New mineralogical, fluid inclusion and sulfur isotope data of ores from the Obří důl skarn-type deposit (Bohemian Massif)

Geoscience Research Reports 49, 2016, pages 47–52

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Published online: 13 August 2016

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Abstract

The Obří důl Fe-Cu-As deposit is located in the Krkonoše Mts. in North Bohemia (Bohemian Massif). The deposit is of skarn-type and is developed at the contact zone of the Krkonoše-Jizera granite (Variscan age). The deposit consists of two sulfide lenses Gustavská and Helenská (1-12 m thick) that resulted from pyrometasomatic alteration (skarnization) of sedimentary dolomitic marble lenses (obr. 1) and also quart-rich gneisses (obr. 2). This metasomatic process resulted in the formation of pyroxene-garnet skarns and garnet-rich rocks and was accompanied by precipitation of sulfides. The deposit has a rich mining history with a monopoly position in the Bohemian market for arsenic in the period 1828-1868 and copper 1830-1853. Recently, the deposit became a heritage site and is accessible for general public. We studied a new set of samples taken from accessible part of the Gustavská ore lense. The oldest ore mineral is arzenopyrite, followed by pyrrhotite, chalcopyrite, sphalerite, stannite and other ore minerals (e.g., native Bi, bismuthinite, Bi-Pb-Ag-S phases, molybdenite, cassiterite, scheelite, malayaite and stokesite). It was shown that arzenopyrite has variable As/S ratio and can contain up to 0.1 wt.% Co. Non-altered pyrrhotite (Fe_1-xS₁) revealed lower Fe values than altered one, with ”x” ranging from 0.12 to 0.16 documented by electron microprobe study (Fe = 57.5-59.2 wt.% and S = 38.3-39.7 wt.%), in places with Pb values of 0.01-0.2 wt.%. Chalcopyrite contains up to 0.17 wt.% Pb, and Zn and Ag concentrations of 0.04 wt.% and 0.1 wt.%, respectively. Locally, it hosts micro-inclusions of Ag-Bi-S phase (matildite?). Sphalerite is mostly associated with chalcopyrite and less frequently with pyrrhotite, and locally bears inclusions of stannite. Sphalerite contains 9-11.6 wt.% Fe, 1.2-1.6 wt. % Cd, 0.8-1.6 wt.% Cu, up to 0.6 wt.% Mn, and up to 0.02 wt.% In. Molybdenite from skarn assemblage showed micro-inclusions of native bismuth and Ag-S phase. Native bismuth is the most abundant accessory mineral which contains up to 0.2 wt.% Zn and up to 0.6 wt.% Cd. Three types of paleofluids were identified: (1) H₂O-type fluids in diopside with Th = 387-424 °C and salinity up to 8 wt.% NaCl equivalent, (2) H₂O-type fluids with low CO₂ values in quartz veins with sulfide mineralization (minimum temperature of sulfide formation Th = 324-358 °C and salinity 4.0-8.8 wt.% NaCl equivalent), and (3) H₂O-type fluids in carbonate veins and younger quartz veins with Th = 113-168 °C and salinity up to 8 wt.% NaCl equivalent. Sulfur isotope data in arzenopyrite, pyrrhotite and chalcopyrite range from 0.9 to 4.8 ‰, suggesting a dominant magmatic source of sulfur.

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