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EXPLORATION AND MINING GEOLOGY JOURNAL EMG Supergene Mineralization at the Kori Kollo Gold Mine, Bolivia Geophysical Signature of the Jinchuan Ni-Cu-PGE Deposit, Gansu Province, China Gahnite Composition: Implications for Base Metal and Rare-element Exploration
PII:S0964-1823(98)00010-5 Abstract—To evaluate geochemical aspects of sheared zones, 35 samples of granitoid rocks from an area of shearing >5 km2 at Derehamzeh, Iran were studied geochemically. Relatively unaltered samples within the shear-zone area average 38 ppm Cu, 1.45 ppm Mo, 4.6 ppb Au, 0.21 ppm Ag, <1 ppm Re, 46 ppm Zn, 28.5 ppm Pb, and 0.34% S. Bleached, hydrothermally-altered samples are anomalous in Cu (>896 ppm), Au(>59.5 ppb), Ag(>24.4 ppm), Re(>1.28 ppm), Zn (>164 ppm), Pb (>218 ppm), and S(>0.9%). Some samples from bleached sheared zones that are highly anomalous in Cu (5000 ppm), Mo(23 ppm), Ag(74 ppm), and Au(200 ppb) are characterized by a typical sericitic alteration assemblage consisting of sericite, pyrite, chalcopyrite, and granular quartz. The main anomalous zone extends over about 0.4 km2 in the southern part of the area. The strong Cu-Mo-Au-Ag association in the sericitic shear zones suggests a possible relationship to porphyry copper mineralization, similar to that found in northern Chile. Further studies on drill-core samples in the southern sericitized shear zones are required to determine if more extensive porphyry-type sulfide mineralization is present. ©1999 Canadian Institute of Mining, Metallurgy and Petroleum. Published by Elsevier Science Ltd. All rights reserved.
PII:S0964-1823(98)00007-5 Abstract — The Kori Kollo gold deposit is hosted by a c.15 Ma sericitized dacite stock in the La Joya district on the Bolivian Altiplano. The mineralization is of the Bolivian polymetallic vein (BPV) type, with a significant amount of the gold reserves contained in the oxide cap. Pyrite d34S values fall in the limited range -3.5 ± 0.6‰, consistent with a magmatic origin. Sulfate d34S values are homogeneous and overlap closely with those of pyrite (-1.2 ± 2.1‰), which supports petrographic evidence that sulfates replaced pyrite. By contrast, sulfate d18O values are inhomogeneous with a large range (alunite +2 to 26‰, jarosite -5 to +18‰). These data are interpreted as reflecting kinetically controlled reactions involving various proportions of atmospheric oxygen and local meteoric water. This is typical of supergene alteration and is due to the seasonally dry climate of the Altiplano. The oxide-zone mineralogy is dominated by sulfates and iron oxy-hydroxide minerals with some phosphates and halides, which indicates that the groundwaters were acidic, oxidizing and relatively saline. Under these conditions the most likely complexes responsible for gold mobilization are halides.Geological data and K-Ar ages indicate that oxidation of the BPV deposits began at least around 11 Ma and possibly up to 15 Ma ago, and that supergene alteration lasted until around 4 Ma. During this period, erosion was minimal. Although supergene processes have been active on the Bolivian Altiplano since the Middle Miocene, in contrast to Northern Chile, enrichment of gold has not occurred. ©1999 Canadian Institute of Mining, Metallurgy and Petroleum. Published by Elsevier Science Ltd. All rights reserved.
PII: S0964-1823(98)00008-7 Abstract—Rare earth element (REE) data are reported for ores, sulfide-bearing rocks and host rocks from massive sulfide deposits and barren sulfide facies iron formations in Proterozoic volcanic rocks of the Flin Flon-Snow Lake area, Manitoba. Some of the rare earth elements (e.g., Eu), are selectively leached in zones of intense alteration and redeposited together with the sulfides and silicates in the uppermost parts of the alteration zones and at the rock-water interface. Chondrite normalized REE profiles of layered sulfide ores and exhalites proximal to the deposits typically have positive europium anomalies and are enriched in light REE. In contrast, barren sulfide facies iron formations exhibit negative or ‘normal’ Eu profiles. REE data are useful to explorationists in determining the presence of ‘off-hole’ metal-bearing sulfide zones, in distinguishing metal-poor sulfidic layers related to ‘economic’ hydrothermal activity from barren sulfide facies iron formation and in some instances as a means of establishing ‘ore-equivalent’ strata. © 1999 Canadian Institute of Mining, Metallurgy and Petroleum. Published by Elsevier Science Ltd. All rights reserved.
PII:S0964-1823(98)00009-9 Abstract—The Jinchuan ultramafic-hosted nickel-copper sulfide deposit in Gansu Province, China, is one of the largest nickel deposits in the world. A range of geophysical methods have been tested at Jinchuan and have proved useful for both mapping the intrusion that hosts the mineralization, and for direct detection of the mineralization itself. Aeromagnetics were successfully used for mapping of the ore-bearing ultramafic intrusion, but ground magnetics can not only do the same job, but may also be able to detect zones of mineralization. The effects of the mineralization can sometimes be seen in residual gravity anomalies. However, this depends on the reliability and accuracy of the regional-residual separation and terrain corrections. This is because the effective anomalous response from a mineralized body is only 0.6 to 0.8 mGal. Time-domain IP and resistivity are also effective at the the Jinchuan deposit, but only where the host intrusion is exposed. The contour maps derived from gradient array surveys define the horizontal distribution of mineralization within the intrusion. The double pole-dipole array is able to indicate the dip-direction of a sheet-like mineralized body. However, in alluvium-covered areas, the effectiveness of IP and resistivity methods is greatly reduced by the highly conductive overburden. The self-potential method defines clear anomalies over the mineralized body within the intrusion as well as over the graphite-bearing country rocks. A trial seismic refraction tomography experiment at the Jinchuan deposit was successful at mapping lithologies and blind orebodies lying between mining tunnels. © 1999 Canadian Institute of Mining, Metallurgy and Petroleum. Published by Elsevier Science Ltd. All rights reserved.
PII:S0964-1823(98)00011-7 Abstract — Gamma-ray spectrometry, an effective tool in geological mapping, was used to define boundaries between various lithological formations in three adjacent areas of Central Syria, namely, the Syrian Desert, Ar Rassafeh Badiayat and the Northern Palmyrides mountains. This paper describes the role of an airborne gamma spectrometric survey which was orginally undertaken to assist uranium exploration. Interpretation of the total count data obtained through the survey has led to significant modifications and corrections to the previously published distribution map of the palaeogene phosphate rocks. Another important result of the survey is the discovery of four previously unknown phosphorite horizons in the Rasm Al-Aawabed area of the Northern Palmyrides. In addition, previously unknown phosphatic horizons are outlined in the other study areas. The importance of this technique as an effective prospecting method, not only for U but also for exploring for the economically important phosphorite formations in Syria, is emphasized. Furthermore, its role in geological mapping is stressed, especially in areas that lack sufficient geological data. ©1999 Canadian Institute of Mining, Metallurgy and Petroleum. Published by Elsevier Science Ltd. All rights reserved.
PII:S0964-1823(98)00012-9 Abstract—Gahnite, a zinc-bearing spinel, is a resistate mineral that may be used as an indicator mineral of polymetallic deposits in high-grade metamorphic terrain. The Ontario Geological Survey recently completed surficial mapping and sampling programs in the Separation Lake area of northwestern Ontario and Kinniwabi Lake area of northeastern Ontario in which gahnite was recovered from samples of surficial sediments. Ternary plots of the composition (ZnO, MgO, FeO[t]) of gahnite recovered from these surficial studies reveal three distinct groups: Group 1 is characterized by low (< 1 wt%) MgO values; Group 2 by higher (>1 wt%) MgO values; and Group 3 by higher FeO(t) values than Groups 1 and 2. The composition of in situ gahnite recovered from volcanogenic massive sulfide mineralization, Zn-rich Sedex-type mineralization and peraluminous rare-element mineralized granite and pegmatite also was determined. The composition of the gahnite obtained from surficial samples was compared with that of in situ material. Gahnite from the peraluminous rare-element granite and pegmatite overlapped very well with surficial gahnite of Group 1. Gahnite from volcanogenic massive sulfide mineralization and Sedex-type mineralization overlapped well with surficial gahnite of Group 2. These results suggest that determination of gahnite composition in indicator mineral surveys may be useful in exploration programs when assessing the mineral potential of a region. ©1999 Canadian Institute of Mining, Metallurgy and Petroleum. Published by Elsevier Science Ltd. All rights reserved. Last updated: |
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