I would suggest that you look carefully at the Zr content of your samples and as you suggest prioritise the Zr-rich samples. It is worth bearing in mind that baddeleyite (ZrO2) is quite common in gabbroic rocks - the distribution of zircon vs baddeleyite is a function of the silicate activity of the melt.

Select the samples with the highest amount of Zr. Crush them and conduct a heavy mineral analysis so as to concentrate Zr minerals and get rid of one of the major components feldspar that may cause some difficulties in finding the Zr silicate in question

It is usually an exhaustive task to separate Zr from the rock types such as gabbro, basalt. However, we can still go for separation of Zr from such samples. If your gabbro is a coarse-grained rock, you need a large amount of sample. Cautiously collect your H1 during density separation. After the sample is dried enough in the furnace, go for magnetic separation. Try to remove those very mafic minerals using a magnetic rode before feeding them into the magnetic separator, this will avoid from choking the machine due to high concentration of mafic minerals. After this stage, you can directly go for heavy liquid separation (go for Diodomethane which has a density of 3.3 g/ml). Now carefully collect those grains settle down during heavy liquid separation. Keep them to dry in a furnace. Now the time has come to hunt zircon crystals from those grains under a double microscope. I hope this will help in some or other ways.

I don't know any common method, but you need more than 20 kg of sample (fresh Gabbro not altered) for crushing and then other steps.

Whole rock chemistry is critical. Zircon (ZrSio2) will not form in a silica deficient (mafic) system, free silica will determine zircon formation no matter how much zirconium is present. If the samples contain relatively high concentrations of zirconium, in a silica deficient system, then there is the possibility of baddeleyite (ZrO) forming. It can also be dated!

So zirconium and silicon oxide are the components necessary for zircon formation.

I would prioritise samples with highest silica to lowest silica concentration first, if the silica concentration is similar, then prioritise samples by highest to lowest zirconium concentration.

If you do happen to find zircon in a gabbro, by just using a large sample, then you should be aware of the possibility that they are inherited from the source rock. If they are inherited they may exhibit resorption textures. Also, be aware of inherited cores in well crystallised grains.

Thanks to all responders. Now I have better idea of zircon separation from gabbro.

I suggest to look at the field relation first, looking for evolved, very coarse-grained to pegmatitic ponds within the gabbroic intrusion. This pegmatitic gabbro is the most promising for you to find zircon suitable for U-Pb geochron work.

Thanks Dr. Yang, this another useful answer.

It would be better if the coarse grained plagioclase ratio is higher.

sincerely,

Samples with the higher model proportion of plagioclase are more suitable for zircon separation.

Before separation, I recommend using SEM (backscattered electron detector) to see if zircon or badeleyite is present at all and what its dimensions are. I have a bad experience with the processing of zircon from coarse-grained analcimic gabroid rocks. They were very small (approximately 5 μm) and xenomorphic.