Subhorizontal Veins Research Articles

Characterization and timing of the different types of fluids present in the barren and ore-veins of the W-Sn deposit of Panasqueira, Central Portugal

The Panasqueira W-Sn deposit is the largest quartz-vein type deposit of the Iberian Peninsula and the most important wolframite deposit in Western Europe. The ore-veins are almost exclusively sub-horizontal. Besides ore-bearing sub-horizontal veins, the Panasqueira mine also contains barren quartz veins. There are essentially two generations of barren quartz: quartz, contemporaneous with the earliest regional metamorphism (QI), and recrystallized quartz, contemporaneous with the thermal metamorphism related to the granite intrusion (QII). Fluid inclusion studies (microthermometry and Raman) were undertaken in order to distinguish fluids contemporaneous with the barren quartz from those contemporaneous with the ore-bearing quartz (QIII). Fluid inclusion data indicate that the barren and ore-bearing quartz fluids are dominantly aqueous (93 to 98 mol% H2O), with a nearly constant bulk salinity (8 to 12 wt% eq. NaCl), with the quantity of volatile component (determined by Raman spectrometry) higher in QIII, but never greater than 5 mol%. However, the CO2/CH4 + N2 ratio is different for each type of quartz. Volatiles are dominated by CH4 (10 to 96 mol% ZCH4 and/or N2 (3 to 87 mol% ZN2) in the barren quartz and by CO2 (60 to 73 mol% ZCO2) in ore-bearing quartz. The bulk chemical composition of the fluids in QIII is comparable to that found commonly in hydrothermal fluids associated with wolframite mineralization, where Na>K>Ca and HCO3>Cl>SO4. A dispersion in TH (226 to 350 °C) found in QIII, together with a variation in the degree of filling (0.5 to 0.7) and with the consequent variation of fluid densities (0.70 to 0.79), may result from changes in the fluid pressure regime below lithostatic pressure, suggesting vein filling related to tectonic events.

Archean gold-bearing quartz veins at the Sigma Mine, Abitibi greenstone belt, Quebec; Part I, Geologic relations and formation of the vein system

The Sigma deposit consists of an Archean gold-bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite, both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes. The veins cut all rock types and are composed chiefly of quartz and tourmaline, with minor amounts of carbonates, pyrite, chlorite, scheelite, and free gold. They were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes. The veins were not significantly affected by any subsequent metamorphism or tectonism.Two principal types of veins occur in the mine; subvertical veins and subhorizontal veins. They are contemporaneous and cogenetic. Subvertical veins occur within steeply dipping, east-west-trending ductile shear zones along which reverse subvertical displacements have taken place. The veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears. Subhorizontal veins occupy extensional fractures which developed between ductile shear zones, preferentially in the more competent host rocks. These veins were formed by one or several episodes of open-space filling.Geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones. A structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation: the Z-axis trending north-south and horizontal, the Y-axis plunging 10 degrees west, and the X-axis plunging 80 degrees east. The deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north-south-trending compression postdating regional deformation.

Archean gold-bearing quartz veins at the Sigma Mine, Abitibi greenstone belt, Quebec; Part II, Vein paragenesis and hydrothermal alteration

The Sigma deposit consists of an Archean gold-bearing vein system in deformed volcanic rocks intruded by two generations of porphyries. The veins cut all rock types, and wall-rock alteration is superposed on greenschist facies metamorphic assemblages. There are two related vein sets: subvertical veins in ductile shears and subhorizontal veins in extensional fractures between shear zones. This paper reports on the paragenesis of extensional veins and related hydrothermal alteration.Tension veins are composed chiefly of quartz and tourmaline, and subordinate amounts of carbonates, chlorite, pyrite, pyrrhotite, scheelite, and free gold. The veins formed by one or more episodes of open-space filling. The paragenetic sequence consists essentially of successive deposition of carbonate-scheelite, tourmaline, pyrite, pyrrhotite-carbonate-chlorite, and quartz. Most of the gold and tellurides was introduced into fractures and along grain boundaries of recrystallized vein minerals during a postfilling episode of deformation.Veins are surrounded by zoned alteration envelopes consisting of an outer zone of cryptic alteration characterized by a chlorite-carbonate-white mica assemblage, and an inner zone of visible alteration characterized by a carbonate-white mica outer subzone and a carbonate-albite inner subzone. Alteration zones represent reaction fronts advancing into the wall rocks, and resulted from progressive interaction of wall rocks with a single hydrothermal fluid. The chlorite-carbonate-white mica zone is characterized by replacement of epidote by calcite and white mica accompanied by fixation of CO 2 and K, and by release of small amounts of Si. In carbonate-white mica and carbonate-albite subzones of visible alteration, chlorite, biotite, and white mica were replaced by calcite, albite, and quartz; variable amounts of gold, tourmaline, pyrite, pyrrhotite, and ilmenite were also introduced. Visible alteration zones are characterized by additions of Ca, CO 2 , Na, B, S, P, and Au, and by important losses of Mg, Fe, Al, and H 2 O. Mass balance calculations show that Ca, Na, Si, CO 2 , S, and B in vein minerals were contributed from the hydrothermal fluid, whereas Fe, Mg, and Al were derived from the walls.The development of alteration envelopes began with the infiltration of hydrothermal fluids into open extensional fractures and it ceased before complete filling of the veins when continuous mineral coatings formed along vein walls. Deposition of carbonate accompanied development of cryptic alteration, including initial CO 2 additions to the wall rocks.Subsequent deposition of tourmaline, pyrite, and pyrrhotite-carbonate-chlorite accompanied the progressive development of visible alteration which encroached on cryptically altered rocks. Visible alteration is characterized by additions of Ca and CO 2 to the wall rocks and by release of Mg, Fe, and Al. Later bulk quartz deposition, which filled the vein openings, generally took place after complete development of the alteration envelopes. Except for this late quartz, deposition of vein minerals is attributed to chemical changes in the fluid resulting from progressive wall-rock alteration. Bulk quartz deposition is attributed to progressive cooling of the hydrothermal fluid.