Title: Minérios de ferro de Goa (Índia Portuguesa)
Abstract: The deposits of iron ores of Goa consist of outcrops spread out in strata towards NW-SE. They are related to folded formations of the Dharwar system, and consist of outcrops of the more important folds.
In this region the relief is generally direct and the zones of exploitation are located in the anthiclinoriums.
At the surface of the deposits there is in many places a lateritic crust (laterite ore) 1 to 8 m thick. Below, and cropping out in several places, are banded layers of coherent and porous martite ore 10 to 30 m
thick. This ore changes deep below to banded layers of friable ore, with the same mineralogical composition, 5 to 8 m visible thick.
It is most likely that in several deposits, below the friable martite ore, there may be other layers of ore, mainly of quartz gangue (martite quartzite with specularite). This quartzite is seen in pit n° 1 at Lamgao de Bicholim.
Detrital lateritic ore is found in the the slopes.
At the Goa iron deposits there are hundreds of millions of tons of reserves of ores.
The martite ore is stratified, porous, and consists of idioblasts of magnetite which are partially or totally martitized. It changes by weathering to goethite and limonite. Rare maghemite is changed from magnetite.
Also accessorily present are some quartz granoblasts and rare granoblasts of albite. This ore has a high proportion of Fe (57 to 70 %), with SiO2 rarely over 5%; S is practically absent and the proportion of P is generally very low.
The friable martite ore («blue dust») is spread, such as the coherent one, in thin porous banded layers. It consists essentially of magnetite crystals partially or totally martitized. It changes by weathering to goethite and limonite. This ore is rich in Fe, non phosphorous, S is practically absent, and there is a very low percentage of SiO2 (generally lower than 5%).
It is mineralogically and chemically similar to the coherent martite ore.
The martite ore of quartz gangue is a martitic quartzite with specularite. It shows a clear stratification. It consists essentially of quartz granoblasts, magnetite idioblasts partially or totally martitized, fine specularite idioblasts and accessorily of some albite and apatite granoblasts.
It changes by weathering to limonite and goethite and, rarely, to maghemite. It is medium in Fe, siliceous, with very little P, and S is practically absent. It can be concentrated.
The laterite ore is vacuolar with a colloformic structure. It consists essentially of goethite and limonite and accessorily of hematite and quartz. The dark brown material of the walls of vacuoles and channellings consists essentially of goethite, limonite and some hematite; the red-wine coloured mass filling the vacuoles and channellings consists of hematite and gibbsite, and ocasionally of goethite and limonite.
The yellow-brown or whitish-yellow material either filling some vacuoles and channelings or found in the red-wine coloured mass consists of gibbsite and goethite, gibbsite being predominant. This is an ore rich in Fe, non-phosphorous, with very little S and a very small percentage of Si02.
Pockets of bauxite (gibbsite, accessorily with kaolinite and goethite) are occasionally found in the laterite ore.
The detrital lateritic ore consists of gravels and grains of coherent martite ore, of martite quartzite and of laterite ore cemented by vacuolar goethite-limonite with or without gibbsite.
Thin layers of serecite-chlorite schist, which are interstratified in the ore, have dominantly evolved towards kaolinite whereas quartz became soluble.
We wish to stress the similarity of the Goanese ores with other ores, mainly of India and Brazil.
The layers of coherent and friable martite ores would represent in most cases the residue of layers of martite quartzite with specularite of the possible Dharwar system.
We are aiming to explain the genesis of martite quartzite with specularite (martite ore of quartz gangue) of Goa : colloidal suspensions of pH between 4 and 6; coagulation and sedimentation of Fe hydroxide and SiO2; evolution of the whole through regional metamorphism towards quartzite rich in magnetite; later martitization of magnetite and precipitation of specularite through the possible action of granitizing hydrothermal fluids.
The large deposits of coherent and friable martite ores of Goa may have resulted from that martite quartzite through removal of quartz and other impurities by dissolution, by infiltrating surface waters
(pH >> 7) under tropical climatal conditions of the monsoon.
In other places the same type of ore may be formed from other rocks, such as amphibolites. The feldspaths are hydrolized and the quartz became soluble; the martitized magnetite (martite) remains as residue.
There is an identical structure between the coherent martite ore and the friable martite ore. The latter is found in the present zone of deep and active circulation of the infiltrating surface waters.
The friable ore above the hydrostatical level would aggregate due to the weight of the layers and some reprecipitation of ferriferous minerals and become coherent. Both types of ore would maintain in their general or detailed aspects the folds present in martite quartzite.
The crust of laterite ore resulted through supergene processes from coherent martite ore. The removability of some Fe towards water with acid properties after the monsoon resulted in the pores of the ore and in the formation of channellings. The increase of pH permitted the precipitation of ferrigel whence goethite and limonite were produced.
A little limonite may have evolved due to loss of water towards hematite. Through increase of pH some alumogel precipitated in the channellings and evolved towards gibbsite.
Detrital laterite was formed as slope deposit. Among the detritus of other types of iron ores ferrigel precipitated and evolved towards goethite and limonite which cemented the detritus.
We emphasize at the end of this work the great economic and industrial value of the iron ores of Goa and the possible interest of the installation of a steel industry in this territory.
Publication Year: 1961
Publication Date: 1961-01-01
Language: en
Type: article
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