Santa Cruz, Argentina
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The small, high grade, intermediate-sulphidation Martha silver-gold deposit is located in Santa Cruz Province, southern Patagonia, Argentina, ~160 km NW of Puerto San Julián on the Atlantic coast and 360 km SSW of Comodoro Rivadavia (#Location: 48° 41' 41"S, 69° 41' 44"W).
Regional exploration reconnaissance conducted by Yamana Inc. in 1997 led to the discovery of the Martha Vein which was up to 4.7 m thick with assays of up to 6.9 g/t Au and 5200 g/t Ag. The drill testing that ensued from January 1998, resulted in an initial Mineral Resource estimation in late 1999. The mineral rights were subsequently transferred to Compañía Minera Polimet S.A (Polimet), a wholly owned subsidiary of Yamana. Mine development started in October 2000 and production commenced from shallow surface pits late in the same year. The first direct shipping ore (DSO) was exported in February 2001. Mining activities lasted until October 2001, although the export of DSO continued to February 2002. During this period 52.9 t of Ag and 71 kg of Au were produced from ~4000 t of ore.
In early 2002, Coeur Mining Inc. purchased Yamana's interest in Polimet and restarted mining that continued until late 2007, initially shipping mined material 900 km by road to its Cerro Bayo mill and concentrator facilities near the town of Chile Chico in eastern Chile. A new mill and flotation plant was constructed on site at Martha in 2008. Production continued until late 2012 when mining and milling were suspended. In 2016, the operation was sold to Cerro Cazador SA (CCSA), an Argentine subsidiary of Hunt Mining Corporation.
Martha lies within the central part of the Deseado Massif in the Chile-Argentina Patagonian Region. The geology of this region is dominated by the volcanic rocks of the Chon Aike large igneous province, represented by the Bahia Laura volcanic complex. This complex comprises a lower package predominantly composed of andesitic volcanic rocks - mainly lavas - and an upper package of rhyolitic volcanic rocks - ignimbrites and lavas. Patagonia has long been known to contain precious and base metal mineral deposits, whilst lead, zinc and copper minerals have been mined from veins and from irregular pods and stratabound bodies hosted in Mesozoic volcanic and sedimentary rocks and in the Palaeozoic metamorphic basement.
The regional setting and geology of the Deseado Massif is described in the Southern Andes and Patagonia and Cerro Vanguardia records.
The rocks exposed in the Martha District are part of the thick, essentially flat lying felsic volcanic pile of the Middle Jurassic Chon Aike Formation, a member of the Bahia Laura volcanic complex. The underlying Triassic sedimentary rocks and Lower to Middle Jurassic andesitic units are not exposed in the area. The felsic sequence is mainly overlain by Cretaceous sandstone, shale and limestone, and by Tertiary basaltic flows that form flat-topped mesas to the west, north and northeast of Martha. The closest capping basalt in the main deposit area is exposed west of the Martha mine and mill site.
The Chon Aike sequence and main host to silver and gold mineralisation at Martha, comprises a series of crystal rich ignimbrite flows that are locally interbedded with, or masked by dacitic flows and tuffs. Intrusive rocks are rare in the area but consist of irregular dykes of porphyritic rhyolite and dacite that intrude the main volcanic units, and by basaltic plugs that pierce the entire sequence.
A semi-circular feature is apparent in geologic and geophysical mapping, possibly representing the rim of a 12 km diameter caldera, known as the Primero de Abril Caldera (Paéz, et al., 2015). Martha lies along a WNW-ESE trend that forms a tangent on the southwestern rim to this feature. The Martha area is also characterised by block faulting. Four main structural trends are recognised in the district, east-west, 300°, 330° and north-south. The first two trends host mineralised veins, whilst the latter two resulted in local, vertical sinistral displacement of the mineralised veins. A prominent north-south to NE-SW, nearly vertical fault bisects the Main Martha area but only has limited, possibly dip-slip, displacement.
Silver and gold mineralisation at Martha occurs within a series of quartz-rich, locally banded and brecciated veins and veinlets, interpreted to have an intermediate-sulphidation in character (Sillitoe, 2005; Paéz et al., 2015). The main trend of the mineralised systems is WNW and east-west which dip moderately to steeply to the south and SW. At surface, the mineralised structures can be up to several metres wide, although they are often much less than a metre in true width but may expand in thickness in the subsurface.
Base and precious metal minerals include galena, sphalerite, chalcopyrite, polybasite [(Ag,Cu)6(Sb,As)2S7][Ag9CuS4], silver-bearing tetrahedrite, pyrargyrite (Ag3SbS3), freieslebenite (AgPbSbS3), acanthite and native silver. Pyrite and arsenopyrite are common. No gold-bearing minerals have been identified at Martha (Paéz, et al., 2015).
Veining is clustered into several discrete domains, the majority of which are concentrated in the main Martha area.
Vein-hosted adularia has been dated at 156.5 Ma (Sernageomin reported in Paéz, et al., 2011), whilst the enclosing host rock has an age of 157.6 Ma.
Four broad alteration assemblages have been defined, ranging from distal propylitic, through silico-argillic, argillic and proximal silicic. Cedillo Frey (2009) noted the following alteration zonation and assemblages associated with the immediate vicinity of the Martha Oeste Vein which is an along-strike westward repetition of the Martha Vein. This alteration involved the formation of sericite, quartz and adularia from pH neutral hydrothermal fluids ranging in temperature from 200 to 250°C, and developed two main zones:
• An outer zone, extending outwards for 1 to 5 m from the vein, where sericite replaced potassium feldspar, plagioclase, biotite and phengite. This alteration increased inward toward the vein margin forming muscovite and illite.
• An Inner zone, extending outwards for up to 1 m from the vein, characterised by massive silicification. Adularia is also developed in both zones, but is more abundant in the inner.
Additional alteration related to a post-hydrothermal cooling event resulted in aluminum and lead phosphate minerals being formed (e.g., plumbogummite - PbAl3(PO4,OH)(OH)6).
Production, Reserves and Resources
Mineral Resources and included Ore Reserves as reported by Coeur Mining Inc. (2010; 2012) were:
Measured + Indicated Resource - 0.037 Mt @ 1579 g/t Ag, 1.93 g/t Au for 58.5 t Ag, 71 kg Au;
Inferred Resource - 0.030 Mt @ 2091 g/t Ag, 2.07 g/t Au for 62.7 t Ag, 62 kg Au;
Measured + Indicated Resource - 0.0836 Mt @ 1264 g/t Ag, 1.55 g/t Au for 105.6 t Ag, 130.6 kg Au;
Inferred Resource - 0.0657 Mt @ 943 g/t Ag, 0.88 g/t Au for 62.7 t Ag, 56 kg Au;
Measured + Indicated Resource - 0.061 Mt @ 1534 g/t Ag, 1.52 g/t Au for 93.5 t Ag, 93.3 kg Au;
Inferred Resource - 0.0194 Mt @ 232.8 g/t Ag, 0.15 g/t Au for 4.5 t Ag, 2.8 kg Au;
Proved + Probable Reserves - 0.0342 Mt @ 1136.3 g/t Ag, 1.27 g/t Au.
Total production between 2000 and 2012 was (Hunt Mining Corporation 2016):
0.5319 Mt of ore mined for 754.25 t Ag and 0.9766 t Au.
This record is largely drawn from: Birak, D.J., 2016 - Martha silver and gold project, Santa Cruz, Argentina - an NI 43-101 Technical Report prepared form Hunt Mining Corp., 59p. and similar Technical Reports between 2006 and 2010 prepared for Coeur d'Alene Mines Corporation and Coeur Mining Inc. quoted by Birak (2016).
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
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