Quartz textures - potential clues to position and or process in LS systems
Primary Growth Textures
chalcedonic, saccharoidal,comb,zoned crystals, colloform, crustiform
Recrystallisation Textures
Moss, microplumose
Replacement Textures
Mold, Bladed
Forms of Silica and Quartz
Amorphous Silica
Lacks crystalline structure
Cryptocrystalline Silica
Crystals too small to distinguish under normal microscope egs chalcedony - agate (banded chalcedony)-opal
Crystalline
Needle Prismatic Equant Aqueous silica concentrations are directly influenced by presence of solid silica phases (AMORPHOUS / CHALCEDONY / QUARTZ) and temperature. In long lived sytems quartz controls silica solubility >1800C and chalcedony controls silica solubility <900C to 1400C.
Slide 1
Buchanan/Morrison/Corbett & Leach
Buckskin Mt
Broadlands Geothermal
broad_geotherm.ppt
Vera Nancy - Textural Paragenesis
Champagne Pool and sinter
Champagne Pool Amorphous silica /mineralised ooze
80 ppm Au 175 ppm Ag 2%As 2%Sb
320 ppm Tl
170 ppm Hg
White Sinter Terraces Waimangu
Sinter - Waiotapu NZ
Columnar Growth Structures perpendicular to Sinter Laminations
Mud cracks in sinter near Ivanhoe
Chalcedonic silica Gwenivere Hg Mine (Ivanhoe)
Coarse carbonate-mold replacement amorphous silica (Calcatreu)
Lattice Bladed Carbonate Replacement Texture - Ovacik
Indicative of boiling
Red Bluff Midas Lattice Bladed Texture
Indicative of boiling
Geothermal Pipe Scale
Colloform chalcedonic silica, gold, sulphide, adularia (Hishikari)
82405g/t Au 34400 g/t Ag
Midas colloform high grade with lattice bladed bands
Indicative of boiling
Hishikari Colloform = Ginguro fine silica-adularia-sulphide
Indicative of boiling
Vein Textural Relationships
Faure et al 2002
Banding and Sedimentary textures in Veins
Simpson et al 1995 describe apparent sedimentary textures such as grading of sulphides/quartz plus ripple marks in banded vertical veins at Golden Cross NZ. Banding within the quartz veins is primarily a result of variation in quartz grain size. Sulphide minerals and kaolinite are most abundant in fine grained quartz bands that may have been deposited in an amorphous silica state. These NZ workers interpret observed textures in the following way. System pressure is breached by fault dilation or hydrothermal eruption resulting in amorphous silica saturation of a rapidly moving fluid. This event is accompanied by upward surge of sulphide rich fluid from deeper in the system. As system pressure resumes via mineral deposition in veins, temperature increases and silica saturation levels return toward quartz. Eventually the higher T/P system deposits sulphide barren quartz from more slowly moving fluid. Repeated system rupturing leads to formation of banded veins.
Schematic Relationship Quartz Grain Size and Sulphide/Kaolinite Abundance
Grain sizes <2 to ~ 60 microns
From Simpson et al 1995
Fluid flow textures in vein from Waihi
Pajingo Scott Lode - Moss texture
Recrystallisation of silica gel globules
Crustiform banding-Adularia stained
(Sodium Cobaltinitrite Solution)
Needle Adularia Mold
Bonanza Ore Karangahake with crustiform comb quartz
Victoria Lepanto Comb Quartz - crustiform banded
Calcatreu Terminal breccia single phase crystalline quartz
Amethyst - Vera Nancy Pajingo
Indicative of vapour phase?
Sacharroidal drusy quartz
Generally associated with late hydrothermal activity
Temperature Overlap Quartz and Chalcedony
Faure et al 2002
Multiphase Breccia (Wirralie Qld)
Cockade texture (Calcatreu V49)
Useful Textures
Sinter- marks paleo-surface and paleo-watertable. Textures distinguish from silicified tuff or lake sediment. NB. no Sinter in HS environment. Chalcedony - low temperature silica (120o-200oC) usually at shallow depths above an upflow zone and possibly overlying mineralisation. Colloform banding -in chalcedonic quartz-kidney like or rounded external surface from original silica gel. Close association with ore. Downgrade potential if these textures dont carry grade. Crystalline quartz -usually occurs in deeper/ hotter part of system at > 180oC though the temperature overlap between quartz and chalcedony means both can occur in same part of a vein. Crystalline Quartz alone likely indicates hot/deep environment > 250oC. Drusy cavities - void spaces partly filled with terminated crystals (calcite or quartz) are commonly last vestiges of hydrothermal activity. Lattice textures are indicative of boiling conditions. Cockade and comb textures - indicate open space filling.
By careful observation a great deal can be learnt about processes operating and likely spatial relationships to ore.
