Exploration map for PCPG review course

Friehaufenbach Exploration Map

The map below illustrates the very basic geology of a fictional place for mineral exploration.  The scales of some of the features are admittedly on the small side, but this is just a conceptual exercise created primarily for geologists expert in other things who are reviewing economic geology.

Based on the mapped geology, what types of mineral resources might one find on the Friehaufenbach Quadrangle?

Some possible answers 

Expand each to read some possibilities - not guaranteed economic discoveries, rather places one might look for things of interest.

PꞒmbl - metamorphosed limestones

Marble might be quarried for dimension stone.  Bodies of pure calcitic marble are potentially mineable as source of calcium for use in making glass, as a flux in making steel, or as pH-neutralizing additive in paper, etc.

PꞒsch - mica schist

Metamorphosed pelitic rocks are a potential source of garnet or graphite, depending on mineralogy.  Metamorphic rocks may also contain corundum (abrasive) or gems (e.g., ruby and sapphire varieties of corundum).

PꞒper - Peridotite

The layered nature of this rock suggests either metamorphic mineral segregation or a primary cumulate-texture.  The black mineral in peridotite may be chromite = primary ore for chromium used in stainless and/or hardened steel

Some peridotite plutons concentrate nickel-copper-(cobalt-PGE) near the base or in bedrock fractures if magma saturated with respect to molten sulfide.  Note: If this peridotite is part of a layered mafic intrusion, and the gabbros are located to the south, the paleo-up direction is to the south and the base of the peridotite intrusion would be the north side.

PꞒgab - Gabbro and Anorthosite

The layered nature of mafic igneous rocks suggest the possibility it is a Layered Mafic Intrusion (LMI).  LMIs are a major source of platinum group elements (PGE), commonly concentrated near the boundary between underlying cumulate ultramafic rocks (peridotite) and overlying mafic rocks (gabbro and anorthosite).  Large LMIs are also mined for vanadium, iron, and titanium.

Okim - Ultramafic volcanic breccia pipe

The ultramafic volcanic rock matrix suggests magma originated in the mantle where diamonds are a stable form of carbon.  Fluidized breccia pipes indicate brecciation was driven by extremely high volatile escape that churned the material (typical of kimberlites – the breccia pipe is called a diatreme).  This is a kimberlite or lamproite diamond target.  

Sss - Fine-grained quartz arenite

Arenites are clastic rocks that have very low concentrations of fine-grained (muddy) matrix.  This quartz arenite has a very high SiO2 concentration, so this sandstone is a potential target for mining silica for making glass, mining sand to use as proppant in hydraulic fracturing (fracking), or even mining silica for reduction to silicon for semiconductors.  If the sand is only weakly-cemented, this could also be a source of construction materials.

Dls - Micrite limestone

This limestone could potentially be mined for aggregate in roadmaking and construction, but bulk composition defined by the volume percentages of minerals corresponds closely to the recipe for Portland cement, which would fetch a higher price than aggregate.  This rock could potentially be mined, tossed into a calcining rotary kiln with some steel-belted tires (for iron), then milled with a bit of gypsum to form a marketable product.  Sometimes commodities have multiple potential uses, so prioritize the use with higher potential payoff.

Pmar - Marine siltstones with apatite

Apatite is the primary ore mineral for phosphate used in fertilizer.  30% P2O5 would be potentially economic ore.  Pure apatite is 40% P2O5, so a rock that is 75% apatite puts us within a desirable range.

The dolomite horizons may be a sellable byproduct as aggregate for making roads, etc.

Jhms - Heavy mineral sand sandstone

The heavy minerals in sandstone could be magnetite (not economic – there are much better sources of iron), ilmenite or rutile (mined for titanium oxide and/or titanium metal), garnet (mined as an abrasive), and/or zircon (mined for zirconium used in ceramics, as well as specialty alloys).  Columbite-tantalite (mined for columbium and tantalum used in super alloys) may be concentrated in heavy sands if the weathered source rocks contain accessory amounts.

Kgr - Granodiorite complex

Granodiorite itself can potentially be mined for dimension stone if it is visually appealing and exceptionally coherent (i.e., not fractured).  Multiple intrusions are favorable for a porphyry deposit target of copper and/or gold with byproduct molybdenum if the intrusions formed at appropriate depths.  Look for porphyritic dikes, rusty gossans formed by weathering of sulfides, and hydrothermal alteration of primary igneous minerals to alkali feldspar and/or very fine-grained micas.

Contact between Kgr and Dls 

This is a potential skarn target (gold, copper, base metals) if the granodiorite is a porphyry deposit that formed a large hydrothermal system.  If the granodiorite did not form a porphyry, groundwater near in the limestone heated by the granite might recrystallize the limestone to form a marble target.

Quartz veins 

Many metal commodities occur in sulfide-bearing quartz veins.  Quartz-carbonate veins with minimal pyrite are a major gold target in metamorphic terrains (“greenstone gold”).  Vuggy quartz veins with pyrite are a classic epithermal gold and/or silver prospect.  These veins dip steeply, so may require underground mining methods if economically metalliferous.

Qal = Alluvium 

Gravel deposits in alluvium are potentially valuable as construction materials if located near construction sites.  Due to the high cost of transporting bulk material, the value of these gravel deposits decreases with distance to where the gravel would be used (e.g., gravel deposits in the middle of Alaska may be worthless, but the identical deposits a few miles from a rapidly growing city might be very valuable).

In recent years, lake sediments have become a potential target for lithium deposits where weathering leached felsic source rocks.

Qaf - Alluvial fan deposits 

Alluvial fan deposits are coarse clastic sediments deposited where mountain streams slow as they enter flat valleys.  Kurt Creek erodes the kimberlite target, so may concentrate placer diamonds.  Friehaufenbach Creek erodes the quartz veins, so may concentrate placer gold in the stream bed or alluvial fan.

Qlac - Playa lake deposits

Graben valleys (note bounding normal faults) may form restricted basins into which streams that chemically weather the surrounding mountains flow, but not flow out.  Such streams feed lakes that can periodically dry to form playas – an important source for evaporite gypsum (used in making plaster and wall board), trona (sodium bicarbonate for making glass), potash (for fertilizer in agriculture and chemical feedstock), or lithium (for batteries).  Some playas concentrate borax (an important flux in glass-making and ceramic glazes).