Strontium isotope geochemistry of barite mineralization at La Minita, S.W. Mexico

http://dx.doi.org/10.18268/BSGM1993v52n1a1

F. Ortigoza-Cruz¹; A. Changkakoti²; R.D. Morron³; J. Gray⁴

¹Zuloaga 380. Col. Los Angeles, Torreon. Coah CP. 2700, Mexico
²P. O. Box 6639.600 St. Kilda Road. Melbourne. Australia, 3004
³Department of Geology, University of Alberta, Edmonton. Canada T6G 2E3
⁴Department of Physics. University of Alberta, Edmonton, Canada T6G 2J1

Abstract

The La Minita Fe-Ba-Zn-Pb-Ag deposits are located in the Michoacan Province of Mexico. Previous fluid inclusion and stable isotope studies showed that these deposits were generated around submarine hot-springs issuing in a Cretaceous island-arc environment.

A strontium isotope study was carried out to determine the source of Sr and Ba. The ⁸⁷Sr/⁸⁶Srratios in barites, mafic volcanics, felsic volcanics and carbonates range from 0.70492 to 0.70535, 0.70451 to 0.70557, 0.70660 to 0.75709 and 0.70571 to 0.70685 respectively. The data suggest that both the volcanics and limestones were the sources of Sr in the mineralizing fluids. On the other hand, trachytes and rhyolites seem to be the most viable sources of Ba, with possible minor contributions from andesite. basalt and limestone.

Introduction

The La Minita mining district (Lat. 18°52'06"N, Long. 103°17'06"W) is located in the Michoacan Province of Mexico (Fig. 1) Although exploration for barite had been carried out in the 1950's, a quest for silver-lead zinc deposits by Peñoles Co. did not begin until 1976 (Gaytan et al., 1979). In 1979 an economic deposit of> 6x10⁶ tonnes, grading 4.0% Zn, 0.33% Pb, 48.0% BaS0₄ and 78 g/tonne Ag was outlined; however, the inventory of reserves as of October 1986 indicated that the ore consisted of over 2x 10⁶ tonnes, grading 4.48% Zn, 0.34% Pb, 34.9% BaS0₄, and 58 g/tonne Ag (Alonso and Jimenez, 1986).

Detailed fluid inclusion and stable iostopic studies on the mineralization of the distric revealed that the deposits formed from circulating seawater which caused replacement of the reefal limestones at low-rock ratios during the pre-ore stage/Fe-oxide mineralization and at higher water-rock ratios during sulfate-sulfide mineralization (Ortigoza-Cruz et al., 1990). The carbon isotope data suggested a marine origin for the carbon. The sulfur isotope suggested seawater sulfate and inorganically-reduced sulfur (and/or igneous sulfur) to be the main sources of sulfur in the system.

The importance of strontium isotopes as tracers for barium in ore deposits is well documented (Kessen et al., 1981: Lange et al., 1983; Kesler et al., 1988; Frimmel and Papesch, 1990). The present investigation on strontium isotopes of barites, carbonates and whole-rocks was undertaken to augment the existing geochemical data base on the deposits, and to shed additional information on the origin of these submarine exhalative deposits.

Regional geological setting

The Southern Sierra Madre of SW Mexico comprises an intricate series of belts whose stratigraphy and structure suggest allochthonous origins. These terranes were accreted to North-America during Late Cretaceous-Early Tertiary times. Mid Tertiary to Quaternary volcanic sequences blanket juxtaposed terranes, preventing interpretation of their boundaries and interrelationships (Campa and Coney, 1983).

Campa and Coney (1983) recognized five allochthonous terranes in the Southern Sierra Madre. The Guerrero terrane is an accreted ancient island arc, related to the Cretaceous subduction of the Pacific Plate, off the western coast of Mexico. The absence of a crystalline basement below this terrane is an indication of the generation of this terrane upon young continental, or oceanic crust. The Guerrero terrane has been further divided into three subterranes of similar stratigraphy but of contrasting structure, namely: the Zihuatanejo, the Huetamo, and the Teloloapan sub-terranes (Campa and Coney, 1983).

The Zihuatanejo sub-terrane, within which the La Minita district occurs, consists of a thick sequence of andesitic volcanics intercalated with Albian-Cenomanian limestone and some shale-sandstone interbeds (more than 3000 m of this unit were recovered from the Jalisco 1 drill-core (Lopez Ramos, 1981). The existence of evaporitic strata intercalated with these limestones has been described elsewhere (Lopez Ramos, 1981). The upper part of this sequence grades into a clastic unit. Folding and faulting have affected this subterrane, but no metamorphism has been detected.

Figure 1. Location of the La Minita District. Mexico.

The Huetamo sub-terrane consists of Upper Jurassic volcaniclastic sediments overlaying by a Neocomian sandstone-shale sequence which grades into an Albian unit of limestone, tuff and redbeds. Late-Cretaceou redbeds and ignimbrites from the upper member of the unit. Tight, upright folds and an absence of obvious metamorphism characterize this terrane.

The Teloloapan sub-terrane consits of a low-grade metamorphic, intensely deformed, andesitic volcanic-sedimentary sequence interbedded with shale, sandstone and limestone of Late-Jurassic to Early Cretaceous age (Campa, 1978: Campa and Coney, 1983).

In both the Zihuatanejo and Huetamo sub-terranes, the lithologic facies define a series of topographic highs with shallow to deep basins. The presence of patch reefs within volcanic-pyroclastic sequences, very common in the La Minita-Coalcoman area, suggests seamount-like environments with fringe reefs. Sediments deposited in shallow-water and transitional environments also suggest a typical island-arc archipelago environment (Campa, 1978).

F1gure 2. Geology of the La Minita District. Mexico.

The presence of the metamorphosed Teloloapan subterrane in between the apparently unmetamorphosed Zihuatanejo and Huetamo sub-terranes, all three being of similar age, has constituted an enigma. Campa (1978) suggested that these sub-terranes were tectonically juxtaposed and that the outcrops of the Teloloapan terrane represent the upper block of a nappe, for in some instances they have been found overlying unmetamorphosed sequences. The presence of these metamorphosed and unmetamorphosed terranes of similar age, could be ascribed to stacking of thrust-sheets into a melange during the process of accretion.

Local geology

Lithologic Setting

The mineral deposits (Vulcano, Tabaquito, Sapo Negro and Ojo de Agua) of the La Minita District are hosted by a Middle Cretaceous volcano-sedimentary sequence, the majority of the mineralized zones being located within or near reefal carbonate horizons.

Three lithologic units can be recognized (Fig.2), namely:

(i) A lower unit, predominantly of mafic, submarine volcanic origin, occurring in the NNE sector of the district. This unit consists of hydrothermally-altered. submarine andesitic volcanics and pyroclastics with autoclastic-hyaloclastic textures. No pillow structures have been observed. The rocks carry propylitized assemblages consisting of albite +chlorite +epidote +calcite + hematite + quartz. Iron-manganese rich-beds within spilitized basalts are present at the Sapo Negro locality.

(ii) A middle unit which hosts all of the mineralized bodies and occurs in the north and central sectors, is composed of hydrothermally-altered mafic to intermediate volcanics (pyroclastics and lavas), reef limestones and thinly-stratified carbonates. This unit consists of volcanic and sedimentary rocks deposited within a hallow marine, sub-tropical environment. The volcanic components are trachyandesitic to trachytic pyroclastic and lavas. Vitric and lithic tuffs predominate, volcanic breccias also being common. At places biohermal patches of fossiliferous limestone are intercalated with volcanic sequences. At Vulcano there is clear evidence of coeval reef growth and sulfate-sulfide/oxide mineralization.

(iii) An upper unit consisting of turbiditic shale-limeslone interbeds which blanket the middle unit in many areas and do not exhibit any hydrothermal alteration. The youngest unit consists of basinal shale-limestone interbeds. Its base consists of coarse to fine-grained turbiditic strata unconformable upon a kaolinized-chloritized tuff which in turn lies on the reef limestone (at Vulcano). No basal conglomerate exists at the unconformity, however the lack of alteration and of volcanic material on the shale-limestone unit suggests a sharp change in the tectonic/volcanic regime of the region. The upper part of the sequence is composed of a black shale-limestone with frequent thin pyritic layers. This unit seems to have been deposited within small graben-like basins of variable depth (Gaytan et al., 1979). This observation is reinforced by the intensely sheared nature of the middle unit in the center of Vulcano pit and the relatively mild deformation of this upper unit, at least south of the pit.

Mineralization

Mineralization in the La Minita district is dominated by iron oxides with occasional associated barite and Zn-Pb sulfide mineralization. The mineralization is of two principal varieties, namely:

(i) Epigenetic, stratabound mineralization carrying iron oxides, barite and Zn-Pb sulfides and replacing carbonate horizons (at Vulcano, Tabaquito and probably Ojo de Agua).

(ii) Syngenetic, stratiform mineralization in the form of massive bedded pyritites (at Vulcano) and iron formations (at Vulcano, Tabaquito and Sapo Negro).

The Vulcano orebody

At Vulcano, the main replacement body occurs at the center of the mine and extends radially outwards in a domal structure (Gaytan et al., 1979). It consists of irregular hematite-magnetite-pyrite zones, principally in the lower part of the body. Most of the orebody contains radial intergrowths of fine-grained barite with disseminated sphalerite, pyrite, galena and tetrahedrite. The top of the body is rich in jasper which occurs mixed with brecciated limestone; this zone grades upwards into recrystallized limestone and finally into fresh limestone. The wholebody is transected by quartz-pyrite-calcite veins. Sporadic occurrences of limestone remnants and of replaced fossils within the ore zone support an epigenetic origin for this body. At depth, there are two limestone-magnetite beds, only detected in the deepest drill-holes, whose thickness reaches up to 20 m thick. These strata are enclosed by a dominantly pyroclastic sequence of highly-chloritized, silicified trachyte underlying the main ore body. Fine grained magnetite, hematite, and jasper are ubiquitously present with minor associated sulfides (sphalerite, pyrite and chalcopyrite). Recrystallized calcite is generally associated with the oxide-rich zones. Veins and irregular replacement of limestone are common.

At the southwestern sector of the pit there is a reefal limestone, largely silicified and replaced by magnetite. Here almost all of the fossils, mainly caprinulid rudists, are replaced by quartz and magnetite. Irregular magnetite-quartz veinlets transect the rock, however no appreciable recrystallization of the limestone occurs. This seems to suggest that the alteration of the limestone occurred shortly after its deposition, perhaps, during the diagenetic stage.

The second variety of mineralization at Vulcano consists of massive and internally-bedded syngenetic pyrite which occurs in the northeast sector of the pit. This ≈ 15x10 m body occurs at a fault zone and is intensely sheared. Graded bedding, pseudo-layering and framboidal textures are present and confirm the exhalative-sedimentary nature of this body. It consists of fine-grained pyrite with minor sphalerite-barite and traces of galena. Laterally this body grades into a zone of pyrite-marcasite-barite-calcite with minor sphalerite. The whole body is overlaying by marine limestone and seems to be a lateral equivalent of the replacement body. It would appear that the aforesaid fault zone existed prior to the mineralizing event, and probably constituted a vent for the ore-solutions (Gaytan et al., 1979) which generated the proximal pyritic body. Further reactivation of the faull disrupted the pyritic ore.

The third variety of mineralizations consists of stratiform iron mineralization which occurs at the southeastern sector of the mine. It consists of at least two strata of jasper-hematite, enclosed within pyroclastic rocks.The unsorted, massive structure of the underlying pyroclastic member, which may constitute an ignimbritic unit, points to the existence of a shallow environment of deposition. Chloritization and baritization of the enclosing tuffs is extensive. Fossils replaced by barite are found in the tuffs underneath the ferriferous layers. Additionally, olistostromes of limestone within the tuffs are, as well, highly altered to jasper and barite, and late barite-sphalerite associations.

It is thus apparent that syngenetic and epigenetic ores were coeval at Vulcano. The former resulted from replacement of reef limestone by sulfate-sulfide-iron oxide mineralization, whilst the latter formed by precipitation onto the seafloor of proximal pyrite and "more distal" jasper hematite beds.

The Tabaquito Deposit

On the whole, the mineralized zone at Tabaquito is stratabound within the volcano-sedimentary sequence. The limestone component is much less than in Volcano. However, the mineralization is again clearly spatially associated with this rock-type. The main ore zone is massive and consists of barite-quartz and minor sulfides (including sphalerite, chalcopyrite, galena and proustite). Although the continuity of the body has been disrupted by a series of normal faults, it seems that its lateral equivalent is a massive reefal limestone which is almost totally replaced by magnetite and carrying silicified-baritized fossils. In the distal sectors, the magnetite content decreases and the limestone shows extensive replacement by barite laths; this peripheral zone then grades into an unaltered limestone. Covering the main replacement body, there is an irregular mass of subtly-layered jasper a few meters in diameter, which is extensively transected by barite veinlets.

The main body is generally fine-grained. It consists of radial aggregates of barite and euhedral, often doubly-terminated quartz. Occasionally, irregular zones of coarse barite are present. In a few cases remnants of highly sericitized K-feldspat are recognizable in the upper part of the body. Occasional, circular structures within this zone could be interpreted as totally replaced fossils. The sulfide content of this body is very low. Sphalerite and galena are generally associated with the most silicified zones and are often replaced by quartz. Coarse-barite veins and hematite-quartz veins frequently transect the body.

The mineralized bodies are underlain by chloritized-silicified-baritized andesitic to trachytic volcanics. The bodies are overlain by a less-altered volcanic-volcaniclastic sequence. Laterally the volcanics are kaolinized and transected by black veins of pyrolusitebraunite which, in a general sense, rim the main mineralized zone.

In short, as it is the case with Vulcano, Tabaquito presents evidence of a paragenetic development of initial iron mineralization followed by barite-sulfide mineralization. The stratabound nature of the body and some relict textures again point to a selective replacement of reefal limestones and of associated volcanic rocks. Further introduction of barite-quartz and hematite through veins affected not only the main body, but the base of the overlying lithologies as well. In contrast, the jasper body seems to have been deposited as a siliceous sinter, prior to the last mineralizing event.

The Sapo Negro Deposit

At this locality a series of Mn-Fe-rich strata (iron-formation) is enclosed by a volcanic unit which grades upwards into a layered sequence of carbonates. These strata extend over an area -200x100 m and are 10 cm to ≈1.0 m thick. The Mn and Fe contents of the richest stratum are 27% and 14% respectively (Castillo-Madrid, 1986). Two sequences may be distinguished, separated by an horizon of kaolinized tuff. The upper member consists of two strata of layered jasper, hematite and Mn-bearing brownish zones, separated by a thin layer of brownish (montmorillonitic) tuff. The lower member consists of four shale horizons, rich in Mn oxides and hematite, with almost negligible jasper. Internal folding due to seismically-induced (?) gravitational slides are common in this lower member.

Above this iron formation is a unit of spilitized, pillowed basalt which grades into layered carbonates. The spilitic rocks carry an aphanitic association of plagioclase (oligoclase), chlorite, calcite and minor tremolite, vesuvianite (melilite?), and zeolites. There are two spilitized flows, the thickest apparently lying directly upon the iron formation. The other is intercalated with layered carbonates which contain japser at the bottom of the sequence. The limestone is micritic and contains poorly preserved microfossils, probably Calcispherulae.

The Ojo de Agua Mineral Ocurrence

At this locality, barite and Fe-Mn oxides replace a biohermal mound of massive limestone. Barite laths occur disseminated and less frequently, concentrated within irregular zones. In some cases, fossils are replaced by barite and by sparry calcite. Hematite occurs in irregular veinlets containing ankerite-siderite and calcite. Mn oxides alter the rock along irregular zones within the micritic matrix.

In contrast to the other areas, the alteration of the limestone is mild at Ojo de Agua. Although there are zones where hematite-silica veinlets are abundant, no major occurrence of mineralization has been yet found. However, recent geophysical data indicate the presence of a magnetic body at depth.

Experimental Techniques

Barite and whole rocks from several localities in the La Minita District were analyzed. Barite was dissolved by the sodium carbonate method. 100 mg of the mineral was fused with 1 g of sodium carbonate (the Sr content of the carbonate was determined by spiking) in a platium vessel. The resulting "cake" was dissolved in distilled water; the insoluble residue centrifuged, and the solution discarded. The residue was dissolved in HNO₃, and the Sr and Ba precipitated as nitrates with an excess HNO₃, the solution being discarded and the precipitate dried. The nitrates were then dissolved in 2N HCl and the solution evaporated to dryness. Next the Sr was extracted by ion-exchange chromatography.

Table 1 Sr-Rb Data for Barite and whole rock.

Notes: Measuring error, as determined with succesive runs of the NBS SRM 987 standard is 0.1% for ⁸⁶Sr/⁸⁷Sr and 0.5% for ⁸⁷Rb/⁸⁶Sr
(T) Tabaquito, (V) Vulcano, (O) Ojo de Agua, (S) Sapo Negro
* error is 2σ
a- measured in MM30 mass·spectrometer; b-measured in VG 354 mass-spectrometer
** Sr spike utilized; 4.865 ppm; ⁸⁴Sr/⁸⁶Sr=185.6; ⁸⁷Sr/⁸⁶Sr=0.417; ⁸⁸Sr/⁸⁶Sr=3.072
*** Rb spike utilized: 10.819 ppm; ⁸⁵Rb=2%(atomlc}; ⁸⁷Rb=98%
⁸⁷Sr/⁸⁶Sr₀ calculated at 100 M.a. using λ=1.42x10^-11y^-1 (Steiger and Jager, 1977)

For whole rock analysis, 200 to 300 mg of finely ground sample was dissolved in three steps. The samples were placed in teflon beakers and moistened with distilled water. A mixture of 2/3 HF+1/3HNO₃ was added, and the beaker was covered with a lid, and left overnight at approximately 80°C. HNO₃ was added to the dried fluoride salts, and left overnight on a hot plate. Next the nitrates were converted to chloride by adding 4N HCl.

Prior to spiking, the dissolved sample was split into two amounts, one for Sr and the other for Rb analyses. The aliquots were weighed and a specific amount of spike was added. Samples which had an insoluble residue were centrifuged to get rid of the solid material. The solution was evaporated, then transferred in 2 ml 1.25N HCl to the Sr columns. Rb was extracted in the same columns.

⁸⁷Sr/⁸⁶Sr ratios were measured on MM30 and VG354 solid source mass spectrometers. Rb measurements were made on a home-built 12" 90° magnet sector solid source mass spectrometer.

Results and discussion

The ⁸⁷Sr/⁸⁶Sr ratios in barites range from 0.70492 to 0.70535. The analytical results are presented in Table 1. The ⁸⁷Sr/⁸⁶Sr ratios in mafic volcanics range from 0.70451 to 0.70557, whilst in felsic volcanics the values range from 0.70660 to 0.75709. The carbonate samples have a ⁸⁷Sr/⁸⁶Sr fa!I$e from 0.70571 to 0.70685. The magnitude of the ⁸⁷Sr/⁸⁶Sr ratio of the volcanics correlates with the amount of Rb in the sample; the more radiogenic Sr occurring in the more Rb-enriched samples.