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Cretaceous tectonics of overpressured vapor from the Guarani aquiferdeforming and mineralizingthe Serra Geral Group, Paraná volcanic province
Léo Afraneo Hartmann
Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500; 91501-970 Porto Alegre, Rio Grande do Sul, Brazil
Abstract
Overwhelming evidence points to widespread tectonic activity in the Serra Geral Group of volcanic rocks, South America, caused by vapor overpressure from volcanic heating of the Guarani aquifer. The main observed structures are hexagonal to polygonal faults (100-500 m large) andlozenge fractures (5-30 km large). Intense hydrothermal alteration (H) occurred continuously, interspersed with two sealing events (S). Initially porous (30 vo.%) lava flow was altered (H1) in the Cretaceous (130 °C), forming smectite and zeolites (e.g., clinoptilolite) and sealed (S1). This nearly impermeable caprock was subjected to vapor overpressure (H2), generating different structural responses in different positions in the lava flow; (1) doming of the hangingwall with generation of lozenge fractures, and (2) fracturing of the hangingwall. In both cases, loose sand from the Botucatu erg was injected as breccias, dikes, sills and flows at the surface. Continued hydrothermal activity again sealed the rock (seal 2); overpressured vapor (H3) intensely altered the rock (which became a mudstone), generated polygonal faults and opened cavities by ballooning (amethyst geodes). The hydrothermal activity also generated mineralizations of copper, kaolin and opal. This is a contribution to the understanding of the tectonics of vapor overpressure from the Guarani aquifer in the Paraná volcanic province.
Introduction
The Cretaceous tectonics of overpressured vapor from the Guarani aquifer deforming and mineralizing the Serra Geral Group, Paraná volcanic province, is presently evaluated. The circulation of fluids in continental, sedimentary-volcanic basins is a major issue in the geology of underground water, oil, gas and metals (e.g., copper, gold). The H2O-rich fluids may be derived from the compaction of the sedimentary formations at depth or from the heating of buried aquifers. The fluids may freely ascend to the surface, intensely altering the porosity of the rocks during their ascent, but the present article concentrates on the tectonics of overpressured water-vapor when confined by basaltic flow seals in the Paraná basin. The interruption of the ascent of hot water and its vapor by a seal caused significant deformation and intense hydrothermal alteration during the Lower Cretaceous in the Serra Geral Group lava flows, as here described. Major structures include polygonal and lozenge faults, whereas the hydrothermal alteration generated the largest deposits of amethyst and agate in the world, and also copper, kaolin and opal mineralization.
A six-year period of investigations of the Serra Geral Group by the “Núcleo de Excelência em Minerais Estratégicos” (UFRGS) led to a better understanding of the origin of the giant amethyst geodes (Duarte et al., 2005, 2009, 2011; Hartmann, 2008, 2010; Hartmann et al., 2011a, 2011b; Rosenstengel and Hartmann, 2011), copper mineralization (Pinto et al., 2011b), the age of volcanism (Pinto et al., 2011a), and of volcanic processes and lava flow correlation (Hartmann et al., 2010, 2011d; Pinto and Hartmann, 2011). This article expands the acquired knowledge into a novel proposal of water-vapor tectonics that integrates all structural, petrological, metallogenic and hydrothermal evidence recognizedin the volcanic group. The interpretation is based on extensive field work, observation of satellite images, rock sample studies with the optical microscope and scanning electron microscope,chemical analyses of minerals with the electron microprobe (major elements) and laser ablation (trace elements), stable isotopes (C, O, S) of rocks and minerals, zircon SHRIMP determination of U-Th isotopes, and numerical simulation of cavity generation in basalt, andesite and rhyodacite.
Structures and alteration
The main structures presently characterized are polygonal and lozenge faults in the volcanic rocks. Hydrothermal alteration features include two sealing events (seals 1 and 2) of the solid, cool lava, the injection and extrusion over the basalts of sand originated in the active eolian dunes (now Botucatu Formation), silicification of the sand, intrusion of breccia sills, continued alteration of the basalt into a claystone and the formation of large cavities and their filling with amethyst and agate.
Large, strike-slip faults cut the Serra Geral Group in several directions. The vertical component of some of these faults was recognized byPinto et al. (2011b) and Rosentengel and Hartmann (2011). Individual blocks were downthrown 50-200 m to the west in the Ametista do Sul mining district.They are commonly ascribed to the distensive thinning of the crust during the opening of the Atlantic Ocean, and have commonly no hydrothermal minerals or only calcite in the fault planes. These faults are not discussed here, because they are younger than the structures and alteration formed in the volcanic group by water vapor tectonics.
This article evaluates older faulting, often associated with intense hydrothermal alteration of the basaltic rocks. Two types of faults are recognized: (1) polygonal faults and (2) lozenge faults. They are smaller (e.g., 0.5-5km-long) than the strike-slip faults (e.g., 100 km-long)but widespread and of great geological and economic significance in the Paraná volcanic province.
The presence of polygonal faultsin the Serra Geral Group was recognized by Hartmann (2008) and related to the formation of amethyst geodes, particularly the hexagonal to irregular, polygonal gossans above amethyst deposits. They are common in the Serra Geral Group, easily recognized by the absence of native forest in the water-saturated soil inside the polygon. In the pampas of Quaraí, the structure is marked by green grass and negative cintilometric (K2O, U, Th) anomaly (Pertille et al., 2011).
Lozenge faults were first identified in two places by Hartmann et al. (2011c) in the Quaraí region and later in Torres (Hartmann et al., 2011d), both in Rio Grande do Sul state. This rare type of fault was first described by Reches (1978) and is typical of distensive geological environments (e.g., Sagy et al., 2003). The presence of lozenge fractures at the surface is confirmed as a tridimensional lozenge structure because lozenge fractures are present in vertical, exposed surfaces of rock at the same exposure. This is the first description of lozenge fractures in the Serra Geral Group.
The intense hydrothermal alteration of the volcanic rocks in event H1 to smectite and zeolites is concentrated in amethyst geode-mineralized portions of the rock, where the loss-on-ignition (LOI) rises to 2-8 wt.%. Unaltered basalt has ˂1 wt.% LOI.Kaolin, opal and copper mineralization were formed during event H1.In event H2, fluidized sand was injected into the cool rocks due to heating of the Guarani aquifer by volcanism; sand also extruded at the surface and breccia sills intruded the rocks. Event H3 caused silicification of the sandstones and volcanic rocks, including the world-class mineralization of amethyst and agate geodes.
Overpressured vapor tectonics
The structural and mineralogical, hydrothermal alteration features described in the Serra Geral Group are all interpreted as caused by the overpressure of hot water and its vapor from the Guarani aquifer, heated by volcanism and confined below sealed basalt (andesite, rhyodacite) lava flows.Experience in Iceland shows that a basalt lava flow has ~30 vol.% porosity immediately after cooling. In Hawaii basalts, the vesicles remain unfilled, but in the Serra Geral Group the vesicles and fractures were filled with smectite and zeolites (some native copper) during hydrothermal event H1. This initial sealing event was required in the Cretaceous for the evolution into overpressured vapor tectonics.The overpressure caused doming of large, horizontal portions of the lava flow, and the cracking of the flow in lozenge fractures. This three-dimensional extension was responsible for this rare type of tectonic feature (e.g., Reches, 1978) in the continental Serra Geral Group. In sequence, fluidized sand was injected into the lozenge fractures and silicified by the fluid.
During H2, cracking of the seal also occurred in other portions of the same flow without doming, with similarinjection of fluidized sand in several structures, including dikes, sills, flows and breccia sills. Continued fluid flow again sealed the sand and the cracks, so this seal 2 again raised the pressure of the confined vapor.
The H3 eventcaused failure of the rocks along hexagonal or irregular polygonal faults and strongly altered the rocks and opened large cavities by ballooning (Hartmann et al., 2011b) later filled by valuable silica minerals (also gipsite and calcite), completing the formation of the geodes.
Large transcurrent faults, presumably associated with the opening of the South Atlantic Ocean (e.g., Jacques et al., 2010), cut the volcanic group and caused vertical displacements of hundreds of meters (200 m in Ametista do Sul; Manara and Hartmann, 2011).
Conclusions
Vapor tectonics was active during the short time span between the cooling to ˂130°C of the lava flows of the Serra Geral Group, Paraná volcanic province, and the development of large transcurrent faults. Vapor tectonics caused a sequence of events of major significance for the understanding of the geology and metallogeny of this major continental volcanic province. The following events were recognized by systematic field work associated with laboratory tests:
(1) Seal 1 – The porous, cool lava was sealed by deposition of smectite and zeolites into vesicles and fractures during hydrothermal event H1;
(2) H2 – Vapor overpressure from heated Guarani aquifer caused the brittle failure of the lava into three-dimensional lozenge fractures, intrusion and silicification of sand; formation of deposits of native copper, opal and kaolin;
(3) Seal 2 formed;
(4) H3 – Overpressure during H3 caused the silicification of the rocks, intense alteration, opening of cavities by ballooning and their filling with silica minerals; brittle failure along polygonal faults;
(5) Weathering of the mineralized portions of the rocks formed gossans;
(6) Mineralized portions of the lavas (amethyst) have high LOI (˃2 vol.%) due to alteration; rocks with lozenge fractures have low LOI, because they remain nearly unaltered – the fluid percolated easily to the surface through the lozenge cracks.
(7) Phreatic breccias with monomict volcanic clasts and sandstone matrix are formed at ˂130°C by vapor overpressure tectonics in several positions in the flow: (a) in the lower amygdaloidal crust; (b) in the upper amygdaloidal crust; (c) below vertically jointed massive core (type II).
(8) Intrusive breccias (sills) can be injected into an overlying, cool flow, in a process akin to diatremes.
(9) Sand dikes and breccias can feed sand volcanos at the surface, and sand can spread laterally to form thin (0.5-3.0 m), little extensive (hundred of meters), silicified sandstone layers, very common at the contact of two different basalt flows in many places in the Serra Geral Group.
Acknowledgements
This investigation was made possible by the systematic support from FAPERGS and CNPq to PRONEX projects on the evolution of strategic minerals from Southern Brazil and Universal projects, and from CAPES, all coordinated by the author. Geological Survey of Brazil (CPRM) geologists participated in many field trips.
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