Page 101 - Petrology of Sedimentary Rocks

Shapes of heavy minerals are sensitive indicators to the intensity of abrasion. In
comparing beach vs. river sands by grain shape, it is much more efficient to use soft
heavy minerals like kyanite or amphiboles rather than hard quartz, as they are affected
much more quickly. Angular and rounded tourmaline in the same specimen indicates a
multiple source; as would, for example, angular hornblende and round yet harder
tourmaline.
Heavy minerals are generally studied in terms of four groups: Opaques, Micas,
UI tra-stables, Metastables.

I. Opaque minerals. These generally have very high specific gravity, because
of their iron content. Little has been done with them so far. Ore
microscopy, in polished sections, may reveal important information, how-
ever.

a. Magnetite and Ilmenite. May form placers of economic value. Very
difficult to tell apart except magnetically; both moderately stable, but
magnetite may alter to hematite or limonite and ilmenite fairly
commonly alters to chalky-looking leucoxene or even fairly large
crystals of sphene, anatase, or other titanium minerals. Magnetite
often alters to specular hematite, which can not be identified in the
microscope; magnetic or X-ray tests must be used.

b. Pyrite is nearly always authigenic; thus occurs in great amounts in
some heavy mineral slides, is absent in most others.

C. Hematite and Limonite are usually alteration products but sometimes
may be detrital. Both dissolve in reducing environments.

d. Leucoxene is an aggregate of extremely fine-grained sphene, rutile or
anatase, and forms as an alteration product usually after ilmenite.
2. Micas. Percentages unreliable because they do not always sink in bromo-
form. Commonly not counted in heavily mineral studies because of their
widely different shape, hence different hydraulic behavior.

3. UI tra-S table Group. Zircon, tourmaline, and rutile. Because the first two
are very hard and inert, (even more so than quartz) they can survive many
reworkings. When older sediments are reworked to form younger ones,
zircon and tourmaline are about the only ones that can survive. Also, in
supermature rocks they are about the only ones that can withstand such
prolonged abrasion. Hence they are the backbone of many heavy mineral
suites. An abundance of tourmal ine and zircon in a heavy suite then means
either (I) prolonged abrasion and/or chemical attack has occurred, or (2) the
minerals are being reworked from older sediments. The beautiful thing
about both tourmaline and zircon is that they are poly-varietal; over 30,000
types of zircon are possible based on color, crystal form, elongation, and
inclusions; and tourmaline (Krynine) has been divided into several dozen
types each believed to be diagnostic of certain source--e.g. pegmatites,
schists, aranites, etc. Both tourmaline and zircon are excellent correlation
indicators, and idiomorphic zircon is an indicator of volcanism (Callender).

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