Lamprophyre Alteration

Texcan of the lamprophyre alteration shell, Thor-248, 121 m depth

What Does Lamprophyre Alteration Look Like?

Lamprophyre alteration at Thor is mainly found in drill holes, but it is extensively exposed in Broadview Creek. These outcrops have a distinct green color, and this is due to the pronounced magnesium alteration. The wall rocks around the lamprophyre dykes are enriched in of grossular-andradite garnets, magnetite and minor amounts of pyrrhotite. There are also other distinctive minerals such as Mg-rich pyroxenes and amphiboles that are able to be detected with a hand-held SWIR spectrometer. The presence of magnetite is important because it allows airborne magnetic geophysical surveys to map the alteration around the lamprophyre dykes.

Lamprophyre alteration in Broadview Creek. The sediments are green in color and contain abundant magnetite.

Carbonate and albite alteration in sediments surround one of the lamprophyre dykes

What is Lamprophyre Alteration?

One of the Most Important Discoveries at Thor

Lamprophyres are complex intrusive rocks and they are described here. Lamprophyres can have alteration zones develop around them, and the mineralogy can be very complex. Alteration around lamprophyre dykes can signal:﻿
Deep, mantle‑linked magmatic activity
Major structural corridors
Hydrothermal fluid flow pathways
Potential mineralization zones, especially where lamprophyres intersect faults or granitic systems

In polymetallic districts like Thor, altered lamprophyres can mark deep feeder structures, fluid mixing zones, or boundaries between mineralizing events.﻿
At Thor, these alteration zones are easily recognized by the presence of garnet, magnetite, chlorite, amphiboles and pyroxenes that are heavily enriched in magnesium and iron. They always occur marginal to Lamprophyre dykes within the host rocks.﻿
Owing to the intense alteration around the lamprophyre dyke at Thor, the rocks are very altered and silicified. This physical property makes this part of the hydrothermal system easily mappeable suing resistivity, and this can be seen on the MT survey.﻿
The geophysical feature is called the Crab's Claw, as it form a hand-like feature around the lamprophyre dyke. The alteration at surface is typically green in color, and this is referred to as the "Green Tuff Unit". These rocks are very altered to a pistachio green color, and are characterized by extensive sodium enrichment, including the mineral paragonite.﻿
A picture of this can be seen to the right, and the original host rocks have become bleached and highly altered. The still preserve the original sedimentary layering however, layers that are enriched in silica retain the original grey color.

In the drilling around the main Thor deposit, there were no drill holes that hit lamprophyre or its associated alteration. It was only when deep drilling was conducted that this unit was encountered. The alteration that is associated with this unit is described below.﻿
Bleached, pale halos
• 	Host rocks right beside the dyke turn light grey to white.
• 	This bleaching is from volatile‑rich fluids (H₂O–CO₂–Cl–F) breaking down feldspars and micas.
Strong carbonate flooding
• 	The margins are often carbonate‑rich (ankerite, calcite, ferroan dolomite).
• 	This gives the wall rock a buff, tan, or creamy look (see the photo above)
• 	It’s one of the most obvious field indicators.
Chlorite–sericite overprint
• 	Outside the carbonate zone, you get a greenish chlorite halo.
• 	Sericite appears where fluids interacted with felsic units.
• 	This creates a green → pale → bleached gradient outward from the dyke.
Sulphidation fronts
• 	Pyrite and pyrrhotite develop along the margins.
• 	These fronts can be sharp, forming a dark, sulphide‑rich rind.
Reaction rims at the contact
• 	The contact between lamprophyre and host rock often has a thin, sharp reaction band.
• 	This is where the mafic, volatile‑rich melt chemically attacked the host rock.
• 	In core, it looks like a dark rind against a pale halo.
Magnetic and conductive alteration
• 	Pyrrhotite + magnetite alteration gives the dyke and its halo a magnetic signature.
• 	This is why the lamprophyres correlate with aeromag highs and deep MT conductors.
Multi‑stage overprint
• 	Early: carbonate + chlorite
• 	Middle: sulphidation + bleaching
• 	Late: sericite + minor silica
﻿
This multi‑pulse alteration is part of what makes the lamprophyres such strong structural markers at Thor.

Lamprophyre alteration shell in sediments. The rocks still retain their sedimentary banding, but have various levels of alteration in the beds.