Mineralogy -- Major: pyrite, sphalerite, chalcopyrite; in some examples pyrrhotite or Minor: bornite, tetrahedrite, electrum, arsenopyrite, marcasite, cubanite, copper-lead-bismuth-silver-su1fosalts, cassiterite, plus many °the in trace amounts.
Mode of Occurrence -- Massive to disseminated stratiform sulfide ores in volcano-sedimentary quences ranging from ophiolite complexes (Cyprus-type deposits) felsic tuffs, vas and sub sea floor intrusions (Kuroko-type deposits) to mudstones with little immediately associated recognizable volcanic material (Bes-’deposits).
Examples -- Kuroko- and Besshi-type deposits of Japan; Timmins, Ontario; Bathurst, I Brunswick; Sullivan, British Columbia; Flin-Flon, Manitoba-Saskatchewai Noranda, Quebec; Mt. Lyell, Australia; Rio Tinto, Spain; Scandinavian C donides; Avoca, Ireland; Parys Mountain, Wales; Troodos Complex depo Cyprus; Bett’s Cove, Newfoundland; Modern Red Sea and East Pacific Rise deposits.
Mineral Associations and Textures -- The deposits range from ores in thick volcanic sequences such as the F ores of Japan and ores directly associated with a volcanic vent (Vanna L Fiji) to ores associated with ophiolite sequences (Cyprus; Bett’s Cove, I found land) to distal ores that are emplaced in dominantly sedimentary sequences (Besshi deposits of Japan) and sequences containing no recognizable volcanic (Sullivan, British Columbia). They thus grade into ores of the type described in Section 10.7. Tn spite of the different settings in which these ores are found, there are similarities among the ore types observed. Zoning within many of these deposits is recognizable and three major ore types occur; the distribu4 ton of the primary minerals in the Kuroko ores is shown in Figures 10.19 and 10.20. Although the major ore types described in the following are those corn manly observed in the Kuroko deposits, they appear in most or all of the ores of this class with only minor variation. These ores, which appear to grade into the ores described in Section 10.7, have frequently been considered in terms of
Cu-Pb-Zn ratios as shown in Figure 10.21. Plimer (1978) has suggested that a trend in ore-type from Cu-dominant to Zn-dominant to Zn-Pb-dominant corresponds to a progression in time and distance from the volcanic Source (i.e., proximal to distal in nature). Jambor (1979) has enlarged on this theme and proposed a classification of the Bathurst-area (Canada) deposits based on their established or assumed displacement from feeder conduits (proximal versus distal) and position of sulfide crystallization (autochthonous versus allochthonous).
Although the ores of the volcanic deposits are members of a continuum, several specific ore types are observed most commonly; the following is a brief discussion of these ore types.
Pyritic (= Cyprus type) These ores, associated with ophiolite complexes are composed of massive banded to fragmental pyrite with small amounts of interstitial chalcopyrite and other base metal sulfides. The pyrite is present as friable masses of subhedral to euhedral, commonly zoned, grains, as colloform banded masses, and as framboids. Marcasite is admixed• with the pyrite and often appears to have replaced the pyrite. Chalcopyrite occurs as anhedral interstitial grains and as inclusions in the pyrite; sphaleritc occurs similarly but is less abun
 |
| From mining engineering |
 |
| From mining engineering |
•dant. Secondary covellite, digenite, chalcocite, and bornité occur as rims on, and along fractures in, pyrite and chalcopyrite.
Siliceous Ore (==Keiko-type of Kuroko Deposits) These ores apparently rep- resent feeder veins and stock works and consist primarily of pyrite, chalcopyrite, and quartz with only minor amounts of sphalerite, galena, and tetrahedrite. The pyrite occurs as euhedral grains, subhedral granular stringers, and colloform masses. The other minerals are minor and occur as anhedral interstitial grains in pyritic masses and gangue. Scttt (pers. commun., 1980) has noted that a black siliceous ore composed of sphalerite and galena is not uncommon in Kuroko de_ts.
Yellow Ore (=Oko-type of Kuroko Deposits) This ore type is characterized in both hand sample and polished section by the conspicuous yellow color resulting from the presence of chalcopyrite interstitial to the dominant euhedral to anhedral pyrite (Figure 10.22a). Minor amounts of sphalerite, galena, tetrahedrite, and lead sulfosalts and trace amounts of electrum are dispersed among the major sulfides. In unmetamorphosed bodies, the pyrite is often quite fine (<0.1 mm), but in metamorphosed ores pyrite commonly recrystallizes to form
euhedral grains which are several millimeters across. These ores and the black ores described later commonly exhibit extensively developed clastic textures that apparently formed at the time of ore deposition or immediately thereafter as a result of slumping.
Black Ore (Kuroko-type) The black ores (Figures 10.22b and 7.4c), the most complex of the common volcariogenic ore types, were so named because of the abundant dark sphalerite within them. Galena, barite, chalcopyrite, pyrite, and tetrahedrite are common but subsidiary to the sphalerite. Bornite, electrum, lead sulfosalts, argentite, and a variety of silver sulfosalts are customary accessory minerals. The black ores are usually compact and massive but primary sedimentary banding is often visible and brecciated and colloform textures are not uncommon. In ores unmodified by metamorphism, pyrite occurs as framboids, rosettes, colloform bands, and dispersed euhedral to subhedral grains. Pyrite grain size increases during metamorphism but growth zoning is often visible either after conventional polishing or after etching. In polished sections, sphalerite appears as anhedral grains that frequently contain dispersed micron-sized inclusions of chalcopyrite. Barton (1978) has shown, by using doubly polished thin sections in transmitted light (see Figure 2.7) that this “chalcopyrite disease” consists of rods and thin vermicular, myrmekiticlike growths, probably formed through epitaxial growth or replacement. He has also shown the presence of growth-band’g and overgrowth textures in sphalerite and tetrahedrite. During metamorphism, the sphalerite is ccmmon1y recrystallized and homogenized, and the dispersed chalcopyrite is concentrated as grains or rims along sphalerite grain-boundaries.
 |
| From mining engineering |
to be continue..
No comments:
Post a Comment