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Awaruite, Heazlewoodite, and Native Copper in Serpentinized Peridotite from the Mineoka District, Southern Boso Peninsula

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Awaruite, Heazlewoodite, and Native Copper in Serpentinized Peridotite from the Mineoka District, Southern Boso Peninsula J. Japan. Assoc. Min, Petr. Econ. Geol. 70, 388-394, 1975 AWARUITE, HEAZLEWOODITE, AND NATIVE COPPER IN SERPENTINIZED PERIDOTITE FROM THE MINEOKA DISTRICT, SOUTHERN BOSO PENINSULA KEIICHIRO KANEHIRA Department of Earth Sciences, Chiba University, Chiba SHOHEI BANNO Department of Earth Sciences, Kanazawa University, Kanazawa SHUNZO YUI Department of Mining Geology, Akita University, Akita Awaruite was found in serpentinized peridotites from the Mineoka district, southern Boso Peninsula. The mineral occurs invariably in serpentinized parts of the rocks, in dicating that it was formed in the course of serpentinization. Heazlewoodite and native copper were formed also in association with awaruite during serpentinization. Chemical analysis of the awaruite yields the formula: Ni2.26 Fe. Native copper associated with heazlewoodite contains about 5 weight percent nickel. The occurrence of awaruite suggests that oxygen fugacity of fluid being responsible to serpentinization was lowered at least locally down to 10-42 bar during serpentinization. The low oxygen fugacity of fluid was resulted from reaction between fluid and fayalite component of olivine producing magnetite and serpentine minerals. native copper in serpentinized peridotites INTRODUCTION from the Mineoka district, southern Boso Awaruite has been found in serpentin Peninsula. In this brief paper we intend to ized peridotites from various localities of describe modes of occurrence and some the world (Ramdohr, 1967). The mineral properties of the minerals and to discuss occurs not in unaltered peridotites but the formation environment of the minerals. invariably in serpentinized rocks. This fact has been considered to indicate that GEOLOGIC SETTING AND HOST ROCKS awaruite was formed in the course of A geologic sketch map of the Mineoka serpentinization of the ultramafic rocks district is shown in Figure 1. In the (Nickel, 1959; Kanehira et al., 1964; Cham Mineoka district probable Paleogene sed berlain et al., 1965; Ramdohr, 1967). The iments called the Mineoka Group are widespread occurrence of awaruite in exposed in a zone extending in E-W direc serpentinized rocks suggests a reducing tion. The Mineoka Group consists of condition of serpentinization. siliceous shale, sandstone, alternation of We found awaruite, heazlewoodite, and shale and sandstone, and subordinate (Manuscript received, October 3, 1975) Awaruite, heazlewoodite, and native copper in serpentinized peridotite from the Mineoka district 389 Fig. 1. Geologic sketch map of the Mineoka district, southern Boso Peninsula. amounts of impure limestone and tuff. The various parts of the ultramafic mass, and folded Mineoka Group is faulted and flank examined them under the reflected light ed by Neogene calstic sediments on both the microscope. Awaruite was found in 5 north and south sides. Ultramafic rocks specimens, heazlewoodite in 4, and native are intruded in the Mineoka Group and the copper in 6. Awaruite localities are shown Neogene sediments, and are exposed either in Figure 1. in two narrow zones or in small isolated Ultramafic rocks are extensively masses. The primary intrusion has taken serpentinized, but primary minerals are place in Miocene age, but later movement preserved to some extent. In most part of cannot be denied. Basic extrusives com the ultramafic mass 70 to 80 volume posed largely of pillowed basalt, pillow percent of the rocks are serpentinized. In breccia, and dolerite dikes are exposed in part the rocks are completely serpentinized. association with the ultramafic rocks. Primary minerals are olivine (Fo88-91), The largest mass of the ultramafic rocks rhombic pyroxene and subordinate amount is about 700m in maximum width and of monoclinic pyroxene. Chromite and a extends about 9km in E-W direction. small amount of pentlandite are contained The mass comprises serpentinized peridotite sporadically. Pentlandite grains are an and a number of lens-shaped bodies of hedral and interstitial against primary various sizes of diorite, gabbro, gabbro silicate minerals. This suggests that the mineral is a primary constituent of the pegmatite, and leucocratic rocks. We ultramafic rocks. Olivine is altered to collected 40 fragments of peridotite from 390 K. Kanehira, S. Banno and S. Yui serpentine which is always accompanied by specimen, awaruite coexists with heazle fine-grained crystals of magnetite. woodite in the inclusions surrounded by magnetite. When awaruite occurs in as MODES OF OCCURRENCE sociation with pentlandite, the former exists along the rim of the latter. In some cases Awaruite is white and bright under the reflected light microscope. Two distinct pentlandite grains are armoured by the thin crusts of awaruite as is shown in modes of occurrence are observed, i.e., the Figure 2-C. The crusts of awaruite are mineral occurs as minute grains included usually about 3 microns thick. in secondary magnetite formed by ser Heazlewoodite is cream-coloured with pentinization of peridotite in some cases (Fig. 2-A), and it occurs also in association yellowish tint, slightly brighter than with pentlandite in other cases (Fig. 2-B). associated pentlandite but darker than The awaruite inclusions in magnetite are awaruite. The mineral occurs in associa usually about a few microns in size. In one tion with awaruite in inclusions within Fig. 2. Electron image (A) and photomicrographs (B, C and D) of awaruite and associated minerals . (A) An inclusion of awaruite (aw) in magnetite (mt). (B) Awaruite (aw) formed in contact with pentlandite (pn). (C) Pentlandite (pn) armoured by awaurite (aw). Minute stringers of native copper (Cu) are formed adjacent to pentlandite and awaruite . (D) Pentlandite (pn) is replaced by unidentified grey minerals along cracks. Heazlewoodite (hz) and native copper (Cu) are formed in the peripheral zone of pentlandite grain. Awaruite, heaziewoodite, and native copper in serpentinized peridotite from the Mineoka district 391 magnetite grains or in association with of the awaruite yields the formula: Ni2.26Fe. pentlandite as is shown in Figure 2-D. In a specimen of serpentinite, an Native copper is pink and considerably aggregate of pentlandite, heazlewoodite, and bright under the microscope. The mineral native' copper was observed (Fig. 2-D). occurs as minute stringers or streaks isolated The shape of the aggregate indicates that the in serpentine aggregate and also adjacent to primary pentlandite is formed in interstices pentlandite grains. among silicate minerals. The pentlandite is altered along cracks to a dark grey CHEMICAL COMPOSITION minerals and partly replaced by heazlewood ite, and an irregular-shaped grain of native A grain of awaruite associated with copper is formed in the rim of the pentland pentlandite, which is shown in Figure 2-B, ite grain. Each of the three metallic min was analyzed with the Hitachi electron erals was analyzed with the JXA-5 electron probe microanalyzer with a take off angle probe microanalyzer with a take off angle of 40•K. Pure metals of iron and nickel of 40•K. A pure metallic nickel and an were used as standards. The measured analyzed crystal of natural chalcopyrite were values were corrected by the procedure as used as standards. The measured values described by Sweatman and Long (1969) were corrected by the procedure similar to modified by the method of Colby (1966) for that mentioned above. The results together F(x), and by that of Springer (1967) for the with normalized atomic composition of the effect of the continuous fluorescence. The result is shown in Table 1. The analysis minerals are tabulated in Table 1. Table 1. Microprobe analyses of native metals and sulphides in serpen tinites from the Mineoka district. * Specimen No. 35 ** Specimen No. 50. Analyses were made in two different grains of each mineral in one and the same mineral aggregate. 392 K. Kanehira, S. Banno and S. Yui Analyses were made for two different awaruite in serpentinized peridotite of the grains of each mineral in the mineral Muskox intrusion in northern Canada. aggregate. The analyses indicate that They considered that the peridotite was each of the minerals is fairly uniform in desuiphurized during serpentinization and chemical composition within one and the pentlandite, a primary constituent of the same mineral aggregate. The atomic ratio peridotite, was altered in its periphery to of nickel to iron in the analyzed pentlandite awaruite. is 1.6. The pentlandite is a nickel-rich In the serpentinite from the Mineoka variety. The number of metal atoms district awaruite occurs either in association contained in the analyzed heazlewoodite with secondary magnetite or along the rim in respect to 2 atoms of sulphur is 3.18. of pentlandite grains. The pentlandite The mineral is somewhat sulphur deficient grains are interstitial against primary as compared with stoichiometric heazle silicates, olivine and pyroxenes. Pentland woodite. The analyzed native copper ite is probably one of the primary con contains 5 to 6 atomic percent nickel and 1.3 stituents. The pentlandite grains are atomic percent iron and forms a homogeneous altered along their cleavage and cracks, alloy. sometimes replaced partly by heazlewoodite The grey minerals filling the cracks of and native copper, and rarely surrounded pentlandite and surrounding heazlewoodite by thin awaruite layer. Nickel and or iron and native copper grains are one silicate of secondary metallic minerals may have phase, possibly serpentine, and an unidenti been derived from pentlandite as suggested fied nickel-iron-bearing mineral.
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