1Geol 481: Geology of Earth ResourcesEl-Shazly, A. K., 2004Base metal lode (vein) depositsBase metal lodes are closely related to porphyry Cu or porphyry Mo deposits, and in manycases are observed to occur on their periphery. As such, the ore is restricted to veins,networks of veins (lodes), or breccia pipes characterized by open space rather thanreplacement textures. Unlike the PCD's, in which the ore occurs either disseminated or inveins in the igneous intrusion and the country rocks, base metal lodes (BML) areconcentrated in the country rock, and are not as disseminated. Their spatial distributionrelative to PCD's as well as data from fluid inclusions seem to suggest that they form at T~ 250 - 350°C, placing them in the category of "mesothermal" deposits of Lindgren.Accordingly, it is perhaps appropriate to list some of the general characteristics ofmesothermal deposits before we describe the base metal lode deposits.General characteristics of Mesothermal deposits:1- Unlike hypothermal deposits which are characterized by replacement type textures,mesothermal deposits tend to have both replacement and open space filling textures, withthe latter more common in shallower deposits.2- Whereas hypothermal deposits tend to have diffuse outlines grading laterally intoigneous or metamorphic rocks, mesothermal deposits have sharper contacts with thecountry rocks which are either igneous, sedimentary or metamorphic. 3- Alteration patterns are very characteristic and well developed, with the propyliticalteration almost ubiquitous.4- The most abundant ore minerals are: chalcopyrite, bornite, enargite, tetrahedrite,sphalerite and galena.5- The source of metals in many cases is unresolved; it could be igneous intrusions or theintruded country rock.Characteristics of Base metal lode deposits:· Source of fluid: Mostly meteoric water· Tonnage and Grade: Grade is generally much higher than PCD's (they are moreconcentrated); e.g. the base metal lodes of Butte, Montana, are characterized by grades of3% for Cu, 5% Pb, and 11% Zn. Tonnage is also very large, with hundreds of millions tobillions of tonnes.· Geological environment, regional distribution and age: similar to PCD's, but lesswidely distributed. They also occur along the peripheries of PCD's, and appear to haveformed at shallower depths.· Mode of occurrence: In veins and fractures characterized by open space fillingrather than replacement textures.- Host rocks: variable- Gangue minerals: Most commonly quartz and/or calcite; somewhat dependenton the host rock type.· Ore minerals and ore mineral zoning: Zoned from Cu -rich central ore zones, toZn rich intermediate zones to peripheral zones rich in Pb, Ag and Mn. This pattern followsclosely that expected from the relative stability of chloride and sulfur salt complexes inbrines and hydrothermal solutions (see previous handout on hydrothermal solutions).Despite the high grade of many base metal lodes, veins in general are not considered a2Geol 481: Geology of Earth ResourcesEl-Shazly, A. K., 2004primary economic source of Cu; the best metal to be extracted from veins remains the Au(and/ or Ag). Ore minerals include spahlerite (which is ubiquitous in all zones), enargite, tennantite,bornite, chalcocite, and galena. Unlike PCD's, ore mineral zonation is from covellite andchalcocite bearing assemblages in the core of the lodes or veins, to pyrite + chalcopyrite inthe fringe. Considering the log fS2 vs. T diagram (Fig. 8 of PCD handout) this zonationcan be explained by either an increase in T from the vein centre towards the periphery ofthe lode, or by an isothermal decrease in fS2. Fluid inclusion data support the latterexplanation.· Alteration zones: As was the case for the PCD's, there is a close spatial, temporaland genetic relationship between alteration and ore mineral formation. However, unlikePCD's, BML's have their own characteristic alteration patterns, which from "core" tofringe can be described as:advanced argillic (with kaolinite, pyrophyllite, quartz and alunite) ® sericitic (withsericite + chlorite) ® argillic (with K-spar + montmorillonite + kaolinite) ® propyliticzone or to a zone of fresh rock. This pattern can be explained by an isothermal increase in aK+/aH+, as can be seen on aHemley diagram (T vs. log aK+/aH+). Such a trend indicates that the fluid, originally quiteacidic with very strong leaching capabilities, becomes progressively more buffered orneutralized by the wall rock towards the periphery.· Origin: BML's form from a fluid with a major meteoric water component, whichmay represent the upper or shallower level expression of PCD's. The source of metal isunresolved, but is believed to be either the igneous intrusions of the associated PCD's orthe intruded country rock leached by the highly acidic nature of the