It has been proved that the blind dacite lava dome being affected by sulfide mineralization at depth underneath known Kuroko ore deposits is a source of IP anomaly which could be detected from the surface. Field IP method is still employed in a part of routine explorations for the field with high potentials of blind Kuroko ore deposits to detect this type of dacite as well as the ore body itself. Our long experiences both for frequency domain type and time domain type IP survey at Hokuroku district have revealed that the latter has an advantage for elimination of electro-magnetic coupling noise. Examinations through the concept of "signal contribution" by Roy and APPARAO (1971) for the gradient array, three electrodes array and dipole-dipole array concerning for the deep anomaly source have proved that sensitivity for the depth mostly depends on the distance of unit electrodes separation. Profiles from dipole-dipole configulation has been slightly modified from 45°plotting to lower angles. Various examples of IP survey at the surface over the known Kuroko ore deposits support the results of the examination. A trial to separate the anomaly of deep origin from noise of shallow origin has been proposed by the compensation of two surveys of different electrodes separation.

View full abstract

The Mamut deposit, which has ever produced about 43 million tons of crude ore at 0.6% Cu, 0.7 gm/ton Au and 4 gm/ton Ag, is of typical gold-rich porphyry copper type of the southwestern Pacific island arc setting. The gold-silver mineralization as well as copper is genetically associated with adamellite porphyry intrusives emplaced in serpentinite and siltstone. Alteration zones exhibit a concentric arrangement around the major intrusive of adamellite porphyry successively from the centre to the margin, namely, a broad zone of potassic alteration from the innermost core slightly enriched in K-feldspar to an intensive quartz-biotite zone, a phyllic zone chara cterized by quartz-sericite and an argillic zone with a peripheral kaolinite-montmorillonite assemblage.
Microscope and electron probe studies have revealed that gold-silver minerals, which are closely associated with chalcopyrite and to a lesser extent with pyrite, cubanite and pyrrhotite, comprise native gold and electrum with an average grain size of 30 to 35 μ in the gold-rich quartz veins and 3 to 6 μ in the normal ore-grade zones.
Based on the geological information from the evaluation of diamond drills and daily blast hole samples, the attitude of gold-silver mineralization is summarized as follows. The gold mineralization as well as copper is best developed in the magnetite-poor quartz-biotite type of potassic alteration giving rise to a high grade ore shell which occurs near the intrusion contact straddling both the host intrusive of adamellite porphyry and wall rocks. However, a relative intensity of gold mineralization in terms of gold to copper ratio is also found within the core of adamellite porphyry characterized by weak potassic alteration, whereas the gold grade, compared with copper, tends to sharply decrease outward in the quartz-biotite zone. Thus, an overall concentric distribution pattern is recognized in the gold to copper ratio which becomes progressively less with increasing distance from adamellite porphyry. Notwithstanding the close association of gold and silver deposition, the best silver grades are mainly formed in the near-surface upper half of the ore shell. The gold to copper ratio of sulfide-poor granodiorite porphyry dykes postdating adamellite porphyry is much higher than that of adamellite porphyry, which may imply a gradational bias of metal contents in ore-forming fluids from copper-rich to gold-rich types.
The study results could offer valuable suggestions for investigation on improvement of gold recovery in flotation processes and lead to the establishment of an ore target criteria of gold-rich zones in further exploration drilling around the present ore body.

View full abstract

Cookes Creek area, Pilbara region, Western Australia is situated in approximately 1, 200 km north northeast of Perth. Several small showings of tungsten mineralization have been known in the area.
The surveyed area (123.5 km²) is mainly occupied by the Salgash Subgroup of the Warrawoona Group of the Archaean Pilbara Supergroup and a biotite adamellite stock (2600 m. y.) called Cookes Creek Granite. The Sal-gash Subgroup chiefly consists of basalt lavas with subordinal interbedded greenschist, chert, acidic volcanics, and dolerite sills, gabbro sills, ect. The Cookes Creek Granite, cropping out over an about 10 km×4 km area, intruded into the Salgash Subgroup.
It was hypothesized that tungsten mineralization has been derived from the Cookes Creek Granite and occurs mainly as sheelite in the Salgash Subgroup and further that the mineralization zones have been controlled by fissure patterns and/or lithological factors. On such hypothesis, the selection of exploration targets was carried out effectively by the following exploration programmes.
(1) Regional geological survey: Shape of the Cookes Creek Granite and regional structure of the Salgash Subgroup were investigated. Southern and western outside areas of the granite were pointed out as most prospective ones.
(2) Geochemical survey: Stream sediments under 40 mesh were collected from the surveyed area and W thereof was analyzed as the indicator element. Eight areas showing over 500 ppm W were selected successfully as the first priority targets in southern and western outside areas of the granite.
(3) Detailed geological survey: Survey works, including ultra-violet lamping, were carried out in the eight areas, and three areas were further selected amongst them and trenched on the basis of the survey results.
As the result of the exploration works, it has proved that there can be defined the followings:
(1) As fissure-controlled mineralization: vein-type mineralizations consisting of scheelite dissemination in thin aplite dykes and in altered zones nearby.
(2) As horizon-controlled mineralization:
(a) Lens-shaped mineralizations consisting of scheelite-plagioclase-phlogopite-quartz-fluorite-tourmaline-margarite assemblage along schistosity of greenschist.
(b) Network mineralizations composed of scheelite-bearing thin aplite dykes which have intruded into a gabbro sill horizon selectively.

View full abstract