The island of Hispaniola, shared by the Dominican Republic and the Republic of Haiti, is located near the central portion of the Greater Antilles. The island, composed geologically of regional metamorphic rocks, sedimentary rocks of Cretaceous to Recent in age, and various kinds of igneous rocks, is well featured by both folds and faults of NW to WNW trends.
Copper ore deposits so far known on the island are grouped into following three types:
1) Chalcopyrite (or chalcocite)-quartz veins, mostly found both in Upper Cretaceous pyroclastics and regional metamorphics immediately adjacent to a quartz-diorite batholith, and very rarely in porphyry or along a contact between quartz-diorite and metamorphics.
2) Contact metasomatic deposits, accompanied in parts by calcite-garnet-epidote-tremolite-wollastonite skarn, in Eocene limestone intruded by quartz-diorite dikes.
3) Disseminated ore deposits (porphyry copper type) in a porphyry mass intruding Upper Cretaceous pyroclastics.
Almost all those mineralizations are believed to have occurred during the Laramide Orogeny in close association with igneous activities of quartz-diorite and porphyry mentioned above.

View full abstract

The ore deposits of the Kamaishi mine are of contact metasomatic origin and the mine is well known as the largest producer of iron-copper ores in Japan. The Nippo ore deposit, comprising five copper ore-bodies, is located at the western extremity of the Kamaishi mining area. These ore-bodies occur in "breccia skarn" consisting of biotite hornfels fragments and intersitial clinopyroxenes and garnets, and in massive clinopyroxenes or clinopyroxenes-garnets skarn. The Shinyama ore deposit, the largest one in the mine, consists of an iron ore-body and seven copper or copper-iron ore-bodies in massive garnets or clinopyroxenes skarn replacing porphyrite and limestone.
Nickel-and cobalt-bearing minerals are commonly found as minor consituents in sulfide ores from the copper ore-bodies. The major sulfide minerals are chalcopyrite, pyrrhotite, cubanite, bornite, and pyrite. The nickel-and cobalt-bearing minerals, identified by ore microscopy, X-ray micro-diffraction and electron micro-probe, are cobaltian or cobalt pentlandite, siegenite, nickel-and cobalt-bearing mackinawite, argentian pentlandite, millerite, and nickelian smythite, in decreasing order. Cobaltian or cobalt pentlandite occurs most abundantly in pyrrhotite and cubanite of the chalcopyrite-pyrrhotite-cubanite association as exsolution products and in chalcopyrite of the chalcopyrite-bornite association. Cobaltian or cobalt pentlandite is extensively altered to siegenite by hydrothermal solution probably related to the alteration of hexagonal pyrrhotite to monoclinic pyrrhotite. Mackinawite and argentian pentlandite are encounterd in chalcopyrite of the chalcopyrite-pyrrhotite-cubanite association. Occurrence of millerite is confined to the chalcopyrite-bornite ore in the Nippo ore deposit. Smythite is closely associated with monoclinic pyrrhotite which seems to have been altered to smythite during the last stage of mineralization.
Although there is much analogy in the paragenesis of sulfide minerals between Kamaishi and "Sudbury type" ores, the following facts in ores from the Kamaishi mine exhibit contrasts to "Sudbury type" ores. 1) extremely low exsolution temperature of pentlandite from Mss estimated from the Fe:Ni:S ratio of the ore; 2) very rare occurrence of pentlandite in chalcopyrite of the chalcopyrite-pyrrhotite-cubanite association; 3) no occurrence of pentlandite in direct conta t with pyrite; 4) high ratio of Co:Ni in the sulfide ores.

View full abstract

Acid volcanic rocks in the Middle Miocene formations of the Green Tuff region, northeast Japan, were analyzed. The majority of the analyzed rocks were collected in the Tsugawa area, Niigata Prefecture. The Middle Miocene sediments in the area are divided into the Tsugawa, Awase, and Tokoname formations in ascending order. Acid volcanic rocks occur either as lava flows intercalated in the Tsugawa formation or as shallow intrusives. The volcanic rocks are contemporaneous or nearly contemporaneous with the Tsugawa formation correlated to the Nishikurosawa stage. Analyzed samples were mainly collected from the intrusives and a few from lava flows.
Normative mineral compositions were plotted on the Q-A-P and Q-Ab-Or diagrams. Many of the analyzed rocks are classified to rhyodacite, and some to dacite and rhyolite in the classification and nomenclature by STREKEISEN (1967). Chemical compositions of the acid volcanic rocks were compared with the rocks from Quaternary volcanoes in northeast Japan and Hokkaido, and with Tertiary volcanics from various areas. Differentiation indices of the rocks concerned are higher than acid volcanics associated with Kuroko deposits in the Kosaka and Tsuchihata areas.
Crystallization of acid volcanic rocks is discussed in terms of Q-Ab-Or-H₂O system, and nomenclature of so-called plagiorhyolite is considered from the viewpoint of classification by STRECKEISEN (1967) and KUNO (1954).

View full abstract