42)Jo(0-19)) and epidote and are spatially associated with porphyritic quartz-diorite to granodiorite intrusions. Endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone. Exoskarn has an outer K- and Na-enriched zone in the volcanosedimentary unit. Gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide-poor vugs and milky quartz veins and veinlets in association with hematite. Fluid inclusion data for the main part of the prograde stage indicate the coexistence of high-temperature (500A degrees C to &gt; 600A degrees C), high-salinity (up to 65 wt.% eq. NaCl), and moderate- to low-salinity aqueous-carbonic fluids interpreted to have been trapped at pressures around 100-120 MPa, corresponding to about 4-km depth. Lower-temperature (510-300A degrees C) and moderate- to low-salinity (23-2 wt.% eq. NaCl) aqueous fluids are recorded in garnet and epidote of the end of the prograde stage. The microthermometric data (Th from 513A degrees C to 318A degrees C and salinity from 1.0 to 23 wt.% eq. NaCl) and delta(18)O values between 6.2aEuro degrees and 11.5aEuro degrees for gold-bearing milky quartz from the retrograde stage suggest that the ore-forming fluid was dominantly magmatic. Pressures during the early retrograde stage were in the range of 50-100 MPa, in line with the evidence for CO(2) effervescence and probable local boiling. The dominance of magmatic low-saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system, which could have delayed the ingression of external fluids until relatively low temperatures were reached. The resulting low water-to-rock ratios explain the weak retrograde alteration and the compositional variability of chlorite, essentially controlled by host rock compositions. Gold was precipitated at this stage as a result of cooling and pH increase related to CO(2) effervescence, which both result in destabilization of gold-bearing chloride complexes. Significant ingression of external fluids took place after gold deposition only, as recorded by delta(18)O values of 0.4aEuro degrees to 6.2aEuro degrees for fluids depositing quartz (below 350A degrees C) in sulfide-rich barren veins. Low-temperature (&lt; 300A degrees C) meteoric fluids (delta(18)O(water) between -10.0aEuro degrees and -2.0aEuro degrees) are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities (about 100A degrees C and 25 wt.% eq. NaCl). The latter are similar to low-temperature fluids (202-74.5A degrees C) with delta(18)O values of -0.5aEuro degrees to 3.1aEuro degrees and salinities in the range of 21.1 to 17.3 wt.% eq. CaCl(2), trapped in calcite of late veins and interpreted as basinal brines. Nambija represents a deep equivalent of the oxidized gold skarn class, the presence of CO(2) in the fluids being partly a consequence of the relatively deep setting at about 4-km depth. As in other Au-bearing skarn deposits, not only the prograde stage but also the gold-precipitating retrograde stage is dominated by fluids of magmatic origin