GEOLOGY AND PETROLOGY OF THE MASINLOC CHROMITE
DEPOSIT, ZAMBALES, LUZON, PHILIPPINE ISLANDS

 


W. C. STOLL

 

 

 

ABSTRACT

 
The Masinloc mine, the world’s largest producer of refractory-grade chrome ore,
is at Coto, Zambales, Luzon, Philippine Islands. Ore production from 1946 through
1952 was 1,605,867.6 long tons, containing 32.33 per cent Cr2O2. The ore occurs with
other chromite deposits in a layered ultramafic complex intruded by microdiorite and
similar dikes and overlapped by Miocene sediments. Near the mine, the complex is
composed of pseudostratified gneissose norite and olivine gabbro- and massive saxonite
serpentine and dunite serpentine. Dunite intrudes olivine gabbro; saxonite may also
be intrusive.


The chrome deposits form a belt 8000 feet long, which lies in saxonite near its
contact with olivine gabbro. The largest body is roughly oval and lies slightly in-
clined at the topographic surface; it is about 1800 feet long, 950 feet wide, and as
much as 175 feet thick. It is composed of dense and disseminated ores with small
quantities of associated dunite, melano-troctolite, and olivine gabbro.
Dense ore forms most of the deposit as well as fragments in dunite at the mar-
gins. It is a mosaic of chromite grains, interpreted as a compact mass of clusters,
with about 10 per cent interstitial gangue, partly dunite serpentine and partly olivine
eabbro. The chromite is fractured and in places anisotropic because of strain. It
contains rare poikilitic enclosures of euhedral olivine and is corroded in contact with
the silicates. Olivine gabbro gangue shows microscopic zonal distribution of minerals
in the ore interstices.


Disseminated ore occurs as a selvage, dike, and marginal fragments, but mostly
as large bodies with flowage banding, adjacent to dense ore. It is 68-80 per cent
chromite, as corroded multigrainular pieces, in a dunite serpentine matrix. Dunitie is
found mainly as a shell at the dense ore contacts. Dark troctolite forms segregations,
pockets, and dikes, and olivine gabbro, similar to that in ore interstices, forms marrow
dikes cutting ores and other rocks. The field relations show that dense chromite is
the oldest and olivine gabbro the youngest component of the ore deposit. Dissemi-
nated ore, dunite, and melano-troctolite are probably almost contemporaneous.
Dense chromite has the formula Cr41Al54(Mg74), similar to Caribbean ores, and
disseminated chromite has nearly the same composition.

 

Norite and olivine gabbron form the upper zones of the layered complex, and their
pseudostratincation and gneissosity originated through crystal sedimentation often
interrupted.

The dense chromite bodies are regarded as xenoliths that have been borne upward
in intrusive saronite. Disseminated ore and associated dunite probably originated
through corrosion and disruption of dense ore by peridotite and they accumulated
next to the dense masses after movement had ceased.

The high Al and Mg contents of the chromite, as well as its reactions with olivine
gabbro and peridotite, which tended everywhere to the generation of the pyroxene
free types troctolite and dunite, indicate that chromite crystallized from troctolitic
magma. The ore composition and texture seem best explained by Bowen’s hypothesis
of the genesis of chromite: that spinel formed first through transformation of glomero-
porphyritic clusters of olivine and basic plagioclase in reaction with magma saturated
with forsterite, and that with falling temperature spinel was converted to chromite,
olivine, and plagioclase. These, together with pyroxene, settled to form a compact
deposit of clusters, beyond reach of liquid sufficient to resorb them.

Such deposits may have formed in the transitional zone of the complex. This
and the subjacent ultramatic zones may have been subsequently heated and disrupted
incidental to orogeny, and peridotite masses with solid inclusions of chrome ore
irrupted into higher levels of the complex. The temperatures then prevailing were
high enough to fuse the olivine-gabbro ore matrix and commence anew the reaction
between chromite and liquid.

 

 

 

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