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Choco 10 - Rosika, Coacia, Pisolita, Villa Balazo, Karolina, El Callao

Venezuela

Main commodities: Au
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Deposit Description

The Choco 10 group of gold deposits are located in the El Callao district, approximately 15 km west of the historic gold mining town of El Callao in Bolivar state, 190 km south-east of Puerto Ordaz, and approximately 650 km SE of Caracas, in Venezuela.

Choco 10 and related deposits are located within the Paleoproterozoic Guasipati Greenstone Belt, some 125 km NNW of the Las Cristinas deposit which is hosted by the adjacent El Dorado-Marwani greenstone belt. The El Callao mines, also within the Guasipati Greenstone Belt, are considered to have been among the most productive gold mines in the world during the latter part of the 19th century, with more than 260 gold-bearing quartz veins known in the district.

These greenstone belts lie within the Guyana Shield, which forms the northern part of the Amazon Craton, located between the Amazon and Orinoco river basins. The Guyana Shield has been subdivided into three major geological terranes:   i). the Archaean Imataca Complex on the northern margin of the Shield in Venezuela;   ii). the Paleoproterozoic Trans-Amazonian granite-greenstone belts hosting the gold deposits, separated from the Imataca Complex by a major ENE-trending ductile shear zone, the Guri Structure; and   iii). the onlapping Paleo- to Mesoproterozoic sedimentary and igneous rocks of the Roraima Group, Uatumã Group and the Avanavero Suite to the south and south-west.

The main gold deposits of the shield are hosted by the greenstone belts; the more important of which include the Pastora Supergroup and Botanamo Group in Venezuela, the contiguous Barama-Mazaruni Groups in Guyana, the Marowijne Group in Suriname, and the Maroni Group in French Guiana.

The metavolcano-sedimentary sequences of the greenstone belts have been dated at from 2250 to 2110 Ma, while the associated granitoid complexes are dated at between 2250 and 1900 Ma. Both suites were metamorphosed, deformed and mineralised during the approximately 2200 to 1900 Ma Trans-Amazonian Orogeny which accreted various volcanic centres (the precursors to the greenstone belts) onto the Archaean palaeo-continents, such as the Imataca Complex in Venezuela. Regionally, many gold deposits in the Guyana Shield are found in close proximity to, but not on, major shear zones. Deformation and peak metamorphism of the Trans-Amazonian Orogeny was from from 2120 until 2090 Ma, with gold mineralisation being formed at from 2000 to 1950 Ma.

The Choco 10 and adjacent deposits comprise oxide and sulphide gold mineralisation hosted within the Paleoproterozoic Guasipati Greenstone Belt in the Pastora Province of the TransAmazonian. The stratigraphic sequence within the Guasipati Greenstone Belt comprises the following succession, from the stratigraphic base to top (west to east):
El Callao Formation - predominantly tholeiitic basalt flows, with local hyaloclastite, auto-breccias, massive zones and a package of mainly pillowed basalt at the base. Sub-units of in situ fractured tholeiitic basalt or flow-top breccias increase in frequency upwards. The fractures are typically filled with calcite-silica-magnetite-pyrite and jasperoidal silica-magnetite-pyrite, with frequent metre-thick lenses of exhalative jasperoidal silica at the top of each sub-unit. In addition, there are upward increasing lenses of conglomerates, breccias, epiclastics and local basaltic-arkosic sandstones;
Cicapra Formation - comprising tholeiitic volcaniclastics, generally chaotic conglomerates and breccias of sedimentary or epiclastic mass flows of basalt and local basaltic-arkosic sandstones, similar to the lenses found in the underlying unit;
Caballape Formation - comprises a lower sub-unit of volcaniclastic conglomerate and chaotic breccias of calc-alkaline andesitic to dacitic composition. This is overlain by and intercalated with fine- to medium-grained calc-alkaline andesitic to dacitic tuffs with interbedded volcaniclastic siltstones and carbonbaceous shale. These sub-units have been dated at 2143±6 and 2144±5 Ma;

This sequence represents a tholeiitic to calc-alkaline volcanic package, overlain by volcaniclastic and epiclastic rocks intruded by 2142±3 Ma gabbroic sills, which form topographic highs and probably follow faults at the base of an over-thrust blocks. All of these rocks have been metamorphosed to lower greenschist facies. Late intermediate dykes, felsic stocks and sills have intruded the greenstone-belt package and probably indicate a deep-seated intrusion that may be the cause of shearing. The trondhjemite granitic pluton/domes and associated dykes of the Supamo Complex intrude the whole sequence and form intrusive uplifts that separate branching synclinoria of the meta-sedimentary and meta-igneous greenstone-belt rock packages. The Supamo Complex comprises biotitic gneiss, migmatites and stocks of K feldspar poor diorite, trondhjemite, granodiorite and monzonite, as well as local felsic porphyritic granodiorite dykes. These greenstone and granitoid rocks are all cut by the NNE-trending doleritic Laguna Dyke. All of the currently known gold deposits in the district occur to the north of this dyke. All of these rock packages have been subjected to intense tropical weathering, resulting in the near-surface mineralisation being contained within saprolitic horizons.

The deposits of the Choco 10 gold complex are hosted by the basal portion of the Guasipati Greenstone sequence (or Pastora Supergroup), and are located in the core of a regional northeast-plunging synclinal hinge zone, characterised by a structural architecture dominated by folds and ductile fabrics. Mineralisation is dominantly structurally controlled and is associated with strain partitioning, represented by large volume low-strain zones that preserve primary features, while deformation is concentrated in high-strain zones of intense development of ductile fabrics. Ductile or brittle shear zones that accommodate large displacements are absent.

The deformation history of the Choco 10 mineralised complex comprises:
i). Early Trans-Amazonian NE-SW to E-W directed bulk shortening that produced a steep S1 fabric and rotated S0 to a moderate dip;
ii). D2 NW-SE bulk shortening resulting in NE to ENE trending folding of S0 and S1;
iii). WNW to ESE shortening during D3 which partitioned deformation into discrete zones of high-strain. During development of D3 shear zones, the S2 fabric was reactivated;
iv). Late Trans-Amazonian NE-SW shortening during D4 which produced weak, kink-like, crenulations.

The main geometric control on mineralisation at the Choco 10 was created during D2 corresponds to the S2 axial planar trend to regional folds. The D2 high-strain zones have been further divided into higher-strain ­ cleavage/limb domains where S0 and S1 have been rotated into parallelism with S2, and by lower-strain ­ microlithons/hinge domains where D1 folds are preserved as a result of strain partitioning. High-grade ore shoots are controlled by the geometry of D2 structures, including D2 fold axes and S2 strain domains which intersect favourable bedding (S0) or foliation (S1) geometry.

The gold mineralisation at the Choco 10 deposits are the result of an interplay between host rock rheology, permeability and geochemistry, proximity to structures and hydrothermal fluid composition. The mineralisation is predominantly structurally controlled, associated with strain partitioning. The higher-grade mineralisation is mostly hosted in lower strain domains comprising spaced or chaotic crenulation and folding, less commonly brecciation and cataclastites, which are coupled with and bounded by high-strain domains which contain well-developed continuous foliations. Mineralisation is generally associated with carbonate alteration, predominantly ankerite and dolomite, with varying amounts of silica, quartz-veining and sericite. Pyrite, which is present in relatively low levels of 2 to 10%, is the most abundant sulphide, correlating closely with the gold tenor. Replacement-style hydrothermal alteration and mineralisation is best developed in the more permeable host-rocks, such as coarse-grained volcaniclastics and flow-top basalt breccias, while vein-hosted mineralisation is more prevalent in the less permeable massive basalts and gabbros, commonly containing visible gold. Magnetite, that occurs as disseminations and stringers between clasts in the original flow-top breccias, is replaced by pyrite which is closely associated with gold mineralisation in the higher-grade lodes.

Gold is very fine grained and occurs in association with aggregates of limonite throughout the oxidised zone with gold particles measuring about 1 µm. In the primary zone, gold particles range in size up to a few tens of µm and occur in fractures within pyrite grains or as isolated grains in quartz.

The characteristics of the individual deposits or zones of the Choco 10 complex is as follows:

Rosika­Coacia­Pisolita - where the controls on mineralisation are related to rheological heterogeneities within a distinct stratigraphic sequence, specifically the coarse intermediate volcaniclastic, the contact between the intermediate and mafic volcaniclastic units, and the flow-top breccia unit at the top of the footwall basalt, although the entire stratigraphy (from hanging-wall gabbro, through volcaniclastics, to footwall basalt) can host economic mineralisation locally. Lithological contacts dip at between 40 and 65°E, on the northern limb of the syncline, and steepen and wrap around the nose of the syncline. Mineralisation broadly, but not strictly, follows this geometry within the most favourable units.

Mineralisation at Rosika and Coacia tends to be related to more brittle features, with abundant hydrothermal brecciation and veining and an alteration assemblage of ankerite-dolomite-silica-pyrite±sericite and 2 to 10% pyrite. At Rosika, high grade ore shoots occur at the intersection between SE-dipping S2 and east-dipping S1 foliations. At Coacia, a major ore shoot is controlled by the steeply NE plunging syncline axis. Mineralisation is predominantly associated with a stack of sub-horizontal quartz veins which were formed in the southwest continuation of the hinge zone of the Coacia syncline. At Rosika, three main lodes, the upper Rosika Main Lode, the middle NNE Cross Cutting Lode and the Footwall Basalt Lode, with grades of 2 to 5 g/t Au and thicknesses of 5 to 45 m are distributed over an up to 200 m thick zone, separated by halo of lower grade mineralisation, dipping at 30 to 60°E. Each lode varies along strike from a single thick development to thinner ribbons separated by low grade mineralisation/anomalous halos. The ore zone at Coacia is similar in character and thickness, but dips more steeply at 60 to 75°E.

Pisolita lies within the basal basaltic unit, with shallow and double plunging D2 fold axes forming open M-shaped folds in all directions with shallow and undulating dips, and mineralisation predominantly within the regolith, mainly hosted within a pisolitic laterite horizon. Where the mineralisation extends into the fresh bedrock, mineralised quartz±albite±dolomite veins are enveloped by ankerite-dolomite-sericite-pyrite alteration. To the north of Pisolita a tronjhemite intrusion occurs, with gold grades usually enhanced along the margins which represents a site of great competency contrast. The weathering profile extends to a depth of up to 50 m below the surface. Within this zone there are a three or more, relatively flat lying lode zones, each of 5 to 15 m thickness with grades of 5 to >15 g/t Au enveloped by lower grade halos. Additional, parallel, comparable grade lodes are found in the un-weathered host for up to 120 m below the lateritised lodes.

Villa Balazo ­ Karolina - where mineralisation is hosted within southeast dipping lower-strain domains which are bounded by high-strain lower grade halos. This trend parallels the axial plane of district scale folds and the general trend of the differentiated crenulation cleavage S2. The mineralised strain domains are present as a stacked series, largely due to the host basalt-dominated stratigraphy which lacks the strong fluid focus of the sequence hosting Rosika-Coacia. Where favourable the host does provides geochemical and rheological contrasts, mineralisation is enhanced at the intersection with the strain domains, particularly in the flow-top breccias. Within the ore zones, the mineralised domains comprise alternating lower- and high-strain domains. The the lower-strain mineralised lodes are dominated by ankerite-dolomite±sericite±silica with a high (typically 1 to15%) pyrite content, while the high-strain halos are predominatly calcite and chlorite altered with only minor pyrite. The highest gold grade correspond with increased silicification and pyritisation, particularly where the main domain 41 lode intersects flow-top basalt. Significant mineralisation is also found in both the footwall and hanging-wall where strain domains dip at relatively shallow angles. The mineralisation tends to pinch-out toward the surface. In schematic cross-section, the mineralised zone comprises seven or more lode zones with intersections of 5 to 50 m of 2.5 to 15 g/t Au, each embraced by a halo of lower grade mineralisation, all within a 4 to 60°SE dipping zone that is up to 150 m thick. Each lode splits and coalesces down dip, with internal lenses of lower grade mineralisation. Other, less closely spaced and less well developed lodes are known in the hangingwall.

From SW to NE respectively, the Pisolita, Coacia and Rosika zones are distributed over a 1500 m length, each separated by an interval of 100 to 200 m. Although the core of the Pisolita deposit lies to the SW of Coacia, it does extend northward to the west of Coacia towards the southern tip of Villa Balazo­Karolina. The larger Villa Balazo­Karolina zone is immediately to the NW of Rosika, separated by a narrow zone of low grade mineralisation.

Reserve and resource statistics as of 30 September, 2007 (Leader, et al., 2007) were:
      Measured resource - 2.6 Mt @ 2.86 g/t Au, for 7.5 t Au,
      Indicated resource - 56.3 Mt @ 2.45 g/t Au, for 137.8 t Au,
    Total measured + indicated resource - 58.9 Mt @ 2.47 g/t Au, for 144.8 t Au,
      Inferred resource - 42.9 Mt @ 2.19 g/t Au, for 93.8 t Au.

      Proved reserve - 2.7 Mt @ 2.70 g/t Au, for 7.28 t Au,
      Probable reserve - 15.0 Mt @ 3.32 g/t Au, for 49.6 t Au,
    Proved + probable reserve - 17.7 Mt @ 3.22 g/t Au, for 57 t Au.

This summary is largely based on Leader, et al., 2007

The most recent source geological information used to prepare this summary was dated: 2007.    
This description is a summary from published sources, the chief of which are listed below.
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