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31744 results in Solid Earth Geophysics

Page 149 of 1588

  • 3 - Gravity and magnetic methods

  • Michael Dentith, University of Western Australia, Perth, Stephen T. Mudge
  • Book:
    Geophysics for the Mineral Exploration Geoscientist
    Published online:
    28 May 2018
    Print publication:
    24 April 2014, pp 85-192

  • Summary

    Introduction

    The gravity and magnetic methods measure spatial variations in the Earth’s gravity and magnetic fields (Fig. 3.1). Changes in gravity are caused by variations in rock density and those in the magnetic field by variations in rock magnetism, which is mostly controlled by a physical property called magnetic susceptibility. Gravity and magnetic surveys are relatively inexpensive and are widely used for the direct detection of several different types of mineral deposits and for pseudo-geological mapping.

    Magnetic measurements made from the air, known as aeromagnetics, are virtually ubiquitous in mineral exploration for wide-area regional surveying, for detailed mapping at prospect scale and for target detection. In areas where exposure is poor, aeromagnetics has become an indispensable component of exploration programmes. Gravity measurements are also used for regional and prospect-scale mapping but, historically, measurements of sufficient accuracy and resolution for mineral exploration could only be made on the ground. The development of airborne gravity systems, known as aerogravity, with precision suitable for mineral targeting, means that aerogravity in mineral exploration is likely to become as common as aeromagnetics.

  • 1 - Introduction

  • Michael Dentith, University of Western Australia, Perth, Stephen T. Mudge
  • Book:
    Geophysics for the Mineral Exploration Geoscientist
    Published online:
    28 May 2018
    Print publication:
    24 April 2014, pp 1-12

  • Summary

    Geophysical methods respond to differences in the physical properties of rocks. Figure 1.1 is a schematic illustration of a geophysical survey. Over the area of interest, instruments are deployed in the field to measure variations in a physical parameter associated with variations in a physical property of the subsurface. The measurements are used to infer the geology of the survey area. Of particular significance is the ability of geophysical methods to make these inferences from a distance, and, for some methods, without contact with the ground, meaning that geophysics is a form of remote sensing (sensu lato). Surveys may be conducted on the ground, in the air or in-ground (downhole). Information about the geology can be obtained at scales ranging from the size of a geological province down to that of an individual drillhole.

    Geophysics is an integral part of most mineral exploration programmes, both greenfields and brownfields, and is increasingly used during the mining of orebodies. It is widely used because it can map large areas quickly and cost effectively, delineate subtle physical variations in the geology that might otherwise not be observed by field geological investigations and detect occurrences of a wide variety of mineral deposits.

  • Geology and geochemistry of the Baijiantan–Baikouquan ophiolitic mélanges: implications for geological evolution of west Junggar, Xinjiang, NW China

  • YONGFENG ZHU, BO CHEN, TIAN QIU
  • Journal:
    Geological Magazine / Volume 152 / Issue 1 / January 2015
    Published online by Cambridge University Press:
    23 April 2014, pp. 41-69
    • Article
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  • We report two newly identified Ordovician ophiolite belts in west Junggar, NW China: Tajin–Tarbahatai–Kujibai–Honguleleng (TTKH) and Tangbale–Baijiantan–Baikouquan (TBB) ophiolitic belts. These two ophiolitic belts provide constraints for the Palaeozoic reconstruction of Central Asia and the geological evolution of this region. The TTKH and TBB ophiolitic belts are dismembered parts of different ophiolitic belts which represent relics of Ordovician oceanic floor; they subducted to the north under the Chingiz–Tarbahatai arc and to the south under the Junggar plate, respectively. The Baijiantan–Baikouquan ophiolite mélanges comprise the major part of the TBB. Flat rare Earth element (REE) patterns with positive Eu anomalies and insignificant depletion of high-field-strength elements (HFSE) relative to melts of primitive mantle suggest a mid-ocean-ridge basalt (MORB) origin for the metagabbro. Lherzolite samples define a Sm–Nd isotopic isochron with age of 474 Ma and ɛ Nd(t) of +8.9. Lherzolite samples with positive ɛ Nd(t) values of +8.8 to +9.1 and initial 87Sr/86Sr ratios of 0.7037–0.7040 are rather homogeneous in Sr–Nd isotopic composition, whereas metagabbro samples show wider Sr–Nd isotopic compositional ranges with ɛ Nd(t) of +5.9 to +11.0. The Sm–Nd isotopic isochron age (c. 380 Ma) for garnet amphibolite samples, consistent with a zircon U–Pb age (c. 385 Ma) for metagabbro, represents a magmatic event prior to subduction. Thermodynamic calculations for garnet amphibolite yield a clockwise pressure–temperature path with peak metamorphic condition of c. 15 kbar and 520–560°C at 342 Ma, indicating a subduction-channel setting. The Rb–Sr isochron ages (335 Ma, 333 Ma) for metagabbro represent a metamorphic event during exhumation.

Page 149 of 1588