Last week there were a few comments that the Quiz on Structural Geology was not difficult enough – this week we have made it more of a challenge! Your challenge is to beat the GPTAPI that got 7/10 correct. Answers at the end.

Siem Reap has 98km of new roads  and new sidewalks much of which has been paved with interesting felsic intrusive rocks from quarries in Shandong Province in China.  The composition ranges from granodiorite to tonalite and is locally granophyric and pegmatitic.  The intrusive consists dominantly of plagioclase, quartz, pyroxene and hornblende. Ovoid structures known as Miarolitic Cavities are evident in the sidewalks to the observant and indicates that the parental magma was hydrous.

Here we have attempted to reassemble a miarolitic cavity over ~1 metre of its length. Just bear in mind that this is dimension stone and its now the pavement. There are many places where there are a large number of slabs that are clearly related. Reassembling them is quite instructive and reveals much about the evolution of these structures.

We have previously published libraries of miarolitic cavities.

The purpose of this post is to delineate the key characteristics of the Mt Isa deposit from an explorer’s pragmatic perspective and then discuss a possible analogue in the Eastern Succession, north of Cloncurry. Beside the fascinating tectonic, geological and structural history of the Mt Isa region,  it represents the largest repository of Pb-Zn metal.

This post is a review and summary of an excellent paper: Bradley Cave, Richard Lilly, Alexander Simpson, Lucy McGee, A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes, Ore Geology Reviews 154 (2023) 105311. https://doi.org/10.1016/j.oregeorev.2023.105311

SUMMARY

1. The George Fisher and Hilton Zn-Pb-Ag deposits are located approximately 20 km north of Mount Isa.
2. A dolerite dyke has been discovered at the George Fisher Zn-Pb-Ag deposit and the dolerite dyke have a large spike in TiO2 and V values.
3. In drill core, the dolerite dykes occur as a light grey to grey-brown coloured rock that is commonly overprinted along its margins by Zn-Pb-Ag mineralisation.
4. In-situ U-Pb geochronology performed on igneous apatite produce a lower intercept age of 1611 +/- 21 Ma and 1619 +/- 22 Ma for the dolerite dykes at the George Fisher and Hilton deposits, respectively.
5. The dolerite dykes have experienced multiple stages of post-emplacement hydrothermal alteration/veining.
6. Monazite from a quartz-albite-K-feldspar vein in the Hilton dyke produces a lower intercept age of 1513 +/- 16 Ma.
7. To assess the timing of alteration in the adjacent George Fisher Zn-Pb-Ag deposit, in-situ Lu-Hf geochronology was performed on pre-mineralisation calcite from a section of stratabound Zn-Pb-Ag mineralisation, and a paragenetically late cross-cutting sphalerite-calcite vein.
8. Calcite from the pre-mineralization alteration assemblage produces a Lu-Hf age of 1501 +/- 32 Ma.
9. Calcite from a cross-cutting vein that post-dates Zn-Pb-Ag deposition produced a Lu-Hf age of 1289 +/- 26 Ma. The 1289 Ma age is associated with late faulting and movement along the adjacent Mount Isa Fault (B. Cave pers comms 2023).
10. The paragenetic equivalents of the hydrothermal alteration/veining in the dolerite dykes are found in the adjacent Zn-Pb-Ag orebodies.
11. The maximum age of alteration within the dolerite dykes is constrained by the monazite age of 1513 +/- 16 Ma, and the maximum age of stratabound Zn-Pb-Ag mineralisation is constrained by the Lu-Hf age of 1501 +/- 32 Ma.
12. The dolerite dykes intruded during the early Isan Orogeny at ca. 1620 Ma, and experienced subsequent hydrothermal alteration during D3 of the Isan Orogeny coeval with Zn-Pb-Ag mineralization. Post mineralization faulting occurred during D2 of the Isan Orogeny, at ca. 1290 Ma.

This post is a summary of the interesting paper on the geodynamic relationships  between Paleozoic arc development along the flanks of the interior (e.g. the Iapetus and Rheic) oceans and the exterior Paleopacific Ocean. Murphy, B., van Staal, C and Collins, W, A comparison of the evolution of arc complexes in Paleozoic interior and peripheral orogens: Speculations on geodynamic correlations, Gondwana Research, 2011. doi:10.1016/j.gr.2010.11.019.