Mineral. Deposita (Berl.) 12, 235 - 238 (1977) MINERALIUM DEPOSITA © by Springer-Verlag 1977

Short Note The Milluachaqui Epithermal Silver District of Northern Peru

V. F. Hollister and L. P. Entwistle Duval International Corporation, Vancouver

The mines in the IV~illuachaqui district, located near Salpo in the northern part of Peru, show a vertically zoned, epitherrnal silver-gold mineralization. Colonial production was obtained from oxidized, high-grade ores, consisting mostly of native silver and cerargyrite. More modern development exposed, from the top of ore shoots to their base, the "bonanza" ores (the pyrargyrite-polybasite-aeanthite zone), silver-copper ores (the tetrahedrite zone), silver-lead ores (the galena-sphalerite zone), and the basal zone ores (the sphalerite zone). The ratio of gold to silver in the "bonanza" ores was about 1 to i00, by weight, Interpretation of mineralogical textures and the composition of minerals in the sulfide zone suggests thai supergene movement and enrichment of silver at Milluachaqui was not a significant factor. Silver tended to be fixed in place during weathering as the chloride and native min- eral. The "bonanza" ores are interpreted to be hypogene sulfide accumulations. These findings contrast to hypotheses proposed for some epithermal districts in the U.S. which assume supergene activity to have been significant in the accumu- lation of high grade, near-surface silver ores.

INTRODUCTION ditional data collected by Mr. Paul Kents were available for this study. The Milluachaqui epithermal silver- gold district is noteworthy for its well- developed hypogene vertical metal zon- GENERAL GEOLOGY ing and for persuasive field evidence Regional against substantial supergene enrichment as a mechanism for the formation of The Milluachaqui district lies near a "bonanza" ores, The district lies in the composite diorite-quartz diorite-grano- Department of La Libertad, Province diorite stock dated by Stewart et al. of Otuzco, in the northern part of Peru. (1974) at 26 m.y. which is intrusive into From all past production, the mines flat-lying Calipuy andesite formation of may have produced in the order of Cossio (1964). It is one of a number of 3, 000, 000 kilograms of silver and young centers of mineralization which 30, 000 kilograms of gold. Its two main occur near the margin of the Calipuy periods of production were from Colonial andesite. oxidized ores (mostly native silver and The Calipuy formation unconformably cerargyrite) and modern (1922 to present) overlies folded Upper Cretaceous and from sulfide ores. older sedimentary rocks on the north Milluachaqui was studied by the authors and east margins and Coastal batholith in 1954, 1959 and again 1962-63. Ad- rocks dated about 43 and 53 m. y. (Ste- 336 V.F. Hollister and L. P. Entwistle wart st al., 1974) on the west. Coarse Propylitization of the walls is much and fine elastic rocks of Cretaceous age more widespread. It consists mostly of underlie the Calipuy at a shallow depth a pervasive development of chlorite, epi- in the vicinity of Milluachaqui and are dote and calcite. Pyrite is not a signifi- penetrated by mine workings. cant component of the propylitically alter- ed rock. Propylitization may be hydro- thermal, but it may also reflect regional Local alteration in the volcanic rocks. Argillic altered rock grades into propylitic alter- Structure: The veins at Milluaehaqui ation away from the vein, but the pro- generally trend northwest and dip 45" to pylitic assemblage is most easily dis- vertically. They are fault structures cerned where the walls are andesitic. which include a component of left lateral movement. Tectonic voids in the fault Hypogene Mineralization: Quartz pre- trace were filled with vein matter to form dominates as vein filling throughout the the ore shoots. These are separated by district. Other gangue minerals ident- heavy fault gouge and breccia where tec- ified in various parts of the district in- tonic openings failed to appear. The ore clude subordinate barite, calcite, rhodo- shoots contain mostly quartz with sub- chrosite, gypsum and fluorite. Sulfide ordinate sulfide and non-quartz gangue minerals ubiquituously scattered through minerals in a variety of structure types. the veins in minor amounts include pyrite, A vuggy quartz breccia is commonly lesser marcasite and rare arsenopyrite. well-developed. This may grade into a No other sulfide has been found thai oc- massive or ribbon vein structure. Sul- curs throughout the veins in all metal fide occurs in the quartz breccia ordi- zones. Neither pyrite nor marcasite narily as a matrix mineral. In the mass- makes up more than 10% of any vein, and ive veins, it occurs as blebs, pods and they may be less than 5% by volume. disseminations. Sulfide appears as Gold was important enonomically, and streaks, veinlets and disseminations in direct shipments of crude "bonanza" ore the ribbon veins. Mineral textures in the sometimes contained 30 to 50 oz per ton, vein suggest that pre- and intro-mineral Although accurate figures are not avail- fault movements provided openings for able, it appeared that the gold to silver the filling by vein matter, but that sub- ratio may have been in the order of 1 to sequent fault movement crushed or dis- I00, by weight; or nearly I to 2 in gross torted the deposited material in some value. Th~ ~ ratio probably remained con- parts of the veins. stant for "bonanza" ores whether they Alteration: Argillization and propylitiza- were oxidized or sulfide. The Au:Ag tion of vein walls were the only mega- ratio appears to have decreased slight- scopically noted alteration at Milluacha- ly with depth, and even in the principal qui. Argillic alteration, including a mine, La Guardia, values in both el- "bleaching" of the host rock, most com- ements nearly disappear in the lowest monly occurs in the vein walls within levels. The economic minerals are gold one meter of the vein. Analytical deter- and sulfides or sulfosalts of silver, cop- minations made by Northern Peru Mining per, lead and zinc. These are arranged Corp. (J. Caceres, personal communi- in a zonal pattern with, from the top cation, 1957) show that clay separates down, pyrargyrite dominant, tetrahedrite from argillized rock contain large amounts dominant, galena dominant and zinc domi- of the alkali metals, but no rnineralogic nant mixtu-res. Each zone grades one into determinations were made. Minor pyrite another vertically. The zones have econ- is also found. The argillized rock includes omic significance because the pyrargyrite both Calipuy andesite and Cretaceous mar- and tetrahedrite zones have provided ine shale and argillaceous quartzite. Ar- most of the past production. Silver con- gillization apparently did not alter the tent drops erratically but sharply from walls of the veins in the shale greatly, zone to zone as depth is gained. Silver District of Northern Peru 237

Mineralization occurs along the vein such a variance in the Ag:Cu ratio was structures for thousands of meters on never determined. strike, but it is a shallow phenomenon. As depth is gained, tetrahedrite de- Little sulfide mineralization extended creased in importance as a vein constitu- more than 400 meters below the present ent, but galena becomes increasingly im- surface. The telescoped nature of the portant. The galena carries an average zoning suggest dumping by boiling. Ore one ounce silver to each one percent textures indicate a simple sulfide para- lead, and the galena zone has been ex- genesis from the tetrahedrite zone down- ploited if the galena is accompanied by ward. Early pyrite and marcasite was enough other sulfides. Sphalerite com- over-lapped and succeeded by sphalerite. monly is present in minor amounts in Sphalerite was in turn overlapped and the galena zone, as are erratic traces succeeded by galena. Tetrahedrite follow- of tetrahedrite. ed the galena. Paragenesis above the tetrahedrite zone is confused. Ore tex- The galena zone grades downward into ture indicate a simple filling of tectonic a sphalerite dominant zone in which ga- openings by sulfides, but vugs lined with lena is either subordinate or missing. silver rich minerals suggest late intro- Iron sulfides become proportionately duction of this metal. more important. The pyrargyrite zone is abruptly super- Supergene Mineralization: Oxidation of imposed on the top of the tetrahedriie the pyrargyrite zone minerals has re- zone, and it was intersected by the pre- sulted in native silver-cerargyrite rich sent erosion surface, Characteristic sul- mixtures. The richness of these second- fide mineralogy is pyrargyrite-polybasite- ary silver mineral accumulations imply acanthite-pyrite intergrowths, with one little leaching or long distance transport or several of these minerals missing of the silver during weathering. Further from parts of a shoot. Marcasite may evidence against substantial transport of occur with the pyrite. Spectacular rug the hypogene silver deposits during linings of large crystals of silver sulfo- weathering may be seen in the leaner salts intergrown with octahedral acanth- ores. Silver present in most oxidized ire suggest original deposition under hypogene ores exposed in surface stopes thermal conditions. The acanthite is in- was fixed in place during weathering, and terpreted as an inversion of originally workings below the oxide section showed deposited argentite. Although the physics no enrichment in the sulfide zone over and chemistry of the silver-antimony- the oxide. The Ipres section is typical sulfur systems (Craig and Barton, 1973; of most veins in Milluachaqui in that sil- Skinner et al., 1972) indicate that the ver grade does not improve when moving "bonanza" ores could have developed from the weathered to sulfide ores. The either as hypogene or supergene systems, secondary minerals do not necessarily the presence of acanthite pseudomorph- form pseudomorphs after sulfides, ing octahedral argentite (intergrown with suggesting partial mobilization of silver polybasite and pyrargyrite) suggests the during weathering. The probable con- sulfosalts are hypogene at Milluachaqui. stancy of the gold-silver ratio in both oxide and sulfide ores also suggests Tetrahedrite, accompenied by lesser minimal supergene mobilization. galena and rare chalcopyrite, character- ises metallic ores underlying the "bonan- Rarely, however, acanthite may be za r' ore zone. These ores accounted for found coating tetrahedrite or other sul- most of the district production, and tetra- fide close to the oxide-sulfide interface. hedrite was volumetrically the most im- This mode of occurrence of the Ag 2 S is portant silver-bearing mineral. Silver: compatible with minor supergene silver copper ratios vary haphazardly in tetra- mobilization near the oxide-sulfide bound- hedrite from 2 to 126 ounces silver to ary. The coatings are the latest mineral each one percent copper. The reason for deposited, and they do not persist into 238 V.F. Hollister and L. F. Entwistle: Silver District of Northern Peru

the sulfide zone more than I0 meters Abundant chloride ion is available in below the oxide zone. The absence of the fine Cretaceous marine clastics which sulfosalts as supergene products coat- underlie the district. A dilute concen- ing sulfides with the acanthite further tration of silver ions would quickly be infers these complex minerals are only precipitated as chloride because of the of hypogene origin. probable pervasive presence of chloride ion from this source. Observation of Supergene copper sulfide coatings are cerargyrite as a dominant secondary also rarely found near the oxide-sulfide mineral in the weathered zone supports interface. The paucity of such coatings such reasoning. Rapid precipitation of sil- implies negligible transport for copper ver as chloride during weathering would in supergene conditions. also have discouraged widespread super- gene movement. CONCLUSIONS Evidence cited above is interpreted to show silver sulfosalts formed entirely REFERENCES as hypogene minerals, and that the sil- Cossio, A. :Geologia de los euadrangulos ver sulfide largely formed under hypo- de Santiago de ehoco y Santa Rosa: gene conditions. The Ag 2 S formed most- Peru Comm. Corta Geol. Bol, N, 8 ly as argentite and inverted to acanthite (1964) as heat was lost from the system. Field Craig, J.R., Barton, P. B. : Thermochemi- evidence against large-scale supergene cal approximations for sulfosalts: transport of silver is compelling. Econ. Geol., 68, 493-506 (1973) Supergene transport of silver has been Stewart, J. W. , Evernden, J. F., Shelling, demonstrated in other districts, however. N. J. : Age determinations from Andean It seems possible that the presence of Peru: Geol. Soc. America Bull. 85, reactive wall rock silicates rich in alkali 1107-1116 (1974) metal bearing clays may have been an Skinner, B. J., Luce, F. D., Makovicky, inhibiting factor at Milluachaqui. Clays E. : Studies of the sulfosalts of Copper and other loosely-bonded hydrated sili- III. phases and phase relations in the cates would have kept the groundwater system Cu-Sb-S: Econ. Geol. 67__, pH at or near 7. If montmorillonite is 924-938 (1972) an important component of the clay frac- tion, it is conceivable that hydration by ground water could also have resulted in Received January 31, 1977 a volume expansion. Any such tendency would have inhibited access of ground water to the vein sulfides and have lessen- V. F. Hollister ed the ability of ground water to trans- Dural International Corporation, port metals. Silver would not have been Suite 204, The Marine Building, able to move substantially under these 355 Burard Street, conditions in solutions with dilute anion Vancouver, B.C. concentrations. V6C 2G8, Canada