Hello PIGs and others:

When I visited the UNM Archives to review their collection of documents on the Harding pegmatite, they gave me permission to publish – i.e., make public – the images that I selected (I still have that permission form). A few of those appeared in my book Pegmatites, but some of the more interesting ones were scanned at a resolution too low to reproduce in print, or else too faint for the same purpose. Here’s one of those. Jahns and Ewing (1977) presented a geologic fence diagram (their Figure 5) of zonation within the Harding pegmatite. That map was a simplified version of the more detailed fence diagram that appears in the attached PDF image. The published map shows zones in stippled patterns, which are similar enough that they are difficult to follow across the diagram. The attachment shows the original nodes where Jahns use core and underground exposures to create the fence diagram, and he shaded in the “bull quartz” zone.

For those who are unfamiliar with the internal zonation of pegmatites, they are granitic in composition, but much of the quartz forms central pure masses that have been regarded as the pegmatite core, or the last-formed zone in most of them. At Harding, the “bull quartz” zone lies just below the border and wall zone along the upper contact, and it was the substrate upon which the coarsely crystalline intermediate zone of spodumene + quartz nucleated. The attached photo (also from UNM archives) shows the contact between the massive quartz layer above and the spodumene zone below. At Harding, the massive quartz layer was an early formed zone, nowhere near the last core unit, which is a very different textural intergrowth of spodumene-albite-lepidolite-beryl-apatite-microlite that is nearly quartz-absent (see Figure 4-9c of London, 2008). Pegmatites, the Devil’s in the details.

David London
Pegmatite Interest Group: https://msaweb.org/pegmatites/

Hi all,
Thanks for sharing this very interesting example, David.
It would be fantastic if someone with the relevant skills (not me, sorry), perhaps as part of a student project, could create a colour 3D model of the Harding area fence diagram to make it easier to interpret.
After reading the description below, I think many of us would be keen to hear David's interpretation of the formation process of the massive quartz layer at Harding; we would be much obliged.
Many thanks,
Ben

From: MSA-Pegmatite Interest Group msa-pegmatite@minlists.org
Sent: 26 June 2025 3:47 PM
To: MSA-Pegmatite Interest Group msa-pegmatite@minlists.org; msa-talk@minlists.org msa-talk@minlists.org
Subject: [MSA-pegmatite] More about Harding mine, UNM Archives

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Hello PIGs and others:

When I visited the UNM Archives to review their collection of documents on the Harding pegmatite, they gave me permission to publish – i.e., make public – the images that I selected (I still have that permission form). A few of those appeared in my book Pegmatites, but some of the more interesting ones were scanned at a resolution too low to reproduce in print, or else too faint for the same purpose. Here’s one of those. Jahns and Ewing (1977) presented a geologic fence diagram (their Figure 5) of zonation within the Harding pegmatite. That map was a simplified version of the more detailed fence diagram that appears in the attached PDF image. The published map shows zones in stippled patterns, which are similar enough that they are difficult to follow across the diagram. The attachment shows the original nodes where Jahns use core and underground exposures to create the fence diagram, and he shaded in the “bull quartz” zone.

For those who are unfamiliar with the internal zonation of pegmatites, they are granitic in composition, but much of the quartz forms central pure masses that have been regarded as the pegmatite core, or the last-formed zone in most of them. At Harding, the “bull quartz” zone lies just below the border and wall zone along the upper contact, and it was the substrate upon which the coarsely crystalline intermediate zone of spodumene + quartz nucleated. The attached photo (also from UNM archives) shows the contact between the massive quartz layer above and the spodumene zone below. At Harding, the massive quartz layer was an early formed zone, nowhere near the last core unit, which is a very different textural intergrowth of spodumene-albite-lepidolite-beryl-apatite-microlite that is nearly quartz-absent (see Figure 4-9c of London, 2008). Pegmatites, the Devil’s in the details.

David London
Pegmatite Interest Group: https://msaweb.org/pegmatites/