Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 142

if cooling was rapid, but nevertheless the curves give useful
indications of the compositions towards which the Fe-Ti

Susceptibility (SI)

Ti
oxides will tend. Introducing SiO 2 to the system allows the
FMQ curve to be added, representing a boundary below
which iron will tend to be mainly in silicate minerals. The
curve shown is for a silica-saturated system. If it is not
silica-saturated the curve is shifted downwards, encour-
aging the formation of the Fe-Ti

-

10 -4 10 -3 10 -2 10 -1 10 0

Gra Medet, Bulgaria

Dexing, China
The Phillipines
Tucson area, USA
Ajo, USA
Bingham & Robinson, USA
Highland Valley, Canada
Butte, USA
Chuquicamata, Chile
Rio Blanco, Chile

Gra

Ton-grd

Grd-gra

Qmd-gra

Qmd

Ton-grd

Gra

-

Ti oxides.
The titanohaematites are all very close to the ilmenite
end-member (less than 10% haematite), but the titanomag-
netites have a much broader range of compositions. More
than about 30% magnetite (Mt 30 ) is required for significant
magnetism (cf. Fig. 3.39b ). Data from felsic lavas almost
entirely plot above the FMQ curve, and magnetic titano-
magnetite is predicted. Basic lavas tend to straddle the
FMQ curve, and ulvospinel-rich titanomagnetites are
expected. Although these grains may be less magnetic than
their equivalents in felsic rocks, they are more abundant,
and exsolution and oxidation to more magnetic species
may occur during cooling. Intermediate lavas plot in an
intermediate position, albeit at high temperatures. Data
from intrusives show a broadly similar pattern, albeit
shifted to lower temperatures parallel to the FMQ curve,
a result of slower cooling. Slower cooling permits the
titanomagnetite to evolve towards more iron-rich varieties,
since the isopleths are oblique to the FMQ curve. Of course
these observations are not de nitive, since a complex
chemical system has been simplified, and the rapid cooling
of extrusives may prevent equilibrium conditions being
attained. However, most points on Fig. 3.44 are above the
Mt 30 isopleth indicating that, in virtually every case, if a
titanomagnetite species occurs it will be ferromagnetic.
Even if this is not the case, subsolidus exsolution is likely
to create magnetic species.

Grd

Qmd

Gra Yangjia-Zhangzi, China

Endako, Canada
Alice Arm, Canada
Climax, USA
Log Cabin, Questa, USA
Questa Mine, Questa, USA
Red River, Questa, USA

Grd-gra
Gra

Gra
Gra
Gra
Gra

Gra Gra Seward Peninsula, USA

Cretaceous granite, Thailand
Triassic granite, Thailand
West Belt, Malaysia
East Belt, Malaysia
Xihuashan, China
Tasmania, Australia
Erzgebirge, Germany
Southwest UK & northern Portugal

Gra

Gra

Gra

Gra

Gra

Grd-gra

Gra

Gra

Ilmenite series Magnetite series

Figure 3.45 Magnetic susceptibilities of granitoids related to
porphyry copper, molybdenum and tin - tungsten deposits.

Ton - tonalite, Qmd - quartz monzodiorite, Grd - granodiorite,

and Gra granite (mostly monzogranite). Redrawn, with

permission, from Ishihara ( 1981 ).

-

basic members of the series. Other types of rhyolite are
usually paramagnetic and have very low susceptibility.
Trachyandesites and trachytes have moderate to high sus-
ceptibility comparable to, or less than, that of related alkali
basalts, whilst corresponding phonolites are weakly mag-
netic. Magnetic properties can be related to geochemistry.
For example, tholeiitic rocks having greater modal titano-
magnetite exhibit higher susceptibility than similar rocks
with lower Fe and Ti contents.
In granitoids, bimodal susceptibility ( Fig. 3.42 ) re

3.9.3.2 Magnetism and igneous rock types
The multitude of controls on rock magnetism means that it
is unwise to use magnetic measurements from igneous
rocks in one area to predict responses of similar rock types
in another. Within the same area some generalisations may
be made because magmatic characteristics, which control
the all-important partitioning of iron between silicates and
Fe

ects two
distinct series of granitoids corresponding broadly, but not
exactly, with the S- and I-type granitoids of Chappell and
White ( 1974 ) . The magnetite-series have strong magnetism
due to abundant magnetite. The ilmenite-series, correspond-
ing roughly with S-type granitoids, have lower levels of mag-
netism. A-type granites are also poorly magnetic. The
differences are due to different temperature

Ti oxides, are more likely to be similar. For example,
magnetic susceptibility generally increases with ma city
for rocks from the same area, and andesites generally have
lower or similar susceptibilities to their related basalts.
Rhyolites show a bimodal susceptibility distribution with
the ferromagnetic rhyolites less magnetic than the more

- fo 2 conditions,

cf. Fig. 3.44 . In both series, susceptibility decreases as silica
content increases. A susceptibility of about 5

-

10 - 3 SI,
corresponding with about 0.1 vol% magnetite, de nes the
class boundary ( Fig. 3.45 ). Their distinctively different sus-
ceptibilities allow the magnetite- and ilmenite-series granites