Summary:
Deposit type - Lateritic titanium accumulation
During chemical weathering and leaching of rocks, constituent elements are dissolved and transported away in approximate order of their solubility. The alkali (Na2O, K2O) and alkaline earth elements (MgO, CaO) are lost early in the process. Silica is also relatively soluble, but due to its abundance, tends to remain longer in the weathering profile. Aluminum and iron are relatively insoluble, remaining in the profile as the weathering products kaolinite clay and goethite, although kaolinite breaks down slowly as the silica is lost. Least soluble among the common elements is titanium, which gradually increases in relative abundance as the other elements are lost.
At Alto Paraná the lateritic weathering products consist of kaolinite, goethite (hydrated iron oxide), ilmenite and titanomagnetite. The original volcanic glass, plagioclase, pyroxenes, and amphiboles have completely weathered and, in the laterite layer at least, no traces of the original rock textures remain. In places there are some quartz fragments in the laterite, probably originating from quartz in diorite intrusions or silicious vesicle-fill in the basalts.
Within the saprolite layer underlying the laterite, weathering is incomplete and some of the original rock textures remain. This horizon may contain other, less weathered clay minerals (chlorite, smectite, sericite) as well as elevated MgO and CaO.
The most common primary titanium mineral is ilmenite, which contains a high percentage of iron in the ferrous state (primary ilmenite contains approximately 50% TiO2 and 50% FeO). During weathering, ilmenite undergoes oxidation, and the iron is progressively converted to the ferric state, requiring changes to the crystal structure and the loss of some of the Fe to maintain a neutral covalent state. Weathered ilmenite often contains complex interior crystal structures such as alternating lamellae or ‘blebs’ of iron-rich and Ti-rich crystal forms, however, such structures appear to be mostly absent at Alto Paraná with ilmenite grains showing little internal structure.
In basalts with high TiO2 levels such as parts of the Alto Paraná Formation, it is common for some of the TiO2 to be bound up with magnetite, forming titanomagnetite. This mineral also undergoes changes during cooling and weathering, where the magnetite progressively becomes titaniferous hematite (titanomaghemite - see chart below).
At Alto Paraná, the grainsize of the source rock is important in determining the amount of recoverable TiO2 in the laterite. The ilmenite and to a lesser extent titanomagnetite particles need to be large enough to be separated, concentrated and processed. The lower grainsize limit for processing is between 38 and 63µm, as grains smaller than this are difficult to separate from the slimes, difficult to clean and are subject to loss during processing and smelting.
Mineralization Deposit/Exploration Model
Titanium-enriched magmas that appeared late in the eruptive cycle of the Alto Paraná Formation in the Paraná Basin were largely deposited in high-titanium basalt flows in the northern portion of the basin (Bacha et al, 2022). In the southern portion of the Paraná Basin, in eastern Paraguay, the older low-titanium basalts predominate. However, the latestage titanium-rich magmas forced their way through these older basalts forming dikes and sills, which cooled more slowly and therefore have coarser grainsize than the surrounding basalts.
Lateritic weathering then obliterated the earlier rock structures and textures near the surface, removing much of the original volume of the rock. Where the source rock was lowtitanium basalt, the resulting barren laterite is red/brown kaolinitic silt/clay with little or no sand-sized particles. Siliceous vesicles are present in the basalt flows and are also preserved in the laterite as silica-cemented lumps and coarse quartz. Ilmenite and titanomagnetite are present in small amounts, but with very fine grainsizes. Whole rock TiO2 values of these basalt-derived laterites are typically less than 6%.
Mineralization
Titanium is present in three different phases in the Alto Paraná laterites:
- ilmenite
- titanomagnetite
- groundmass or ilmenite and titanomagnetite finer than 45µm
Ilmenite
lmenite is present as fine to medium grains. QEMScan sizing analyses of the bulk sample BSE204 show the ilmenite is clearly finer-grained and better sorted than the titanomagnetite, with a D50 of about 100µm.
In comparison to titanomagnetite in the laterite layer, ilmenite has a relatively uniform composition and interior structure. This is rare for weathered ilmenite, as ilmenite in heavy mineral deposits typically exhibits zoning, lamellae and exsolution rims.
Titanomagnetite
Titanomagnetite is the second-most important titanium-bearing mineral at Alto Paraná. As noted in the previous section, it is coarser than the ilmenite, poorly sorted and more angular. The titanomagnetite D50 measured in the QEMScan sizing analysis is approximately 230µm. Another notable feature of the titanomagnetite grains are the cavities on many of the grains and crystals faces - reminiscent of ‘hopper’ crystals with cavities resulting from the crystal growing faster at the edges of each face than at the center.
Magnetic response of titanomagnetite
Generally, titanomagnetite can be separated from ilmenite using low intensity magnetic separation (LIMS). However, in some areas the titanomagnetite has reduced magnetic susceptibility, presumably due to the magnetite converting to hematite. From the work completed to date, the areas where titanomagnetite has reduced magnetism are near the tops of hills, possibly reflecting more lengthy oxidizing weathering conditions.
The loss of magnetism in the titanomagnetite in some zones makes it more difficult to separate ilmenite and titanomagnetite in the ‘standard’ way with magnetic separation, because their magnetic response is similar. However, testing of a short duration reducing roast showed that the magnetism of the titanomagnetite can be easily restored. As well, there are grainsize differences that can also be used to separate the minerals.
Block A mineralization
Block A mineralization has been tested in detail in an area approximately 5km north of Minga Pora, extending for about 3km east-west and 4km north-south. Mineralization is hosted in the near-surface soil horizon and the underlying laterite horizon.
Mineralization is apparently controlled by two different source rocks in the tested area:
- A lower grade, probably basaltic source under the main hill in the test area. The laterite in this area is relatively thick (8–10 meters), but the TiO2 content is generally low (below 6.4%) and gives a low yield of ilmenite when processed.
- At least one type of higher grade source, probably a high-TiO2 gabbro or gabbro-diorite, that, at least in the detailed tested area and nearby areas, lies under some of the drainage channels and the nearby lower and middle flanks of some of the hills. Laterite in these areas is generally thinner than lower-grade zone, averaging about 7 meters thickness. Most of the laterite in the channel areas (bosques) has been eroded, leaving little or no potentially mineable material in these zones.
Block E1 mineralization
Drilling in 2022 targeted a zone of mineralization towards the north of Block E1. The mineralized laterite horizon lies around a low hill forming a zone 3.3km long (east-west), and 1.4km wide (north-south) and averaging 7.5 meters of depth. In this area it is likely that the underlying source rock is a gabbro or gabbro-diorite with high TiO2 . The mineralized laterite consists of red-brown iron-stained kaolinite with sand-sized ilmenite and titanomagnetite.