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Book Chapter

Chemical Processes of Kuroko Formation

By
H. Ohmoto
H. Ohmoto
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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M. Mizukami
M. Mizukami
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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S. E. Drummond
S. E. Drummond
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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C. S. Eldridge
C. S. Eldridge
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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V. Pisutha-Arnond
V. Pisutha-Arnond
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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T. C. Lenagh
T. C. Lenagh
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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Published:
January 01, 1983

Abstract

This paper attempts to evaluate the causes of the regularities and variations in the mineralogic and chemical constituents of volcanogenic massive sulfide deposits and to put constraints on the origin and the hydrology of the ore-forming systems. This attempt has been carried out mostly through examination of the chemistry of sulfur and metals in the Kuroko ore-forming fluids and of depositional processes.

Sulfur and oxygen isotopic characteristics of sulfides and sulfates from the Kuroko deposits suggest that the precipitation of these minerals was caused by rapid and disequilibrium mixing of hot (T ≥ 200°C) hydrothermal fluids with cold seawater. The δ 34 S and δ 18 O values place constraints on the total sulfate contents of the ore-forming fluids. Concentrations of aqueous sulfur species were also estimated from thermodynamic calculations by employing a simple assumption that the fluids were saturated with pyrite before interaction with cold seawater but were undersaturated with other sulfides and sulfates. The calculations indicate a continuous increase in the H 2 S content from ~10 –4.3m for the early stage fluids (T ≃ 200°C) to ~10 ~15 m for the later fluids (T ≃ 350°C), but a constant ∑SOforumla content of ~10 -3m. The isotopic and chemical characteristics of the fluids suggest that these fluids obtained most of their sulfur from disseminated gypsum-anhydrite which had been formed through interaction of the pore fluids (seawater) and volcanic rocks during the diagenetic and the early hydrothermal stages at T ≤ 150°C. Some of these sulfate ions derived from anhydrite were reduced to form H 2 S through reactions with Fe +2 and organic C in the country rocks at elevated temperatures. The fluids of T ≥ 300°C also acquired some sulfur through dissolution of pyrite in the country rocks.

From this sulfur chemistry of the Kuroko fluids and solubility data of minerals, we have put constraints on the metal contents in the ore-forming fluids and on the amounts of minerals precipitating from the mixtures with normal seawater. The amounts and proportions of minerals in the ores depend on the oxygen content of cold seawater, relative rates of reactions among various oxidized and reduced aqueous species, and rates of chemical reactions vs. rates of fluid mixing, as well as on the concentrations of reduced sulfur, oxidized sulfur, and individual metals in the ore-forming fluids. Rapid mixing above the sea floor of discharging ore-forming fluids with normal seawater causes quantitative precipitation of Ba in the fluids as barite but no precipitation of quartz and native gold, which explains low Si/Ba and Au/Ag ratios in the massive black ores. The calculated metal ratios for the precipitates from the mixtures of the Kuroko fluids of T tnitial = 200° to 300°C with normal seawater are Zn/Pb ≃ 1, Zn/Fe &same 10, Zn/Cu ≥ 10, and Zn/Ba ≃ 1 (wt ratio), which agree well with the observed metal ratios for the massive black ores. The amounts of metals precipitated from a metric ton of the Kuroko ore-forming fluids were on the order of ~1 to ~10 g (~1 to ~10 ppm) each for Zn, Pb, Fe, and Ba, and ~0.1 to ~1 g for Cu.

Continuous reactions of the black ore with the later, hotter hydrothermal fluids lead to conversion of the black ore to the yellow ore and then to the pyrite-rich ore. The key to the process was that the hydrothermal fluids before interactions with the black ore were under-saturated with respect to major sulfides (e.g., sphalerite, chalcopyrite) except pyrite. The principal factors that determine the amounts and proportions of economic metals in massive sulfide deposits are suggested to be the thermal history of an ore-forming system, the dominant rock type in a region, and the composition of seawater (particularly its oxygen and sulfate contents). The differences in these factors can explain both the contrasts between Archean and Phanerozoic deposits and between those associated with basaltic rocks and those with felsic rocks.

The results of examination of the regional metal zoning patterns and of the location of volcanic centers in the Hokuroku district, together with computations of the mass of fluids and rocks based on material balance of Zn, Ba, Fe, S, Mg, Ca, and oxygen isotopes, of the size of heat sources based on heat balance, and of the duration of ore-forming activity based on the δ 34 S values of diagenetic pyrite, suggest that an average Kuroko ore-forming system discharged ~10 11 metric tons of fluids at T ≥ 200°C in an area of ~1.5 × ~3 km and deposited 0.5 to 1 million metric tons of Zn (3 to 20 million metric tons of Ore) in 200 to 50,000 years. The thermal anomalies responsible for these ore-forming systems appear to have been established by a relatively small (~10 km 3) intrusion. It is also, suggested that the rate of accumulation of clastic (and volcanic) rocks on the sea floor is an important factor in determining the formation and preservation of massive sulfide deposits.

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Contents

Economic Geology Monograph Series

The Kuroko and Related Volcanogenic Massive Sulfide Deposits

Hiroshi Ohmoto
Hiroshi Ohmoto
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Brian J. Skinner
Brian J. Skinner
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Society of Economic Geologists
Volume
5
ISBN electronic:
9781629490007
Publication date:
January 01, 1983

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