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Focker

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05/02/18

13:01

Post #: 30824587
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Originally posted by Focker ↑

OVERVIEW REPORT SHOULD BE READ ON POLARX WEBSITE,BACKGROUND - DEPOSIT STYLES - A BRIEF OVERVIEW

The styles of mineralisation being explored for by PolarX are well understood, and are widespread globally, in addition they are major sources of the sought for metals.

　
Porphyry and Porphyry-Related Cu-Au-Mo
Geology

Porphyry copper deposits are the main source of copper globally, and are found in numerous regions, including North, Central and South America, SE Asia and Oceania and Mongolia amongst others.

They form at convergent plate margins - these include island arc settings (e.g. the Philippines, Indonesia) and continental margins (e.g. the current South American western continental margin and the historical North American plate margin in Alaska, Figure 4).

Copper production from the world’s largest producer, Chile, is largely from porphyry copper deposits, as is that from Peru and the US, both major global copper producers.

Molybdenum and gold are major by-products, again with Chile, the US and Peru being major producers.

Although the primary ore is generally low grade, with a mean copper grade of ~0.5%, and gold grades ranging from 0.05g/t to ~1g/t, this is more than made up by size, with many containing 100’s of millions to billions of tonnes of mineralisation.

The key copper minerals are chalcopyrite and bornite, with mineralisation generally being disseminated and in stockwork vein zones - in the continental margin porphyries (such as those in South America) mineralisation commonly forms a shell to a barren core or a cupola over the top of the high-level, sub-volcanic porphyry intrusions.

Notable examples include Pebble (Alaska, 10Bt), Chuquicamata (Chile) and Grasberg (Irian Jaya).

Magmatic arc porphyry deposits are related to a number of other mineralisation styles, including gold-copper skarns, epithermal gold and mesothermal base metal carbonate vein gold - the relationship between these mineralisation styles is shown conceptually in Figure 16.

Skarns, with an example being Zackly, are formed by the alteration of reactive rocks, such as limestone and marl with metal and volatile rich fluids emanating from an intrusive (Figure 16) - common metals include copper, gold, tungsten tin, lead, zinc and iron, with the metal types dependent on those enriched in the source intrusive.

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Skarns commonly exhibit metal zonation and numerous mineralising events - these include prograde and retrograde skarns, with the former due to alteration by the original hot fluids, and the latter due to alteration by cooling fluids during the waning of the hydrothermal system.

As discussed earlier, the Zackly skarn is interpreted as being at least partly related to a buried porphyry intrusive, with the possibility that this is overprinting an earlier skarn related to the older, outcropping diorite.

Major global skarn deposits include Ertsberg in Irian Jaya, paragenetically related to the nearby Grasberg porphyry copper deposit, and Ok Tedi, again with skarn mineralisation related to the adjacent porphyry mineralisation - skarn also forms within the porphyritic intrusives (endoskarn).

These major deposits show a clear relationship between skarns and porphyries - another example includes the Cadia complex in New South Wales, Australia.

Figure 16: Conceptual model - magmatic arc related mineralisation styles
Source: Corbett and Leach, various publications
Exploration

Porphyry copper deposits have distinctive geochemical and geophysical signatures, which are exhibited by the targets with the Alaska Range Project.

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Zoned geochemical signatures include a core of copper +- gold+- molybdenum, with this grading out to possibly lead or zinc on the peripheries.

Other pathfinder elements commonly include tin, bismuth arsenic and antimony, with these also showing zonation.

The two major geophysical tools are magnetics and IP (chargeability) - they are often (but not always) characterized by a bulls eye magnetic anomaly, due to the presence of a core of magnetite-rich potassic alteration, surrounded by an annular low, due to the magnetite destructive propylitic and phyllic zones - this will particularly show up where the porphyry has intruded into magnetic rocks, such as basalts and andesites.

One of the features of particularly the phyllic alteration zone is large quantities disseminated pyrite, which provides a perfect target for IP chargeability surveying.

Geologically, porphyries are associated with distinctive alteration styles in the wallrocks, which can be recognised in outcrop.

In skarns metal associations include proximal base metal, grading through precious metal to distal Pb-Zn-Ag veining - the metals present will depend upon the skarn style.

Typical geophysical tools include magnetics (skarns, due to the presence of magnetite, can be highly magnetic) and EM, due to the common presence of massive sulphides.

　
Intrusion Related Gold Systems
Geology

IRGS deposits span a gamut of mineralisation styles, however are all associated with post orogenic intrusives, largely situated along the continental side of a convergent plate margin, as is seen in the Tintina Belt in Alaska and British Columbia (Figures 17 and 18).

The mineralisation style was first postulated in 1999, with steady advances in the understanding subsequent to this.

Figure 17: Global distribution of IRGS belts
Source: Adapted Lang and Baker (2001)

Figure 19 shows the styles of mineralisation within the IGRS grouping - mineralisation forms over a wide depth range, from <1km to >8km, with one of the most common form of mineralisation being low sulphide sheeted veins within the host intrusion, with the intrusive event and mineralisation largely being contemporaneous.

These styles are also associated with other broad mineralisation groupings, including orogenic gold, with which IRGS systems are often confused.

Differentiating features are tectonic setting (post orogenic vs orogenic), location of mineralisation within and around the cupola of the host intrusions and geochemical signatures and zonation; in addition, IRGS mineralisation is generally related to smaller intrusive bodies including stocks - they are not related to batholiths as is commonly the case with orogenic gold deposits.

Geochemically the deposits lack significant copper, and also there is concentric mineral zoning

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Bought more at 12 today,just cant help myself
PXX Price at posting: 11.5¢ Sentiment: Buy Disclosure: Held
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0 Great analysis

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