Precious Metals Deposits

Petrographic and CL data quantify and enhance observations made in the field. They provide important elements for deposit interpretation and modeling:

  • Identification of episodes of quartz, carbonate, and adularia deposition, and their
    relationship to metallization
  • Determination of ore mineral assemblages, parageneses, and textural disposition
  • Identification and distribution of potentially key gangue mineral phases
  • Characterization of alteration and zoning patterns
  • Determination of detailed stratigraphy within intensely altered lithologies

    • Cathodoluminescence techniques can make it possible to identify multiple episodes of Q and feldspar deposition within a vein system, correlate specific episodes with modal gold and gold grade, and use the distribution of CL Q and feldspar to predict proximity to lode mineralization within the mineralization/alteration system.

    Epithermal Au-Ag breccia veins (RL)
    The left-hand reflected light photomicrograph shows a complexly intergrown assemblage of electrum, pyrite-marcasite intergrowth, chalcopyrite, and an Ag sulfosalt phase. The right hand photo contains naumannite with threads of native gold in edge contact with, and as inclusions within, pyrargyrite. A porous mesh of tetrahedrite is present to the right of the large pyrargyrite crystal.
    (TLX) Multiple epithermal quartz types (CL)
    An epithermal quartz vein is shown in transmitted light with crossed polars (TLX;left) and under cathodoluminescence (CL; right). The TLX photo illustrates early microcrystalline, mosaic-textured quartz and late comb-textured quartz void fill. The CL photo shows two varieties of microcrystalline quartz (tan CL Q1 and dull red CL Q2), while the comb-textured quartz has fine-scale growth zoning in shades of yellow, red, gray, and blue CL.
    Late comb-textured epithermal quartz (CL)
    The comb-textured Q3 shows very fine-scale growth zoning under CL, while the microcrystalline Q2 is nearly non-luminescent.
    (TLX) Complexly banded Q-Chalcedony Vein (CL)
    Complex epithermal Q-chalcedony vein shown in TLX (left) and in CL (right). The CL photo highlights a quartz vein stratigraphy with early non-luminescent microcrystalline quartz (Q1), an intermediate stage of microcrystalline quartz with moderate yellowish-brown CL, and late-stage chalcedony and quartz with strong yellow CL (Q3). The main vein is cut by a narrow veinlet of Q2 and orange-luminescent calcite.
    (TLX) Lattice-bladed Quartz-Adularia Vein (CL)
    An epithermal quartz-adularia vein is shown under both TLX (left) and CL (right). The bladed structures are interpreted to represent adularia replacement of primary calcite. The adularia has tan CL, while the quartz (Q) is non-luminescent.
    Disseminated epithermal Au-Ag deposits
    Two different CL photomicrographs of samples from epithermal, volcanic-hosted Au-Ag deposits. The left photo shows a silicified ash flow tuff. Primary resorbed pyrogenic quartz has weak blue CL, and it is rimmed by dull red-luminescent hydrothermal quartz, which has also silicified the matrix. The right-hand photo is an RVC chip of quartz vein material. The hydrothermal quartz has dull red CL and contains disseminated fine hydrothermal apatite with pinkish-white CL. Disseminated apatite haloes high-grade gold mineralization in this system.
    (TLX) Carlin-type Gold Deposits (CL)
    Photomicrographs in TLX (left) and CL (right) show non-luminescent jasperoidal quartz (Q). Two luminescent varieties of quartz are noted. Q’ has dull red CL, while Q’’ has weak yellowish brown CL. The Q’’ veinlet appears to cut the Q’ veinlet. Late ore-stage calcite has bright yellowish orange CL. Some of the calcite is disseminated, and some is more coarsely crystalline and fills void spaces. The apparent luminescence in realgar (R) may be imparted by proximity to the brightly luminescent calcite.
    (TLX) Carlin-type Gold Deposits (CL)
    Photomicrographs showing two phases of open-space filling calcite under transmitted light with crossed nicols (TLX; left) and cathodoluminescence (CL; right). The earliest sparry calcite has moderate dark orange CL, while later calcite (calcite’) has brighter orange CL. Realgar (R) is non-luminescent.