Abstract

The Masinloc mine, the world's largest producer of refractory-grade chrome ore, is at Goto, Zambales, Luzon, Philippine Islands. Ore production from 1946 through 1952 was 1,605,867.6 long tons, containing 32.33 per cent Cr2O3. 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 inclined 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 margins. 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 gabbro. 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 60–80 per cent chromite, as corroded multigranular pieces, in a dunite serpentine matrix. Dunite 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 narrow 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. Disseminated 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 gabbro form the upper zones of the layered complex, and their pseudostratification and gneissosity originated through crystal sedimentation often interrupted.

The dense chromite bodies are regarded as xenoliths that have been borne upward in intrusive saxonite. 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 glomeroporphyritic 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 ultramafic 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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