Class 6 Volcanic Rocks & Volcanic-Related Deposits - Notes

Objective: 

To become familiar with the general features of volcanic rocks, their origins, and the types of ore deposits which are associated with them.

Volcanic Rocks
Volcanic rocks are rocks formed from magma crystallizes at the surface, often called “lava”.  The cooling is very quick due to the relatively low temperature of the air or water in contact with the lava.  This “quenching” often happens too quickly to allow large crystals to form.  This explains the fine-grained textures which are characteristic of volcanic rocks.  Some volcanic rocks are quenched so quickly that no crystals form, leading to the formation of a volcanic glass.  An example is the black volcanic glass called “obsidian”.  Glass is a common constituent of most but not all volcanic rocks.  It may not be readily apparent.  Sometimes the glass is only present in the “groundmass”, or the fine-grained matrix material between larger, obvious grains.  The only way it can be observed is by using a microscope. 

The timing of the crystallization is also quite variable.  The magma linger any length of time before eruption, allowing for any percentage of crystals to form.  There may be be sparse or they may be abundant.  When larger crystals are suspended in a fine-grained groundmass, resulting in a texture called is called “porphyry”. 

There are three principle main modes of occurrence of volcanic rocks, including pyroclastics/tuffs, lava flows, and shallow level dikes and other feeders of the eruptive materials.  Pyroclastics are materials ejected into the air during an explosive eruption.  Lava flows may be either from a circular vent, or an elongate fracture.  Dikes occur in many different structural positions.  Lava intruding along high angle circular fracture or fault systems are called “ring dikes”.  Or they may form in sets of parallel, near vertical fractures, in which case they are called “dike swarms”.  If the magma encounters groundwater at shallower depths, large volumes of steam can be generated, causing explosive eruptions to occur.

Rocks formed from pyroclastic material are generally classified by the size of the fragments (Figure 6-2).  The three main types of fragments are :  1)  ash (< 2 mm), 2)  lapilli (2 ­ 64 mm), and bombs (> 64 mm).  A “tuff” is a volcanic rock consisting mostly of ash.  Due to the often violent and chaotic conditions or explosive eruptions, breccia is very common.  Pyroclastic rocks are usually further subdivided based on the relative proportion of vitric material to crystals to lithic fragments (or rock fragments).  For example a tuff which is mostly glass but contains some crystals would be called a crystal lithic tuff. 

Pending Permission to Use

Figure 6-1.  Classification of pyroclastic volcanic rocks by size of the fragments (from Williams & McBirney, 1979).

Some magmas contain a large amount of gas.  The gas accumulates in pockets as it is expelled from the magma, particularly in lava flows at the surface.  Upon cooling, spherical hollows remain in the volcanic rock, forming a texture called “vesicular”

Volcanic Rock Compositions

Table 5-1 below characterizes the four principle types of volcanic rocks in terms of their physical appearance and chemical composition.

BASALT	ANDESITE	DACITE	RHYOLITE
Low Silica	Moderate Silica	High Silica	Super high Silica
High Iron - Magnesium	Moderate Iron - Magnesium	Low Iron - Magnesium	Low Iron - Magnesium
Black	Gray to Green	Gray to Green	Light colored
Low viscosity	Moderate viscosity	High Viscosity	Very High Viscosity

Note that basaltic and rhyolitic magmas are very different.  Rhyolitic magma contains abundant silica while basalt low silica.  High silica in a magma causes it to be extremely viscous (thick and difficult to flow).  Note also that “obsidian” is an exception to the normal color scheme mentioned above.  Although it is black, obsidian actually classifies as a rhyolite due to its high silica content.  The reason for this is because of the numerous inclusions of microscopic grains of the mineral magnetite, and some other black minerals called iron-magnesium silicates. 

Study the triangular diagram shown in Figure 6 -1.  The diagram classifies volcanic rocks in terms of the three major minerals quartz (Q), alkali feldspar (A) and plagioclase feldspar (P).    In the volcanic rocks called basalt and andesite, at least 65 % of the total feldspar is plagioclase.   Dacite and rhyolite both contain 20 % to 60 % quartz, but can be distinguished by the ratio of alkali feldspar to plagioclase feldspar.  Dacite contains mostly plagioclase feldspar and rhyolite contains mostly alkali feldspar. 

By far the most widespread volcanic rock type on the surface is basalt, for two reasons.  Rhyolites, dacites, andesites and other rocks with higher silica and alkali content are far less common.  These felsic to intermediate types of volcanic rocks most often are associated with continental, subaerial volcanic settings, while the basaltic rocks dominate oceanic, submarine settings.

Volcanic-Related Ore Deposits

Volcanic-related ore deposits are those which form as a result of volcanic activity, either in a oceanic, submarine environment or in a continental, subaerial environment.  Examples of submarine environment types of volcanic-related deposits include copper and other base metal deposits which locally contain anomalous gold and silver.  Seawater circulation through fractures in the ocean floor crust is probably a factor in the precipitation of metals from hydrothermal solutions.  This occurs either along a rift or around the flanks of a submarine volcano.  The three major types of volcanic-related deposits are 1) volcanogenic massive sulfide deposits (or “VMS” deposits), and 2) stratabound shale-hosted deposits (“SS” deposits), and 3) epithermal deposits. 

Volcanogenic Massive Sulfide Deposits (VMS)

VMS deposits are sulfide-rich deposits hosted in submarine volcanic and sedimentary rocks.  VMS deposits are polymetallic, but are chiefly recognized for their rich copper, lead and zinc values.  The classic example of a VMS deposit is “Kuroko” type deposits, named after a locality in northeast Japan.  At this locality, there were explosive eruptions which laid down pyroclastic rocks, which were interspersed with layers of sediments such as sands and muds.  At the vent locations there is typically a plug or dome ­shaped intrusion of rhyolite or dacite composition.  The ore itself is occurs in the form of massive sulfides or dense concentrations of disseminated sulfide minerals of various types. 

Kuroko ore which occurs as abundant massive pyrite hosted in siliceous tuffaceous rocks is referred to as “yellow ore”.  It tends to occur near the vent site and therefore tend to be hosted in brecciated rocks intruded by fine-grained intrusive rocks.  “Black ore” is another type of Kuroko ore which is characterized by concentrations of sphalerite and galena, which are typically much finer grained than the minerals of yellow ores.  The circulation of waters, both seawater and “juvenile” (from the magma itself) is thought to play a major role in distribution of the metals.  As a result, there is a zonation of metals from a proximal zone of copper-rich mineralization to a distal zone of lead- and zinc-rich mineralization.  Barium in the form of the mineral barite forms the most distal portions of the zonation.

Stratabound, Sediment-hosted Deposits (SS)

Stratabound, sediment-hosted deposits (SS) have few or no volcanic rocks associated with them, but they are included because it is assumed that some process involving magma is involved.  They typically form on the ocean floor, typically around the edges of large, deep sedimentary basins.  SS deposits are characterized by having very high zinc, lead, silver and barium values.  Zinc and lead occur mainly as sphalerite and galena, and silver occurs primarily as argentiferous galena.  The SS deposits in Alaska carry base metal sulfides or barite or both, and they are located in the Brooks Range, with other localities in southwestern, southeastern and east-central Alaska.

They are typically hosted in sequences of dark, fine-grained clastic sedimentary rocks, particularly in black shales or mudstones (in the upper sections) and in turbites (or layered, deep-water siltstones and mudstones, in lower sections).  Less commonly the deposits are hosted in limestone units forming a shelf-like layers near the edges of the basins.  Deep water varieties contain extremely high zinc values as well as abundant lead, silver and barium.  Limestone-hosted varieties are characterized by their higher copper and cobalt values. 

Shale-hosted varieties tend to be laminated, and consist of zones of fine-grained, disseminated sulfides.  Limestone hosted deposits tend to occur more as veins and be associated with breccias.  Banded or layered deposits, semi-concordant to the stratigraphic layering, are generally rich in zinc.  The sphalerite occurs as dense accumulations of fine-grained sulfides in parallel layers or bands.  This banding is generally parallel to layering in the overall stratigraphy.

Althought the origin of the metals is uncertain, but the origin of the is fairly well agreed to be involved with dewatering of the basinal sediments resulting from compaction by the thick overlying column of water.  During transport the fluids ae heated by the geothermal gradient or by coming into close contact with some type of magma body.  High angle fault structures, near the edges of the basin, appear to have channeled metal rich, hot water brines.  As the fluids migrated away from the feeder zone, chemical and physical changes of the brines resulted in precipitation of the metals.  The hydrothermal fluids precipitated ore in several ways.  Sometimes ore is formed by replacement of wall rocks adjacent to a deep fissure.  Ore can also be deposited near the top of the fracture where it will typically be in the form of a breccia.  Lastly, the metal-rich waters migrate away from the fracture or vent and travel for some distance.  The brines accumulate in the deeps, and precipitate metals.  Evidence suggests some of these deposits formed under anoxic (lack of oxygen) conditions.

General zoning sequence:   

Proximal	>           >	Distal	>
Source	Copper	Lead-Zinc	Barite
Vent/Rift

  

Alaskan Examples:

Red Dog, Brooks Range, Alaska:  Hosted in Mississippian shale.   Barite lens caps the deposit (Figure 6-4). 

Abundant replacement textures.  Highly deformed and metamorphosed during the Cretaceous.

Epithermal Deposits

Epithermal deposits are those which form at relatively low temperatures typically in the 100 ­ 300 deg. C range, and at very shallow depths or even at the surface.  They are most well known for occurrences of gold and silver, but also have very high mercury, lead, zinc, copper, antimony, uranium and vanadium values.  The vast majority of known deposits are known to be related to Tertiary or recent volcanic rocks, although a few appear to be caused by heating deeper plutonic sources.

They are almost always localized near volcanic centers, such as calderas, stratovolcanoes, volcanic necks, breccia pipes and shallow intrusions (a “caldera” is a large circular region which is downdropped along a circular fault system).  They also tend to be associated with regional doming.   The deposits may occur in the volcanic rocks themselves, or the rocks the are extruded onto.  The extrusions can be in any form of extruded volcanic rock, and also as shallow intrusions of various types (dikes, sills, pipes), or pyroclastic rocks.   In certain cases they are hosted in sedimentary rocks (Carlin). Epithermal deposits occur in a wide range of geometries, ranging from tabular veins to pipe- or funnel-shaped. 

Ore minerals include native gold and silver, and telluride and sulfosalt minerals containing variable proportions of gold, silver, lead and antimony.  The ore minerals occur in a gangue of quartz, chalcedony, carbonate minerals, fluorite, barite, sericite, adularia and clay minerals (Figure 6-5).  Banding of the ore and gangue minerals is common.  Other textures include drusy, comby, crusty, vuggy and colloform.  There is widespread alteration of rocks around the deposits, especially to the minerals chlorite, sericite, quartz, pyrite, and locally to carbonates and feldspar minerals. 

A common metal zoning pattern seen in many examples shows high base metal values in the lower portion of veins and high precious metal values in the upper portions.  The ratio of gold to silver ratio typically decreases outward or upward in the deposits.  The decreasing temperatures of the hydrothermal waters away from the igneous rock sources is the biggest influence on how the metals are precipitated.  Meteoric water influence, during later stages of the volcanism, appears to be common in many examples.

Figure 6-5.  General cross sectional model through epithermal vein deposit showing composition and geomgetry of various alteration envelopes.(from SME Mining & Engineering Handbook)

Examples: