Deposit Types & Mineralization
Deposit Types
Quartz veins in the Dabolava region consist of long, linear shear or fault controlled mesothermal systems
containing iron and base metal sulphides. They are similar to vein gold systems found worldwide though
the high level of metamorphism of the host rock is a distinguishing feature. The metamorphic grade is
similar to the Homestake mine in South Dakota. Geological conditions at the Kolar Mine in Karnataka
State, India closely resemble those seen in Precambrian rocks in the Dabolava region of Madagascar.
Gold at the Kolar mine is associated with persistent, tabular gold bearing quartz veins that form ore
bodies at the intersections of cross-cutting structures. This mine would have been located less than
three hundred kilometers off the east coast of Madagascar before the Indian land mass split from the
parent sub-continent.
Copper mineralization, in the Dabolava region, is often association with amphibolitic schists as
weak disseminations, but geochemical studies done by the BRGM in 1988 encountered unexplained, strongly
anomalous copper in soils. The strong copper anomalies and untested gossans within a metamorphosed region
suggest a potential for base metal VMS (volcanic massive sulphide) with similarities to the Broken Hill
Mine in Australia.
Mineralization (Mineralized Zones on the Property)
Placer and Alluvial Gold
Stream placers and associated alluvial deposits, were worked during the early 1900's by several small
companies, producing about 2.5 tonnes of gold between 1901 and 1921. Guigues estimated that at the
time, the region was still producing 15 to 20% of the country's gold, but by 1964 only three or four
significant operations remained, and these were marginally profitable. Visual evidence indicates that
most operations focused on lag mineralization in saprolite. Mines often worked the banks of small to
medium sized streams but processed saprolite rather than gravel. Gold is related to underlying
fault-controlled mineralization.
Several attempts at large scale dredging of major rivers failed to recover commercial gold though
it is not clear why they failed.
Gold in Veins and Stringers
The Dabolava area has been known for its gold bearing quartz veins since the late 1800's. Though many
veins have been worked, controls to mineralization are not well documented. Besairie reported that
most gold bearing veins are remarkably persistence along strike and some E-W, near vertical veins
were traced continuously for >2 km. Most veins underlie lag deposits and were not well exposed by
mining. Quartz stringers are found as a series of parallel veinlets over one or two meters widths
or as distinct veins separated by several meters of barren country rock. A few veins have sub-horizontal
layered or ribboned veins. Host rocks are mainly granitic gneiss with schistose to sub-massive textures
though other rock types are also mineralized. Veins mined from magnetite bearing quartzites lack detailed
descriptions. The influence of host rock characteristics on gold grades and vein widths has not been studied.
The regions main production came from operations treating saprolites and laterites carrying lag
material derived from gold bearing veinlets and stringers (Plate 3). Several operations extended to
the enriched bedrock or several tens of meters into weathered bedrock. Very little true underground
mining occurred in consolidated bedrock. A few informal miners continue to work many of these properties,
but rarely to depths in excess of 50 meters (Plates 2 & 4).
Gold grades reported by Besairie and others, range between 10 and 50 g/t but it is not certain how
much of this relates to primary mineralization. Since old workings have collapsed, there are very few
opportunities to examine mineralization in bedrock. Recent BRGM reports indicate that veins carried
significant pyrite and occasionally arsenopyrite and sulphides often extended into the wall rocks.
Gold occurred as free grains in both the veins and altered wall rocks and pyrite also carried gold.
Vein patterns indicate that emplacement postdates the intense island-scale deformation, but the absence
of chloritic margins implies that they predated at least part of the high-grade thermal event that followed.
Structural controls to gold mineralization have not been studied, but nearly all gold mineralization
follows linear E-W fracture systems and crosscutting veins appear locally constrained. This dominant
direction controls major streams in the region and is consistent with the overall attitude of the
'Antananarivo Virgation'. Well developed but unmineralized faulting or shearing are oriented N-S
and northwest. Some of these appear to be late events associated with Cretaceous dolerite dykes but
others are describes as strongly developed mylonites that may have acted as conduits to mineralizing
fluids. Available data indicates that gold is present in high-grade east-west structures and that it
was introduced during the later stages of the latest tectonic event.
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