Lamprophyre Dykes

Two contrasting types of lamprophyres - the upper on is a photo from a drill hole and it shows Mg-rich fluid streaming around fragments of what are probably I-1 porphyritic intrusive rock

What Do Lamprophyres Look Like?

1. Dark, mafic, and visually distinct
• 	They’re dark green‑black to brown‑black intrusive dykes.
• 	Fine‑grained groundmass with big mafic phenocrysts (amphibole, biotite/phlogopite).
Steep, blade‑like intrusions
• 	They form narrow, steeply dipping dykes that cut straight through the Thor system.
• 	In 3D they look like long, vertical tusks slicing the deposit.
Strong alteration halos
• 	The margins are bleached, carbonate‑altered, and sulphidized.
• 	They often have reaction rims where the lamprophyre meets the host rock.
Magnetic
• 	They’re the source of the aeromagnetic highs and deep MT conductors beneath Thor.
• 	This makes them easy to track geophysically.
They physically divide the deposit
• 	The main lamprophyre dyke splits the Thor epithermal deposit into two halves:
• 	Upper epithermal block (Thor)
• 	Lower Borr Zone block
They’re younger than the veins
• 	They cut across the epithermal veins, so in core you see veins truncated by dark lamprophyre.

Tescan completed on the lamprophyre at Thor completed at the Colorado School of Mines.  This scan details the mineralogy of the lamprophyre and it includes mafic minerals such as amphibole, pyroxene and albite.

Ternary diagram showing K2O-MgO-Al2O3 variation in some of the lamprophyre drill core samples at Thor (Thor-256).  This is the classic field that lamprophyre rocks lie within.

What is a Lamprophyre?

Understanding Lamprophyres

A lamprophyre dyke is a small‑volume, ultrapotassic igneous intrusion that forms when volatile‑rich magma rises along fractures and solidifies as a narrow, steeply dipping sheet. These rocks are unusual because they contain abundant biotite or amphibole phenocrysts, high magnesium, and usually high potassium, and they commonly occur as dikes, sills, or small intrusions rather than large bodies.﻿
Lamprophyres have an outsized importance in economic geology because they often appear in structurally active, deeply rooted conduits—the same pathways that focus mineralizing fluids.
In younger terranes, they appear in districts with:﻿
Au–Ag epithermal systems
Sn–W greisen systems
Polymetallic Ag–Pb–Zn deposits
Skarn and porphyry environments

Their presence often signals fertile magmatic systems.

Strongly mafic, Mg‑rich
• 	High MgO and FeO — classic for lamprophyres derived from a mantle‑influenced melt.
• 	Abundant biotite/phlogopite and amphibole reflect this chemistry directly.
High in volatiles (H₂O, CO₂, F, Cl)
• 	These dykes carry lots of fluid, which is why they produce:
• 	Intense carbonate alteration
• 	Bleached halos
• 	Local sulphidation
Enriched in incompatible elements
• 	Elevated K, Ba, Rb, Sr, and LREEs (light rare earth elements).
• 	This is typical of lamprophyres sourced from a metasomatized mantle wedge.
Sulphide‑bearing chemistry
• 	They commonly contain pyrite, pyrrhotite, and minor chalcopyrite.
• 	This sulphide content contributes to:
• 	Their conductivity
• 	Their magnetic signature (especially where pyrrhotite is present)
Chemically reactive with host rocks
• 	Their mafic, volatile‑rich chemistry makes them highly reactive, producing:
• 	Carbonate flooding
• 	Sericite–chlorite halos
• 	Sharp reaction rims
• 	This reactivity is why they create such strong alteration contrasts at Thor.
Distinct from the epithermal vein chemistry
• 	Lamprophyres are mantle‑derived, mafic, volatile‑rich.
• 	Epithermal veins are felsic‑fluid, metal‑rich, silica‑dominated.
• 	The contrast makes cross‑cutting relationships easy to see in core.