Pyrite, enargite, and other sulfides are associated with the highest Au values below the base of oxidation. A large area of gusano texture is shown in Figure 5. Illustrated by Figure 6c-d, this alteration texture completely obliterates original rock texture. Gusano texture is characterized by segregation of soft, white patches of pyrophyllite, typically with minor diaspore and as much as percent alunite, in a hard siliceous matrix. In thin section, the siliceous matrix consists of granular quartz containing approximately 5 to 30 percent interstitial aluminosilicate minerals similar to the soft patches that generally contain less than 10 percent quartz.
The siliceous matrix is hard but generally scratchable with a steel blade. Table 3 summarizes defining characteristics of gusano texture and other features discussed below. At Tantahuatay 2, pervasive gusano-textured rock covers an area of at least by m and extends in drill holes to a depth of at least to m.
Smaller amounts are present in the other mineralized and advanced argillic-altered dome centers in the district. In places it crops out only in structural zones several meters or less in width. More intensely silicified breccia and vein zones cut this rock, are more brittle, and have localized more intense sulfide mineralization.
Photographs of the Tantahuatay district: a. Looking southeast, across terraced ferrocrete in drainage, to Cerro Tantahuatay; Tantahuatay 2 at center skyline. Breccia dike cutting more silicified breccia. Gusano texture; texture-destructive alunite-pyrophyllite-diaspore-quartz alteration.
Enargite-pyrite follows permeable zones of intense silicification cutting denser gusano-textured rock with weakly disseminated mineralization. Two sides of same piece of core near e , with patchy quartz grading into linked wormy quartz veinlets; drill hole T This relatively late assemblage is mostly pyriteenargite-covellite. Copper and Au contents vary directly with the abundance of enargite, which is by far the most abundant copper mineral. Enargite-pyrite alone rarely contains more than 0. These intervals are typically in the same siliceous zones of breccia and veins that contain the most enargite.
Weak supergene chalcocite enrichment locally. In the overlying oxide zone the Au remains, with local enrichment very close to surface, whereas Cu has been strongly leached during surficial oxidation. Substantial sulfide Cu-Au resources exist at Tantahuatay 2 and probably at other centers within the volcanic complex, but exploration has been insufficient to quantify them.
Most drilling has been confined to the oxide zone, where a resource of Although several deep holes were drilled at Tantahuatay, no potentially economic porphyry-style mineralization was recognized. Two holes did penetrate below the unconformity at the base of the volcanic rocks and encountered skarn in the limestone that hosts massive pyrite-enargite mineralization a few hundred meters farther east Paredes, The skarn contains pale diopside, epidote, and clay with minor amounts of pale sphalerite, galena, and chalcopyrite.
In the dome complex with only rare dikes, only advanced argillic- to sericitic-altered volcanic rock and pyrite-enargite mineralization with minor amounts of pyrite-bornite-chalcopyrite and pyrite-covellite were seen at to m below surface. Only in one drill hole, T41 at the northeastern edge of Figure 5, has clear evidence for a deeper porphyry deposit been encountered.
Granular quartz veins, similar to A veins at Minas Conga and porphyry deposits throughout the world, occur from 50 m to the bottom of the hole at m. Only vapor-rich fluid inclusions occur above the top of halite-bearing inclusions at m, and anisotropic daughter minerals are common in polyphase brine inclusions from there downward. The quartz contains traces of minute chalcopyrite inclusions, although the bulk sulfides are pyritic, high-sulfidation assemblages with sharply diminishing grades of gold and copper at depth.
Although potassic alteration is invariably associated with A veins in porphyry systems, any preexisting potassic alteration has been completely obliterated by the overprinted advanced argillic and sericitic alteration.
Within a few meters of the top of A quartz veins in this hole, gusano texture, which is dominant in quartz-pyrophyllite- alunite —altered rock in the upper part of the hole, undergoes an abrupt change. As shown in Figure 6e, the highest A vein cuts a more silicified rock in which the siliceous matrix to the rounded patches in the surrounding gusano texture is harder and segregated into irregular, locally contorted veinlike masses Fig. This modified texture is much more abundant at Yanacocha, where it has been termed patchy quartz and wormy veins, which are described below and in Table 3.
Tantahuatay 2 is only one of several centers with strongly anomalous Mo, but the values are scattered with little consistent pattern at surface within these centers. This zone overlaps the southern and western edge of the 0.
Beneath this area are rare occurrences of euhedral molybdenite in quartz veins, which further support the interpretation of a porphyry system at depth. Yanacocha The Yanacocha district is the central and economically most important in the Cajamarca region Fig. With 2. With more than 10 Moz already produced and Various aspects of the discovery and geology of Yanacocha have been presented by Turner , Harvey et al.
The district contains at least ten distinct near-surface, highsulfidation epithermal deposits formed in the middle Miocene Epithermal gold mineralization is associated with large volumes of massive to vuggy quartz and lesser advanced argillic clay alteration.
Associated alunite has been dated at The oxidized gold mineralization is underlain by multiple centers of porphyryassociated Cu-Au sulfide mineralization, which have been incompletely explored. At least two distinct periods of mineralization are recognized: an early, premain-stage gold event at Kupfertal, Yanacocha Sur, and Maqui Maqui, and a later postmain-stage gold event contemporaneous with the emplacement of the Yanacocha diatreme Teal et al.
Intermediate-sulfidation mineralization with high gold grades is peripheral to central high-sulfidation Cu-Au mineralization.
From , exploration of deep sulfide resources has expanded knowledge of the Kupfertal porphyry system Pinto, , as well as other occurrences below Yanacocha Norte and Maqui Maqui. Porphyry Cu-Au alteration and mineralization at Kupfertal affect rocks of the early eruptive cycle of Longo , whereas most of the epithermal gold orebodies are at higher elevations in overlying explosive-cycle rocks. These xenoliths are tentatively interpreted as having been derived from the Kupfertal system, which is considered to be earlier than the main Au mineralization in the district Teal et al.
The most distal indications of the proximity of the porphyry are seen in the south wall of the valley, roughly m laterally from and 75 m above the collar of drill hole CLL In rock altered to alu-. Location of the Kupfertal porphyry deposit relative to the gold orebodies at Yanacocha. From Pinto In the halo, soft white clots of alunite-pyrophyllite are surrounded by siliceous matrix similar to the gusano texture at Tantahuatay.
This alteration becomes pervasive closer to drill hole CLL-5 and in one outcrop is cut by a discontinuous finegrained dike Fig.
The dike is altered to pyrophyllite, appears to have been an aphanitic igneous dike rather than tuffisite, and was apparently intruded after formation of the gusano texture. Within several meters of the drill collar, the quartz-pyrophyllite alteration is more ragged, with irregular quartz patches harder than typical gusano matrix that are more or less isolated in pyrophyllite and locally linked to form very irregular, sinuous veinlike shapes Fig. This texture is called patchy by Yanacocha geologists Table 3 , who consider our gusano texture to be a variety of patchy texture.
The rock is also cut by quartz veins that are both sinuous and planar.
Particularly in the underlying drill core Fig. These wormy quartz veins abruptly decrease and disappear at about 90 m in this drill hole Fig.
A similar transition in alteration textures and shape of quartz veins occurs in nearby drill hole KUP-6 Fig. Rock at the top of the hole has well-rounded gusano texture. Wormy quartz veins appear at 27 m and A veins near 50 m. Wormy veins do not extend below 85 m, whereas A veins continue to depth.
All these phases, except quartz, were precipitated from solution that were supersaturated with respect to quartz, and then recrystallised to quartz with time because they were metastable at low temperatures Fournier, Veinlets up to 10 cm in thickness with similar gangue mineralogy occur commonly within 10 m of the principal veins. References Carlile, J. The nature of the host rock plays an important role in formation of different textures such as granular silica , massive quartz and quartz-alunite textures. Thistexture involves successive, narrow upto a few.
Some A veins are irregular and discontinuous similar to the earliest A veins formed at El Salvador; Gustafson and Hunt, , making the distinction between A and wormy veins locally ambiguous. However, in this and other drill holes at Kupfertal wormy veins are consistently restricted to an interval of m or less that overlaps the top of relatively continuous and planar A veins.
Wormy veins also consistently form only within patchy or gusano-textured alteration. Note that the wormy veinlet indicated with the arrow in Figure 8f at The dark wormy vein is composed of hard, granular quartz with only traces of interstitial pyrophyllite Fig. Granular quartz in both patchy alteration and wormy veins is essentially identical to that of A quartz veins in these rocks and contains the same high concentrations of fluid inclusions, which are overwhelmingly vapor dominant at this elevation Fig.
In different drill holes, the abundance and distribution of patchy quartz in the vicinity of the top of A quartz veins varies considerably, and because of gradational characteristics, it is not always possible to distinguish patchy quartz from gusano and wormy veins. In drill hole CLL-5 Fig. A vein quartz contains trace inclusions of relict anhydrite, chalcopyrite, and bornite, apparently relics from an earlier stage of mineralization. Present alteration. Photographs of the Yanacocha district. Alunite vein with irregular, siliceous gusano-textured alteration halo in volcanic rock, about 50 m above c.
Gusano-textured volcanic rock cut by discontinuous fine-grained dike, about m laterally and 50 m above quartz veining of the Kupfertal porphyry system; both rocks altered to pyrophyllite. Outcrop at collar of drill hole CLL5; patchy quartz and large wormy quartz vein, quartz-pyrophyllite alteration. Wormy quartz veins cut by more planar A quartz veins in top 21 to 77 m of drill hole CLL Transition from gusano texture to patchy quartz and wormy quartz veins at top of A quartz veins in drill hole KUP-6, This is about m from drill hole CLL5 Fig. The kaolinite may well be supergene after chlorite, which is abundant at deeper levels.
At this depth, texturally unmodified feldspar porphyry with only trace amounts of small A veins is present. Weak chlorite-sericite is superimposed on weak potassic alteration, manifested by biotite replacing hornblende. Thin section with cross-polarized light, showing granular quartz of wormy quartz vein in drill hole KUP Very abundant fluid inclusions, overwhelmingly lowdensity vapor-rich, with only traces of high-salinity inclusions, some of which have anisotropic daughter crystals; these are typical of high-level quartz in wormy, banded, and A quartz veins and also patchy quartz; thin section, bar is 0.
Relatively large 1. Cross section, looking northwest, across the Kupfertal porphyry deposit, showing drill holes and the alteration pattern; the propylitic alteration corresponds to a late intramineral porphyry intrusion. Hydrothermal breccia Texture obliterated - coarse granular Texture obliterated - f.
Alteration and mineralization in drill hole CLL-5, Kupfertal.