Análisis estructural y litológico del distrito la paloma. Metalogénesis de la veta sulfuro, macizo del deseado, Patagonia

Abstract

La Paloma, a vein-hosted Au-Ag and base metal epithermal district of the Don Nicolás Mine, is located in the northeast of the Deseado Massif, Patagonia, Argentina. The mineralization is hosted in the Middle Jurassic volcanic rocks of the Bajo Pobre Formation which comprises three main lithological groups: (1) lavas and intrusive rocks, (2) pyroclastic rocks, and (3) reworked volcaniclastic rocks. The first group, composed of andesite to basaltic-andesite coherent and autobrecciated flows as well as shallow subvolcanic (stocks and lacoliths) intrusions, is the most representative in the district. The second group, either intercalated between andesitic lava flows or intruded by basaltic-andesite stocks and lacoliths, is represented by pyroclastic surge, fall and flow deposits. Reworked volcaniclastic sequences comprise debris and hyperconcentrated flow deposits and reworked pyroclastic deposits scarcely preserved and restricted to topographic highs that overlie primary volcanic units or are intercalate between them. The La Paloma district displays a complex vein network dominated by NNW to NW (with local N-S and NNE deflections) striking major veins (Sulfuro, Esperanza, Rocio, Princesa, Reyna and Verde), ENE to E-W (Duquesa) and NE (Arco Iris) striking structures. The NE-SW striking El Molino fault separates the Sulfuro Vein System (Sulfuro, Esperanza and Rocío veins) from the remaining veins located towards the north-northwest of the district. The normal to oblique- to strike-slip reactivation of pre-existing joints led to the hybrid extension-shear fractures hosting the mineralization of economic interest. Paleostress analysis unravels how all the resulting four sets of hybrid extension-shear fractures form two groups of structures that are kinematically compatible with a sub-vertical σ1 and either NNW (Group A) or ENE (Group B) trending σ3, within the framework of an extensional stress field characterized by multiple permutations between the σ3 and the σ2 axes. Such permutations are interpreted to have resulted from the reactivation of some of the NNW striking joints as normal faults in response to regional ENE stretching, and by orthogonal (i.e. NNW) extension in the hanging wall of such laterally-terminating normal faults associated with along-strike stretching produced by the differential downthrow of the hanging-wall block. Within this framework, further (i.e. NE and NW striking) joint sets were variably reactivated as oblique-slip dilational faults. The NNW trending hybrid extension-shear fractures (i.e. Sulfuro Vein System, Princesa–Reyna and Verde; Group B) were the most effective for hydrothermal fluid circulation and are characterized by the development of multi–episodic veins with exceptional width, strike continuity and ore grades whereas ENE to E-W striking structures (i.e. Duquesa; Group A) exhibit minor opening and filling events mainly represented by late stages. NE and NW striking oblique-slip dilational faults (i.e. Arco Iris Vein and El Molino fault; Group A and B, respectively) record no major vein opening history, and characterized by brecciated vein textures with minor silica cement and/or vein clasts within a fault gouge matrix. Later fault reactivation appears to be associated with far a field propagation of Andean stresses, probably related with the late Early Cretaceous shortening event known to have affected the studied sector of the Deseado Massif. This event is marked by superposed strike-slip and reverse fault slip components recording roughly E-W subhorizontal compression (σ1). The subsequent tectonic evolution of the study area, recorded by apatite fission track cooling ages, was characterized by steady-state exhumation at a rate of c. 1 °C/Ma during the last 90 Ma. The Au-Ag (Zn, Pb, Mo and Cu) Sulfuro Vein, the main ore body in the district (174.251 Oz of gold and 525.985 Oz of silver and an average grade of 6.6 g/t Au and 20 g/t Ag), exhibits a sub-horizontal ore-shoot geometry, as are the areas of greater vein thickness, consistent with dominant extensional faulting during mineralization except in the northern sector where Au, Ag and Cu shoots and the thickest areas reflect a larger strike-slip component of motion and depletion of the fluid in these metals. This strikeslip component seems not to have exerted an important control in the distribution of Zn and Pb in this sector, where these metals continued to precipitate at lower temperatures favoured by the permeability of the volcaniclastic units. The highest values of Au, Ag, Cu, Mo, Pb, Zn and Sb are concentrated at depths between 50 and 100 m a.s.l. High Mo values occur also at greater depths in the southern sector of the vein and its distribution in shallower sectors is controlled by remobilization processes. The geochemical distribution of metals shows a slight vertical zonation and a distinct lateral zonation, which suggests upward hydrothermal fluid flow from deep zones in the southern termination of the vein where ore shoots exhibit steeper plunges, to the north through the NNW-NW striking fractures. Three mineralizing stages contributed to the final metal endowment of the Sulfuro Vein. The first, which corresponds to stage 3, introduced Mo ± Au (Ag) ± Cu (molybdenite ± Au (electrum) ± chalcopyrite ± tennantite-tetraedrite) as millimetric dark sulfide-rich band at the beginning of the vein infill during the episode E1. The second and the third mineralizing stages, which correspond to stages 7 and 8, respectively, introduced Fe ± Au (Ag) and Fe ± Au (Ag) ± Cu ± Zn ± Pb ± As ± Sb, respectively, during the episode E2 and generated limited brecciation in the vein sealed by minerals of the E1 episode. Pyrite ± chalcopyrite ± tennantite-tetrahedrite ± sphalerite ± galena containing Au and/or electrum micro-inclusions, formed during this episode (E2) as breccia cement and filling veinlets. Based on the metal content and accompanying minerals, the Sulfuro Vein can be classified as a Au-Ag low sulfidation epithermal deposit enriched in Zn + Pb and unusually rich in Mo (up to 1.5 %). Quartz textures (colloform-crustriform banded, cockade, mosaic and brecciated) suggest several opening and subsequent vein filling events and the deposition of a silica gel from a boiling hydrothermal fluid which must have been responsible for the metal deposition. Chlorite ± albite ± epidote formed distal (>20 - 30 m) to the Sulfuro Vein could represent a regional propylitic alteration related to the magmatic heat source in the district prior to the onset of the Sulfuro Vein hydrothermal system. Host-rock interaction with boiling, neutral to slightly alkaline fluids formed chlorite ± albite ± adularia ± carbonates (dolomite, siderite and calcite) ± pyrite during the vein-related pre-ore stage. Syn-ore stage potassic dioctahedral phyllosilicates ± quartz ± pyrite overprint early formed minerals. Different grain size of dioctahedral potassic phyllosilicates were identified: – coarse-grained dioctahedral potassic phyllosilicates (>2 μm) or mica-like and – fine grained (<2 μm) potassic dioctahedral phyllosilicates represented by illite (or illite-richillite-smectite mixed layers) and by subordinated mixed-layers with variable amounts of illite. The composition and distribution of phyllosilicates near the vein (≤5 m) reflect paleotemperature gradients of the fluid with time, from >220°C (mica-like and illite-richillite-smectite mixed-layer) to <220°- 150° C (mixed-layer illite-smectite superimposed). In the post-ore stage, Al-bearing silicates were partially dissolved and replaced by widespread kaolinite. At shallow depths, above the central upflow zone, kaolinite ± aluminum phosphate-sulfate ± dickite dominate and represent an advanced argillic environment caused by steam-heated acid-sulfate waters. Smectite was the last mineral to form during the collapse of the hydrothermal activity. The chemical compositions of some hydrothermal minerals are potential indicators of paleo depths and proximity to the vein: - dolomite (Mg) was identified only at depth, while calcite (Ca) and siderite (Fe) formed at intermediate and shallow zones, respectively; - chlorites in the lower andesite shows more Fe than Mg near the vein and at greater depths; - secondary feldspars are scarce and relictic proximal to the vein compared to potassic dioctahedral phyllosilicates but are prominent alteration minerals with increasing distance from the vein. Relict adularia dominates in strongly altered rocks proximal to the vein whereas albite dominates in distal moderate to weakly altered rocks; and - the abundance and pervasiveness of potassic dioctahedral phyllosilicates + quartz + pyrite near the vein (≤5 m) constitute an excellent indicator of the mineralized structure as well as coarse-grained potassic dioctahedral phyllosilicate (>2 μm) confined to the contact with the vein and in deeper zones. The isotopic composition of fluids in equilibrium with quartz (δ18Ofluid –5.8 to –1.5‰) and phyllosilicates (δ18Ofluid 0.1 to -2.4‰; δ2Hfluid –48 to –69.5) deposited in the vein during episodes E1 and E2, and fluids in equilibrium with phyllosilicates (δ18Ofluid –1‰; –2.3‰; δ2Hfluid –62 to –65) in the host rock, suggest their formation by mixture of meteoric Jurassic water with magmatic water and/or meteoric water that have isotopically exchanged with basement rocks. The carbon isotopic signatures (δ13C in solution as H2CO3: –7.5 and –7.1‰) and the sulfur isotopic composition of sulfides (δ34S ‰ –0.1 to 3.3‰) indicate a magmatic source. Cooling and the increase in the fO2 of the hydrothermal fluid through mixing with meteoric water in the northern sector and in shallow parts of the central sector, could have resulted in lower δ34S values of sphalerite, galena and chalcopyrite (δ34S ‰ –3.1, –5.3 and –11.1‰). The Pb isotopic compositions of the galenas: 206Pb/204Pb (18.37 and 18.39), 207Pb/204Pb (15.61 and 15.62), 208Pb/204Pb (38.32 and 38.39) are similar to the isotopic composition of the volcanic rocks of the Bajo Pobre Formation and suggest a mixed sources of Pb, with contributions from the mantle and crust. Ar–Ar ages (168.48 ± 1.21 Ma) obtained for illite-rich illite-smectite mixed layer and Re–Os molybdenite age (169.7 ± 0.9 Ma), indicate that the formation of the potassic dioctahedral phyllosilicates in the alteration halos of the Sulfuro Vein was coeval with Au-Mo mineralization in the vein. These radiometric ages are among the oldest ages known for Middle-Upper Jurassic epithermal deposits of the Deseado Massif.