Magnetic petrology: Application of Integrated Rock Magnetic and Petrological Techniques to Geological Interpretation of Magnetic Surveys

Clark, David; French, David; Lackie, Mark; Schmidt, P. W.

doi:10.1071/eg992065

Cited by 64 publications

(61 citation statements)

References 5 publications

(4 reference statements)

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Aeromagnetic anomalies over porphyry deposits depend strongly on the alteration history of the deposit, with sericitic and phyllic alteration usually described as magnetite-destructive, and potassic alteration described as both magnetitedestructive and magnetite-focusing or generating (Clark et al, 1992;Cox et al, 1995;Thoman et al, 2000;Blakely and McPhee, 2010). Although annular or otherwise shaped magnetic lows have been observed at some porphyry deposits, quantitative models suggest that corresponding anomalies are likely to be small compared to those associated with the surrounding geology and thus difficult to discern in a regional map (Blakely and McPhee, 2010).…”

Section: Discussionmentioning

confidence: 95%

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

et al. 2013

View full text Add to dashboard Cite

We combined aeromagnetic, induced polarization, magnetotelluric, and gravity surveys as well as drillhole geologic, alteration, magnetic susceptibility, and density data for exploration and characterization of the Cu-Au-Mo Pebble porphyry deposit. This undeveloped deposit is almost completely concealed by postmineralization sedimentary and volcanic rocks, presenting an exploration challenge. Individual geophysical methods primarily assist regional characterization. Positive chargeability and conductivity anomalies are observed over a broad region surrounding the deposit, likely representing sulfide minerals that accumulated during multiple stages of hydrothermal alteration. The mineralized area occupies only a small part of the chargeability anomaly because sulfide precipitation was not unique to the deposit, and mafic rocks also exhibit strong chargeability. Conductivity anomalies similarly reflect widespread sulfides as well as water-saturated glacial sediments. Mineralogical and magnetic susceptibility data indicate magnetite destruction primarily within the Cu-Au-Mo mineralized area. The magnetic field does not show a corresponding anomaly low but the analytic signal does in areas where the deposit is not covered by postmineralization igneous rocks. The analytic signal shows similar lows over sedimentary rocks outside of the mineralized area, however, and cannot uniquely distinguish the deposit. We find that the intersection of positive chargeability anomalies with analytic signal lows, indicating elevated sulfide concentrations but low magnetite at shallow depths, roughly delineates the deposit where it is covered only by glacial sediments. Neither chargeability highs nor analytic signal lows are present where the deposit is covered by several hundred meters of sedimentary and volcanic rocks, but a 3D resistivity model derived from magnetotelluric data shows a corresponding zone of higher conductivity. Gravity data highlight geologic features within the deposit, including shallow diorite sills that locally contain higher-grade mineralization. The results thus show ways in which an integrated survey approach might be used to distinguish zones of potentially economic mineralization.

show abstract

Section: Discussionmentioning

confidence: 95%

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

et al. 2013

View full text Add to dashboard Cite

show abstract

“…According to Clark et al (1992Clark et al ( . 2004, the induced magnetic anomalies caused by gold-rich copper porphyry systems have specific semi-predictable characteristics ( Fig.…”

Section: Idealised Magnetic Response Of a Porphyry Mineralising Systemmentioning

confidence: 97%

Automatic identification of responses from porphyry intrusive systems within magnetic data using image analysis

Holden¹,

Fu²,

Kovesi³

et al. 2011

Journal of Applied Geophysics

View full text Add to dashboard Cite

“…Susceptibility to the geomagnetic field or to different applied fields during surveying is of considerable commercial value (CLARK, 1983;1997;CLARK et al, 1988). In the laboratory, comparison of low-and high-field responses aids in discriminating the importance of paramagnetic and diamagnetic material versus RBM (HROUDA and JELINEK, 1990).…”

Section: Application and Uses Of Low-field Susceptibility And Its Animentioning

confidence: 99%

Induced Magnetization of Magnetite-titanomagnetite in Alternating Fields Ranging from 400 A/m to 80,000 A/m; Low-field Susceptibility (100 – 400 A/m) and Beyond

Borradaile

Stupavsky²,

Metsaranta

2008

Pure appl. geophys.

View full text Add to dashboard Cite

For remanence-bearing minerals (RBM) such as magnetite-titanomagnetite, susceptibility to induced magnetism (M) measured in alternating fields (H AC ) is field-dependent. However, for fields B 400 A/m, measured in an AC induction coil instrument (at 19,100 Hz), susceptibility k 0 = M/H AC is sufficiently linear to provide a reproducible rock (or mineral) magnetic characteristic and its anisotropy may be related to arrangements of minerals in rock, or for single mineral grains to their crystalline or shape anisotropy. For any remanence-bearing mineral at higher fields k HF (¼ M/H AC ) is not constant and the term susceptibility is not normally used. This study bridges the responses between traditional low-field susceptibility measurements and those due to high applied fields, for example when studying hysteresis or saturation magnetization of RBM. Where |k HF | is measured in alternating fields that peak significantly above 400 A/m the M(H AC ) relation is forced to follow a hysteresis loop in which |k HF | > k 0 for small |H AC | and where |k HF | decreases to zero for very large fields that achieve saturation magnetization. Hysteresis nonlinearity is due to remanence acquired with one field direction requiring a reverse field for its cancellation. We investigate the transition from initial, traditional ''lowfield'' susceptibility (k 0 ) measurements at 60 A/m, through 24 different fields from 400 A/m to 40,000 A/m (for very high k 0 to 80,000 A/m). This reveals M(H AC ) dependence beyond from conventional k 0 through the range of hysteresis behavior in fields equal to and exceeding that required to achieve saturation magnetization (M S ). We show k HF increases with peak H AC until the peak field is slightly less than saturation magnetization in natural rock samples rich in magnetite (TM 0 ¼ Fe 3 O 4 ) and TM 60 (Fe 2.4 Ti 0.6 O 4 ). All sample suites predominantly contain multidomain grains with subordinate pseudo-single domain and single-domain grains. k/k 0 increases by B 5% for fields up to 2 kA/m. Above 4 kA/m k/ k 0 increases steeply and peaks, usually between 24 kA/m and 30 kA/m where all grains magnetic moments are activated by H AC since this exceeds the coercive force of most grains. For higher peak H AC , k/k 0 declines sharply as increased H AC values more effectively flip M with each field-direction switch, leading to the low gradient at distal portions of the hysteresis loop. For M 0 -TM 60 bearing rocks, susceptibility peaks for fields *12 kA/m and for magnetite rich rocks up to 24 kA/m. These values are approximately 10% of saturation magnetizations (M S ) reported for the pure minerals from hysteresis DC field measurements. Both the field at peak k/k 0 and the peak k/k 0 value appear to be controlled by the dominant domain structure; multidomain behavior has larger k/k 0 peaks at lower H AC . Stacked k/k 0 versus H AC curves for each sample suite (n = 12 to n = 39) were successfully characterized at the 95% level by a polynomial fit that requires the cubic form k/k 0 = a + bH + cH 2 + dH 3 . Thu...

show abstract

Magnetic petrology: Application of Integrated Rock Magnetic and Petrological Techniques to Geological Interpretation of Magnetic Surveys

Cited by 64 publications

References 5 publications

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

Automatic identification of responses from porphyry intrusive systems within magnetic data using image analysis

Induced Magnetization of Magnetite-titanomagnetite in Alternating Fields Ranging from 400 A/m to 80,000 A/m; Low-field Susceptibility (100 – 400 A/m) and Beyond

Contact Info

Product

Resources

About