Alto Chicama

Other Names: Las Lagunas Norte, Alto la Bandera, Tres Cruces, Laguna West
District: Peruvian Andes
Commodities :   Gold

The Alto Chicama Project and the Lagunas Norte high-sulphidation epithermal gold deposit, are located in the Quiruvilca district, Santiago de Chuco Province, in Northern Peru, roughly half way between the Yanacocha and Pierina deposits. It is approximately 90 kilometres east of the coastal town of Trujillo. 

The Alto Chicama District is part of a 290 km long gold-rich belt containing world class deposits such as the Yanacocha mine in the north, and the Pierina mine to the south in central to northern Peru. This gold rich belt is located in the western Cordillera of the Peruvian Andes, between the Coastal Batholith to the west, and the Marañon fold and thrust belt to the east. 

The regional geological setting in North Peru comprises:  i) the Proterozoic metamorphic terrane intruded by Palaeozoic granitoids of the Marañon Complex to the east; and  ii) the Mesozoic to Early Tertiary Coastal Batholith to the west; separated by  iii) a NNW trending basin, predominantly filled by Upper Jurassic and Lower Cretaceous marine and deltaic clastic sediments of the Chicama and Chimu Formations. 

The Cretaceous sequence is unconformably overlain by stratified volcanic and volcaniclastic rocks of the Calipuy Group.   Most of the gold occurrences of northern Peru, including Yanacocha and Pierina are hosted by the Chimu and Calipuy formations.   NNW trending Paleogene and Neogene units, predominantly composed of intrusive rocks, are well exposed to the east of the Pierina mine, in the Cordillera Blanca. 

The Lagunas Norte epithermal gold deposit is hosted at the contact between the Cretaceous Chimu Formation sedimentary rocks and the Tertiary Calipuy Group volcanic rocks. 

The Cretaceous Chimu Formation sediments are characterised by thick sandstone beds intercalated with fine-grained siltstones and mudstones, locally grading into coal beds, all of which were folded by the Andean orogenesis. 

The Tertiary Calipuy Group volcanic pile has a restricted thickness and is characterised by a cluster of volcanic vents related to phreato-magmatic activity, which is believed to be closely related to the gold mineralisation. 

The exposed mineralisation is surrounded by extensive areas of post-mineral volcanic cover which may hide potential blind satellite deposits. 

In plan, the Lagunas Norte orebody has a 2x1 km, elliptical, NNW elongated shape which is coincident with the regional structural lineaments, folds and the regional stratification of the Mesozoic sedimentary hosts which have also localised other gold bearing magmatic and hydrothermal centres in the region.   Over 75% of the ore body outcrops and is exposed in its northern portion, while to the south it dips below post-mineral volcanics. 

The exposed mineralisation extends to a depth of at least 200 metres vertically, occuring as near horizontal tabular bodies, although at depths of 300 to 350 m below surface, both west and east-dipping mineralised structures have been intersected, interpreted to represent feeder structures to the main ore body. 

Gold mineralisation is disseminated, following both structural and stratigraphic controls.   Economic grades accompany broad zones of vuggy residual silica within the Tertiary volcanics, and irregular zones of fracturing, brecciation, and silicification in the underlying Cretaceous sediments.   Primary gold is contained within pyrite, iron-oxides, pyrite/quartz and iron-oxides/quartz, primarily contained within the pyrite and iron-oxides. 

The hydrothermal alteration pattern is characterised by a central core of silica surrounded by advanced argillic alteration grading outwards to clays and a propylitic zone. 

At the end of 2002, on the basis of 120 000 metres of diamond drilling, in 445 diamond drill holes, on a 50 metre centres grid, the declared resource at Lagunas Norte was:
    123.5 Mt @ 1.83 g/t Au, for 226 tonnes (7.3 Moz) of gold, including 192 tonnes (6.2 Moz) in oxides and 34 tonnes of gold in sulphides. 

At December 31, 2010, reserve and resource figures were (Barrick Gold Annual Report 2010):
    Proven + probable reserves - 218 Mt @ 0.95 g/t Au, for 205 tonnes (6.62 Moz) of gold;
    Measured resource - 1.23 Mt @ 0.61 g/t Au, for 0.75 tonnes (0.024 Moz) of gold;
    Indicateded resource - 40.92 Mt @ 0.58 g/t Au, for 22.8 tonnes (0.732 Moz) of gold;
    Inferred resource - 8.27 Mt @ 0.46 g/t Au, for 3.64 tonnes (0.117 Moz) of gold. 
 

(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 2010)

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