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Mineral Textures and Their Bearing on Formation of the Kuroko Orebodies

By
C. Stewart Eldridge
C. Stewart Eldridge
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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Paul B. Barton, Jr.
Paul B. Barton, Jr.
U. S. Geological Survey, National Center, Mail Stop 959, Reston, Virginia 22092
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Hiroshi Ohmoto
Hiroshi Ohmoto
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16S02
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Published:
January 01, 1983

Abstract

The widely accepted depositional model for Kuroko-type deposits postulates precipitation of most ore minerals on or above the sea floor from plumes of hydrothermal fluids and simple superposition of ore minerals to accumulate stratified massive ore piles; syndepositional slumping is commonly recognized. Our observations of ore textures using doubly polished thin sections suggest that most of the sphalerite, pyrite, galena, tetrahedrite, barite, and quartz in the deposits once formed in the open ocean water but were recrystallized when insulated in the middle and lower portions of the accumulating ore pile. Furthermore, most of the chalcopyrite in the lower part of the massive ores formed by replacing the other sulfides.

The depositional history for Kuroko-type deposits is divisible into five stages, all of which may have taken place nearly contemporaneously but at different sites within the accumulating sulfide deposits: (1) precipitation of primitive or facies 1 (black ore) minerals which are fine grained (<50 µm in size) and often colloform; sphalerite, galena, pyrite, tetrahedrite, barite with minor chalcopyrite, and quartz at the sites of mixing of hot hydrothermal fluids with cold seawater (i.e., on or near the sea floor); (2) resolution of facies 1 minerals by hotter hydrothermal fluids, resulting in the formation of coarser grained facies 2 minerals in the lower parts and the reprecipitation of facies 1 minerals in the outer parts of growing orebodies; (3) introduction of hotter and Cu-rich hydrothermal solutions into the ore pile, resulting in the replacement of facies 2 sulfides by facies 3 chalcopyrite (forming yellow ore) in the lower parts of orebodies and the migration of facies 2—and facies 1—zones outward; (4) introduction of hotter (and undersaturated with chalcopyrite) fluids into the ore pile, resulting in the dissolution of chalcopyrite and formation of pyrite ores in the lowermost part of the ore pile and migration of chalcopyrite, facies 2, and facies 1 ores upward and outward of the ore pile; and (5) precipitation of tetsusekiei (chert-hematite) ores on top of the massive ores.

Some deposits experience only the stages 1, 2, and 5, thus leading to the variation in the metal ratios among deposits. In most deposits, mechanical disruption of the ores takes place throughout all stages of ore formation, resulting in ores with spectacular clastic textures.

After sediments and volcanic rocks have covered the deposit, the waning stages of the thermal system heat and alter the hanging-wall rocks. During the subsequent heating stages, there occurs a local partial reequilibration of the sulfides so that some of the heterogeneous assemblages (such as mixtures of high and low sulfur copper mineral assemblages) are able to react, the only unreacted remnants being those encapsulated in pyrite or quartz.

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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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