Skarn zonation and fluid evolution in the Groundhog Mine, Central mining district, New Mexico

Abstract

The Groundhog mine contains one of the largest and best exposed Zn-Pb-Cu-Ag skarn deposits in the United States. Skarn is formed in Carboniferous limestones spatially associated with a nearly vertical swarm of Early Tertiary granodiorite porphyry dikes. Metal ratios, mineralogy, composition, and fluid inclusion characteristics of the skarn are systematically zoned from northeast to southwest along the >3-km strike length of the system and relative to the original dike-limestone contact. Zn/Cu, Zn/Ag, Pb/Cu, Pb/Zn, and Pb/Ag ratios all increase toward the distal southwest end of the mine where highest Zn, Pb, and total metal grades also occur. Endoskarn is zoned in terms of the relative proportions and manganese content of epidote and chlorite. In the Northeast zone, skarn is zoned away from dikes in the sequence: garnet to pyroxene to pyroxenoid to marble. In the Central skarn zone, garnet is absent and pyroxene begins the zonation sequence at the dike contact. In the Southwest zone, pyroxene is less abundant and pyroxenoid locally begins the zonation sequence. Sphalerite-galena-pyrite-quartzcarbonate are present in all skarn zones and in carbonate replacement mantos and chimneys beyond skarn. Distal mineralogical zones (e.g., pyroxenoid and manto) are more extensive to the southwest. In general, the skarn zonation sequence is wider and more fully developed in pure, coarse-grained limestone than in silty, carbonaceous, or fine-grained limestone. High permeability and water/rock ratio are considered the dominant controls on skarn size. Most skarn minerals, including garnet, pyroxene, pyroxenoid, ilvaite, amphibole, chlorite, carbonate, and sphalerite, are enriched in manganese. Pyroxene becomes more manganese rich (up to 85 mole % johannsenite):(1) toward marble, (2) along strike to the southwest, (3) at higher elevations, and (4) in coarser grained, cleaner limestones. The more manganese-rich pyroxenes formed from fluids which are distal in the overall zonation sequence. In contrast, pyroxene formed in proximal locations (dike contact, Northeast skarn zone, low elevation, fine-grained, impure limestone) is enriched in magnesium. Pyroxene-forming fluids are depleted first in Mg, then in Fe, and finally in Mn, as proposed by Burt (1977). Fluid inclusion homogenization temperatures are lower than for most zinc skarns. In the Northeast skarn zone the average homogenization temperature is 337øC and salt daughter minerals are present in garnet and pyroxene (inclusions without daughter minerals average 8.5 wt % NaCI), whereas salt daughter minerals are absent in the Central and Southwest skarn zones and average homogenization temperatures and salinities are 320øC, 7.3 wt percent NaC1, and 293øC, 6.2 wt percent NaC1, respectively. Pressure estimates from rare clusters of fluid inclusions with evidence for boiling are between 155 and 175 bars. Skarn thermal gradients calculated from fluid inclusion homogenization temperatures are 35øC from dike to marble front and 50øC from dike to the limit of alteration (carbonate repla...

Fluid inclusion evidence for the physicochemical conditions of sulfide deposition in the Olympias carbonate-hosted Pb-Zn(Au, Ag) sulfide ore deposit, E. Chalkidiki peninsula, N. Greece

“…The estimated formation-pressure conditions of the Olympias deposit are similar to those recorded in the lower pressure part of the "granite"-related Keno Hill, Canada (P: 468 675 bar), the lower-temperature distal (Erwood et al 1979 and data in Megaw et al 1988), Providencia, Mexico (Sawkins 1964 and data in Megaw et al 1988), Groundhog, USA (Meinert 1987), Washington Camp, USA (Surles 1978in Kwak 1986), Hidalgo, Mexico (Simone 1951in Kwak 1986), Uchucchacua, Peru (Bussel et al 1990), carbonate-replacement skarn-fi'ee Pb-Zn (Aq) deposits (Taebaek, Korea (So et al 1993), Bluebell, Canada (Ohmoto and Rye 1970), epithermal polymetallic Ag-Pb-Zn (Au) deposit {Creede, USA (Woods et al 1982 in Hedenquist and Henley 1985), Finlandia, Peru (Kamilli andOhmoto 1977, Sunnyside, USA (Casadeval and, and polymetallic Ag-Ph-Zn(Au) systems associated with "granitic" imrusions (Keno Hill, Canada (Lynch et al 1990) Beaverdell, Canada (Godwin et al 1986) with the Olympias deposit parts of skarn systems as in Providencia, Mexico (P: 300 bar), Naica, Mexico (P: 300 bar), and Groundhog, N. Mexico (P: 155-175 bar); and also the limestone replacement Bluebell, Canada (P: 300-800 bar), and Taebaek, Korea (P: 210-420 bar), and the epithermal Sunnyside, USA (P: 110 220 bar). The CO2 content of the Olympias inclusions is high enough to form a distinct liquid CO2 phase.…”

Fluctuation model for compositional heterogeneity in skarn clinopyroxenes.

“…Einaudi and Burt, 1982;Einaudi, 1977). Compositional variations within an in dividual grain may even occur, although zona tion patterns within single grains are not always systematic (Burton et al, 1982;Nakano, 1982;Meinert, 1987 Samples analyzed in this study were collected from seven Japanese metalliferous skarn deposits. Most of these were clinopyroxene skarn with small amounts of secondary am phibole replacing clinopyroxene and trace amounts of interstitial calcite and/or quartz.…”

Çataltepe (Lapseki-Çanakkale) Pb-Zn±Cu±Ag Yatağı'nın Jeolojisi ve Alterasyon Özellikleri

“…Prograd evreye özgü ve mikrotermometrik ölçümlere uygun birincil kapanımlar içeren bir granat mineralinden elde edilebilen homojenleşme sıcaklıklarının (Th) 300-353 o C arasında ve son buz ergime sıcaklıklarının (Tm) ise % 0,5 -1,4 NaCl eşdeğeri tuzluluklar arasında değiştiği görülmüştür (Demirela, 2011). Elde edilen bu değerler Meinert (1987) ve Boni vd. (1990) tarafından Pb-Zn skarnlar için ortaya konulmuş homojenleşme sıcaklıkları ve % NaCl eşdeğeri tuzluluk değerleri ile karşılaştırıldığın-da, benzer homojenleşme sıcaklık aralıklarında kaldığı ancak % NaCl eşdeğeri tuzluluk değer-lerine göre ise oldukça düşük değerler içerdiği ve magmatik kökenli sıvılardan da (%7,3-8,5 NaCl eşdeğeri tuzluluk) belirgin şekilde ayrıldı-ğı görülür.…”