EXPLORATION AND MINING GEOLOGY JOURNAL

EMG
Volume 6, No. 1 (January, 1997)

Lateritization as a Major Process of Ore Deposit Formation in the Amazon Region
MARCONDES LIMA COSTA

Improved and New Uses of Natural Radioactivity in Mineral Exploration and Processing
R.J. DE MEIJER, C. STAPEL, D.G. JONES, P.D. ROBERTS, A. ROZENDAAL, W.G. MACDONALD

Geology and Oxide Mineralization of the Pipestone Lake Anorthosite Complex, Manitoba
L.S. JOBIN-BEVANS, N.M. HALDEN, D.C. PECK and H.D.M. CAMERON

Mineral-potential Assessment by Consistency-driven Pairwise Comparisons
WALDEMAR W. KOCZKODAJ, WILLIAM O. MACKASEY

The Tectonic, Magmatic and Mineralization History of the Sudbury Structure
DON H. ROUSELL, HAROLD L. GIBSON, IAN R. JONASSON

Sedimentary Cycles and Selective Dolomitization in Limestones Hosting
GIANCARLO RIZZI, C.J.R. BRAITHWAITE

PII:S0964-1823(97)00005-6
Lateritization as a Major Process of Ore Deposit Formation in the Amazon Region
MARCONDES LIMA COSTA
Geosciences Center, Federal University of Pará
C.P. 1611 66075-110, Belém-PA, Brazil.
Received March 10, 1996; accepted January 10, 1997.

Abstract—The Amazon region is characterized by the development of deep chemical weathering represented by widespread lateritic covers and soils. Two distinct periods of laterite formation can be distinguished: mature laterites from the Eocene-Oligocene (and locally from the end of the Cretaceous), and immature laterites from the Pleistocene. The older laterites occur on plateau landscapes, and the younger ones on widespread hilly to flat lowlands. Both types show complete or truncated profiles, generally covered by deep yellow to brown topsoils.

The mature laterite profiles contain high concentrations of gibbsite and/or aluminum phosphate, whereas the immature ones are poor in these minerals. The Amazon region is rich in mineral deposits related to lateritic profiles. Some mineral deposits are of primary origin but were concentrated to economic grades by lateritic processes. The most important ore deposits are of iron, aluminum (bauxite), kaolin, manganese, gold, nickel, copper and phosphate. These are mainly related to mature profiles.

The wide spectrum of ore deposits and mineralogical and geochemical complexity of the Amazon laterites has been promoted by: (1) the prevalence of ideal conditions for laterite formation throughout much of the Tertiary; (2) the presence of varied basement lithologies due to contrasting geological environments; and (3) a variety of epigenetic alteration types. The lateritic profiles show a well-developed ore zonation which is related to specific lateritic horizons. Iron and gold deposits occur in the ferruginous horizon, which is nearest the surface; the central aluminous horizon hosts bauxites, aluminum phosphate, strontium and gold; and the lower clayey horizon contains deposits of manganese, nickel, copper and kaolin.

Resistate phases which residually accumulate throughout entire profiles include titanium (as ilmenite and anatase), chromium (as chromite), tin (as cassiterite), yttrium (as xenotime), and niobium (mainly as ilmenorutile). Epigenetic alteration, caused by swamp environments developed above truncated lateritic profiles, has led to the formation of high-grade kaolin and refractory bauxites. The laterites in the Amazon region contain most of all known kinds of ore mineralization related to laterite processes. © 1997 Canadian Institute of Mining, Metallurgy and Petroleum.

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PII: S0964-1823(97)00009-3
Improved and New Uses of Natural Radioactivity in Mineral Exploration and Processing
R.J. DE MEIJER, C. STAPEL
Environmental Radioactivity Research and Consultancy Group
Kernfysisch Versneller Instituut, Rijksuniversiteit Groningen
Zernikelaan 25, 9747 AA Groningen, The Netherlands
D.G. JONES, P.D. ROBERTS
British Geological Survey
Keyworth, Nottingham NG12 5GG, United Kingdom
A. ROZENDAAL, W.G. MACDONALD
Department of Geology, University of Stellenbosch
P.O. Box X01, Matieland 7602, Stellenbosch, South Africa
Received December 10, 1996; accepted June 9, 1997.

Abstract—Measurement of natural radioactivity has been used in both a qualitative and a quantitative way in mineral exploration, particularly in the search for uranium. In the last five years, the Kernfysisch Versneller Instituut (KVI) and British Geological Survey (BGS) have designed, built and tested a new detector system that greatly improves quantitative applications in mineral exploration, especially on the seafloor and in the nearshore zone. The new system is an enhancement of an earlier BGS design. The major improvements comprise the use of a highly sensitive g-ray detector, together with new data processing, from which concentrations of natural radionuclides may be deduced in real time. After laboratory analysis of samples, these concentrations can be converted into mineral percentages. Once the mineralogy of an area has been characterized, this conversion can also be done in real time.

Thus far the system has mainly been used to map heavy-mineral concentrations in coastal subtidal and intertidal sediments. For such work the multi-sensor detector probe is towed on the seafloor and various parameters are recorded continuously. The probe contains, in addition to the gamma-ray detector, a water-pressure sensor (giving water depth) and an acoustic device to measure bottom roughness. The system, when used together with high-precision positioning systems such as D-GPS, enables the production of detailed maps of bathymetry, bottom roughness, and seafloor radioactivity or mineral distribution. These maps allow mineral concentrations to be readily correlated with sea-bed topography and sediment type. In the same way, g-ray detectors could be used advantageously during dredging operations to provide on-line assessment of the mineral content of the dredged sediment as well as to guide the direction of dredging. Additionally, the same technique could be used to monitor the subsequent fate of a dredge spoil after dumping on the seafloor.

Natural radioactivity arising from potassium and the thorium and uranium decay series has potential use for the fast and reliable assessment of siliciclastic and phosphatic impurities in limestones. Detection of such impurities may preclude the use of the limestone for chemical or cement production.

Apart from its use in exploration, the technique has significant potential in mineral processing, particularly of heavy minerals. It can be applied as on-line quality control in dry and wet separation processes and for process control, by using decisions based on the results to control valves and other instrumentation. In addition, it can also be used in environmental monitoring of waste arising from mineral extraction and processing. In conclusion, the technique described has a diversity of applications in exploration, mining and mineral processing. © 1997 Canadian Institute of Mining, Metallurgy and Petroleum.

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 PII: S0964-1823(97)00008-1
Geology and Oxide Mineralization of the Pipestone Lake Anorthosite Complex, Manitoba
L.S. JOBIN-BEVANS, N.M. HALDEN
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2
D.C. PECK and H.D.M. CAMERON
Manitoba Energy and Mines, Geological Services Branch, Thompson, Manitoba, Canada, R8N 1X4
Received February 3, 1997; accepted June 5, 1997.

Abstract—The late Archean Pipestone Lake anorthosite complex (PLAC) is a 0.9 x 17 km sill-like layered intrusion that is part of a suite of coeval megacrystic anorthosite bodies in the Cross Lake region, northwestern Superior Province. Exploration for Ti-V-Fe oxide mineralization and concurrent geological investigations have contributed to a greater understanding of the lithostratigraphy of the PLAC and its oxide mineralization. Mapping and drill core studies have established a laterally continuous igneous stratigraphy comprising six leucogabbro/anorthosite zones and three melagabbro/gabbro zones.

Field evidence, including disrupted blocks of layered units, scour contacts, and size-graded and modally-graded layering, suggests younging towards both the northern and the central parts of the complex. A decrease in the anorthite content of the feldspars from An75 in the south to An30 in the north suggests that the magmas become more evolved to the north. Macrorhythmic layering and gross morphological features suggest that the PLAC crystallized from four major influxes of geochemically similar basaltic magmas. The paucity of magmatic erosional features in the PLAC indicates that layering developed in a relatively static magma chamber, so that in-situ crystallization processes are required to account for the observed petrologic diversity. Drilling has confirmed that a significant Ti-V-Fe resource is contained in two of the four discrete mineralized subzones in the PLAC. Textural evidence and the stratiform nature of the oxide-enriched subzones are consistent with a magmatic origin for the oxide mineralization. © 1997 Canadian Institute of Mining, Metallurgy and Petroleum.

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Abstract — A consistency-driven pairwise comparisons method for mineral potential assessment is presented, using a simplified case of volcanic-associated massive sulfide type deposits as an example. Geological, geochemical, and geophysical criteria are considered on two levels: local and regional. The local geological criteria are subdivided into stratigraphy, lithology, alteration and/or mineralization, and structure. The concept of geomerit index and a procedure for computing this index are introduced.

The method of consistency-driven pairwise comparisons can be combined with other quantitative and qualitative assessment methods, enhancing them by incorporation of nonmeasurable geological criteria (e.g., stratigraphy or lithology) and by computation of weights. The consistency-driven approach allows experts to refine at-times subjective judgments concerning exploration and land-use planning, and contributes to the improvement of mineral-potential assessments. © 1997 Canadian Institute of Mining, Metallurgy and Petroleum.

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PII: S0964-1823(97)00006-8
The Tectonic, Magmatic and Mineralization History of the Sudbury Structure
DON H. ROUSELL, HAROLD L. GIBSON
Mineral Exploration Research Centre
Department of Earth Sciences, Laurentian University,
Sudbury, Ontario, Canada P3E 2C6
and
IAN R. JONASSON
Mineral Deposit Division, Geological Survey of Canada
601 Booth Street, Ottawa, Ontario, Canada K1A 0E8
Received, July 1, 1996; accepted March 7, 1997.

Abstract — The Sudbury Structure consists of three major components: 1) the Sudbury Basin;
2) the Sudbury Igneous Complex which surrounds the basin as an elliptical collar; and 3) an outer zone of shatter-coned and intensely brecciated footwall rocks. Although the Sudbury Event which formed the structure may be unique, the area has, in fact, been shaped by a series of tectonic, magmatic and mineralization events which can be considered in terms of two Wilson cycles of continental extension and closure. The first cycle, from 2500 to 1700 Ma, included the following events: doming (uplift of Levack Gneiss Complex); NE-SW extension (Matachewan dikes); N-S to NW-SE rifting (faults, mafic intrusions, sedimentation, and Ni-Cu-PGE and U mineralization); NW-SE extension (Nipissing diabase, and Ni-Cu-PGE, Ag, Co mineralization); and NW-SE and NE-SW closure (Penokean Orogeny, 1900 to 1700 Ma) with superimposed meteorite impact (1850 Ma, Ni-Cu-PGE, Zn-Cu-Pb deposits). The second cycle, from 1700 to 1000 Ma, included: N-S extension (alkali metasomatism, Au); N-S extension (hornblende diabase dikes along the Murray fault set); NNE-SSW extension (olivine diabase dikes); NE-SW extension (Fecunis Lake fault set); and NW-SE closure (Grenvillian Orogeny).

The rich and diverse Ni-Cu-PGE and subordinate Zn-Pb-Cu and Au mineralization of the Sudbury region is related to endogenic and impact-triggered crustal extension and magmatism. The Sudbury Structure apparently was the site of a triple junction or hot-spot. The Sudbury ores, although located within an impact structure, are analogous in terms of age and tectonic setting to Ni-Cu-PGE and Zn-Pb-Cu ores elsewhere in the world. Meteorite impact accentuated on-going ore forming processes and magmatism at Sudbury. © 1997 Canadian Institute of Mining, Metallurgy and Petroleum.

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PII: S0964-1823(97)
Sedimentary Cycles and Selective Dolomitization in Limestones Hosting
the Giant Navan Zinc-Lead Ore Deposit, Ireland
GIANCARLO RIZZI and C.J.R. BRAITHWAITE
Department of Geology and Applied Geology
University of Glasgow, G12 8QQ, Scotland
Received November 13, 1995; accepted October 12, 1996.

Abstract—The Navan zinc-lead ore deposit (70 million tonnes) is hosted principally (97%) by the Meath Formation (Lower Carboniferous, Courceyan, Navan Group), comprising a lower, mainly carbonate mudstone unit, the Stackallan Member, and an upper grainstone-dominated unit. The Stackallan Member, about 60 m thick, comprises about 35 peritidal cycles including an oolitic grainstone interval. Grainstones forming the upper part of the formation, about 150 m thick, comprise at least six shallowing-upward ramp cycles. Emergence horizons are indicated by subaerial dissolution surfaces, in situ breccias, and a single palaeosol.

Most dolomitization at Navan is confined to the Meath Formation. The linear dolomite body, trending NE-SW, has a flattened, laterally limited, tabular geometry in cross-section, suggesting that dolomitizing fluids initially rose vertically, cross-cutting stratigraphic and sedimentological boundaries, although fluid flow was subsequently controlled by these features.

Three stages of dolomitization are indicated by textural relationships, cathodoluminescence (CL), oxygen isotopes, and fluid inclusion data. Stage 1 crystals, with generally dully luminescing cores and up to six overlying subsidiary zones, are the main replacive phase. Crystal surfaces are commonly corroded and overlain by bright stage 2 rhombs (which also form new nuclei lining molds and fractures). Three subsets of zones are present in stage 2 crystals: bright, non-luminescent, and dull; these are locally separated by additional corrosion surfaces. Stage 3 comprises relatively large baroque crystals, generally dully luminescent but with up to seven sub-zones. Isotopic and fluid inclusion data suggest growth of successive stages from waters which became progressively hotter (60°–160°C).

The distribution of the ores follows the dolomite trend. Ores were precipitated between Stages 1 and 2 of dolomite growth, following Stage 2, and also during the nucleation and growth of Stage 3 crystals. Dolomitization and mineralization appear to have been temporally and genetically related. The cyclic sedimentation of the host rocks, which accentuates lithological and petrographical contrasts between depositional units, controlled dolomitization. The relationship between dolomitization and mineralization, which is closely associated with faults, provides powerful exploration criteria. The recognition of these features in limestones elsewhere in the Irish ore field may point toward unrecognized zinc-lead prospects. © 1997 Canadian Institute of Mining, Metallurgy and Petroleum.