Eyebrow-raising trench results, favourable geology and limited but enticing historic drill intercepts have whetted
Channel samples from a newly excavated trench in the Trapia 1 West zone yielded 84 metres grading 2.8 grams of combined platinum and palladium (Pt+Pd) per tonne. This included a 51-metre section that ran 4.4 grams Pt+Pd. The highest assay cut 4.1 metres averaging 14.5 grams Pt+Pd.
Altoro has $700,000 in its coffers and, beginning in mid-November, will carry out 1,000 metres of drilling, combined with surface work, to define additional shallow drill targets.
“What is required is extensive aerial photography and geological mapping over the entire complex,” Altoro President David O’Connor told The Northern Miner during a recent site visit. “Our initial drilling will give us information on metallurgy and the style of mineralization, as well as confirm previous drill results.”
Pedra Branca is in central-western Ceara state in an area known as “Land of the Sun.” The region is 6 degrees south of the equator, where the climate is semi-arid and temperatures average 26C on the coast and 33C in the interior.
The main platinum and palladium showings — Trapia, Ipueiras and Esbarro — all lie within 15 km of this small town of Capitao Mor, which is a 3-hour drive southwest of Fortaleza, the state capital. The entire project comprises 80 sq. km of low rolling hills and poor-quality grazing land.
Altoro can earn up to a 70% interest in Pedra Branca from
The company can also acquire an additional 30 sq. km in the Pedra Branca ultramafic complex. In total, Altoro expects to have acquired a 25,000-ha position in this northerly trending U-shaped body, which has a strike length of 30 km. The western limb dips gently toward the east and hosts all the known mineral occurrences, whereas the eastern limb dips to the west and is largely unexplored. The northern portion of the western limb curls toward the east and dips gently towards the south.
Altoro’s land package incorporates all the known chromitite mineral occurrences discovered to date. (A rock composed dominantly of chromite is referred to as a chromitite.)
Platinum and palladium mineralization is associated with chromitite bands and with disseminated sulphide zones in the Pedra Branca ultramafic complex. The chromitites contain, in rock samples, values of up to 57 grams per tonne platinum plus palladium. The sulphide zones grade between 2 and 10 grams platinum-palladium and are associated with elevated copper, nickel and cobalt values.
Platinum group element (PGE) deposits in layered intrusions are classified into two groups: those that are primarily associated with sulphides and those that have a dominant association with chromitite. The latter group includes deposits of the Middle and Upper Group chromitites of the Bushveld complex in South Africa, as well as the “A” chromitite of the Stillwater complex in Montana.
At Pedra Branca, platinum group metals (PGMs) exist in layered mafic intrusive rocks associated with chromitite. The layering is believed to be the result of segregation and gravity settling of sequential pulses of fresh magma. As individual layers cool, they crystallize certain minerals — a process known as fractionation. If a fresh pulse of magma passes through and mixes with resident magma that has already undergone significant fractionation, the resulting mixture may become oversaturated in sulphides. Dense sulphide droplets settle out of the layer towards a lower-density section of the intrusion. PGMs tend to segregate and are subsequently concentrated in the settling sulphides. Depending on the chemistry of the two mixing magmas, chromite may also be deposited as layers with the PGM-sulphide material.
The most northerly chromitite occurrence at Pedra Branca is a pair of showings known as Esbarro 1 and 2, which lie within 400 metres of each other in an area 15 km northeast of Capitao Mor.
Tremolite schists, olivinites and serpentinites (all of which host chromitites) dip gently to the south at Esbarro. The surface is strewn with massive chromitite rubble derived from these chromitite layers.
In 1987, Rio Tinto (rpt-n) explored for nickel by drilling 42 shallow diamond drill holes into Esbarro 1 and 2. Highlights included hole 17, which intersected 32 metres averaging 1.15 grams Pt+Pd, including a 5-metre section of 6.27 grams; hole 35, which cut 35 metres of 1.52 grams, including 3 metres of 9.44 grams; and hole 26, which returned 23 metres of 2.6 grams, including 5 metres of 9.6 grams.
Preliminary exploration by Altoro indicates that numerous ultramafic outcrops unexplored by Rio Tinto host platinum and palladium mineralization. Three recently excavated trenches returned 1 gram Pt+Pd over 13 metres, 1.3 grams over 14.6 metres, and 0.9 gram over 11.3 metres.
The junior also tested six old shafts, with results ranging from 0.6 gram Pt+Pd over 1.5 metres to 10.6 grams over 2.3 metres.
Altoro plans to drill 5-6 holes into the Esbarro showings to verify previous work and test extensions of the host rock along strike, as well as downdip.
The Trapia 1 and Trapia 1 West showings are 5 km southeast of Capitao Mor, whereas Trapia 2 is 4 km to the east. This area was explored by Gencor of South Africa in the mid- to late 1980s. Then, at the end of the decade, the company abandoned all of its activities in South America.
Gencor had carried out soil-sampling, geophysical surveys, trenching and a shallow diamond drilling at Trapia 1 and 2.
Trapia 1 features limited outcropping, though previous channel sampling yielded up to 1.72 grams Pt+Pd over 44 metres. One hole drilled by Gencor, No. 18, tested this site and intersected 11.3 metres averaging 4.37 grams Pt+Pd, as well as 3.29 metres of 9.78 grams. Altoro plans to collar a hole downdip of hole 18 to determine if the high-grade intercepts continue at depth.
At the Trapia 2, outcropping chromitite horizons have been exposed by trenching. Gencor’s hole 20 intersected 7.8 metres averaging 2.67 grams Pt+Pd, including 1.26 metres of 4.87 grams.
Based on its limited drilling at Trapia 1 and 2, Gencor estimated a platinum-palladium ratio of 0.3-to-1 down to a maximum depth of 70 metres.
The Trapia 1 West prospect is an ultramafic outcrop traceable 700 metres along a north-south strike length with a width of up to 150 metres. The general trend of the stratigraphy strikes toward the northeast; however, chromitite layers strike east-west, suggesting local folding. Early trenching by Gencor identified broad zones of mineralization along an east-west trend. Results averaged 1.85 grams Pt+Pd over 122.7 metres. Altoro excavated a north-south trench perpendicular to the chromite layers, with assays averaging 2.8 grams Pt+Pd over 84 metres, including 51 metres of 4.4 grams.
Trapia 1 West was discovered toward the end of the Gencor program and was not drill-tested. Altoro plans to drill 3-4 holes.
The third main platinum and palladium showing, Ipueiras, 15 km southeast of Capitao Mor, has yet to be explored. However, analyses of three chip samples yielded values of 2.9, 4.2 and 6.5 grams Pt+Pd. Altoro has targeted the site for systematic mapping, trenching and sampling. Following the initial ground work, the company intends to drill a couple of test holes.
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