Deposit Type
- Vein / narrow vein
- Hydrothermal
Summary:
Hydrothermal tin Iodes typically have very high ore grades (half of the deposits grade> 1.3 % Sn, and a tenth ofthem > 2.3 %), and most ofthem contain in excess of 1,000 t Sn, with sorne hosting > 50,000 t Sn (Menzie et al., 1988; Singer et al., 1993). Compared to most tin Iodes, the Peruvian San Rafael Sn-Cu deposit, the subject of this thesis, is a giant member of this deposit clan. Hosting a total resource of ~ 1,000,000 t Sn (metal) and having an average grade of 4.7 wt.% Sn (Minsur, unpub. data, December 2002), it is responsible for ~13% of the annual global hard-rock tin production (J. Carlin, USGS, writlen comm. 2000) and is considered the world's richest hydrothermal tin Iode.
In the San Rafael deposit, the high-grade mineralization is of the cassiteritesulfide type, and the bulk of it is hosted by a large, vertically extensive, single complex vein, referred to as the San Rafael Iode. This Iode is part of a vein-breccia system, which is centered on a Late Oligocene granitoid stock, shallowly emplaced in Lower Paleozoic c1astic metasedimentary rocks. The tin ores form cassiterite-quartz-chlorite-bearing veins and breccias, hosted by several large fault jogs at depth in the Iode. By contrast, the copper ores, which contain disseminated acicular cassiterite, are localized in the upper part of the system. Both ore types are associated with a very distinctive strong chloritic alteration, which was preceded by intense sericitization, tourrnalinization and tourmaline veining.
The San Rafael Iode is not only remarkable because of its unusually rich tin resource, but also because it is very young (~25 Ma), therefore, remained undisturbed by any later processes, and exhibits rich tin-copper ores over a vertical extent of> 1,300 m, allowing a fresh, unprecedented insight into a deposit of this type.
The San Rafael vein, referred to as Iode because of its structural complexity, on average, strikes -330°, dips 40 to 75° to the northeast and can be traced on surface for a distance of -3.5 km. It is mineralized over an unusually large vertical extent, exceeding 1,200 m (from 5100 to 3900 m above sea level), and hosts 98 % of the tin reserves of the San Rafael deposit (Minsur, unpub. data 1999). The width of the vein is < 2 m in the upper part of the deposit (Fig. 2b), but, at depth, the structure dilates into a series of subvertical shoots, sorne of which are as much as 50-m-wide (Figs. 6 and 7). Field observations show that there was movement along the San Rafael fault synchronous with vein filling. Kinematic indicators,such as fault-vein relationships, drag folds, and stratigraphie offsets, consistently indicate a normal-sinistral sense of displacement (Sherlock 1999). Slickensides are poorly developed, but where seen are steeply plunging.
The surface expression of the San Rafael Iode consists of a 1- to 2-m-wide network of anastomosing quartz veinlets and quartz cemented-breccias, surrounded on both sides by a zone of chloritization that is -20-m-wide. The Iode is planar (striking 328° and dipping 65°, on average; Fig. 8a) and its hangingwall and footwall contacts are delimited by narrow (-2 cm), gouge-filled fauIt breccias. Mineralization consists of narrow, semi-continuous bands of massive chalcopyrite, generally developed along the hangingwall and footwall contacts.
The Iode continues to be planar and narrow (<2 m) down to the 4600 m level (striking 332° and dipping 68°, on average; Fig. 8b), and displays a geometry identical to that of the surface exposures. It generally has sharp contacts with the adjacent unmineralized wallrock, is commonly brecciated, and exhibits extensive texturaI evidence of open-space filling. The mineralization consists of narrow zones (~20 cm-wide) of massive to semi massive chalcopyrite with minor cassiterite.
Below the 4533 m level, two major fault jogs are developed, producing a major change in the style of mineralization (Figs. 6 and 7). These jogs localize dilational zones, where the sinistral-normal San Rafael fault has stepped to the west and steepened (Fig. 9). The jogs widen considerably at depth (up to ~50 m) and form the Ore Shoot and Contact orebodies, elongated sinuous zones, marked by sharp hangingwall and footwall contacts, and characterized by an abundance of NW striking (Fig. 8c), cassiterite-mineralized quartz veins and breccia zones. The two orebodies display a similar style of mineralization and host the highest concentration of cassiterite in the deposit. There are also several smaller orebodies at depth, which include the South Contact, Breccia 150-S, Ramp 410, 150, 310-S and 250-S orebodies. Significantly, all the orebodies in the lower part of the Iode are confined to the granitoid intrusion (Figs. 6 and 7), and the veins become irregular where they are hosted by the sedimentary rocks. Cassiterite associated with quartz and chlorite dominates the lower part of the Iode. It occurs mainly as open fracture filling, breccia and replacement bodies (Figs. 2g-i and 1 Oa-e), as well as disseminations in strongly chloritized wallrock (Fig. 2d). Repeated opening of the veins, evident from crack and fi11 textures, as well as multiple episodes of brecciation, produced a very complex Iode morphology (Fig. 2e, f).
Cassiterite is most abundant where veins branch and intersect or where they deftect in strike or dip. The highest ore grades (as much as 45 wt.% Sn) occur in breccias that form zones severai meters wide, and in the major veins in the footwa11 and hangingwa11 ofthe Iode structure, he veins composing the San Rafael Iode have been subdivided, based on their mineralogy (Fig. Il). Crosscutting relationships among the different vein types clearly demonstrate that the oldest veins are tourmaline- and quartz bearing (Fig. 2f, g). These veins form a conjugate set (one set striking ~330° and NE-dipping, the other striking ~295° and SW-dipping). By contrast, all main- (ore) and late-stage (post-ore) veins have the same general orientation (strike ~330° and NE-dipping), eological mapping on the surface identified tourmaline-quartz veins orthogonal to the San Rafael Iode, occupying dilational zones in thrust faults that have been subsequently eut and offset by the Iode. These tourmaline-quartz veins are, therefore, very early and may be unrelated to the tin-copper mineralization. However,underground mapping showed that there are major tourmaline-quartz veins, and volumetrically important tourmaline-quartz breccia dykes (described below), which are concordant with the strike of the San Rafael vein-breccia system. Some of these veins were subsequently reopened and filled by the main- and late-stage veins (Fig. 2f). It is thus possible that there was more than one generation of tourmaline bearing veins and tourmaline crystallization could have partly overlapped tin deposition.
Tourmaline-quartz veins and breccias have, therefore, been included in the early paragenesis of the Iode, and a genetic link between these and the younger tin-and copper bearing veins is tentatively inferred.