In this post I would like to share some key points for conceptual mapping of hydrothermal alteration in High Sulfidation (HS) epithermal systems.
In previous posts, I shared some useful tools for targeting and vectoring in hydrothermal alteration mapping, please take a look, Hydrothermal Alteration System, Mapping of Alteration with Focus in Targeting and Vectoring, and Alteration Vectoring in Epithermal Low & Intermediate Sulfidation Au-Ag Veins.
HS systems are emplaced over shallow intrusive magmas and its related volcanic vents, and alteration patterns are produced by reaction of magmatic fluids in conduits and permeable lithologies. Therefore HS mineralization are typically emplaced in pre and sin-volcanic sequences, (breccias, lavas and tuffs), even in older formations. Near surface hydrothermal alteration is represented by footprints kilometric in size, with advanced argillic and argillic mineral assemblages.
My intention in this post is to focus discussion on field conceptual geology useful for vectoring, so geochemistry won’t be boarded in detail. Aimed at easy understanding, I will use a schematic picture to describe and represent the typical patterns in quartz ledges, permeable lithology trap, and diatreme breccias.
Groundwater
Groundwater table location is a very important concept to have in mind when mapping is under way, because of the table location has influence over the vertical and horizontal extension of the alteration footprints.
When magmatic vapor ascending finds a groundwater table, the vapor condensates and the consequence is the acid leaching and residual silica. The Resulting texture proximal to conduits is vuggy silica, and massive silicification is expected in distal locations. Subsequently, both textures occurrences are important vectors in lateral and vertical location in the hydrothermal system. The shape of resultant silicic body will be defined by hydraulic gradient of groundwater in permeable lithologies.
Finally, over this point in shallow levels of the system the steam heated zone is formed, containing characteristic kaolinite (powdery) and cristobalite.
Structural Vector
Faults are the magmatic fluids conduits for excellence, connecting the magmatic source to shallow levels of volcanic vents, and during the up flow great changes affects pH, composition and temperature of fluids
In the field, conduits are generally represented by Quartz ledges, and mineralization related, could be hosted as hydrothermal breccias, or in different traps, like permeable lithologies or geological contacts. For that reason, spatial location of Quartz-ledges structures need the attention during mapping.
Mineralogical Vector
Mineral assemblages usually have well defined haloes representing the pH and temperatures of fluids during crystallization. Alteration minerals has stretch relation to depth and the distance from fluids source.
Mapping supported with spectrometer (i.e. ASD terraspec) permit to differentiate mineral specimens and even composition in some cases, very useful for vectoring the fluids source. For example, potassic, sodic and calcic Alunite.
In the picture is explained typical crystallization conditions for alteration minerals in HS systems.
Lithological Vector
Host rock composition and permeability are important characteristics in HS systems, permeable lithologies or composition contrast in rocks (i.e. ultramafic igneous rocks or carbon rich sediments) could be favorable traps for ore crystallization and precipitation. Also mineral assemblages and extension of haloes in alteration will depend of those characteristics.
Highly active hydrothermal systems are capable to generate a secondary permeability during hydraulic fracturing or breccia formation (for example, diatreme or crackle breccias).
Mineralization
Metals are transported by HS2 or Cl2- solutions, and then are precipitated during changes in fluids conditions (i.e. temperature, mix of fluids, pH).
Some HS systems are hydrothermally actives during long time (for example, 3 m.y. in Veladero and 5 m.y. in Yanacocha), consequently multiple mineralization/alteration pulses and overprinting of those events is very common.
Multiple hydrothermal events favoring the mineralization in HS systems, therefore evidences of mineralization/alteration overprints, and physical conditions need to have the attention during mapping. For example, vuggy silica alteration transform the host rock like a sponge, favoring mineralization in a later hydrothermal pulse.
The same as previous post, "Every projects has their own DNA, so is very important to find the mineralogy/geochemistry ratios showing a better response for vectoring."
Supergene alteration
Supergene alteration generates important leaching and oxidized profiles in many HS systems, sometimes favorable for oxidized ore formation.
The overprinted alteration, generally is characterized by OxFe (Hematite, Goethite), supergene sulfates like jarosite or alunite, and kaolinite (i.e. halloysite) too.
These mineral associations masks many times the hypogene alteration minerals, so is very important the training in hand lens identification of alteration minerals (hypogene and supergene). With spectrometer analysis is easy to define differences between those.
"Developing targeting and vectoring technics to support your next discovery"
PETRA GAIA "Geology and Mining Exploration"
Luciano M.Bocanegra
Consultant Economic Geologist