Why do igneous rocks have different minerals?
Looking at an igneous rock, such as the granite along Stockton Hill Road at milepost 10, you’ll see a variety of minerals. Some are black, others pink and some look glassy. If the rock was formed by the cooling of a molten mass, why are there different minerals and not just a rock consisting of one mineral? There should be a reason for different minerals forming from the same molten mass.
Think of minerals as the Lego blocks of nature, building different rock types. The minerals in an igneous rock are formed by the cooling of a molten mass. Which minerals are formed depends on the chemistry of that melt. As a molten mass starts to solidify, minerals begin to grow. However, the chemistry changes as one type of mineral forms. The first minerals formed remove certain elements from the melt. The mechanism by which this happens is complicated. As the mass cools, two phases form - a molten phase (molten rock) and a solid phase (crystals). Each time minerals form, the chemistry changes, so the final rock has a variety of minerals and not just one type of mineral. Observations of existing rocks and tests where rocks were melted and allowed to solidify identified a particular sequence of formation. This sequence of formation of silicate minerals from a molten mass is called Bowen’s Reaction Series.
Ok, no doubt this is of scientific interest but what good does that do me? I am trying to find gold. Just so happens, that as a molten mass cools and the chemistry changes through Bowen’s Reaction Series, the last bit of molten material is quartz. Gold and quartz commonly occur together so the gold that was contained in the molten mass must have concentrated in the last remaining part. Geologists call this last remaining part “hydrothermal fluids”. And, it’s the hydrothermal fluids that transport the metals such as gold, copper, lead, zinc, etc.
Because the first formed minerals have a chemical composition based on the chemistry of the molten mass at the time they were formed, they are unstable with the remaining molten mass. Some changes occur by the interaction between the crystals and molten mass, but the greatest changes occur with the last remaining liquid phase because this phase has the greatest difference with the first minerals formed. This process is called alteration. If it’s caused by hydrothermal fluids, it’s called “hydrothermal alteration”.
Some of the common types of alteration related to mineral deposits are biotite and hornblende altering to chlorite (propylitic alteration); feldspar altering to sericite (phyllic alteration); and, when feldspar is altered to clay, (argillic alteration).
Hydrothermal alteration is one of the guides explorationists use to look for mineral deposits. That’s why they use a hand lens to better see the mineral grains and detect if they are altered. But mother nature doesn’t play fair. Alteration can also be caused by factors unrelated to mineralization including weathering of the minerals and the oxidation of sulfides. The crumbly and clay-rich nature of the granite found at Hualapai Mountain park is due to weathering of a very old granite - decomposing granite - and not to hydrothermal fluids.
The acid released by the oxidation of sulfides alters the rocks giving them a bleached-white look stained by iron oxide. If you look at the Mineral Park area in the Cerbat Mountains from Highway 93, you’ll see a “blondish” color of the rocks to the north and south of Mineral Park. This is a color anomaly caused by the alteration of the rocks by hydrothermal fluids and by the oxidation of sulfides staining them orangish-brown. Beyond this alteration halo are dull green rocks with no mineral potential.
To further complicate matters, each type of mineral deposit has its own alteration signature. Porphyry copper deposits have “zoned alteration types”, with a center of potassic going out to phyllic, argillic and finally propylitic. Low-sulfidation vein deposits have a center of quartz-calcite followed by sericite and illite. It takes careful study of the rocks to decipher if the alteration is of interest or just a natural process unrelated to a mineral deposit. Kind of like trying to put together a jigsaw puzzle under a blanket but I guess that’s why geologists make the big bucks.
An explorationist should pay special attention to the sequence of minerals formed from a molten mass. If the mineral sequence and alteration of those minerals indicate that the molten mass that solidified could result in a mineral deposit being formed, then stake claims and order the drills. If not, walk away and don’t break your pick (geologist speak for going broke) on a bad prospect.
Granite with black biotite minerals having well-formed boundaries, feldspar minerals with somewhat well-formed boundaries and quartz filling in the remaining spaces- Bowen’s Reaction Series in action.
