Hiroaki Kaneda

Stability of pentlandite in the Fe-Ni-Co-S system

Pentlandite has a wide compositional range in the Fe-Ni-Co-S system and the Fe-Ni-S system. The metal : sulfur atomic ratio is approximately 9 : 8. The Co content of pentlandite from the Kamaishi ore deposit and one from the Outokumpu ore deposit (Knop et al. 1965) varies from a Co-free to nearly a Co9S8 composition. Pentlandite forms a complete solid solution between (Fe, Ni)9±xS8 and Co9±xS8 in the 600°–300°C temperature range. On the other hand, it is presumed that the pentlandite solid solution decomposes into two fields toward the (Fe, Ni)9S8 and Co9S8 members at 200°C. The field of solid solution at 500°C is the most extensive and includes all of the other solid-solution fields. The metal : sulfur ratio of the solid solution varies systematically, i.e., the Fe-rich field sinks toward the Fe-Ni-Co plane away from the M9S8 section, and contrarily the Ni-rich field shifts toward the sulfur apex. The d044 value decreases with increasing S, Ni, or Co contents in pentlandite. The Kamaishi pentlandites, Iwate Prefecture, Japan, lie within or close to the solid-solution field at 300°C, showing a very good correlation with the Co : Ni ratio of the homogeneous natural pyrrhotite phase.

Ore value-tonnage diagrams for resource assessment

An ore value-tonnage diagram has been proposed for assessing mineral resources. Diagrams of W+Mo, and Pb+Zn deposits show a good linearity between ore value and logarithms of cumulative ore tonnage. Diagrams of the massive sulfide, orthomagmatic, placer, porphyry, replacement, and stratabound types are also linear. It is assumed, therefore, that deposits of each of these commodities and these types belong to a single population. In contrast, the ore value-tonnage relations of all the deposits analyzed here is approximated by the combination of two exponential functions. The same feature is seen for deposits of the Cu+W+Mo, Cu+Pb+Zn, and Au+Ag commodities, and of the vein and unconformity-related types. This suggests that deposits belonging to each of such categories are divided into the high and low value groups. We can expect, accordingly, to find high value deposits of such categories.

Mineralogy and geochemistry of the Au-Ag ore deposits of the South Korean Peninsula

Pyrite and arsenopyrite are the predominant ore minerals in the Korean Au-Ag deposits of this study. The XNipy, XCopy, XNiapy, and XCoapyvalues range between 100 and 3,000 ppm, 200 and 6,000 ppm, 200 and 8,200 ppm, and 100 and 10,200 ppm, respectively. Most XNipy/XCopyvalues fall in the field lower than values varying 0.16–1.30. Arsenopyrite also tends to prefer cobalt rather than nickel showing XNiapy/XCoapyvalues between 0.20 and 1.40. The concentrations of minor elements in ores and gangue minerals vary 1–55 ppm Au and 1–1,120 ppm Ag for the former and 4–57 ppm Ni and 2–45 ppm Co for the latter. The Au/Ag ratio in ore has a good correlation to the Ni/Co ratio of arsenopyrite to gangue. The (Ni/Co)py-(Ni/Co)gangue and (Ni/Co)apy-(Ni/Co)gangue diagrams revealed that the values from the Korean Au-Ag deposits plot in the field lower than 900 °C which is the lowermost temperature determined by previous partitioning experiments.

Availability of GPS in Areas where Topographic Maps are not Prepared.

The availability of GPS (Global Positioning System) has been examined in three areas. The first is a fixed point on the roof of a building of the University of Tokyo. The second area is around the Hishikari gold mine, southwestern Japan. The area was selected because topographic maps (1/25, 000) are published. The topography consists of gentle hills. The third area is Yunnan Province, southwestern China, where foreigners are prohibited to use topographic maps. The surveyed area is mostly mountainous. Measured values at the fixed point are scattered within 100m from the true position. The average point is located 30m south from the true position. In the Hishikari area, each GPS value is compared with the coordinates given by a topographic map. The deviation between the GPS value and the coordinates is within 300m. The reason why the deviation is larger than the error at the fixed point is that the coordinates given by a map includes error. In Yunnan, the accuracy was checked by the reproducibility. At many sites, satellites were not found because of steep slopes and high trees. A relatively wide space was located, however, within 300m from each site. When 4 satellites were found in wide areas, values were scattered in a circle with a diameter of 200m. When 4 satellites were found in narrow areas, the diameter was 500m. When only 3 satellites were found, the diameter was 1, 000m. These values are larger than the error of the pacing (less than 10 m for 100m distance). The result suggests that the GPS can provide location data with the accuracy enough to geoscience discussions in regional scales.

An interactive system to assist mineral identification in ore microscopy

An interactive computer system has been developed to assist the mineral identification in ore microscopy. The reference file of the system consists of optical, mechanical, and chemical properties of about 130 ore minerals. The properties are name, chemical formula, color, bireflectance, anisotropism, internal reflection, reflectance at wavelengths of 470, 546, 589, and 650 nm, and polishing hardness and micro-indentation hardness. All the properties except reflectance and microindentation hardness are qualitative or semi-qualitative. Most of the properties are given as characters relative to the more common minerals. This implies that most of the identification processes advance on the basis of the comparison between a subject mineral and coexisting minerals. For this reason, the system asks a user at first to input already identified mineral names. This is quite different from the mineral identification procedures used in petrographic microscopy. To reduce the number of possible minerals, the system presents a series of questions to a user, and the user selects any of the prepared answers according to his observation. The user can also choose any desired question independently of the sequence. The user is expected to be able to recognize some common minerals, such as pyrite, chalcopyrite, galena, and hematite, without the assistance of the system.