Near-infrared effectiveness on degraded core in tropical climates
- Authors: Jansen, Nicholas , Cooke, David , Harris, Anthony , McKnight, Stafford
- Date: 2017
- Type: Text , Journal article
- Relation: Economic Geology Vol. 112, no. 4 (2017), p. 1011-1019
- Full Text: false
- Reviewed:
- Description: In tropical climates, postdrilling oxidation of sulfide-rich core can severely degrade drill core, producing lowtemperature iron oxyhydroxides, sulfates, and clays. Variable growth of these secondary minerals in exposed drill core, combined with the hydration and degradation of primary hydrothermal minerals, may lead to the production of spurious results in near-infrared (NIR) spectroscopic studies. However, the NIR technique can remain an effective tool in assessing hydrothermal alteration, even in extremely degraded core. We have assessed the usefulness of the NIR technique on degraded core at the Ladolam gold deposit, Papua New Guinea. Here, we seek to determine whether the primary alteration mineralogy had been significantly transformed by postdrilling oxidation over several years of weathering. In doing so, the study tested whether NIR analysis can be an effective tool in the discrimination of primary hydrothermal minerals in degraded core. Our study was made possible using semiquantitative X-ray diffraction (QXRD) analyses of a drill hole in 2004, where samples were collected at 50-m intervals. We subsequently repeated NIR and QXRD analyses on the same drill core in 2012. After nine years of storage, the drill core had degraded considerably, with the growth of jarosite and other sulfates. Despite this, XRD results from 2004 and 2012 show no major differences in the primary alteration mineralogy. Closely spaced NIR analyses were conducted at 1-m intervals to increase the chance of obtaining a spectrum of the primary mineralogy and to exclude secondary oxidation minerals. The drill core, where possible, was broken immediately prior to analysis to obtain a fresh surface. On average, over a 10-m interval, approximately 25% of the NIR spectra did not contain secondary minerals and relict primary alteration minerals could be detected. The remaining spectra were affected by the occurrence of secondary jarosite, gypsum, and/or residual water, but in most cases, the primary alteration mineralogy could be determined. We conclude that NIR analyses remain an effective tool in the construction of geological deposit models when logging degraded historic core, even for sulfide-rich core that has degraded in tropical environments. © 2017 Society of Economic Geologists, Inc.
Petrogenesis of a Neoproterozoic magmatic arc hosting porphyry Cu-Au mineralization at Jebel Ohier in the Gebeit Terrane, NE Sudan
- Authors: Bierlein, Frank , Reynolds, N. , Arne, Dennis , Bargmann, C. , McKeag, S. , Bullen, W. , Al-Athbah, H. , McKnight, Stafford , Maas, Roland
- Date: 2016
- Type: Text , Journal article
- Relation: Ore Geology Reviews Vol. 79, no. (2016), p. 133-154
- Full Text: false
- Reviewed:
- Description: The ca. 730 Ma porphyry Cu-Au deposit at Jebel Ohier in the Red Sea Hills of northeastern Sudan represents a rare example of a preserved Neoproterozoic magmatic-hydrothermal system which bears many similarities to major mineral-hosting ('productive') Tertiary-Cenozoic porphyries in circum-Pacific metallogenic belts. Petrographic, lithogeochemical and Sm-Nd isotope systematics confirm that the deposit formed in a supra-subduction setting and during the constructional stage of an evolving intra-oceanic magmatic arc. The calc-alkaline melts were sourced predominantly from juvenile reservoirs and received comparatively little input from continental character material. Comparison with igneous rocks from barren intrusions elsewhere in the region point to the absence of major crustal breaks but indicate that the ore-forming granodiorite-dacite porphyry complex at Jebel Ohier is the result of 'abnormal' and prolonged multi-phase arc plutonism. This process involved the formation of relatively hydrous and oxidized melts via the fractionation of magmas, which possibly had ponded at the bottom of the thickening lithosphere for a protracted period prior to their ascent. The tectonic trigger for the emplacement of the productive pluton into a pre-existing volcanic edifice at Jebel Ohier remains unconstrained. Preservation of what is considered the first documented porphyry Cu-Au deposit in the NE African portion of the Arabian Nubian Shield can probably be related to the accretion of the magmatic arc to a stable continental margin within a few million years of mineralization, thus enabling the deposit to escape excessive uplift, erosion and structural dismemberment (C) 2016 Elsevier B.V. All rights reserved. DELSALAM MG, 1993, JOURNAL OF THE GEOLOGICAL SOCIETY, V150, P1065
Tectonic and economic implications of trace element, Ar-40/Ar-39 and Sm-Nd data from mafic dykes associated with orogenic gold mineralisation in central Victoria, Australia
- Authors: Bierlein, Frank , Hughes, Martin , Dunphy, J. , McKnight, Stafford , Reynolds, P. , Waldron, H.
- Date: 2001
- Type: Text , Journal article
- Relation: Lithos Vol. 58, no. 1-2 (Aug 2001), p. 1-31
- Full Text: false
- Reviewed:
- Description: Mafic to intermediate dykes are spatially and temporally closely associated with major post-tectonic granitic complexes in the western Lachlan Orogen of SE Australia. These dykes, which range petrographically from basaltic to andesitic, are concentrated within several, north- to northwest-trending zones and were emplaced during two broad intervals of extensive magmatic activity during the Silurian–Devonian period. Geochemical and Sm–Nd isotopic characteristics of these mafic intrusives are consistent with their formation in a complex subduction-related tectonic setting. Interaction between mantle-wedge material, sinking oceanic crust and input from the overlying continental crust resulted in the petrological and geochemical variations displayed by these and more felsic dykes throughout the study region. Field evidence and 40Ar/39Ar data show that in the eastern part of the Stawell Zone and in the northwest portion of the Bendigo Zone, mafic dyke were intruded between 410 and 400 Ma (Late Silurian/Early Devonian). Further emplacement in the Bendigo Zone and the eastern part of the Melbourne Zone took place at between 375 and 365 Ma (Middle to Late Devonian). Episodic mantle-derived magmatism was possibly related to step-wise rollback, slab detachment or changes in the angle and rate of westward subduction in response to periodically occurring accretionary pulses. A close spatial and temporal relationship also exists between the dykes and orogenic gold mineralisation in the central Victorian gold province. Mafic to intermediate dykes both crosscut, and are host to, mineralisation in a number of goldfields. Although there is little evidence for a direct genetic association, the two processes are linked by the common utilisation of translithospheric structures, which facilitated the rapid ascent into shallow crustal levels of both mantle-derived magma and crustal-scale ore-forming fluid systems. Previous studies have suggested that transfer of heat into the crust via ascending mafic mantle magmas can provide a thermal engine which triggers and sustains extensive crustal melting, thus explaining the commonly observed close association of mafic to intermediate and felsic intrusive suites. This study supports the viability of this mechanism and in addition, indicates that a causal link exists between the formation of mantle magmas in collisional zones and the generation of orogenic gold deposits.
- Description: C1
Tectonic and economic implications of trace element, Ar-40/Ar-39 and Sm-Nd data from mafic dykes associated with orogenic gold minerals in central Victoria, Australia : reply
- Authors: Bierlein, Frank , Hughes, Martin , Dunphy, J. , McKnight, Stafford
- Date: 2002
- Type: Text , Journal article
- Relation: Lithos Vol. 63, no. 1-2 (Jul 2002), p. 119-123
- Full Text: false
- Reviewed:
The Jebel Ohier deposit - a newly discovered porphyry copper-gold system in the Neoproterozoic Arabian-Nubian Shield, Red Sea Hills, NE Sudan
- Authors: Bierlein, Frank , McKeag, S. , Reynolds, N. , Bargmann, C. , Bullen, W. , Murphy, F. C. , Al-Athbah, H. , Brauhart, C. , Potma, W. , Meffre, S. , McKnight, Stafford
- Date: 2016
- Type: Text , Journal article
- Relation: Mineralium Deposita Vol. 51, no. 6 (2016), p. 713-724
- Full Text: false
- Reviewed:
- Description: Ongoing exploration in the Red Sea Hills of NE Sudan has led to the identification of a large alteration-mineralization system within a relatively undeformed Neoproterozoic intrusive-extrusive succession centered on Jebel Ohier. The style of mineralization, presence of an extensive stockwork vein network within a zoned potassic-propylitic-argillic-advanced argillic-altered system, a mineralization assemblage comprising magnetite-pyrite-chalcopyrite-bornite (+/- gold, silver and tellurides), and the recurrence of fertile mafic to intermediate magmatism in a developing convergent plate setting all point to a porphyry copper-gold association, analogous to major porphyry Cu-Au-Mo deposits in Phanerozoic supra-subduction settings such as the SW Pacific. Preliminary U-Pb age dating yielded a maximum constraint of c. 730 Ma for the emplacement of the stockwork system into a significantly older (c. 800 Ma) volcanic edifice. The mineralization formed prior to regional deformation and accretion of the host terrane to a stable continental margin at by c. 700 Ma, thus ensuring preservation of the deposit. The Jebel Ohier deposit is interpreted as a relatively well-preserved, rare example of a Neoproterozoic porphyry Cu-Au system and the first porphyry Cu-Au deposit to be identified in the Arabian-Nubian Shield. DELSALAM MG, 1993, JOURNAL OF THE GEOLOGICAL SOCIETY, V150, P1065