Las Cristinas - Km88, El Dorado
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The Las Cristinas deposit is located ~750 km SE of Caracas in eastern Venezuela, close to the Guyana border (#Location: 6° 12' 4"N, 61° 27' 56"W).
The deposit comprises a disseminated, stringer and vein gold-copper deposit, developed within the Botanamo (or El Dorado-Marwani) greenstone belt of the Guyana Shield in eastern Venezuela. It falls within the Km 88 or El Dorado district, near the junction of two deep seated crustal fractures with NE and NW trends respectively.
The initial discovery and mining of gold in the km 88 district was most likely by British companies, over a short period of several years, at the beginning
of the twentieth century. However, mining in the district was apparently not reactivated until the 1980s when several thousands of itinerant miners (garimpeiros), located and surficially mined numerous gold-rich lateritic and saprolitic deposits. In addition, a significant quantity of gold was also recovered from the alluvial sediments of major streams such as the so-called Quebrada Amarilla. Systematic exploration and evaluation programs carried
out by Placer Dome de Venezuela revealed the presence at depth of significant primary gold-copper bearing sulphide mineralisation (Bernasconi 1999).
The geology of the El Dorado - Km 88 District may be summarised as follows:
• Palaeoproterozoic Carichapo Group of the Botanamo greenstone belt - intermediate meta-lavas and pyroclastics comprising andesite, andesite porphyry and lapilli tuffs, with associated chlorite and sericite schists believed to represent tuffaceous sediments. Regionally this group comprises amphibolitic schists, epidote rich rocks, basic tuffs and other metavolcanics and metasediments (sandstones) and greywackes.
• Paleoproterozoic intrusive granitoids of the Supamo Complex - biotitic gneiss, migmatites and stocks of K feldspar poor diorite, trondhjemite, granodiorite and monzonite, as well as local felsic porphyritic granodiorite dykes. This complex may have been emplaced as part of a Transamazonian age tectono-thermal event.
• Palaeo- to Mesoproterozoic Roraima Supergroup rocks. These include largely flat lying sequences of:
i). metasedimentary oligomictic conglomerate, breccia, quartzite, subgreywacke, phyllite, and locally, felsic tuff;
ii). hematitic quartzarenite, reddish phyllitic siltstone, feldspathic arenite, conglomeratic arenite, quartz-sericite phyllite, and red laminated fine-grained meta-arenite; and
iii). a major postorogenic magmatic episode comprising volcanic and plutonic rocks of calc-alkaline composition composed of widespread crystal- and lithic-rich rhyodacitic to rhyolitic ash-flow tuff and dacitic and andesitic lava flows, intruded by andesitic to basaltic dykes and rhyodacitic granophyre.
• Mesozoic intrusions of sills, dykes and plugs of dolerite (diabase) and gabbro.
These rocks have been folded about NE-SW trending fold axes. The mineralised area lies along and its distribution and geometry is controlled by a major NW trending, deep seated, brittle-ductile composite shear zone, principally within the metamorphosed and altered tuffs and pyroclastics.
According to the USGS (Bull 2062, 1993) the Las Cristinas deposit, which is in the central part of the Kilometro 88 area, is predominantly underlain by strongly weathered felsic to intermediate metavolcanic and metasedimentary volcanic-derived greywacke, locally metamorphosed to schist of the Palaeoproterozoic greenstone sequence. These rocks include strongly foliated massive pyroclastic deposits, including debris flows, breccia, pumice-rich tuff, thinly bedded tuff and volcanic sandstone. The foliation dominantly trends NNW and dips at 50°N to 80°SW. This volcanic sequence is intruded by quartz diorite, granodiorite and intermediate porphyry stocks that crosscut foliation and define a NNE trending corridor ~850 m wide. The margins of the stocks are commonly altered and brecciated and locally contain stockwork veining with mainly associated iron oxide minerals. Veins in the weathered metavolcanic rocks comprise a gangue assemblage of hematite (with or without goethite) and quartz (with or without free gold). Less weathered impermeable granitic stocks are variously altered to sericite, tourmaline and argillic assemblages. Sulphide-bearing veins, mainly containing pyrite and chalcopyrite, extend below the saprolite zone into unweathered lithologies. Mineralised samples of sericite-quartz schist contain an assemblage of pyrite, chalcopyrite and scattered covellite, whilst more quartz rich schists contain the assemblage pyrite and molybdenite, with or without chalcopyrite.
Below the weathered zone, primary gold and copper sulphide mineralisation (pyrite and chalcopyrite with lesser covellite and molybdenite, and specks of gold) occurs as disseminations and stringers. Four types of mineralisation have been recognised, namely: i). tourmaline and copper rich breccias, ii). sulphide (pyrite) bearing lodes, iii). disseminations and stringers of sulphide, and iv). quartz veins.
The mineralising process is described as being a multistage event involving an early high temperature phase of brecciation and tourmalinisation (both as replacement and veining), followed by a phase comprising the introduction of Au-Cu bearing and silica rich hydrothermal fluids, possibly with additional tourmalinisation. The final phase has decreased copper, silica and tourmaline and comprises gold bearing pyrite stringers and disseminations. Most of the hydrothermal mineralisation and alteration halos represent carbonate alteration, silicification and propylitisation superimposed on a greenschist facies regional metamorphic assemblage. The high temperature tourmalinisation is best developed in the higher gold grade Mesones-Sofia part of the deposit.
According to Bernasconi (1999), igneous activity related to this mineralisation may belong to either the Palaeoproterozoic Supamo Complex, intrusive phases of the Roraima Supergroup or even the Cretaceous mafic suites found in the area.
Reserves/resources in the two main deposits in 1999, as delineated by Placer Dome de Venezuela, were (Bernasconi 1999):
Conductora-Cuatro Muertos - 255.4 Mt @ 1.16 g/t Au, 0.12% Cu
Mesones-Sofia - 14.3 Mt @ 1.67 g/t Au, 0.437% Cu
Due to the insecurity of title to the deposit, it had not been developed by 2017.
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1999.
Record last updated: 21/1/2017
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Bernasconi A 1999 - The geology of Las Cristinas gold deposit - Km 88, Venezuela: in Global Tectonics and Metallogeny v7 pp 91-94|
Rodriguez S E 1997 - Tectonic setting of the large Au deposits of the Km 88 area, Bolivar State, southern Venezuela: in Global Tectonics and Metallogeny v6 pp 147-150|
Voicu G, Bardoux M and Stevenson R 2001 - Lithostratigraphy, geochronology and gold metallogeny in the northern Guiana Shield, South America: a review : in Ore Geology Reviews v18 pp 211-236|
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