Abstract
The tin-tungsten deposits of Panasqueira, Portugal, are the leading source of tungsten in western Europe and are geologically important as a major example of hydrothermal tin-tungsten mineralization of plutonic association and Hercynian age. Integrated field, mineralogic, fluid inclusion, stable isotope, and other studies were undertaken to illuminate the character and origin of the ores.The deposits consist of a large number of near horizontal, ferberite-bearing quartz veins which cut sharply across the steep bedding and foliation of their Beira Schist host rocks. The vein openings were created by purely vertical dilation of the flattest sets of available, preore joints; development of the jointing postdated regional metamorphism, overlapped introduction of granite, and terminated prior to mineralization. The jointing is thought to have been caused by erosional unloading and concomitant release of residual, post-tectonic stress in the schists. It is proposed that the unusual flat vein openings were created, and subsequently supported, by hydraulic pressures of the early tintungsten vein fluids. Fluid inclusion data indicate that the vein fluid pressures were at times adequate to lift the existing rock load. Economic veinage occurs in a laterally extensive but vertically restricted zone, 100 to 300 m thick. The geometry of this zone is interpreted as being clue to hydraulic dilation over a limited depth range in which fluid pressures exceeded lithostatic values.The tin-tungsten veins are spatially associated with one or more greisenized granite cupolas that probably represent high points on an underlying batholith. Isotope studies suggest that the granite is of the S-type associated with tin-tungsten mineralization elsewhere. Although both are of late Hercynian age, the tin-tungsten veins are distinctly younger than the Panasqueira Granite. The veins were mineralized at temperatures of 360 degrees C or less. Several features--such as the increasing tin content of the veins and the increasing abundance of CO 2 -rich fluid inclusions in vein quartz as one approaches the known or suspected granite cupolas--suggest that these intrusives served as structural conduits for the introduction of vein fluids. An unusual, lensoid cap of quartz, resembling a stockscheider, occurs at the apex of the one known cupola, separating it from the overlying schists. All lines of evidence indicate that this silica cap is a hydrothermal filling of an open chamber formed by slight contraction or withdrawal of the granitic melt, accompanied by block caving of the overlying schist. Subsequent to granite crystallization, the cap opening was filled by vein matter of subeconomic grade.Mineralogical studies were undertaken chiefly as necessary groundwork but have also revealed significant new information such as the widespread occurrence of iron-rich chlorite associated with pyrrhotite alteration, the rare occurrence of hypogene Ag 2 S in the veins, and the presence of a unique assemblage of rare hypogene phosphates including althausite, wolfeite, and a new mineral named thadeuite. Typical vein apatites are fluorapatites, and all white micas studied--from the greisen, veins, and altered wall rocks--are fluoromuscovites of remarkably uniform composition.The vein paragenesis is complicated by repeated deposition of some minerals like quartz, muscovite, and tourmaline, but four distinct depositional stages can be recognized at most points in the vein system: (1) oxide-silicate stage, (2) main sulfide stage, (3) pyrrhotite alteration stage, and (4) late carbonate stage. The precise manner in which these stages or facies spread in time and space through the large vein system is not firmly established. Fluid inclusion data indicate that the tin-tungsten vein fluids were NaCl-dominated brines that were well below their critical temperatures throughout the mineralization. During the oxide-silicate, main sulfide, and pyrrhotite alteration stages, fluid temperatures were in the range of 230 degrees to 360 degrees C, salinities in the range of 5 to 10 equivalent weight percent NaCl, and fluid densities in the range of 0.74 to 0.93 g/cc. During the closing late carbonate stage, temperatures decreased to 120 degrees C or lower and salinities to values below 5 percent, while fluid densities increased to values around 1.00 g/cc. Carbon dioxide contents of the vein liquids declined through the depositional sequence; early fluids of the oxide-silicate stage were saturated and at times effervesced CO 2 , but such "boiling" did not continue into the later stages. Values as high as 9 mole percent CO 2 were attained during early filling of the silica cap over the cupola and at rare times during vein quartz deposition, but throughout most of the vein filling the CO 2 contents in the liquids were below 2 mole percent. Fluid pressures followed a comparable path, reaching values close to 1 kb when CO 2 contents were high but remaining below about 100 bars during most of the vein mineralization. These low pressures indicate shallow depths of formation of the tin-tungsten veins--depths on the order of 600 to 1,300 m below the ground-water table existing during mineralization.Various oxygen and sulfur isotope thermometers were tested on the vein assemblages, but they generally gave inconsistent or unreasonable results. The FeS contents of typical Panasqueira sphalerites require their deposition below 300 degrees C. Such low temperatures, supported by the fluid inclusion data, raise questions as to the origin of pyrrhotite, chalcopyrite, and stannite apparently exsolved from these sphalerites.The 18 O contents of fluids of the late carbonate stage require predominance of meteoric water in the veins at that time, but the record is ambiguous for the earlier stages. The delta 18 O (sub H 2 ) O values for the earlier fluids indicate extensive exchange with the schist and/or granite host rocks and are nonspecific as to the original water source(s). The high degree of oxygen isotope equilibration, coupled with a lack of correlation between fluid inclusion salinities and temperatures, suggests that any fluid mixing at this time took place outside of the observed vein system--probably at depth.The warm meteoric waters of the late carbonate stage derived both their carbon and sulfur from a shallow, heterogeneous source, probably the Beira Schist. The delta 34 S values for sulfides of this stage range widely from --13.2 to 12.2 per mil. The delta 13 C values range from --9.3 to --12.9 for dolomites and from --13.2 to --14.2 for calcites. These low values imply a graphitic or organic component for the hydrothermal carbon. The delta D values of these late waters, known to be meteoric from the 18 O contents, fall in a range of --43 to --55 per mil which, combined with paleomagnetic evidence of an equatorial setting, suggests only moderate topographic elevations in the meteoric recharge area.The isotopic data present a different picture for earlier fluids in the tin-tungsten veins. Throughout the oxide-silicate, main sulfide, and pyrrhotite alteration stages, the delta 34 S values of all sulfides remained in a narrow range of --0.1 to --0.9 per mil, indicating an H 2 S-dominated hydrothermal solution and suggesting a deep-seated, possibly magmatic source of the sulfur. Siderites formed during the pyrrhotite alteration stage yield negative delta 13 C values in the range of --10.9 to --13.1 per mil, again suggesting carbon derivation from, or at least equilibration with, an organic or graphitic source, probably the Beira Schist complex. This carbon signature could have been acquired at depth and in association with anatexis of the pelitic schists. The delta 18 O (sub H 2 ) O values of the earlier vein waters were fairly uniform and in the range 6 + or - 2 per mil which is compatible with either a predominantly magmatic water or a highly exchanged meteoric water or some mixture of the two. The hydrogen isotope chemistry has not resolved this question but instead introduces additional complications unique among ore deposits studied in this manner to date. Apparently, two waters were present in the oxide-silicate stage, one with delta D values of from --67 to --124 per mil and the other of from --41 to --63 per mil. In later stages of the mineralization, the lighter water progressively diminished in amount. Each of the two early waters apparently deposited different minerals, but texturally the different minerals appear penecontemporaneous. Various explanations of the data are attempted, but none fully solve the enigma.The flat, Hercynian tin-tungsten veins are cut by steep, so-called Alpine faults that locally contain subeconomic base metal mineralization of uncertain age. The combined fluid inclusion and stable isotope data indicate that these faults were mineralized by tepid (ca. 100 degrees C) meteoric water similar to, but distinctly later than, waters responsible for the closing late carbonate stage of the Hercynian tin-tungsten mineralization. Aside from these late veins and obvious effects of recent weathering and erosion, there is little field evidence to reconstruct the post-Hercynian history of the Panasqueira ore deposits. There has been no postore pervasive deformation or metamorphism, and the retention of original late Hercynian dates by the vein muscovites suggests that the ores were never reheated to temperatures as high as about 250 degrees C in post-Hercynian time. The ore deposits were passively rotated about 35 degrees counterclockwise, along with the entire Iberian Massif, in Mesozoic time. Fission-track dating of the tin-tungsten vein apatite gives subtle evidence of two postore thermal events--a reheating to temperatures in excess of 150 degrees C in Late Jurassic time, about 152 m.y. ago, and a possible second reheating to temperatures below 150 degrees C in Late Cretaceous time about 79 m.y. ago. Preliminary paleomagnetic studies of the Beira Schist and dolerite dikes performed in this study give tentative support to the Late Cretaceous event in that the rocks of the district acquired a very local secondary magnetization at some time after rotation of the Iberian Peninsula but before recent weathering. The authors correlate the Late Jurassic event with the period of high heat flow preceding major rifting in this part of Europe, and the Late Cretaceous event with mineralization along the late Alpine faults. Given the shallow original origin of the tin-tungsten deposits and the evidence for major post-ore uplifts in the general region, the authors propose--but cannot prove--that survival of the 290-m.y.-old tin-tungsten deposits reflects periodic subsidence beneath a protective cover of volcanic and/or sedimentary rocks. The interval between late Hercynian and Late Jurassic time is one likely time for such foundering. All data presented as relating to the original tin-tungsten mineralization were scrutinized for possible alterations by the Mesozoic thermal events, but such postore overprints are judged to be negligible.Future research will concentrate on the minor gas content and detailed chemical analysis of the vein fluids needed as a basis for further intepretation and experimental tin-tungsten solublity studies. Also, coordinated paleomagnetic, fission-track, and fluid inclusion studies merely explored in the present study offer a promising approach to actual mapping of post-Hercynian thermal effects either in limited areas like the Panasqueira district or in the Iberian Massif as a whole.
