MINING GEOLOGY, 27, 355365, 1977 Sulfide Minerals and Their Assemblages of the Besshi Deposit Studies on Sulfide Minerals in Metamorphosed Ores of the Besshi and Hitachi Copper Deposits (1)* Katsuo KASE** Abstract: The contact metamorphism was superimposed on the regional glaucophanitic one in the deeper levels of the Besshi deposit. The mineral assemblages change progressively with increasing depth by this event. Sulfide mineral assemblages were studied on regional and contact metamorphosed ores of the deposit. Pyrite, chalcopyrite and sphalerite assemblage is the majority of ores suffered the regional metamorphism. Pyrite-chalcopyrite-bornite and pyrite-hematite-magnetite assemblages,which constitute univariant equilibria in Cu-Fe-S and Fe-S-O systems, are sometimesobserved in these ores. T and fS2of the metamorphism were discussed from the mutual relation of these univariant equilibria. T and fS2 were also obtained at high pressures from FeS contents of sphalerite (0.48-0.90 mole %) associated with pyrite-chalcopyrite-bornite assemblage. They are 280°-350°C and 10-8.71-10-6.4 atm, supposing 5 Kb lithostatic pressure. By contact metamorphism, pyrrhotite is formed from pyrite in the levels deeper than 18 L. Fe contents of pyrrhotite increase with increasing depth, from monoclinic modification containing 46.7 atomic % Fe and hex agonal one with 47.3 atomic % Fe in 18-25 L to 47.5-48.0 atomic % Fe in 26-33 L. FeS contents of sphalerite associated with pyrrhotite are in the range of 16 to 20 mole %, which does not correspond to pyrrhotite with 47.5- 48.0 atomic % Fe. Hexagonal pyrrhotite coexists commonly with pyrite, which shows sometimes euhedral shape. This association is incompatible in the light of the low temperature phase relation of Fe-S system.hitherto estab lished. Pyrrhotite alone may have changed its composition during the retrogressive metamorphism, reflecting the T-fS2 environments. Sphalerite and pyrite, on the contrary, may preserve the high temperature states. It is there fore difficult to obtain metamorphic T and fS2 from the compositions of coexisting sphalerite and pyrrhotite, unless detailed study is made on the compositional change of these minerals in the retrogressive metamorphism. The association of hexagonal pyrrhotite and euhedral pyrite may be showing that pyrrhotite of high temperature type and pyrite were once in equilibrium at the climax of the metamorphism. Sphalerite has no exsolution dots in the contact metamorphosed ores. The absence of exsolution dots will be due to the low fS2 environments in the retrogressive metamorphism, corresponding to pyrrhotite with 47.5- 48.0 atomic % Fe. latter in the southern Abukuma metamor 1. Introduction phic terrain of andalusite-sillimanite type The Besshi and Hitachi copper deposits (Fig. 1.). are the representative strata-bound massive Both deposits suffered the intense regional sulfide deposits in Japan. The former is metamorphisms. The Besshi deposit suffered situated in the Sambagawa glaucophanitic the contact metamorphism by hidden terrain of the central Shikoku island and the granitic intrusion in the deeper levels of the deposit (KASE,1972; MWYAZAIUet al., 1974). * Received June 30, 1977; in revised form September The granodiorite intruded in the northern 20, 1977. area of the Hitachi district, and gave the ** Department of Earth Sciences, Faculty of Science, contact metamorphism to surrounding rocks Okayama University, Tsushima Naka 3-1-1, and ores. Okayama Mineralogical studies have been carried Key words: Besshi deposit, Strata-bound massive sulfide deposit, Sulfide mineral assemblage, Regional metamo out on ores of the Besshi deposit by KASE rphism, Contact metamorphism. (1972, 1974) and MIYAZAKIet al. (1974), 355 356 K. KASE MINING GEOLOGY: vations are practically quite similar to those of MIYAZAKIet al. (1974), but an inter pretation different from MIYAZAKIand his coworkers will be given to the observed mineralogy in this paper. The Besshi deposit occurs conformably in pelitic schists between the thick piles of basic schists of Paleozoic age (KASE, 1972). The deposit extends about 1800 m along the strike and more than 2500 m along the dip. It consists of two layers of massive sulfide ores and another layer of banded ores between them. The banded ores comprise repeated fine bandings of sulfide-rich and silicate-rich layers. The total thickness of ores is usually 1-2 m but reaches 10 m in some places. Main sulfide minerals observed in the Besshi deposit belong to Cu-Fe-Zn- S system. The primary main sulfides are supposed to be pyrite, chalcopyrite and sphalerite. Pyrrhotite was formed by the desulfurization of pyrite in the process of Fig. 1 Location map showing the Besshi and the contact metamorphism. The variation Hitachi deposits. of sulfur fugacity (fS2) in the metamorphisms and the variation of mineral assemblages is therefore stressed in this paper. A special with increasing depth (equals to the in attention is paid to the change of the sulfide creasing grade of the contact metamorphism) mineral assemblages during the retrogressive was clarified. IZAWAand MUKAIYAMA(1972) metamorphisms. summarized briefly the mineralogical char- acteristics of ores from the contact metamor- 2. Metamorphisms and Classifica tion of Ores phosed strata-bound massive sulfide deposits in Japan, including the Besshi and Hitachi The mineral deposit suffered the regional deposits. They concluded that contact glaucophanitic metamorphism (high pres metamorphism is characterized by the sure, low temperature) at late Mesozoic irreversible desulfurization reactions, for time. The metamorphic grade of ores example, the formation of pyrrhotite from corresponds to the transitional facies between pyrite. the glaucophane schist and epidote- Our knowledge on the progressive miner amphibolite facies (BANNO, 1964; KASE, alogical changes of sulfide minerals in 1972). The assemblages of epidote-chlorite- regional and contact metamorphisms, how hornblende and epidote-chlorite-hornblende- ever, is still very incomplete. It is therefore glaucophane with albite, quartz, muscovite, interesting to compare the behaviors of calcite, pyrite and hematite characterize. sulfide minerals in regional and contact the basic schists of this facies. Biotite does metamorphosed ores of the Besshi and not occur in basic schists as well as pelitic Hitachi deposits. As a part of this compara schists. Ilmenite occurs only as exsolution tive study, sulfide mineral assemblages of intergrowth in hematite. Pyrrhotite is very the Besshi deposit are discussed in relation rarely observed in basic schists. In ores, to the regional and contact metamorphisms pyrite, chalcopyrite and sphalerite are the in this paper. The present author's obser- main constituent sulfides. 27(6), 1977 Sulfide Minerals and Their Assemblages of the Besshi Deposit 357 The contact metamorphism caused by chalcopyrite, bornite and sphalerite. hidden granitic intrusion was superimposed Pyrite is the predominant sulfide mineral on the regional one in the deeper levels of but pyrrhotite becomes predominant locally the deposit (KASE, 1972; MIYAZAKIet al., in this zone. In ores free from pyrrhotite, 1974). The silicate mineral assemblages as the sulfide mineral assemblages and their well as sulfide and oxide ones change pro- textures are similar to those in ores of the gressively with increasing depth by this regional metamorphosed zone. event (KASE, 1972). In ores and basic schists Pyrrhotite is observed in 18 and 22 L suffered the contact metamorphism, pyr along the fault zones. It occurs in the rhotite, ilmenite, bigtite and diopside ap ordinary sulfide ores in levels deeper than pear. Pyrrhotite becomes a predominant 23 L. In pyrrhotite-poor ores, it is usually sulfide mineral instead of pyrite in the levels formed along cracks of pyrite grains. In deeper than 26 L. Glaucophane and hem pyrrhotite-rich ores found locally, this atite disappear. Chlorite in banded ores mineral is generally granular in shape and becomes rich in MgO with increasing the grain size is about 0.1-0.2 mm. Pyr amount of pyrrhotite, from 0.5-0.6 of rhotite of this zone shows stronger reflection atomic ratio Fe/Fe + Mg in 6-14 L to 0.3- pleochroism and anisotropism than those 0.4 in 26-32 L. observed in the contact metamorphosed The deposit was classified from these zone. variation of mineral assemblages into the Chalcopyrite and sphalerite are mixed regional metamorphosed (1-17 L), transi with pyrrhotite in pyrrhotite-rich ores. tional (18-25 L) and contact metamor- Minute dots of chalcopyrite in sphalerite phosed zones (26-33 L). The boundaries of are commonly observed in this zone (Fig. each zone were defined by the appearance 5b). of pyrrhotite, and predominance of pyr Bornite included in pyrite grains is also rhotite relative to pyrite. often observed, coexisting with pyrrhotite. The occurrence of interstitial bornite is 3. Sulfide Minerals of Cu-Fe-Zn-S very rare and restricted to pyrrhotite-free System and Their Assemblages ores. 3.1 Regional Metamorphosed Zone Mackinawite is sometimes found in chal The observed sulfide minerals of Cu-Fe- copyrite and along boundaries between Zn-S system are pyrite, chalcopyrite, chalcopyrite and other minerals such as bornite and sphalerite. sphalerite and silicates,. having a needle- Pyrite is a predominant sulfide mineral. like shape. The length is less than 0.1 mm. It is always euhedral to subhedral in The mineral always coexists with pyrrhotite. shape, and the grain size is usually 0.1 to 3.3 Contact Metamorphosed Zone 0.5 mm. Pyrrhotite becomes predominant instead Chalcopyrite and sphalerite occur filling of pyrite in the levels deeper than 26 L. The the interstices of pyrite grains. Very minute observed sulfide minerals of this system are dots of chalcopyrite are always observed in pyrrhotite, pyrite, mackinawite, chalco sphalerite under magnified ore-microscope pyrite and sphalerite. (Fig. 5a). Lamellar pyrrhotite is commonly observed Bornite occurs sometimes filling the inter in this zone. The grain size is generally stices of pyrite grains. This mineral appears very small, and from 0.01 to 0.2 mm in more frequently as minute inclusions in length.