Learn me about granite(s)

I've stayed away from the granite in the past due to inexperience but am getting and interest and info in it.

I've started looking in the granites, and comparing to tell what each one is.

Can someone help as to how to analyze / describe a type of granite I find?

Its hard to identify feldspars, mica, hornblende, halite etc..how do I tell what the white grain in a granite is easier as there's soo many.

What exactly is porphyry granite as I've read about 3 different explanations.

It seems that dark pockets of rust or black are in the granite, should I be looking for vugh's or metamorphic intrusions..?

Here's two I brought home. On the top is fine granied and very solid / hard. A constant pattern of the same sized particles. On the bottom from the same area but this ones had acid wear and is breaking up to the touch.

Any interesting facts or pointers on granite(s) would be great!

AtomRat

I am not sure why you want to try to classify the different types of Granites??
I place more importance on knowing how the rocks are formed & the actions which lead them to be where they are!
Granites are termed "Igneous" meaning (laymans terms) they are formed deep below the surface then when they get forced up through to the surface they form an intrusion, Exploration company's look for circular feature (on landsat imagery etc) which indicate intrusions, these intrusions or outcrops where they come in contact with "Metamophic Rocks" eg Schist, form a "Contact zones" which can increase the likely hood of mineralisation/ deposits, veins can be fromed within the granite & it is this sort of thing one should seek out, along with fault lines & shear zones.
cheers
Lee

Was a bit of a mystery to me too the first time I came across the term Atomrat. It basically describes the formation of the granite is all, granites vary so much that the easiest way to classify them was by size then composition then age.
Spend a few hours one evening on google searching the different terms and how granites form, that will go a long way to helping you understand them.
Hard to tell from yout photo but it looks like youve got a granite with pyrites through it, the top dark green/grey one which im assuming is the hard one?
The bottom one looks to be mostly feldspar and quartz which is the soft crumbly one, that would be called a disseminated granite where the bonds between crystals have been broken down by age and weathering or chemicals.
Porphyric granites are irregular in the size of crystals present ie you would have lots of quartz with large crystals of feldspar and odd sized crystals of other minerals that were present when the granite formed.

Are you talking about the difference in granitic rocks or in different granite themselves?
Granite itself will have 65% feldspar (with potassium feldspar in excess of plagioclase) 25% quartz and 10% mafic (dark colored) minerals in its makeup. I think where granite start to differ from each other is in crystal size, the mafic mineral assemblage, if xenoliths are present or not, magnetic susepatability and accessory minerals.

Got to pick kids up from school but will pull some of my notes out when i get back, and see if i can help out a bit.

Im not too far from the familiar Woods Point ( though it is still a but of a drive ) where the granites there have intrusive dykes within the granite and I am trying to learn how it formed, what ages and and when. Not soo much for gold or gems prospecting, but basically learning granite in general.

My location puts me in the middle of at least 10 different granites ( or forms ) and if I can learn the granite, I can learn the gold more as well.

Much close where I got this granite from in my pic was Brittania Creek caves where hundreds of rounded granite boulders lay in a valley like a lava lamp has "blobbed" them out like a wax. As I was in the hill I discovered a filled in tunnel that goes straight into solid granite, frozen in time like lava flowing down the hill forming a crust. It interested someone at some point where im looking.

Powelltown has a granite thats constantly decomposing and consisting of a feldspar in massive form mixed with smokey quartz. Im looking for smokey veins within the granite in this area and also have found amethyst which is formed within the granite as well.

From what I know its not pyrite in the granite, possible common mistake. Supposibly its mica as the iron sulfate decomposes as soon as it hits oxygen... dunno, but yes its all through the granite, clays, sedimentary here.

Ok so porphyric holds one large crystal mineral like feldspar and then the rest of the matrix is smaller, just what im looking at in powelltown.

The "pockets" of concentrated mineral seems interesting in the granite and it leads me to think that crystals can be found within similar pockets.

Have not seen dykes running through the granite yet, only veins either clear quartz, smokey or a feldspar.

Theres a sedimentary rock all around the mountain, I dont know if the granite is through it like a vein or if its a cap ontop but I should have a look where the two meet and see what I can look at there.

Ooohh xenoliths, a new word to add to the brain engine! I THINK I have seen the nearby country sedimentary within the granite as balls or a bit of a vein section.

Tryed to answer all your text in one go guys, cheers for the help! Ill most likely keep this thread going as I take the journey. Will have some good photos!

On top if prospecting, I am also having fun sculpting the granite with rock chisels and trying to find a nice smooth textured granite to shape. ( If the hand tools last )

( Think ive written some terms wrong in this, give me a bit to get the hang of it )

pegmatites (pegs) are a good place to find pockets of nice crystal clusters sometimes.. google it up mate..

Thats another word defining what im looking for cheers Kawman! Sometimes ive just forgotten the correct term.

Ok, so in the upper hills in Powelltown and Big Pats Creek I have seen many signs of a pegmatite in the granite and soo large in crystal size that it starts to look like a smooshed vein.

This is more what got me onto the subject was finds like these.

I guess the most interesting one is the granite consisting of a large smokey quartz pegmatite ( if thats the correct way to say it ), also holds a large feldspar pegmatite as well.

Ill get photos of these up tomorrow as ill be heading into the hills then

Ok, if you are wanting to have a closer look at your granite, you should really have a 10X hand lens, a pocket knife and internet/book/notes to reference mineral properties. You then need a fresh clean surface. If you don't have one, crack your rock so you are not looking at a weathered surface, as weathering (especially chemical) will change the minerals makeup and make cleavage hard to see.
Telling the difference between minerals in the rock can be hard, as things like quartz and feldspar can be quiet similar starting out. The best visual way to tell the difference is that quartz has conchoidal or subconchoidal fractures (looks like how glass breaks) where as feldspar has two well developed cleavage planes (like shiney flat surfaces). Cleavage planes can be mistaken for crystal shape sometimes, so it is important to try to tell if you are looking at a broken surface or a crystal surface.
Another visual way is that feldspar will normally show twinning (plagoclase multiple twinning, orthoclse simple twinning), though this will not always be visible.
Also the quartz will be slightly harder than the feldspar, if you have a good steel pocket knife, it should just be able to scratch the feldspar, where as the quartz will be too hard.
The mafic minerals (dark minerals) would most likely be horneblende and biotite ( a mafic type of mica). The biotite you should be able to scratch with your finger, the hornblende should have perfect cleavage greater then 90 degrees.

So with the feldspar,quartz,hornblende and biotite you have you average granite. Accessory minerals will be dependent on what minerals were around when it formed and what happened to it after it formed. I picked up a nice granite sample on our last UNI outing that had some nice chalcopyrite and malachite showing, but this did not change the general makeup of the granite.

As for differing types of granite the closest i can really think of would be S-type and I-type granite's. Really just meaning if they came from igneous or sedimentry protolith (parent rocks). This type of classification could get tricky without knowing the surrounding geology...... There was another field trip we had where we were looking at an I-type granite, but it had sedimentary xenoliths, as it had intruded into sedimentary rock.

As for porphyry granite.... from my notes, A porphyry is an igneous texture in which there is a bimodal grain size (i.e 2 distinct sizes, usually in the same mineral)
The isolated large grains are called phenocrysts
This texture is produced through two distinct cooling episodes.
One slow cooling episode occurs below the surface, with the crystallization of one or more minerals, followed by a rapid cooling episode that may occur near or at the surface.
Yes the phenocryst in porphyritic granite would normally be the feldspar.

Granite normally occurs as large, irregular shaped masses called batholith and also stocks. Less commonly as dykes and sills.This will normally intrude sedimentary and metamorphic rocks.

I did have some more but accidently deleted some and forgot what i had said
hope that helps a bit 8)

Thanks again for the info. I yet again probably knew 50% of the info you gave but ive been giving myself an information overload and forgetting some as I go along.

A few tips I had not picked up on yet though. Absolute bummer on the accidental deletion, I hate that.

Ok, well before I say anymore, the lesson was very useful and resourceful mate and will put it all into play over the next 2 weeks. Ill go around and inspect, photograph and analyze the granites a bit and re-post here with an update.

I had already been making a clean break to study the rock with hand tools, meant to be using the star shape tip for drilling but dont have one so using a spike instead to drive several holes in a line around the rock. I know theres much easier ways but im enjoying the experience this way Had not properly used the hardness test yet, so thank you for those tips too and reminding me.

From what ive found, theres many forms of granite here ( everything thats been mentioned so far all over the area here ) and ive never had much of an interest until recently.

Also the relation between gold and granites interest me a lot as well.

if you find unusal black spots in grannite your near hornfels as for crystal deposits they may be a bit harder to find due to the host rock grannite being weathered away and the crystals and in or bellow the weathered zone of the grannite but usually you will find quartz or with kynite-whit pipe clay when your near a possiple crystal deposit or as they call them pegmatites or vugs

Yea i kind of guessed you would have already known, or read about most of what i said, but had to jostle the memory banks before i rummaged through my notes and needed somewhere to start. I am still learning all about this myself and am no expert but love talking about rocks.

This was the bit that got me interested in granite, when i split it i thought id struck gold at first, but then figured it was chalcopyrite (not really visible in this photo) as there was a lot of malachite as well. It was still something the lecturer was not expecting

Id be very happy with a malachite find ..though with what I know about smelting and copper, malachite to be in granite means that there was no carbon dioxide otherwise it would have formed copper? But wouldnt granite be full of CO2 or is it mostly Hydrogen within the process.. This doesnt need to be answered, just rambling in my head ..( damb..or was it just CO that makes copper...back to google I go lol )

G'day AtomRat

I too have bee trying to get my head around basic geology. With granite coming into play just recently.
Kawman posted a link in this section a little below this thread called (great set of talks about how gold and minerals are formed) ORE DEPOSITS 101 which is a great watch on Youtube. Its more comprehensive than just granite formations but it might help put the pieces together. Another great watch on Youtube is from a group called GEOLOGY FILMS, Victorian gold geology stuff and there's one short talk on the Morning Star Mine at woods point, which is part of the gold belt your looking looking at.
Ive been to Britannia Creek and have been looking into our Goldfields over this side of the city, especially through the YV as I'm only half an hour away from Britannia Creek. I'm in the Dandenong Ranges.

I hope this wasn't too off topic as its not specifically about granite. The Youtube 101 stuff is more about the relationship between granite and the concentration factors for the deposition of gold.

I'd been keen to come out for a few recon missions through the YV if your interested.

PM me if your keen.
Cheers, Lee.

Gday ATR

Most important are the contact zones where the Metamorphosed rock meets the intrusive igneous.

This is where the fluids will be trapped and the treasure you seek will cool and crystallize.

Meta

Cheers for the extra info guys, the more I get the more I start seeing it all too. Now I know what I'm sort of looking for.

I'll pm ya neogeo.

From what I can tell metamorphic that layer is 20m down, luckily its hydro sluiced and I can see the whole wall.

At the same time the sluicing has made it very hard to see what was where apart from a few seconds left

Bugger!!

Thats a long way down dude....not going to be much shed near surface in detector range.

If your plan is too dig and wash the pay, I hope you got a strong pick and bicep

I'm just learning mostly in this area or mountain as its geology is all over the place with many examples of all choas to study. Not so much looking for gold myself but looking at where the old timers got it and how they knew or traced it.

Does look pretty alluvial though, having issues uploading the vids so I have to wait for a new SD card in the mail.

Stupid phone spell check keeps changing my words.. In above post its meant to say sections, not seconds

Studying this area will explain many of the other hills and formations here. Just tracing history before it gets buried

In the area I prospect, diorite dykes are a important host for gold mineralization in quartz veins within and adjacent to the intrusive body. On the sandy creek goldfield , gold was found in ferrugnious seams within the granite dykes in the absence of quartz stringers.

AtomRat said:

I'm just learning mostly in this area or mountain as its geology is all over the place with many examples of all choas to study. Not so much looking for gold myself but looking at where the old timers got it and how they knew or traced it.

Does look pretty alluvial though, having issues uploading the vids so I have to wait for a new SD card in the mail.

Stupid phone spell check keeps changing my words.. In above post its meant to say sections, not seconds

Studying this area will explain many of the other hills and formations here. Just tracing history before it gets buried

Click to expand...

I love it mate....not enough bother to learn and study geological formations in their areas of choice to prospect.

It pleases me to see someone wanting to learn from the old timers and i feel sure it will greatly benefit you when your turn comes.

Keep at it ATR