Download Ore geology field trip NW-Mexico 11 – 21 January 2009 Guidebook

Ore geology field trip NW‐Mexico 11 – 21 January 2009 Guidebook Editor: Honza Catchpole sponsored by: Program of the Mexico Excursion, 11‐21 January 2009 Day Mine Deposit type Sunday 11 January Monday 12 Tuesday 13 Wednesday 14 Thursday 15 Friday 16 Saturday 17 Sunday 18 Monday 19 Tuesday 20 Wednesday 21 ‐Arrival in Hermosillo La Herradura Au(‐Ag) Milpillas Bismark Santa Eulalia Ag Naica Pb‐Zn‐Ag Santa Barbara Ag‐Pb‐Zn ‐Free day and travel Pinos Altos Au‐Ag Pinos Altos Au‐Ag Carlin?‐orogenic? Porphyry Cu Polymetallic skarn Polymetallic skarn Polymetallic skarn Polymetallic skarn Epithermal LS Epithermal LS Night at Hermosillo Cananea, Sonora La Ascensión, Chihuahua Chihuahua Chihuahua/Delicias Parral Chihuahua Pinos Altos Pinos Altos Pinos Altos Hermosillo 2
Field trip leaders: Victor A. Valencia Massimo Chiaradia Participants Miguel Ponce Melissa Ortelli José A. Perez Anne Chevalier Mauricio Ibanez‐Mejia Edina Vago Johannes Mederer Honza Catchpole Aldo Bendezu Robert Moritz Lluís Fontboté Fernando Barra – University of Arizona – University of Geneva – University of Geneva (student M.Sc.) – University of Geneva (student M.Sc.) – University of Geneva (student M.Sc.) – University of Geneva (student M.Sc.) – University of Arizona (student Ph.D.) – University of Geneva (student Ph.D.) – University of Geneva (student Ph.D.) – University of Geneva (student Ph.D.) – University of Geneva (student Ph.D.) – University of Geneva – University of Geneva – University of Arizona La Herradura 3
Index p.4 ‐ Geological introduction and regional tectonic evolution Melissa Ortelli p.8 ‐ “La Herradura” ore deposit in NW Mexico: An orogenic type gold mineralization Mauricio Ibanez‐Mejia p.12 ‐ Milpillas Porphyry Copper deposit Johannes Mederer p.16 ‐ The skarn and carbonate replacement Pb‐Zn deposit of Bismark in northern Mexico Edina Vago p.20 ‐ Santa Eulalia Mine Aldo Bendezú p.23 ‐ Naica Mine (Pb‐Zn‐Ag) Anne Chevalier p.25 ‐ Santa Barbara mine (Mexico Group) José Agustín Pérez p.27 ‐ Summary of observations on the Pinos Altos epithermal gold & silver bearing veins, Mexico Miguel Ponce Acknowledgments A big Thank You to all those who made this field trip possible, especially Victor Valencia who organised a major part of our trip sharing both his knowledge and passion of the area with us. We would like to thank Minera Penmont S.A, Industrias Peñoles, Mexico Group, Agnico‐Eagle and Newmont for the permission to visit their mines and the kind support of mine geologists, engineers, and managers during the visit. Newmont and Barrick and the Society of Economic Geologists are gratefully acknowledged for financial support
4
Geological introduction and regional tectonic evolution Melissa Ortelli Mines visited in NW‐Mexico (Figure 1): La Herradura (Carlin-Orogenic (?) Au deposit)
Milpillas (Porphyry Cu)
Bismark (Carbonate-hosted polymetallic skarn)
Santa Eulalia (Carbonate-hosted polymetallic skarn)
Naïca (Carbonate-hosted polymetallic skarn)
Santa Barbara (Carbonate-hosted polymetallic skarn)
Pinos Altos (Epithermal LS deposit)
Naïca
Santa Barbara
Figure 1: Location map of study area with symbol shape and shading indicating deposit type and mineralization age with sources for districts in Staude (1995). Larger symbols are labeled districts. Mines visited on this trip are circled red (modified from Staude J.‐M. G. & Barton M.D. (2001) 5
Regional tectonic evolution of northwestern Mexico: The northwestern part of Mexico was constructed by the accretion of different terranes south of the North American Craton, intruded by younger igneous rocks since the mid‐Mesozoic (Figure 2). Proterozoic to Paleozoic sequence The La Herradura ore deposit is situated in the Caborca terrane, 50 km west of the Mojave megashear (see below) and at the boundary with the North American terrane. The Caborca terrane is composed of an assemblage of metamorphic rocks (schists, gneisses, amphibolites, and quartzites) of middle Proterozoic age, such as outcropping in the La Herradura mine area. In the northeastern part of Sonora, shallow marine deposits overlie the crystalline basement belonging to the North American craton. The tectonic evolution from Proterozoic to Paleozoic is described by Fries (1962) as the Sonoran orogeny. Figure 2: Simplified stratigraphic column for northern Chihuahua. Data from Clark and Ponce (1983), Mauger et al. (1983). Compiled by Ciudad Juárez (University of Texas at El Paso) Jurassic‐ Middle Cretaceous (Figure 3a) 6
The first magmatism associated with mineralization occurred during a translational tectonic context (subduction?) which led to the formation of the Mojave‐Sonora megashear. This magmatism is concentrated within the coastal belt containing greenschist facies metamorphosed units (ophiolite suites and mafic to intermediate volcanic rocks) and the interior belt. This interior magmatic belt with calc‐alkaline to alkaline volcano‐plutonic complexes is interpreted to have formed in an extensional arc setting. Thrust and strike‐slip structures and the secondary northwest striking shearing related to the megashear juxtaposing Mesozoic and Paleozoic rocks, could have produced deformation in the Chihuahua formations. These were later intruded by Cretaceous batholiths. These tectonic structures can host mineralization and are reactivated as conduits for hydrothermal fluid flow (e.g. San Francisco mine, Sonora). Figure 3a Figure 3a‐d: Cross sections from Jurassic to present, showing major fault systems during each period at the latitude of Hermosillo, Sonora, from west to north (left to right). Section not restored so as to more clearly represent superimposed features. Structures are generalized and represent major fault sets. A‐Away, T‐Toward, SMO‐ Sierra Madre Occidental (Staude & Barton, 2001) Late Cretaceous – Early Tertiary (Figure 3b) During this period, a compressive tectonic phase produced a crustal shortening with major thrusts in the Sonora area, the Laramide magmatism, and also some extensional basin‐structures. Development of igneous rocks parallel to the subduction front will form a calc‐alkaline granodioritic to granitic batholitic belt intruded into the previous volcano‐
sedimentary rocks. The volcanic sequence of this Laramide magmatism can be observed in the Sierra Madre Occidental volcanic province in Chihuahua. In Sonora, thrusting ended before this magmatic event, as the plutons generally were not affected by these faults. Figure 3b Middle Tertiary (Figure 3c) The more felsic calc‐alkaline magmatism of the Sierra Madre Occidental succeeded and mostly covered the Laramide magmatism. We can observe the Eocene transition from mainly andesitic and hydrothermally altered rocks to dominantly dacitic and rhyolitic rocks of these two magmatic stages. 7
The orogenic collapse begins in northwestern Mexico. Extension and formation of core‐complexes permit the localization of new volcanism centers within normal faults. Later northwest trending high angle faults commonly host the middle Tertiary mineralization. Figure 3c Late Cenozoic (Figure 3d) Volcanism evolved to a bimodal composition bounded to normal faults, with a mafic‐
dominated (rift‐type) volcanism associated with the generation of the Gulf of California. During the Miocene, volcanism concentrated around the Gulf with some sparse centers in northern Mexico. The faulting was syn‐ and post exhumation of the core complex and allowed the crustal attenuation. Miocene high angle extensional faults preceded the strike‐slip (transtensional) faults associated with the rifting. Figure 3d References : Ayala C.J. & Clark K.F., Lithology, structure and gold deposits of Norhtwestern Sonora, Mexico; in Clark, K.F. (ed), Gold deposits of northern Sonora, Mexico: Soc. Econ. Geol. Guidebook, v. 30, 203‐248. Fries C.Jr. ,1962, Reseña de la geología del Estado de Sonora, con énfasis en el Paleozoico : Bol Asoc. Мех.Geólogos Petroleros, v. 14, p. 257‐273 Staude J.‐M. G. & Barton M.D., 2001, Jurassic to Holocene tectonics, magmatism, and metallogeny of northwestern Mexico, GSA Bulletin v.113 n°10, p.1357‐1374. Mauger R. L. & Dayvault R. D.1983, The Tertiary volcanic rocks in lower Santa Clara Canyon, central Chihuahua, Mexico; in Clark, K.F. Goodell P.C. (eds) Geology and mineral resources of north‐
central Chihuahua, vol 14; Pages: 175‐186. Clark K.F. & Ponce S.B.F., 1983, Summary of the lithologic framework and contained mineral resources in north‐central Chihuahua, n Clark, K.F. Goodell P.C. (eds) Geology and mineral resources of north‐central Chihuahua, vol 14; Pages: 76‐93. Ciudad Juárez (University of Texas at El Paso), http://www.geo.utep.edu/pub/barud/homepage.html 8
“La Herradura” ore deposit in NW Mexico: An orogenic type gold mineralization Mauricio Ibanez‐Mejia Introduction: La Herradura deposit is located in the Sonora state in northwestern Mexico, approximately 80 km northwest of the town of Caborca (Figure 1). Producing an average of 210.000 Au ounces per year and Ag as a sub‐product (www.penoles.com.mx), this is the largest gold mine operating in the country. Up to date, around 2 Moz of gold have been extracted from this mine, and the remaining reserves are calculated around 3.4 Moz with an average grade of 0.1g/t. The deposit is owned by a joint venture between Minera Peñoles and Newmont. Figure 1: Political map of N Mexico and SW United States showing the approximate location of the “La Herradura” deposit. Regional Geology: In terms of crustal provinces, this region corresponds to the “Caborca block” (Dickinson & Lawton, 2001) which is a Laurentian affine terrane with an underlying Proterozoic basement ~ 1.7‐1.8 Ga old. However, its proximity to the proposed Mojave‐
Sonora megashear in the northeast, the structure that sutures the Caborca block and the North American Block, results in pervasive deformation and complex faulting in the mine area. As would be expected in this tectonic setting, the deposit exhibits a strong structural control. The mineralization is confined to a NW‐SE trending slice of amphibolite‐facies Proterozoic gneisses that is bounded to the NE by the Victoria fault and to the SW by the Ocotillo fault (Figure 2). These shear zones separate the Proterozoic rocks from a low‐grade metavolcanic/metasedimentary Jurassic sequence to the E and an incipiently metamorphosed sequence of limestones and quartzites of upper Paleozoic age to the W. 9
Figure 2. Simplified geological map of the mine area (from Quintanar‐Ruiz, 2008). The host rocks of the mineralization, the Proterozoic gneisses, are shown in orange. These rocks define a NW‐SE trending belt, bounded by the Victoria and Ocotillo faults to the NE and SW respectively. Topographic contours define the actual sizes of the Centauro and Yaqui pits, and the dotted yellow line defines the perimeter of the projected megapit. Mineralization: Gold rich veins are restricted to the Proterozoic rocks which consist of biotite‐
feldspar‐quartz and quartz‐feldspar gneisses of sedimentary and plutonic origin. The hydrothermal system is structurally controlled and the higher grade mineralization occurs in quartz‐sulfide veins filling tensional fractures. Quartz‐sericite‐albite alteration is widespread in the core of the deposit and grades to propylitic in the outer parts where the gneisses are more biotite rich. The average grade is calculated to 1g/t of Au with a cut‐off of 0.35 g/t. Geochronological results indicate that the age of the mineralization is lower Tertiary, confirming its Laramidic origin. Mining of the mineralized body is being conducted in two adjacent open pits, the Centauro and the Yaqui pits (Figure 2). The principal pit (Centauro) is approximately 1km long and 0.65km wide (Figure 3), but the projected megapit will join the two smaller ones and will extend another 1.2 km to the NW following the gneissic host rocks along strike. 10
Figure 3a: Satellite view of the Centauro open pit (image form Google Earth). 3b: Picture of the Centauro pit taken during our visit to the mine (from the SE road of Figure 3a, and looking WNW) The visit: After a 4 hours drive from Hermosillo‐Sonora and a flat tire on a dirt road, we got to the mine on a Monday morning, the 12th of January / 2009. We spent the rest of the morning assisting a talk given by the Peñoles geologists about the deposit model, and the afternoon visiting the Centauros pit. Pictures presented in this report (Figures 3a, 4 and 5) were taken during our visit to the pit. Minera Penmont S.A, Industrias Peñoles and Newmont are gratefully acknowledged for allowing us a visit to the mine. References: Dickinson, W.R., and Lawton, T.F. 2001. Carboniferous to Cretaceous assembly and fragmentation of Mexico. GSA Bulletin 113 (9), p. 1142‐1160 Quintanar‐Ruiz, F.J. 2008. La Herradura ore deposit: An orogenic gold deposit in northwestern Mexico. M.Sc. thesis, Department of Geosciences, University of Arizona. 97 pp. 11
Figure 4: Visit to the Centauro pit. While the Geneva crew discusses the deposit model with the Peñoles geologists (group to the left), Victor (center of the picture) tries desperately to identify the alteration paragenesis and mineral assemblage in the gneisses. Figure 5: Visit to the Centauro pit. Mineralized hydrothermal breccias in the mining front. The mineralogy consists of quartz‐hematite‐galena‐gold. 12
Tuesday, 13th of January 2009 – Milpillas Porphyry Copper deposit Johannes Mederer Overview: •
Departure from Magdalena de Kino, where we had spent the night, early in the morning and arrival at the mine site around 9:30 AM •
Safety introduction and a short presentation of the local geology and the mine operation •
Visit of the surface operation: collecting minerals from the ore stockpile (mostly azurite, malachite, brochantite) •
Study of drill cores at the core shack •
Visit of the processing plant •
Drive to Ascensión to spend the night there Introduction and geology of deposit The secondarily enriched porphyry copper deposit Milpillas is situated in the Northeast of the Mexican state Sonora, 30 km south of the border to the USA and 20 km northeast of Cananea, the next major city. It was discovered after having found porphyry deposit style veins in scarce outcrops of barren rocks with abundant “live hematite”, indicating potentially economic copper mineralization. After an intensive exploration program by Minera Cuicuilco in the 70’s Grupo Peñoles took over the project in 1998 and started mining it in 2006. Milpillas is situated in the Cananea mining district, which has copper reserves of over 11 million tons (Long, 1995). The district lies within the important NW‐SE trending metallogenic Laramidian (late Cretaceous to Eocene) copper belt of Southwest North America which extends from Sonora over Arizona to New Mexico, hosting world class porphyry copper deposits as La Caridad in Sonora or Silver Bell in Arizona (Noguez‐Alcántara et al., 2007). Descriptive work on Milpillas was performed by de la Garza et al. (2003). Noguez‐
Alcántara et al. (2007) carried out mass balance analyses concerning the supergene enrichment. Valencia et al. (2006) constrained the age of the mineralization. Each of the cited papers includes a good overview of the regional and local geology. The information below is taken from these works, as well as from a presentation which was held for our group at the mine site by the engineer Alfonso Ingas. The Milpillas deposit, located in an extensional zone called Cuitaca Graben is covered mostly by Tertiary gravels and Quaternary alluvium. Only very scarce outcrops, which show altered, leached and oxidized volcanic rocks (with abundant “live hematite”) give evidence of the deeply buried treasures. Host rocks are volcaniclastic rocks from the Jurassic Henrietta formation as well as from the Laramide Mesa formation. Small monzonitic to quartz‐
monzonitic stocks intrude the above mentioned volcaniclastic rocks. A sample from the quartz‐monzonite yields a crystallization age of 63.9 ± 1.3 Ma (U‐Pb LA‐ICPMS‐MC on zircons) and the age of mineralization was determined with 63.1 ± 0.4 Ma by Re‐Os on molybdenite 13
(both ages by Valencia et al., 2007). The sercitically altered stocks as well as the intruded volcaniclastic rocks host the main copper mineralization. Supergene enrichment A brief introduction to the processes which control the supergene enrichment of copper is given by Robb (2005). These include oxidation and hydrolysis of the primary hypogene sulfide minerals in the upper portion of the weathering profile. Their destabilization by oxidized acidic groundwater produces acid which alters and leaches the rocks, resulting in residual copper contents typically in between 0.01 and 0.02 wt. %. A mineral assemblage typically enriched in Fe‐oxides and hydroxides characterizes the leached rocks. Cu2+ is released mostly from primary chalcopyrite and transported by the oxidized fluids until reaching more reducing and/or higher pH environments (due to neutralization of the fluids or by reaching the water table). Secondary copper minerals form in two ways: by direct precipitation from the fluids, giving rise to a suite of “Cu‐oxides” (see figure 1a), mostly sulfates, carbonates, oxides and arsenates or by the replacement of Fe2+ in primary hypogene sulfide minerals like pyrite and chalcopyrite, leading to high metal/sulfur sulfides such as chalcocite, covellite or bornite. In Milpillas primary potassic and propylitic alteration as well as phyllic‐argillic alteration towards the center of the system can be observed. Nevertheless, it is only poorly recognizable due to its superposition by the secondary supergene alteration. Primary hypogene mineralization is made up by 1 to 10 % pyrite and chalcopyrite which is the main primary ore mineral in the deposit. However, the primary hypogene mineralization is non‐
economic, with average copper grades between 0.1 and 0.15 wt. %. Supergene alteration and enrichment caused the now economic ore bodies with ore grades of > 1 to over 10 wt. % Cu found in depths in between 150 and 700 meters, covered by subhorizontal gravel layers 20 to 350m thick. 14
Figure 1: Pictures (1a‐d) from the Milpillas operation in northeastern Sonora; a) hand specimen from the oxide zone with azurite and malachite as main ore minerals b) preparation of the leach heaps with black impermeable plastic foil at the base c) the cathodes coated by 99.9 % pure copper are taken out of the copper rich solution by crane d) copper plates ready for shipping Supergene Profile The supergene profile in Milpillas is made up of 4 principal zones, within which certain overlap exists. They are from top to the bottom as follows: 1. The leached cap is characterized by oxidized rocks: goethite, hematite and jarosite are abundant, copper values are very low (between 0.01 and 0.05 %) with a common thickness of 100 ‐ 300m. Montmorillonite, sericite and kaolinite dominate as secondary alteration minerals of the volcanic hostrock. 2. The oxide zone above the water table is situated at the lowermost part of the leached zone: sub‐horizontal mineralized blankets, made up mostly by copper oxides, copper carbonates and sulfates formed mostly by oxidation of preexisting chalcocite. Minor amounts precipitated from oversaturated groundwater. The mix of different copper species is a result of at least three supergene enrichment cycles in Milpillas due to uplift or changes in the water level. 3. Supergene enrichment zone: situated at the base of the weathering profile with reducing conditions underneath the paleo water level. In this zone the copper solubility is decreased, which results in the replacement of iron in the primary hypogene minerals: chalcocite dominates together with covellite when copper enrichment is less pronounced. Native copper can be found at the borderline of reduced and oxidized conditions. 4. Hypogene primary mineralization: the surface between this zone and the supergene enriched one is irregularly shaped. In a transition zone different “copper oxide” minerals as well as sulfides can be abundant. Underneath the transition zone the mineralization is primary as it is described above. The current exploitation depth at Milpillas is about 600 meter below the surface. At the moment of our visit only copper minerals from the oxide zone were mined out. The ore at Milpillas is crushed to an average grain size of 2 cm from where it goes directly to the heaps on the mine area for heap‐leaching. At Milpillas the copper from the oxide ore is released by sulfuric acid, yielding a recovery of 91 % after having remained for 200 days on the heap. Copper goes into solution in the acid which is sprinkled on the ore. The resulting copper‐enriched acid is canalized by gravitational forces towards the processing plant. After four further enrichment steps of copper in solution, the metal is recovered by electrolytic deposition of copper on stainless steel cathodes. After having remained for 7 days in the copper rich acid, the cathodes are removed by crane and the deposited copper can be taken off in form of copper plates, ready to be cleaned, weighed, and packed for shipping. 15
References: Long, K.R., 1995: Production and reserves of cordilleran (Alsaka to Chile) porphyry copper deposits, in Pierce, F.W., Bolm, J.G. (eds.) Porphyry Copper Deposits of the American Cordillera: Tucson, Arizona Geological Society Digest, 20, 35‐68. Noguez‐Alcántara, B., Valencia‐Moren, M., Roldán‐Quintana J., Calmus T., 2007: Enriquecimiento supergénico y análisis de balance de masa en el yacimiento de pórfi do cuprífero Milpillas, Distrito Cananea, Sonora, México: Revista Mexicana de Ciencias Geológicas, 24(3), 368‐388. Valencia, V.A., Noguez‐Alcántara, B., Barra, F., Ruiz, J., Gehrels, G., Quintanar, F., Valencia‐Moreno, M., 2006: Re‐Os molybdenite and LA‐ICPMS‐MC U‐Pb zircon geochronology for the Milpillas porphyry copper deposit: insights for the timing of mineralization in the Cananea District, Sonora, México: Revista Mexicana de Ciencias Geológicas, 23(1), 39‐53. Robb, L, 2005: Introduction to ore‐forming processes, Blackwell Science, 238‐245. 16
The skarn and carbonate replacement Pb‐Zn deposit of Bismark in northern Mexico Edina Vago Introduction The Bismark Pb‐Zn mineralization district, property of the Peñoles Corporation is located in northern Mexico (~N31 04 05; ~W107 54 02) near the city of Ascensión, Chihuahua state. Ore deposits similar to Bismark (Carbonate Replacement Deposits) are very important in the world’s metal production representing a major source of Pb, Zn, Ag +/‐ Cu and also Au. Mining history The history of the mining activity at Bismark has probably started at the end of XIXth century. First precise data about mining activities come from the neighboring area of La Florencia, San Pedro from 1885. In 1979, a series of geophysical surveys were performed for a prospection project of northwestern Chihuahua including magnetometry, resistivity and induced polarization around the contact zone of the Bismark intrusion. Diamond drilling exploration was carried out between 1981 and 1984 with 55 drill holes discovering two continuous ore bodies within the contact skarn of the Bismark intrusion. The first inferred resource assumption was 12.8 Mt ore with 52 g/t Ag, 11% Zn, 0.6% Pb and 0.4% Cu. During the last 3 years the average diamond drilling produced ~22.5 km core from the surface and ~16.5 km from underground, which is definitely higher (double) than in the last 20 years. The daily production is generally 2500t. Since 2002, the proved‐, probable‐, possible‐ and potential resources are slightly increasing. The proven resources of 2008 was approximately 2 Mt and the extraction of 1679t Pb, 76360t Zn and 8961t Cu. Between 1999‐
2002 there was no Pb production. Since 2003, it has been increasing very slowly again. The last peak in Zn production was in 2005 with an average of 15% Zn. The Cu production varied in the last ten years between 1.67 % and 0.92 % (Figure 1). 17
Pb
Pb, Zn and Cu production
Zn
Cu
Metal %
16
12
8
4
0
91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08
19 19 19 19 19 19 19 19 19 20 20 20 20 20 20 20 20 20
Year
Figure 1: Average grade (%) of Pb, Zn and Cu from the Bismark Mine, northern Chihuahua, Mexico (based on calculation from the data what we were showed in a ppt presentation by the responsible geologist on 14th of January 2009) Geological setting and mineralogy The major part of the Chihuahua geological province is composed of early Cretaceous sedimentary rocks. The quartz‐monzonitic Bismark intrusion was emplaced into the Cretaceous limestone during the Tertiary (~42 Ma). The intrusion has two different textural appearances, one is equigranular, which is a texture occurring in the later dykes crosscutting the intrusion itself and the other has the texture of the main intrusion which is porphyrytic. The Bismark fault strongly controls the ore bodies (Figure 2). The mineralisation is related to the contact skarn zone, postdating the grossular and andradite exoskarn, and also occasionally mineralises the intrusion at the contacts, as well as and the host rock (Figure 3). The ore mineralisation is composed of massive sulfide bodies of sphalerite‐galena‐
chalcopyrite‐pyrrhotite controlled by the Bismark fault. They appear as subvertical chimneys and manto type ore bodies (Figure 4). The mantos have more banded structures than the chimneys and contain higher Pb and Ag concentrations. Pyrrhotite is more abundant in the more westerly contact skarn. The mineralization also contains Bi, but its mineralogical position in the paragenesis is not fully understood as yet.
Figure 2: A cross‐section with chimney type ore bodies marked in red. 18
Figure 3: Sulfide veinlets at the contact of the Bismark intrusion and the skarn zone of the host rock. Figure 4: Chimneys type ore bodies following the strike and dip of the Bismark fault (SW‐NE section) The mineralizing fluids were probably circulating along the fault and provoked a replacement type Zn‐, Pb‐, Cu‐ mineralization within the limestone and the exoskarn at both sides of the fault. Textural evidences of reaction fronts are visible in the skarn. Layering of sulphides within the skarn shows the replacement mechanism related to diffusion‐
precipitation equilibria (Figure 5). 19
Figure 5: reaction front of sphalerite within garnet skarn. References Baker and Lang 2003, Reconciling fluid inclusion evidence for a magmatic source of metals in porphyry copper deposits, in Thompson, J.F.H., ed., Magmas, fluid and ore deposits: Mineral Association of Canada Short Course Series, v. 23, p. 139‐152. Baker et al. 2004, Composition and evolution of ore fluids in a magmatic‐hydrothermal skarn deposit, Geology v. 32; no. 2; p. 117‐120. 20
Santa Eulalia Mine Aldo Bendezú General geology The geologic setting of the Santa Eulalia district is similar to other high‐
temperature, carbonate‐hosted Ag‐Pb‐Zn deposits of northern Mexico. The host rocks are Cretaceous limestones intruded by felsite and diabase dikes and sills (Figure 1). Mineralization is associated in time and space with the felsite intrusions in two zones known as the East and West camps, which lie on opposite flanks of a broad, doubly plunging anticline. The felsites are texturally and compositionally indistinguishable and have identical REE patterns. West camp mineralization consists of massive sulfide mantos and chimneys with lesser mineralized breccias and skarn bodies. East camp mineralization consists of dike contact skarns with subordinate massive sulfide bodies. Structural controls on mineralization in both camps include reactivated fold‐related fracturing and faulting as well as intrusive contacts. Host‐rock lithology (specially secondarily enhanced permeability) controls ore distribution in the upper and peripheral parts of the district. Ten different ore types have been recognized in the district, but only the normal sulfides, silicates (calcic‐iron skarns), and calc‐silicate skarn ores were examined in this study. The ore mineralogy throughout the district is relatively simple, consisting of pyrrhotite, pyrite, sphalerite, and galena with minor amounts of calcite, quartz, fluorite, and in places calc‐silicate gangue. Despite stark differences in trace metal contents and the amount and composition of the skarns in the two camps, the overall geologic character, fluid inclusion compositions, sulfur, oxygen and carbon isotope characteristics and alteration assemblages are similar enough to indicate that the ore fluids for both camps had a common origin. Figure 1: Generalized geological map and cross section of the Santa Eulalia mining district. West camp mineralization Manto and chimney deposits are the most common types of mineralization in the West camp. The ore mineralogy consists of pyrrhotite, galena, sphalerite, and pyrite, replacing limestone with little alteration of the host rock (Figure 2a). Skarn mineralization is more limited, present as either silicate or calc‐silicate orebodies (Figure 2b). Silicate orebodies consist of Ca‐Fe silicates with Mn‐rich compositions. The silicate orebodies are reported to have higher silver values than the manto ores. Ca‐silicate orebodies consist of calcic skarn silicate minerals and an ore assemblage of galena, sphalerite, arsenopyrite, pyrite, and pyrrhotite. Trace amounts of chalcopyrite and pyrrhotite, argentopyrite, polybasite, and stephanite are also reported as discrete minerals in the calc‐silicate ores. A
sphalerite
chlorites
pyrite
B
garnets
Quartz,calcite
veins
sulphides
Figure 2a: Sulphide mineral assemblage with host rock altered to chlorites. b: Calc‐silicates (green and brown garnets) cut by quartz, calcite and sulfide veins . 22
East camp mineralization Mineralization in the East camp is controlled by structures related to the San Antonio graben and the contacts of a series of felsite dikes that follow and cut across the graben faults. Skarn mineralization dominates and calc‐silicate mineralogy is bilaterally and symmetrically zoned outward from the dikes. The dikes are widely converted to calc‐
silicates. Most of the sulfides in the East camp occur within the skarn, but podiform sphalerite + pyrite ± pyrrhotite bodies (± galena) commonly occur between the skarns and the enclosing limestone. Massive sulfide mantos composed of galena, sphalerite, and pyrrhotite, are texturally similar to the West camp ores and occur in contact with, and peripheral to, the skarns. Reference: Lueth V.W., Megaw P.K.M., Pingitore N.E., Goodell P.C., 2002: Systematic Variation in Galena Solid‐Solution Compositions at Santa Eulalia, Chihuahua, Mexico; Economic Geology, v.96, n. 8, p.1673‐1687 23
Naica Mine (Pb‐Zn‐Ag) Anne Chevalier Geography The Naica mining district belonging to the Peñoles Corporation is located about 100 kilometers southeast of the city of Chihuahua, which is the capital of the state with the same name. This semi desert region is characterized by elongate 1’300 meters high mountain ranges. These mountain ranges are separated by large alluvium basins. Geology setting The Naica mine, which constitutes one of the most important lead deposits in Mexico, is situated on a structural dome measuring about 12 by 7 km, elongated in a NW‐SW direction (John G. Stone, 1958). The country rock consists mainly of Cretaceous limestones crosscut by Tertiary felsic dykes. The area is cut by 3 main faults: Naica and Gibraltar fault with a 60°NW dipping and the Montana fault which cuts the Gibraltar fault. Ore deposits are found between these structures, which are also the main channel ways of ore‐bearing fluids reaching a temperature of 55°C. They could emanate from a magma intrusion located few kilometers below (Figure 1). Figure 1 : Scematic cross-section of the
mine (information by Peñoles)
Ore bodies Ore deposits are polymetallic skarn type and associated carbonate replacement ore bodies. The skarn hosted mineralization is usually sub‐horizontal while replacement chimneys are sub‐vertical often consisting of massive sulfide. The lead and zinc content in the chimneys reach about 12% and 180 ppm for silver. From these orebodies, the mine extracts 5‐6% of lead and zinc and 120 ppm of silver (also see figure 2 with production data). This underground mine is highly automated with computer controlled production processes guaranteeing continuity of operation. The rock is crushed underground and brought up to the surface by a conveyer belt. A big challenge for the mine operators is the circulation of high quantities of groundwater. This water has to be pumped at 60’000 l/min to avoid flooding of the mine. At the present time, a project is under consideration in order to minimize the pumping charges. The company’s objective is to use the water pressure to reduce pumping costs. 24
Figure 2. : Production overview (source: http://www.penoles.com.mx/penoles/ingles/) THE GIANT SELENITE CRYSTALS (Cave of the Crystals ‐ Cueva de los Cristales) In 2000 the excavation of a communication tunnel at 290 meters below the surface unveiled the presence of a cave containing giant selenite (CaSO4∙2H2O) crystals reaching up to 14 meters in length and 2 meters in diameter (Figure 3). These crystals formed underwater, in a cave where the hot and calcium carbonate and sulfides saturated fluids got in touch with another colder fluid that infiltrated from the surface. The Naica fault which played the role of the channel way for the fluid and the fluid chemistry created a favorable environment to precipitate these minerals. To reach this size, the crystals must have grown during about one million years. In this cave, the temperature is approximately 48° C and the air is saturated with humidity making a visit to these caves a “being cooked in a steamer” experience. Figure 3: Published Picture of the Naica giant selenite crytals on the left (source: http://www.naica.com.mx/galeria_pc.htm); on the right: geological cross‐section through a selenite forming cavitiy (from Peñoles geological staff). Reference : John G. Stone, 1959: Ore genesis in the naica district, chihuahua, Mexico, in Economic Geology, vol.54, pp.1002‐1034. 25
Santa Barbara mine (Mexico Group) José Agustín Pérez Introduction and geology The Santa Barbara mine is located a 45 minutes drive southwest of Parral. Our group was welcomed by by Hugo Soto (Geology CEO) and José Porfirio Pérez, (Mine Geologist). The commodities in Santa Barbara are lead, copper, zinc and silver. The ore occurs as veins, being mainly massive structures of 400 meters extension (in average) and up to 25 meters thick, hosted by Cretaceous and Oligocene shales, andesites and rhyolites. Rhyolithic dikes crosscut mineralization, in other cases they are mineralized. All hostrocks are discordantly covered by Pliocene conglomerates and basalts, and Quaternary sediments. The Santa Barbara mine comprises 29 veins, dipping close to 90° and oriented in three main directions (NS, N25W and N20‐23E) into a NW‐SE structural trend with dextral movement, the NS group being the economically most important. Some of these structures outcrop. The upper 300 meters of the veins are dominated by oxides, followed by 100 meters with presence of other secondary Cu minerals (e.g. covellite) and about 650 meters dominated by primary sulfides (galena, chalcopyrite and sphalerite). With depth, the lead and silver content is lower than in the more shallow zones. Copper and zinc values are more or less constant in depth and higher in the east than in the west. Visit to the mine After a short talk about safe and security (Figure 1) and a slideshow introductory presentation of the geology and mine operation at Santa Barbara, we all moved to the San Diego mining area, in the easternmost sector of the mine.
Figure 1.: Security at Santa Barbara mine; Victor Valencia breaking off loose wall rock San Diego Mine Cegovedad is the mining sector located in the center, from where 70% of the total ore is processed and transported to surface from the underground production areas 26
with the help of modern technology and a refined process. The westernmost part of the mine is named Tecolotes and is presently not in production. After fitting our lamps and security gear, we were ready to visit one of the areas in production. A double‐cab elevator brought us from surface to 1200 m depth where two “modified” tractors picked us up for our visit to the ores of Santa Barbara (Figure 2). In there we recognized the sulfide ore, compound by galena, sphalerite and chalcopyrite veins of different thickness (1‐2cm to 25 m) hosted by carbonates and lutites. After checking out the ore and locking at the different features present in this production area we visited the high tech facilities present underground (Figure 2). A primary crusher reduces the size of the ore fragment for transport by conveyer belt to surface in the Cegovedad area. We concluded our visit after a brief discussion at the surface. Figure 2: Ores of Santa Barbara; veins rich in sphalerite, galena and chalcopyrite; green bands of pyroxene skarn, (left); visiting the mine production facilities (right) 27
Summary of observations on the Pinos Altos epithermal gold & silver bearing veins, Mexico Miguel Ponce Introduction and geology The Pinos Altos gold & silver project is located 280 km west of Chihuahua, NW Mexico, in the Sierra Madre gold belt (Figure 1). This project is owned by the Canadian company Agnico‐Eagle (AEM). Probable reserves are estimated to 2.5 Moz Au and 73.1 Moz Ag, and expecting to start production on third quarter 2009. Currently, most operations are focused on the preparation for production. An open pit and a ramp for underground mining are being prepared, as well as the setting and building of the processing and treatment plants, among others. Figure 1: Pinos Altos location map Figure 2: Location of Pinos Altos in relation to the Ocampo caldera Structurally controlled by regional NE trending faults and by a ring fracture, Pinos Altos is located on the NE border of the Ocampo caldera, which was defined after geomorphologic features. This tertiary caldera has a diameter about 30 km (Figure 2) and has been closely related to other neighboring mineralized areas. The host rock of the mineralized hydrothermal system is represented by an acidic volcanic sequence (Buenavista & Victoria ignimbrites) and the Frijolar andesites (Figure 3). This volcanic sequence is probably the result of activity of the Ocampo caldera. 28
At the project scale, several mineralized structures are defined up to 800m along strike. So far, the most important structure is called Santo Niño. Other areas with high potential like San Eligio, Cerro Colorado and Oberon de Weber, are found in the NNE, WNW, and ESE side of the Santo Niño area respectively (Figure 3). Some ancient small workings from the 19th century are still preserved nearby. Drilling campaigns have been carried out mostly in the Santo Niño, San Eligio, Cerro Colorado and Oberon areas, so the geological and structural information described here is based on these data. Structural control and mineralization
Structures trend WNW‐ESE, and have variable width, showing also variability in gold grades (i.e. up to 15g/t Au on surface samples and thickness to 40m on outcrops at the Santo Niño area) with at least 600m depth extension. All these changes in grade and thickness can be best represented on a longitudinal section (Figure 4) where the contours represent grade x thickness (g/t x m). The longitudinal section roughly shows that the development of the structure and the higher gold grades occur near the surface levels, probably related either to a primary event (boiling level) or to a supergene enrichment process. 29
Figure 5: Several kinds of occurrences of structures and mineralization on the Pinos Altos project. a) Vein‐
breccia (Bx) showing subangular to subrounded clasts; b) Liesegang textures on ignimbrites; c) Stockwork‐
Bx near the vein‐Bx; d) Jigsaw‐Bx; e) Achantyte (>20 oz/t Ag); f) Stockwork development in the host rock of deposits. The beside the main green‐colored silica vein; g) Pyrite veinlets together with hematite patches; h) mineralization is Crackle‐Bx beside the vein‐Bx The Santo Niño structures dip southwestwards while the San Eligio structures dip northeastwards. In San Eligio surface samples yield up to 5g/t Au and locally at depth more than 300g/t. All those structures are best defined as vein‐breccia channels, surrounded by stockwork‐breccias (silica, sulfides, oxydes) as far as 20m from the main structure (Figure 5). In both Santo Niño and San Eligio, the mineralization comprises native gold (Au), acanthite (Ag2S), native silver (Ag) and probably other Ag‐bearing sulfides and selenides which are common on this kind of deposit. The mineralization is hosted by several types of silica, among which we can identify green cryptocrystalline chalcedonic silica (showing massive fine grained and coloform textures) which is closely related to mineralization (Figure 6), as well as drusy (comb) amethyst quartz, milky drusy quartz, and locally opaline silica. 30
Those latter seem to have less relationship with the mineralizing events. The fine grained silica indicates it was formed under low temperature conditions and high cooling rates. Also the presence of acanthite is indicative of temperatures less than 300°C. Other gangue minerals comprise pyrite, hematite, calcite and minor chrysocolla. Alteration minerals were identified mainly as clays (argillic alteration) outwards from the main silica‐mineralized structures. Illite and interlayered illite‐smectite is located close to the vein‐breccia structures, while the identified kaolinite is possibly related to diagenetic alteration of the volcanic glass and feldspar of the ignimbrites. The high porosity of ignimbrites together with the sulfide oxidation produces widespread hematite‐colored band and rings, known as liesegang textures. Based on mineralogy, structural setting, and alteration assemblages, we could classify this deposit as a low sulfidation epithermal system. It is a good example of an epithermal system possibly related to a volcanic caldera and it demonstrated well the structural control on Au‐Ag mineralization. 31